NMAP(1) Nmap Reference Guide NMAP(1)

NAME nmap - Network exploration tool and security / port scanner

SYNOPSIS nmap [Scan Type…] [Options] {target specification}

DESCRIPTION Nmap (“Network Mapper”) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many systems and network administrators find it useful for routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.

   The output from Nmap is a list of scanned targets, with supplemental
   information on each depending on the options used. Key among that
   information is the “interesting ports table”. That table lists the port
   number and protocol, service name, and state. The state is either open,
   filtered, closed, or unfiltered. Open means that an application on the
   target machine is listening for connections/packets on that port.
   Filtered means that a firewall, filter, or other network obstacle is
   blocking the port so that Nmap cannot tell whether it is open or
   closed.	Closed ports have no application listening on them, though
   they could open up at any time. Ports are classified as unfiltered when
   they are responsive to Nmap’s probes, but Nmap cannot determine whether
   they are open or closed. Nmap reports the state combinations
   open|filtered and closed|filtered when it cannot determine which of the
   two states describe a port. The port table may also include software
   version details when version detection has been requested. When an IP
   protocol scan is requested (-sO), Nmap provides information on
   supported IP protocols rather than listening ports.

   In addition to the interesting ports table, Nmap can provide further
   information on targets, including reverse DNS names, operating system
   guesses, device types, and MAC addresses.

   A typical Nmap scan is shown in Example 14.1, “A representative Nmap
   scan”. The only Nmap arguments used in this example are -A, to enable
   OS and version detection, -T4 for faster execution, and then the two
   target hostnames.  Example 14.1. A representative Nmap scan.sp
   # nmap -A -T4 scanme.nmap.org playground

   Starting nmap ( http://insecure.org/nmap/ )
   Interesting ports on scanme.nmap.org (205.217.153.62):
   (The 1663 ports scanned but not shown below are in state: filtered)
   PORT    STATE  SERVICE VERSION
   22/tcp  open   ssh     OpenSSH 3.9p1 (protocol 1.99)
   53/tcp  open   domain
   70/tcp  closed gopher
   80/tcp  open   http    Apache httpd 2.0.52 ((Fedora))
   113/tcp closed auth
   Device type: general purpose
   Running: Linux 2.4.X|2.5.X|2.6.X
   OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
   Uptime 33.908 days (since Thu Jul 21 03:38:03 2005)

   Interesting ports on playground.nmap.org (192.168.0.40):
   (The 1659 ports scanned but not shown below are in state: closed)
   PORT	STATE SERVICE	    VERSION
   135/tcp	open  msrpc	    Microsoft Windows RPC
   139/tcp	open  netbios-ssn
   389/tcp	open  ldap?
   445/tcp	open  microsoft-ds  Microsoft Windows XP microsoft-ds
   1002/tcp open  windows-icfw?
   1025/tcp open  msrpc	    Microsoft Windows RPC
   1720/tcp open  H.323/Q.931   CompTek AquaGateKeeper
   5800/tcp open  vnc-http	    RealVNC 4.0 (Resolution 400x250; VNC TCP port: 5900)
   5900/tcp open  vnc	    VNC (protocol 3.8)
   MAC Address: 00:A0:CC:63:85:4B (Lite-on Communications)
   Device type: general purpose
   Running: Microsoft Windows NT/2K/XP
   OS details: Microsoft Windows XP Pro RC1+ through final release
   Service Info: OSs: Windows, Windows XP

   Nmap finished: 2 IP addresses (2 hosts up) scanned in 88.392 seconds

   The newest version of Nmap can be obtained from
   http://insecure.org/nmap/. The newest version of the man page is
   available from http://insecure.org/nmap/man/.

OPTIONS SUMMARY This options summary is printed when Nmap is run with no arguments, and the latest version is always available at http://insecure.org/nmap/data/nmap.usage.txt. It helps people remember the most common options, but is no substitute for the in-depth documentation in the rest of this manual. Some obscure options aren’t even included here.

   Nmap 4.20RC1 ( http://insecure.org )
   Usage: nmap [Scan Type(s)] [Options] {target specification}
   TARGET SPECIFICATION:
 Can pass hostnames, IP addresses, networks, etc.
 Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
 -iL <inputfilename>: Input from list of hosts/networks
 -iR <num hosts>: Choose random targets
 --exclude <host1[,host2][,host3],...>: Exclude hosts/networks
 --excludefile <exclude_file>: Exclude list from file
   HOST DISCOVERY:
 -sL: List Scan - simply list targets to scan
 -sP: Ping Scan - go no further than determining if host is online
 -P0: Treat all hosts as online -- skip host discovery
 -PS/PA/PU [portlist]: TCP SYN/ACK or UDP discovery to given ports
 -PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
 -n/-R: Never do DNS resolution/Always resolve [default: sometimes]
 --dns-servers <serv1[,serv2],...>: Specify custom DNS servers
 --system-dns: Use OS’s DNS resolver
   SCAN TECHNIQUES:
 -sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
 -sU: UDP Scan
 -sN/sF/sX: TCP Null, FIN, and Xmas scans
 --scanflags <flags>: Customize TCP scan flags
 -sI <zombie host[:probeport]>: Idlescan
 -sO: IP protocol scan
 -b <ftp relay host>: FTP bounce scan
   PORT SPECIFICATION AND SCAN ORDER:
 -p <port ranges>: Only scan specified ports
   Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
 -F: Fast - Scan only the ports listed in the nmap-services file)
 -r: Scan ports consecutively - don’t randomize
   SERVICE/VERSION DETECTION:
 -sV: Probe open ports to determine service/version info
 --version-intensity <level>: Set from 0 (light) to 9 (try all probes)
 --version-light: Limit to most likely probes (intensity 2)
 --version-all: Try every single probe (intensity 9)
 --version-trace: Show detailed version scan activity (for debugging)
   OS DETECTION:
 -O: Enable OS detection (try 2nd generation w/fallback to 1st)
 -O2: Only use the new OS detection system (no fallback)
 -O1: Only use the old (1st generation) OS detection system
 --osscan-limit: Limit OS detection to promising targets
 --osscan-guess: Guess OS more aggressively
   TIMING AND PERFORMANCE:
 Options which take <time> are in milliseconds, unless you append ’s’
 (seconds), ’m’ (minutes), or ’h’ (hours) to the value (e.g. 30m).
 -T[0-5]: Set timing template (higher is faster)
 --min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
 --min-parallelism/max-parallelism <time>: Probe parallelization
 --min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
     probe round trip time.
 --max-retries <tries>: Caps number of port scan probe retransmissions.
 --host-timeout <time>: Give up on target after this long
 --scan-delay/--max-scan-delay <time>: Adjust delay between probes
   FIREWALL/IDS EVASION AND SPOOFING:
 -f; --mtu <val>: fragment packets (optionally w/given MTU)
 -D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
 -S <IP_Address>: Spoof source address
 -e <iface>: Use specified interface
 -g/--source-port <portnum>: Use given port number
 --data-length <num>: Append random data to sent packets
 --ip-options <options>: Send packets with specified ip options
 --ttl <val>: Set IP time-to-live field
 --spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
 --badsum: Send packets with a bogus TCP/UDP checksum
   OUTPUT:
 -oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
    and Grepable format, respectively, to the given filename.
 -oA <basename>: Output in the three major formats at once
 -v: Increase verbosity level (use twice for more effect)
 -d[level]: Set or increase debugging level (Up to 9 is meaningful)
 --open: Only show open (or possibly open) ports
 --packet-trace: Show all packets sent and received
 --iflist: Print host interfaces and routes (for debugging)
 --log-errors: Log errors/warnings to the normal-format output file
 --append-output: Append to rather than clobber specified output files
 --resume <filename>: Resume an aborted scan
 --stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
 --webxml: Reference stylesheet from Insecure.Org for more portable XML
 --no-stylesheet: Prevent associating of XSL stylesheet w/XML output
   MISC:
 -6: Enable IPv6 scanning
 -A: Enables OS detection and Version detection
 --datadir <dirname>: Specify custom Nmap data file location
 --send-eth/--send-ip: Send using raw ethernet frames or IP packets
 --privileged: Assume that the user is fully privileged
 --unprivileged: Assume the user lacks raw socket privileges
 -V: Print version number
 -h: Print this help summary page.
   EXAMPLES:
 nmap -v -A scanme.nmap.org
 nmap -v -sP 192.168.0.0/16 10.0.0.0/8
 nmap -v -iR 10000 -P0 -p 80

TARGET SPECIFICATION Everything on the Nmap command-line that isn’t an option (or option argument) is treated as a target host specification. The simplest case is to specify a target IP address or hostname for scanning.

   Sometimes you wish to scan a whole network of adjacent hosts. For this,
   Nmap supports CIDR-style addressing. You can append

   /numbits to an IP address or hostname and Nmap will scan every IP
   address for which the first numbits are the same as for the reference
   IP or hostname given. For example, 192.168.10.0/24 would scan the 256
   hosts between 192.168.10.0 (binary: 11000000 10101000 00001010
   00000000) and 192.168.10.255 (binary: 11000000 10101000 00001010
   11111111), inclusive. 192.168.10.40/24 would do exactly the same thing.
   Given that the host scanme.nmap.org is at the IP address
   205.217.153.62, the specification scanme.nmap.org/16 would scan the
   65,536 IP addresses between 205.217.0.0 and 205.217.255.255. The
   smallest allowed value is /1, which scans half the Internet. The
   largest value is 32, which scans just the named host or IP address
   because all address bits are fixed.

   CIDR notation is short but not always flexible enough. For example, you
   might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
   .255 because they are commonly broadcast addresses. Nmap supports this
   through octet range addressing. Rather than specify a normal IP
   address, you can specify a comma separated list of numbers or ranges
   for each octet. For example, 192.168.0-255.1-254 will skip all
   addresses in the range that end in .0 and or .255. Ranges need not be
   limited to the final octets: the specifier 0-255.0-255.13.37 will
   perform an Internet-wide scan for all IP addresses ending in 13.37.
   This sort of broad sampling can be useful for Internet surveys and
   research.

   IPv6 addresses can only be specified by their fully qualified IPv6
   address or hostname. CIDR and octet ranges aren’t supported for IPv6
   because they are rarely useful.

   Nmap accepts multiple host specifications on the command line, and they
   don’t need to be the same type. The command nmap scanme.nmap.org
   192.168.0.0/16 10.0.0,1,3-7.0-255 does what you would expect.

   While targets are usually specified on the command lines, the following
   options are also available to control target selection:

   -iL <inputfilename> (Input from list)
      Reads target specifications from inputfilename. Passing a huge
      list of hosts is often awkward on the command line, yet it is a
      common desire. For example, your DHCP server might export a list
      of 10,000 current leases that you wish to scan. Or maybe you
      want to scan all IP addresses except for those to locate hosts
      using unauthorized static IP addresses. Simply generate the list
      of hosts to scan and pass that filename to Nmap as an argument
      to the -iL option. Entries can be in any of the formats accepted
      by Nmap on the command line (IP address, hostname, CIDR, IPv6,
      or octet ranges). Each entry must be separated by one or more
      spaces, tabs, or newlines. You can specify a hyphen (-) as the
      filename if you want Nmap to read hosts from standard input
      rather than an actual file.

   -iR <num hosts> (Choose random targets)
      For Internet-wide surveys and other research, you may want to
      choose targets at random. The num hosts argument tells Nmap how
      many IPs to generate. Undesirable IPs such as those in certain
      private, multicast, or unallocated address ranges are
      automatically skipped. The argument 0 can be specified for a
      never-ending scan. Keep in mind that some network administrators
      bristle at unauthorized scans of their networks and may
      complain. Use this option at your own risk! If you find yourself
      really bored one rainy afternoon, try the command nmap -sS -PS80
      -iR 0 -p 80 to locate random web servers for browsing.

   --exclude <host1[,host2][,host3],...> (Exclude hosts/networks)
      Specifies a comma-separated list of targets to be excluded from
      the scan even if they are part of the overall network range you
      specify. The list you pass in uses normal Nmap syntax, so it can
      include hostnames, CIDR netblocks, octet ranges, etc. This can
      be useful when the network you wish to scan includes untouchable
      mission-critical servers, systems that are known to react
      adversely to port scans, or subnetworks administered by other
      people.

   --excludefile <exclude_file> (Exclude list from file)
      This offers the same functionality as the --exclude option,
      except that the excluded targets are provided in a newline,
      space, or tab delimited exclude_file rather than on the command
      line.

HOST DISCOVERY One of the very first steps in any network reconnaissance mission is to reduce a (sometimes huge) set of IP ranges into a list of active or interesting hosts. Scanning every port of every single IP address is slow and usually unnecessary. Of course what makes a host interesting depends greatly on the scan purposes. Network administrators may only be interested in hosts running a certain service, while security auditors may care about every single device with an IP address. An administrator may be comfortable using just an ICMP ping to locate hosts on his internal network, while an external penetration tester may use a diverse set of dozens of probes in an attempt to evade firewall restrictions.

   Because host discovery needs are so diverse, Nmap offers a wide variety
   of options for customizing the techniques used. Host discovery is
   sometimes called ping scan, but it goes well beyond the simple ICMP
   echo request packets associated with the ubiquitous ping tool. Users
   can skip the ping step entirely with a list scan (-sL) or by disabling
   ping (-P0), or engage the network with arbitrary combinations of
   multi-port TCP SYN/ACK, UDP, and ICMP probes. The goal of these probes
   is to solicit responses which demonstrate that an IP address is
   actually active (is being used by a host or network device). On many
   networks, only a small percentage of IP addresses are active at any
   given time. This is particularly common with RFC1918-blessed private
   address space such as 10.0.0.0/8. That network has 16 million IPs, but
   I have seen it used by companies with less than a thousand machines.
   Host discovery can find those machines in a sparsely allocated sea of
   IP addresses.

   If no host discovery options are given, Nmap sends a TCP ACK packet
   destined for port 80 and an ICMP Echo Request query to each target
   machine. An exception to this is that an ARP scan is used for any
   targets which are on a local ethernet network. For unprivileged UNIX
   shell users, a SYN packet is sent instead of the ack using the
   connect() system call. These defaults are equivalent to the -PA -PE
   options. This host discovery is often sufficient when scanning local
   networks, but a more comprehensive set of discovery probes is
   recommended for security auditing.

   The -P* options (which select ping types) can be combined. You can
   increase your odds of penetrating strict firewalls by sending many
   probe types using different TCP ports/flags and ICMP codes. Also note
   that ARP discovery (-PR) is done by default against targets on a local
   ethernet network even if you specify other -P* options, because it is
   almost always faster and more effective.

   By default, Nmap does host discovery and then performs a port scan
   against each host it determines is online. This is true even if you
   specify non-default host discovery types such as UDP probes (-PU). Read
   about the -sP option to learn how to perform only host discovery, or
   use -P0 to skip host discovery and port scan all target hosts. The
   following options control host discovery:

   -sL (List Scan)
      The list scan is a degenerate form of host discovery that simply
      lists each host of the network(s) specified, without sending any
      packets to the target hosts. By default, Nmap still does
      reverse-DNS resolution on the hosts to learn their names. It is
      often surprising how much useful information simple hostnames
      give out. For example, fw.chi is the name of one company’s
      Chicago firewall. Nmap also reports the total number of IP
      addresses at the end. The list scan is a good sanity check to
      ensure that you have proper IP addresses for your targets. If
      the hosts sport domain names you do not recognize, it is worth
      investigating further to prevent scanning the wrong company’s
      network.

      Since the idea is to simply print a list of target hosts,
      options for higher level functionality such as port scanning, OS
      detection, or ping scanning cannot be combined with this. If you
      wish to disable ping scanning while still performing such higher
      level functionality, read up on the -P0 option.

   -sP (Ping Scan)
      This option tells Nmap to only perform a ping scan (host
      discovery), then print out the available hosts that responded to
      the scan. No further testing (such as port scanning or OS
      detection) is performed. This is one step more intrusive than
      the list scan, and can often be used for the same purposes. It
      allows light reconnaissance of a target network without
      attracting much attention. Knowing how many hosts are up is more
      valuable to attackers than the list provided by list scan of
      every single IP and host name.

      Systems administrators often find this option valuable as well.
      It can easily be used to count available machines on a network
      or monitor server availability. This is often called a ping
      sweep, and is more reliable than pinging the broadcast address
      because many hosts do not reply to broadcast queries.

      The -sP option sends an ICMP echo request and a TCP packet to
      port 80 by default. When executed by an unprivileged user, a SYN
      packet is sent (using a connect() call) to port 80 on the
      target. When a privileged user tries to scan targets on a local
      ethernet network, ARP requests (-PR) are used unless --send-ip
      was specified. The -sP option can be combined with any of the
      discovery probe types (the -P* options, excluding -P0) for
      greater flexibility. If any of those probe type and port number
      options are used, the default probes (ACK and echo request) are
      overridden. When strict firewalls are in place between the
      source host running Nmap and the target network, using those
      advanced techniques is recommended. Otherwise hosts could be
      missed when the firewall drops probes or their responses.

   -P0 (No ping)
      This option skips the Nmap discovery stage altogether. Normally,
      Nmap uses this stage to determine active machines for heavier
      scanning. By default, Nmap only performs heavy probing such as
      port scans, version detection, or OS detection against hosts
      that are found to be up. Disabling host discovery with -P0
      causes Nmap to attempt the requested scanning functions against
      every target IP address specified. So if a class B sized target
      address space (/16) is specified on the command line, all 65,536
      IP addresses are scanned. That second option character in -P0 is
      a zero and not the letter O. Proper host discovery is skipped as
      with the list scan, but instead of stopping and printing the
      target list, Nmap continues to perform requested functions as if
      each target IP is active.

   -PS [portlist] (TCP SYN Ping)
      This option sends an empty TCP packet with the SYN flag set. The
      default destination port is 80 (configurable at compile time by
      changing DEFAULT_TCP_PROBE_PORT in nmap.h), but an alternate
      port can be specified as a parameter. A comma separated list of
      ports can even be specified (e.g.
      -PS22,23,25,80,113,1050,35000), in which case probes will be
      attempted against each port in parallel.

      The SYN flag suggests to the remote system that you are
      attempting to establish a connection. Normally the destination
      port will be closed, and a RST (reset) packet sent back. If the
      port happens to be open, the target will take the second step of
      a TCP 3-way-handshake by responding with a SYN/ACK TCP packet.
      The machine running Nmap then tears down the nascent connection
      by responding with a RST rather than sending an ACK packet which
      would complete the 3-way-handshake and establish a full
      connection. The RST packet is sent by the kernel of the machine
      running Nmap in response to the unexpected SYN/ACK, not by Nmap
      itself.

      Nmap does not care whether the port is open or closed. Either
      the RST or SYN/ACK response discussed previously tell Nmap that
      the host is available and responsive.

      On UNIX boxes, only the privileged user root is generally able
      to send and receive raw TCP packets. For unprivileged users, a
      workaround is automatically employed whereby the connect()
      system call is initiated against each target port. This has the
      effect of sending a SYN packet to the target host, in an attempt
      to establish a connection. If connect() returns with a quick
      success or an ECONNREFUSED failure, the underlying TCP stack
      must have received a SYN/ACK or RST and the host is marked
      available. If the connection attempt is left hanging until a
      timeout is reached, the host is marked as down. This workaround
      is also used for IPv6 connections, as raw IPv6 packet building
      support is not yet available in Nmap.

   -PA [portlist] (TCP ACK Ping)
      The TCP ACK ping is quite similar to the just-discussed SYN
      ping. The difference, as you could likely guess, is that the TCP
      ACK flag is set instead of the SYN flag. Such an ACK packet
      purports to be acknowledging data over an established TCP
      connection, but no such connection exists. So remote hosts
      should always respond with a RST packet, disclosing their
      existence in the process.

      The -PA option uses the same default port as the SYN probe (80)
      and can also take a list of destination ports in the same
      format. If an unprivileged user tries this, or an IPv6 target is
      specified, the connect() workaround discussed previously is
      used. This workaround is imperfect because connect() is actually
      sending a SYN packet rather than an ACK.

      The reason for offering both SYN and ACK ping probes is to
      maximize the chances of bypassing firewalls. Many administrators
      configure routers and other simple firewalls to block incoming
      SYN packets except for those destined for public services like
      the company web site or mail server. This prevents other
      incoming connections to the organization, while allowing users
      to make unobstructed outgoing connections to the Internet. This
      non-stateful approach takes up few resources on the
      firewall/router and is widely supported by hardware and software
      filters. The Linux Netfilter/iptables firewall software offers
      the --syn convenience option to implement this stateless
      approach. When stateless firewall rules such as this are in
      place, SYN ping probes (-PS) are likely to be blocked when sent
      to closed target ports. In such cases, the ACK probe shines as
      it cuts right through these rules.

      Another common type of firewall uses stateful rules that drop
      unexpected packets. This feature was initially found mostly on
      high-end firewalls, though it has become much more common over
      the years. The Linux Netfilter/iptables system supports this
      through the --state option, which categorizes packets based on
      connection state. A SYN probe is more likely to work against
      such a system, as unexpected ACK packets are generally
      recognized as bogus and dropped. A solution to this quandary is
      to send both SYN and ACK probes by specifying -PS and -PA.

   -PU [portlist] (UDP Ping)
      Another host discovery option is the UDP ping, which sends an
      empty (unless --data-length is specified) UDP packet to the
      given ports. The portlist takes the same format as with the
      previously discussed -PS and -PA options. If no ports are
      specified, the default is 31338. This default can be configured
      at compile-time by changing DEFAULT_UDP_PROBE_PORT in nmap.h. A
      highly uncommon port is used by default because sending to open
      ports is often undesirable for this particular scan type.

      Upon hitting a closed port on the target machine, the UDP probe
      should elicit an ICMP port unreachable packet in return. This
      signifies to Nmap that the machine is up and available. Many
      other types of ICMP errors, such as host/network unreachables or
      TTL exceeded are indicative of a down or unreachable host. A
      lack of response is also interpreted this way. If an open port
      is reached, most services simply ignore the empty packet and
      fail to return any response. This is why the default probe port
      is 31338, which is highly unlikely to be in use. A few services,
      such as chargen, will respond to an empty UDP packet, and thus
      disclose to Nmap that the machine is available.

      The primary advantage of this scan type is that it bypasses
      firewalls and filters that only screen TCP. For example, I once
      owned a Linksys BEFW11S4 wireless broadband router. The external
      interface of this device filtered all TCP ports by default, but
      UDP probes would still elicit port unreachable messages and thus
      give away the device.

   -PE; -PP; -PM (ICMP Ping Types)
      In addition to the unusual TCP and UDP host discovery types
      discussed previously, Nmap can send the standard packets sent by
      the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
      request) packet to the target IP addresses, expecting a type 0
      (Echo Reply) in return from available hosts. Unfortunately for
      network explorers, many hosts and firewalls now block these
      packets, rather than responding as required by [1]RFC 1122. For
      this reason, ICMP-only scans are rarely reliable enough against
      unknown targets over the Internet. But for system administrators
      monitoring an internal network, they can be a practical and
      efficient approach. Use the -PE option to enable this echo
      request behavior.

      While echo request is the standard ICMP ping query, Nmap does
      not stop there. The ICMP standard ([2]RFC 792) also specifies
      timestamp request, information request, and address mask request
      packets as codes 13, 15, and 17, respectively. While the
      ostensible purpose for these queries is to learn information
      such as address masks and current times, they can easily be used
      for host discovery. A system that replies is up and available.
      Nmap does not currently implement information request packets,
      as they are not widely supported. RFC 1122 insists that “a host
      SHOULD NOT implement these messages”. Timestamp and address mask
      queries can be sent with the -PP and -PM options, respectively.
      A timestamp reply (ICMP code 14) or address mask reply (code 18)
      discloses that the host is available. These two queries can be
      valuable when admins specifically block echo request packets
      while forgetting that other ICMP queries can be used for the
      same purpose.

   -PR (ARP Ping)
      One of the most common Nmap usage scenarios is to scan an
      ethernet LAN. On most LANs, especially those using
      RFC1918-blessed private address ranges, the vast majority of IP
      addresses are unused at any given time. When Nmap tries to send
      a raw IP packet such as an ICMP echo request, the operating
      system must determine the destination hardware (ARP) address
      corresponding to the target IP so that it can properly address
      the ethernet frame. This is often slow and problematic, since
      operating systems weren’t written with the expectation that they
      would need to do millions of ARP requests against unavailable
      hosts in a short time period.

      ARP scan puts Nmap and its optimized algorithms in charge of ARP
      requests. And if it gets a response back, Nmap doesn’t even need
      to worry about the IP-based ping packets since it already knows
      the host is up. This makes ARP scan much faster and more
      reliable than IP-based scans. So it is done by default when
      scanning ethernet hosts that Nmap detects are on a local
      ethernet network. Even if different ping types (such as -PE or
      -PS) are specified, Nmap uses ARP instead for any of the targets
      which are on the same LAN. If you absolutely don’t want to do an
      ARP scan, specify --send-ip.

   -n (No DNS resolution)
      Tells Nmap to never do reverse DNS resolution on the active IP
      addresses it finds. Since DNS can be slow even with Nmap’s
      built-in parallel stub resolver, this option can slash scanning
      times.

   -R (DNS resolution for all targets)
      Tells Nmap to always do reverse DNS resolution on the target IP
      addresses. Normally reverse DNS is only performed against
      responsive (online) hosts.

   --system-dns (Use system DNS resolver)
      By default, Nmap resolves IP addresses by sending queries
      directly to the name servers configured on your host and then
      listening for responses. Many requests (often dozens) are
      performed in parallel to improve performance. Specify this
      option to use your system resolver instead (one IP at a time via
      the getnameinfo() call). This is slower and rarely useful unless
      you find a bug in the Nmap parallel resolver (please let us know
      if you do). The system resolver is always used for IPv6 scans.

   --dns-servers <server1[,server2],...>  (Servers to use for reverse DNS
   queries)
      By default Nmap will try to determine your DNS servers (for rDNS
      resolution) from your resolv.conf file (UNIX) or the registry
      (Win32). Alternatively, you may use this option to specify
      alternate servers. This option is not honored if you are using
      --system-dns or an IPv6 scan. Using multiple DNS servers is
      often faster, especially if you choose authoritative servers for
      your target IP space. This option can also improve stealth, as
      your requests can be bounced off just about any recursive DNS
      server on the internet.

      This option also comes in handy when scanning private networks.
      Sometimes only a few name servers provide proper rDNS
      information, and you may not even know where they are. You can
      scan the network for port 53 (perhaps with version detection),
      then try Nmap list scans (-sL) specifying each name server one
      at a time with --dns-servers until you find one which works.

PORT SCANNING BASICS While Nmap has grown in functionality over the years, it began as an efficient port scanner, and that remains its core function. The simple command nmap target scans more than 1660 TCP ports on the host target. While many port scanners have traditionally lumped all ports into the open or closed states, Nmap is much more granular. It divides ports into six states: open, closed, filtered, unfiltered, open|filtered, or closed|filtered.

   These states are not intrinsic properties of the port itself, but
   describe how Nmap sees them. For example, an Nmap scan from the same
   network as the target may show port 135/tcp as open, while a scan at
   the same time with the same options from across the Internet might show
   that port as filtered.

   The six port states recognized by Nmap

   open   An application is actively accepting TCP connections or UDP
      packets on this port. Finding these is often the primary goal of
      port scanning. Security-minded people know that each open port
      is an avenue for attack. Attackers and pen-testers want to
      exploit the open ports, while administrators try to close or
      protect them with firewalls without thwarting legitimate users.
      Open ports are also interesting for non-security scans because
      they show services available for use on the network.

   closed A closed port is accessible (it receives and responds to Nmap
      probe packets), but there is no application listening on it.
      They can be helpful in showing that a host is up on an IP
      address (host discovery, or ping scanning), and as part of OS
      detection. Because closed ports are reachable, it may be worth
      scanning later in case some open up. Administrators may want to
      consider blocking such ports with a firewall. Then they would
      appear in the filtered state, discussed next.

   filtered
      Nmap cannot determine whether the port is open because packet
      filtering prevents its probes from reaching the port. The
      filtering could be from a dedicated firewall device, router
      rules, or host-based firewall software. These ports frustrate
      attackers because they provide so little information. Sometimes
      they respond with ICMP error messages such as type 3 code 13
      (destination unreachable: communication administratively
      prohibited), but filters that simply drop probes without
      responding are far more common. This forces Nmap to retry
      several times just in case the probe was dropped due to network
      congestion rather than filtering. This slows down the scan
      dramatically.

   unfiltered
      The unfiltered state means that a port is accessible, but Nmap
      is unable to determine whether it is open or closed. Only the
      ACK scan, which is used to map firewall rulesets, classifies
      ports into this state. Scanning unfiltered ports with other scan
      types such as Window scan, SYN scan, or FIN scan, may help
      resolve whether the port is open.

   open|filtered
      Nmap places ports in this state when it is unable to determine
      whether a port is open or filtered. This occurs for scan types
      in which open ports give no response. The lack of response could
      also mean that a packet filter dropped the probe or any response
      it elicited. So Nmap does not know for sure whether the port is
      open or being filtered. The UDP, IP Protocol, FIN, Null, and
      Xmas scans classify ports this way.

   closed|filtered
      This state is used when Nmap is unable to determine whether a
      port is closed or filtered. It is only used for the IPID Idle
      scan.

PORT SCANNING TECHNIQUES As a novice performing automotive repair, I can struggle for hours trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to the task at hand. When I fail miserably and tow my jalopy to a real mechanic, he invariably fishes around in a huge tool chest until pulling out the perfect gizmo which makes the job seem effortless. The art of port scanning is similar. Experts understand the dozens of scan techniques and choose the appropriate one (or combination) for a given task. Inexperienced users and script kiddies, on the other hand, try to solve every problem with the default SYN scan. Since Nmap is free, the only barrier to port scanning mastery is knowledge. That certainly beats the automotive world, where it may take great skill to determine that you need a strut spring compressor, then you still have to pay thousands of dollars for it.

   Most of the scan types are only available to privileged users. This is
   because they send and receive raw packets, which requires root access
   on UNIX systems. Using an administrator account on Windows is
   recommended, though Nmap sometimes works for unprivileged users on that
   platform when WinPcap has already been loaded into the OS. Requiring
   root privileges was a serious limitation when Nmap was released in
   1997, as many users only had access to shared shell accounts. Now, the
   world is different. Computers are cheaper, far more people have
   always-on direct Internet access, and desktop UNIX systems (including
   Linux and MAC OS X) are prevalent. A Windows version of Nmap is now
   available, allowing it to run on even more desktops. For all these
   reasons, users have less need to run Nmap from limited shared shell
   accounts. This is fortunate, as the privileged options make Nmap far
   more powerful and flexible.

   While Nmap attempts to produce accurate results, keep in mind that all
   of its insights are based on packets returned by the target machines
   (or firewalls in front of them). Such hosts may be untrustworthy and
   send responses intended to confuse or mislead Nmap. Much more common
   are non-RFC-compliant hosts that do not respond as they should to Nmap
   probes. FIN, Null, and Xmas scans are particularly susceptible to this
   problem. Such issues are specific to certain scan types and so are
   discussed in the individual scan type entries.

   This section documents the dozen or so port scan techniques supported
   by Nmap. Only one method may be used at a time, except that UDP scan
   (-sU) may be combined with any one of the TCP scan types. As a memory
   aid, port scan type options are of the form -sC, where C is a prominent
   character in the scan name, usually the first. The one exception to
   this is the deprecated FTP bounce scan (-b). By default, Nmap performs
   a SYN Scan, though it substitutes a connect scan if the user does not
   have proper privileges to send raw packets (requires root access on
   UNIX) or if IPv6 targets were specified. Of the scans listed in this
   section, unprivileged users can only execute connect and ftp bounce
   scans.

   -sS (TCP SYN scan)
      SYN scan is the default and most popular scan option for good
      reasons. It can be performed quickly, scanning thousands of
      ports per second on a fast network not hampered by intrusive
      firewalls. SYN scan is relatively unobtrusive and stealthy,
      since it never completes TCP connections. It also works against
      any compliant TCP stack rather than depending on idiosyncrasies
      of specific platforms as Nmap’s Fin/Null/Xmas, Maimon and Idle
      scans do. It also allows clear, reliable differentiation between
      the open, closed, and filtered states.

      This technique is often referred to as half-open scanning,
      because you don’t open a full TCP connection. You send a SYN
      packet, as if you are going to open a real connection and then
      wait for a response. A SYN/ACK indicates the port is listening
      (open), while a RST (reset) is indicative of a non-listener. If
      no response is received after several retransmissions, the port
      is marked as filtered. The port is also marked filtered if an
      ICMP unreachable error (type 3, code 1,2, 3, 9, 10, or 13) is
      received.

   -sT (TCP connect scan)
      TCP connect scan is the default TCP scan type when SYN scan is
      not an option. This is the case when a user does not have raw
      packet privileges or is scanning IPv6 networks. Instead of
      writing raw packets as most other scan types do, Nmap asks the
      underlying operating system to establish a connection with the
      target machine and port by issuing the connect() system call.
      This is the same high-level system call that web browsers, P2P
      clients, and most other network-enabled applications use to
      establish a connection. It is part of a programming interface
      known as the Berkeley Sockets API. Rather than read raw packet
      responses off the wire, Nmap uses this API to obtain status
      information on each connection attempt.

      When SYN scan is available, it is usually a better choice. Nmap
      has less control over the high level connect() call than with
      raw packets, making it less efficient. The system call completes
      connections to open target ports rather than performing the
      half-open reset that SYN scan does. Not only does this take
      longer and require more packets to obtain the same information,
      but target machines are more likely to log the connection. A
      decent IDS will catch either, but most machines have no such
      alarm system. Many services on your average UNIX system will add
      a note to syslog, and sometimes a cryptic error message, when
      Nmap connects and then closes the connection without sending
      data. Truly pathetic services crash when this happens, though
      that is uncommon. An administrator who sees a bunch of
      connection attempts in her logs from a single system should know
      that she has been connect scanned.

   -sU (UDP scans)
      While most popular services on the Internet run over the TCP
      protocol, [3]UDP services are widely deployed. DNS, SNMP, and
      DHCP (registered ports 53, 161/162, and 67/68) are three of the
      most common. Because UDP scanning is generally slower and more
      difficult than TCP, some security auditors ignore these ports.
      This is a mistake, as exploitable UDP services are quite common
      and attackers certainly don’t ignore the whole protocol.
      Fortunately, Nmap can help inventory UDP ports.

      UDP scan is activated with the -sU option. It can be combined
      with a TCP scan type such as SYN scan (-sS) to check both
      protocols during the same run.

      UDP scan works by sending an empty (no data) UDP header to every
      targeted port. If an ICMP port unreachable error (type 3, code
      3) is returned, the port is closed. Other ICMP unreachable
      errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as
      filtered. Occasionally, a service will respond with a UDP
      packet, proving that it is open. If no response is received
      after retransmissions, the port is classified as open|filtered.
      This means that the port could be open, or perhaps packet
      filters are blocking the communication. Versions scan (-sV) can
      be used to help differentiate the truly open ports from the
      filtered ones.

      A big challenge with UDP scanning is doing it quickly. Open and
      filtered ports rarely send any response, leaving Nmap to time
      out and then conduct retransmissions just in case the probe or
      response were lost. Closed ports are often an even bigger
      problem. They usually send back an ICMP port unreachable error.
      But unlike the RST packets sent by closed TCP ports in response
      to a SYN or connect scan, many hosts rate limit ICMP port
      unreachable messages by default. Linux and Solaris are
      particularly strict about this. For example, the Linux 2.4.20
      kernel limits destination unreachable messages to one per second
      (in net/ipv4/icmp.c).

      Nmap detects rate limiting and slows down accordingly to avoid
      flooding the network with useless packets that the target
      machine will drop. Unfortunately, a Linux-style limit of one
      packet per second makes a 65,536-port scan take more than 18
      hours. Ideas for speeding your UDP scans up include scanning
      more hosts in parallel, doing a quick scan of just the popular
      ports first, scanning from behind the firewall, and using
      --host-timeout to skip slow hosts.

   -sN; -sF; -sX (TCP Null, FIN, and Xmas scans)
      These three scan types (even more are possible with the
      --scanflags option described in the next section) exploit a
      subtle loophole in the [4]TCP RFC to differentiate between open
      and closed ports. Page 65 says that “if the [destination] port
      state is CLOSED .... an incoming segment not containing a RST
      causes a RST to be sent in response.”  Then the next page
      discusses packets sent to open ports without the SYN, RST, or
      ACK bits set, stating that: “you are unlikely to get here, but
      if you do, drop the segment, and return.”

      When scanning systems compliant with this RFC text, any packet
      not containing SYN, RST, or ACK bits will result in a returned
      RST if the port is closed and no response at all if the port is
      open. As long as none of those three bits are included, any
      combination of the other three (FIN, PSH, and URG) are OK. Nmap
      exploits this with three scan types:

      Null scan (-sN)
	     Does not set any bits (tcp flag header is 0)

      FIN scan (-sF)
	     Sets just the TCP FIN bit.

      Xmas scan (-sX)
	     Sets the FIN, PSH, and URG flags, lighting the packet up
	     like a Christmas tree.

      These three scan types are exactly the same in behavior except
      for the TCP flags set in probe packets. If a RST packet is
      received, the port is considered closed, while no response means
      it is open|filtered. The port is marked filtered if an ICMP
      unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is
      received.

      The key advantage to these scan types is that they can sneak
      through certain non-stateful firewalls and packet filtering
      routers. Another advantage is that these scan types are a little
      more stealthy than even a SYN scan. Don’t count on this though
      -- most modern IDS products can be configured to detect them.
      The big downside is that not all systems follow RFC 793 to the
      letter. A number of systems send RST responses to the probes
      regardless of whether the port is open or not. This causes all
      of the ports to be labeled closed. Major operating systems that
      do this are Microsoft Windows, many Cisco devices, BSDI, and IBM
      OS/400. This scan does work against most UNIX-based systems
      though. Another downside of these scans is that they can’t
      distinguish open ports from certain filtered ones, leaving you
      with the response open|filtered.

   -sA (TCP ACK scan)
      This scan is different than the others discussed so far in that
      it never determines open (or even open|filtered) ports. It is
      used to map out firewall rulesets, determining whether they are
      stateful or not and which ports are filtered.

      The ACK scan probe packet has only the ACK flag set (unless you
      use --scanflags). When scanning unfiltered systems, open and
      closed ports will both return a RST packet. Nmap then labels
      them as unfiltered, meaning that they are reachable by the ACK
      packet, but whether they are open or closed is undetermined.
      Ports that don’t respond, or send certain ICMP error messages
      back (type 3, code 1, 2, 3, 9, 10, or 13), are labeled filtered.

   -sW (TCP Window scan)
      Window scan is exactly the same as ACK scan except that it
      exploits an implementation detail of certain systems to
      differentiate open ports from closed ones, rather than always
      printing unfiltered when a RST is returned. It does this by
      examining the TCP Window field of the RST packets returned. On
      some systems, open ports use a positive window size (even for
      RST packets) while closed ones have a zero window. So instead of
      always listing a port as unfiltered when it receives a RST back,
      Window scan lists the port as open or closed if the TCP Window
      value in that reset is positive or zero, respectively.

      This scan relies on an implementation detail of a minority of
      systems out on the Internet, so you can’t always trust it.
      Systems that don’t support it will usually return all ports
      closed. Of course, it is possible that the machine really has no
      open ports. If most scanned ports are closed but a few common
      port numbers (such as 22, 25, 53) are filtered, the system is
      most likely susceptible. Occasionally, systems will even show
      the exact opposite behavior. If your scan shows 1000 open ports
      and 3 closed or filtered ports, then those three may very well
      be the truly open ones.

   -sM (TCP Maimon scan)
      The Maimon scan is named after its discoverer, Uriel Maimon. He
      described the technique in Phrack Magazine issue #49 (November
      1996). Nmap, which included this technique, was released two
      issues later. This technique is exactly the same as Null, FIN,
      and Xmas scans, except that the probe is FIN/ACK. According to
      RFC 793 (TCP), a RST packet should be generated in response to
      such a probe whether the port is open or closed. However, Uriel
      noticed that many BSD-derived systems simply drop the packet if
      the port is open.

   --scanflags (Custom TCP scan)
      Truly advanced Nmap users need not limit themselves to the
      canned scan types offered. The --scanflags option allows you to
      design your own scan by specifying arbitrary TCP flags. Let your
      creative juices flow, while evading intrusion detection systems
      whose vendors simply paged through the Nmap man page adding
      specific rules!

      The --scanflags argument can be a numerical flag value such as 9
      (PSH and FIN), but using symbolic names is easier. Just mash
      together any combination of URG, ACK, PSH, RST, SYN, and FIN.
      For example, --scanflags URGACKPSHRSTSYNFIN sets everything,
      though it’s not very useful for scanning. The order these are
      specified in is irrelevant.

      In addition to specifying the desired flags, you can specify a
      TCP scan type (such as -sA or -sF). That base type tells Nmap
      how to interpret responses. For example, a SYN scan considers
      no-response to indicate a filtered port, while a FIN scan treats
      the same as open|filtered. Nmap will behave the same way it does
      for the base scan type, except that it will use the TCP flags
      you specify instead. If you don’t specify a base type, SYN scan
      is used.

   -sI <zombie host[:probeport]> (Idlescan)
      This advanced scan method allows for a truly blind TCP port scan
      of the target (meaning no packets are sent to the target from
      your real IP address). Instead, a unique side-channel attack
      exploits predictable IP fragmentation ID sequence generation on
      the zombie host to glean information about the open ports on the
      target. IDS systems will display the scan as coming from the
      zombie machine you specify (which must be up and meet certain
      criteria). This fascinating scan type is too complex to fully
      describe in this reference guide, so I wrote and posted an
      informal paper with full details at
      http://insecure.org/nmap/idlescan.html.

      Besides being extraordinarily stealthy (due to its blind
      nature), this scan type permits mapping out IP-based trust
      relationships between machines. The port listing shows open
      ports from the perspective of the zombie host.  So you can try
      scanning a target using various zombies that you think might be
      trusted (via router/packet filter rules).

      You can add a colon followed by a port number to the zombie host
      if you wish to probe a particular port on the zombie for IPID
      changes. Otherwise Nmap will use the port it uses by default for
      tcp pings (80).

   -sO (IP protocol scan)
      IP Protocol scan allows you to determine which IP protocols
      (TCP, ICMP, IGMP, etc.) are supported by target machines. This
      isn’t technically a port scan, since it cycles through IP
      protocol numbers rather than TCP or UDP port numbers. Yet it
      still uses the -p option to select scanned protocol numbers,
      reports its results within the normal port table format, and
      even uses the same underlying scan engine as the true port
      scanning methods. So it is close enough to a port scan that it
      belongs here.

      Besides being useful in its own right, protocol scan
      demonstrates the power of open source software. While the
      fundamental idea is pretty simple, I had not thought to add it
      nor received any requests for such functionality. Then in the
      summer of 2000, Gerhard Rieger conceived the idea, wrote an
      excellent patch implementing it, and sent it to the nmap-hackers
      mailing list. I incorporated that patch into the Nmap tree and
      released a new version the next day. Few pieces of commercial
      software have users enthusiastic enough to design and contribute
      their own improvements!

      Protocol scan works in a similar fashion to UDP scan. Instead of
      iterating through the port number field of a UDP packet, it
      sends IP packet headers and iterates through the 8-bit IP
      protocol field. The headers are usually empty, containing no
      data and not even the proper header for the claimed protocol.
      The three exceptions are TCP, UDP, and ICMP. A proper protocol
      header for those is included since some systems won’t send them
      otherwise and because Nmap already has functions to create them.
      Instead of watching for ICMP port unreachable messages, protocol
      scan is on the lookout for ICMP protocol unreachable messages.
      If Nmap receives any response in any protocol from the target
      host, Nmap marks that protocol as open. An ICMP protocol
      unreachable error (type 3, code 2) causes the protocol to be
      marked as closed Other ICMP unreachable errors (type 3, code 1,
      3, 9, 10, or 13) cause the protocol to be marked filtered
      (though they prove that ICMP is open at the same time). If no
      response is received after retransmissions, the protocol is
      marked open|filtered

   -b <ftp relay host> (FTP bounce scan)
      An interesting feature of the FTP protocol ([5]RFC 959) is
      support for so-called proxy ftp connections. This allows a user
      to connect to one FTP server, then ask that files be sent to a
      third-party server. Such a feature is ripe for abuse on many
      levels, so most servers have ceased supporting it. One of the
      abuses this feature allows is causing the FTP server to port
      scan other hosts. Simply ask the FTP server to send a file to
      each interesting port of a target host in turn. The error
      message will describe whether the port is open or not. This is a
      good way to bypass firewalls because organizational FTP servers
      are often placed where they have more access to other internal
      hosts than any old Internet host would. Nmap supports ftp bounce
      scan with the -b option. It takes an argument of the form
      username:password@server:port.  Server is the name or IP address
      of a vulnerable FTP server. As with a normal URL, you may omit
      username:password, in which case anonymous login credentials
      (user: anonymous password:-wwwuser@) are used. The port number
      (and preceding colon) may be omitted as well, in which case the
      default FTP port (21) on server is used.

      This vulnerability was widespread in 1997 when Nmap was
      released, but has largely been fixed. Vulnerable servers are
      still around, so it is worth trying when all else fails. If
      bypassing a firewall is your goal, scan the target network for
      open port 21 (or even for any ftp services if you scan all ports
      with version detection), then try a bounce scan using each. Nmap
      will tell you whether the host is vulnerable or not. If you are
      just trying to cover your tracks, you don’t need to (and, in
      fact, shouldn’t) limit yourself to hosts on the target network.
      Before you go scanning random Internet addresses for vulnerable
      FTP servers, consider that sysadmins may not appreciate you
      abusing their servers in this way.

PORT SPECIFICATION AND SCAN ORDER In addition to all of the scan methods discussed previously, Nmap offers options for specifying which ports are scanned and whether the scan order is randomized or sequential. By default, Nmap scans all ports up to and including 1024 as well as higher numbered ports listed in the nmap-services file for the protocol(s) being scanned.

   -p <port ranges> (Only scan specified ports)
      This option specifies which ports you want to scan and overrides
      the default. Individual port numbers are OK, as are ranges
      separated by a hyphen (e.g. 1-1023). The beginning and/or end
      values of a range may be omitted, causing Nmap to use 1 and
      65535, respectively. So you can specify -p- to scan ports from 1
      through 65535. Scanning port zero is allowed if you specify it
      explicitly. For IP protocol scanning (-sO), this option
      specifies the protocol numbers you wish to scan for (0-255).

      When scanning both TCP and UDP ports, you can specify a
      particular protocol by preceding the port numbers by T: or U:.
      The qualifier lasts until you specify another qualifier. For
      example, the argument -p U:53,111,137,T:21-25,80,139,8080 would
      scan UDP ports 53,111,and 137, as well as the listed TCP ports.
      Note that to scan both UDP & TCP, you have to specify -sU and at
      least one TCP scan type (such as -sS, -sF, or -sT). If no
      protocol qualifier is given, the port numbers are added to all
      protocol lists.

   -F (Fast (limited port) scan)
      Specifies that you only wish to scan for ports listed in the
      nmap-services file which comes with nmap (or the protocols file
      for -sO). This is much faster than scanning all 65535 ports on a
      host. Because this list contains so many TCP ports (more than
      1200), the speed difference from a default TCP scan (about 1650
      ports) isn’t dramatic. The difference can be enormous if you
      specify your own tiny nmap-services file using the --datadir
      option.

   -r (Don’t randomize ports)
      By default, Nmap randomizes the scanned port order (except that
      certain commonly accessible ports are moved near the beginning
      for efficiency reasons). This randomization is normally
      desirable, but you can specify -r for sequential port scanning
      instead.

SERVICE AND VERSION DETECTION Point Nmap at a remote machine and it might tell you that ports 25/tcp, 80/tcp, and 53/udp are open. Using its nmap-services database of about 2,200 well-known services, Nmap would report that those ports probably correspond to a mail server (SMTP), web server (HTTP), and name server (DNS) respectively. This lookup is usually accurate – the vast majority of daemons listening on TCP port 25 are, in fact, mail servers. However, you should not bet your security on this! People can and do run services on strange ports.

   Even if Nmap is right, and the hypothetical server above is running
   SMTP, HTTP, and DNS servers, that is not a lot of information. When
   doing vulnerability assessments (or even simple network inventories) of
   your companies or clients, you really want to know which mail and DNS
   servers and versions are running. Having an accurate version number
   helps dramatically in determining which exploits a server is vulnerable
   to. Version detection helps you obtain this information.

   After TCP and/or UDP ports are discovered using one of the other scan
   methods, version detection interrogates those ports to determine more
   about what is actually running. The nmap-service-probes database
   contains probes for querying various services and match expressions to
   recognize and parse responses. Nmap tries to determine the service
   protocol (e.g. ftp, ssh, telnet, http), the application name (e.g. ISC
   Bind, Apache httpd, Solaris telnetd), the version number, hostname,
   device type (e.g. printer, router), the OS family (e.g. Windows, Linux)
   and sometimes miscellaneous details like whether an X server is open to
   connections, the SSH protocol version, or the KaZaA user name). Of
   course, most services don’t provide all of this information. If Nmap
   was compiled with OpenSSL support, it will connect to SSL servers to
   deduce the service listening behind that encryption layer. When RPC
   services are discovered, the Nmap RPC grinder (-sR) is automatically
   used to determine the RPC program and version numbers. Some UDP ports
   are left in the open|filtered state after a UDP port scan is unable to
   determine whether the port is open or filtered. Version detection will
   try to elicit a response from these ports (just as it does with open
   ports), and change the state to open if it succeeds.  open|filtered TCP
   ports are treated the same way. Note that the Nmap -A option enables
   version detection among other things. A paper documenting the workings,
   usage, and customization of version detection is available at
   http://insecure.org/nmap/vscan/.

   When Nmap receives responses from a service but cannot match them to
   its database, it prints out a special fingerprint and a URL for you to
   submit if to if you know for sure what is running on the port. Please
   take a couple minutes to make the submission so that your find can
   benefit everyone. Thanks to these submissions, Nmap has about 3,000
   pattern matches for more than 350 protocols such as smtp, ftp, http,
   etc.

   Version detection is enabled and controlled with the following options:

   -sV (Version detection)
      Enables version detection, as discussed above. Alternatively,
      you can use -A to enable both OS detection and version
      detection.

   --allports (Don’t exclude any ports from version detection)
      By default, Nmap version detection skips TCP port 9100 because
      some printers simply print anything sent to that port, leading
      to dozens of pages of HTTP get requests, binary SSL session
      requests, etc. This behavior can be changed by modifying or
      removing the Exclude directive in nmap-service-probes, or you
      can specify --allports to scan all ports regardless of any
      Exclude directive.

   --version-intensity <intensity> (Set version scan intensity)
      When performing a version scan (-sV), nmap sends a series of
      probes, each of which is assigned a rarity value between 1 and
      9. The lower-numbered probes are effective against a wide
      variety of common services, while the higher numbered ones are
      rarely useful. The intensity level specifies which probes should
      be applied. The higher the number, the more likely it is the
      service will be correctly identified. However, high intensity
      scans take longer. The intensity must be between 0 and 9. The
      default is 7. When a probe is registered to the target port via
      the nmap-service-probesports directive, that probe is tried
      regardless of intensity level. This ensures that the DNS probes
      will always be attempted against any open port 53, the SSL probe
      will be done against 443, etc.

   --version-light (Enable light mode)
      This is a convenience alias for --version-intensity 2. This
      light mode makes version scanning much faster, but it is
      slightly less likely to identify services.

   --version-all (Try every single probe)
      An alias for --version-intensity 9, ensuring that every single
      probe is attempted against each port.

   --version-trace (Trace version scan activity)
      This causes Nmap to print out extensive debugging info about
      what version scanning is doing. It is a subset of what you get
      with --packet-trace.

   -sR (RPC scan)
      This method works in conjunction with the various port scan
      methods of Nmap. It takes all the TCP/UDP ports found open and
      floods them with SunRPC program NULL commands in an attempt to
      determine whether they are RPC ports, and if so, what program
      and version number they serve up. Thus you can effectively
      obtain the same info as rpcinfo -p even if the target’s
      portmapper is behind a firewall (or protected by TCP wrappers).
      Decoys do not currently work with RPC scan. This is
      automatically enabled as part of version scan (-sV) if you
      request that. As version detection includes this and is much
      more comprehensive, -sR is rarely needed.

OS DETECTION One of Nmap’s best-known features is remote OS detection using TCP/IP stack fingerprinting. Nmap sends a series of TCP and UDP packets to the remote host and examines practically every bit in the responses. After performing dozens of tests such as TCP ISN sampling, TCP options support and ordering, IPID sampling, and the initial window size check, Nmap compares the results to its nmap-os-fingerprints database of more than 1500 known OS fingerprints and prints out the OS details if there is a match. Each fingerprint includes a freeform textual description of the OS, and a classification which provides the vendor name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type (general purpose, router, switch, game console, etc).

   If Nmap is unable to guess the OS of a machine, and conditions are good
   (e.g. at least one open port and one closed port were found), Nmap will
   provide a URL you can use to submit the fingerprint if you know (for
   sure) the OS running on the machine. By doing this you contribute to
   the pool of operating systems known to Nmap and thus it will be more
   accurate for everyone.

   OS detection enables several other tests which make use of information
   that is gathered during the process anyway. One of these is uptime
   measurement, which uses the TCP timestamp option (RFC 1323) to guess
   when a machine was last rebooted. This is only reported for machines
   which provide this information. Another is TCP Sequence Predictability
   Classification. This measures approximately how hard it is to establish
   a forged TCP connection against the remote host. It is useful for
   exploiting source-IP based trust relationships (rlogin, firewall
   filters, etc) or for hiding the source of an attack. This sort of
   spoofing is rarely performed any more, but many machines are still
   vulnerable to it. The actual difficulty number is based on statistical
   sampling and may fluctuate. It is generally better to use the English
   classification such as “worthy challenge” or “trivial joke”. This is
   only reported in normal output in verbose (-v) mode. When verbose mode
   is enabled along with -O, IPID Sequence Generation is also reported.
   Most machines are in the “incremental” class, which means that they
   increment the ID field in the IP header for each packet they send. This
   makes them vulnerable to several advanced information gathering and
   spoofing attacks.

   A paper documenting the workings, usage, and customization of OS
   detection is available at http://insecure.org/nmap/osdetect/.

   OS detection is enabled and controlled with the following options:

   -O (Enable OS detection)
      Enables OS detection, as discussed above. Alternatively, you can
      use -A to enable both OS detection and version detection. 2nd
      generation OS detection is tried first. If that fails, Nmap will
      either print out the host fingerprint and ask you to submit it
      (if you are certain about what the target host is running), or
      Nmap will fall back to the 1st generation OS detection system in
      case its larger database has a match.

   -O2 (2nd Generation OS Detection Only)
      Enables 2nd generation OS detection, but never falls back to the
      old (1st generation) system, even if it fails to find any match.
      This saves time and can reduce the number of packets sent to
      each target.

   -O1 (1nd Generation OS Detection Only)
      Tells Nmap to only use the old OS detection system. If -O2 just
      gives you a fingerprint to submit, but you don’t know what OS
      the target is running, try -O1. But in that case, don’t submit
      the fingerprint as you don’t know for sure whether -O1 guess
      correctly. If it was perfect, we wouldn’t have bothered to
      create -O2.

      This option, and all other vestiges of the old OS detection
      system, will likely be removed in late 2006 or in 2007.

   --osscan-limit (Limit OS detection to promising targets)
      OS detection is far more effective if at least one open and one
      closed TCP port are found. Set this option and Nmap will not
      even try OS detection against hosts that do not meet this
      criteria. This can save substantial time, particularly on -P0
      scans against many hosts. It only matters when OS detection is
      requested with -O or -A.

   --osscan-guess; --fuzzy (Guess OS detection results)
      When Nmap is unable to detect a perfect OS match, it sometimes
      offers up near-matches as possibilities. The match has to be
      very close for Nmap to do this by default. Either of these
      (equivalent) options make Nmap guess more aggressively. Nmap
      will still tell you when an imperfect match is printed and
      display its confidence level (percentage) for each guess.

   --max-os-tries (Set the maximum number of OS detection tries against a
   target)
      When Nmap performs OS detection against a target and fails to
      find a perfect match, it usually repeats the attempt. By
      default, Nmap tries five times if conditions are favorable for
      OS fingerprint submission, and twice when conditions aren’t so
      good. Specifying a lower --max-os-tries value (such as 1) speeds
      Nmap up, though you miss out on retries which could potentially
      identify the OS. Alternatively, a high value may be set to allow
      even more retries when conditions are favorable. This is rarely
      done, except to generate better fingerprints for submission and
      integration into the Nmap OS database. This option only affects
      second generation OS detection (-O2, the default) and not the
      old system (-O1).

TIMING AND PERFORMANCE One of my highest Nmap development priorities has always been performance. A default scan (nmap hostname) of a host on my local network takes a fifth of a second. That is barely enough time to blink, but adds up when you are scanning tens or hundreds of thousands of hosts. Moreover, certain scan options such as UDP scanning and version detection can increase scan times substantially. So can certain firewall configurations, particularly response rate limiting. While Nmap utilizes parallelism and many advanced algorithms to accelerate these scans, the user has ultimate control over how Nmap runs. Expert users carefully craft Nmap commands to obtain only the information they care about while meeting their time constraints.

   Techniques for improving scan times include omitting non-critical
   tests, and upgrading to the latest version of Nmap (performance
   enhancements are made frequently). Optimizing timing parameters can
   also make a substantial difference. Those options are listed below.

   Some options accept a time parameter. This is specified in milliseconds
   by default, though you can append ‘s’, ‘m’, or ‘h’ to the value to
   specify seconds, minutes, or hours. So the --host-timeout arguments
   900000, 900s, and 15m all do the same thing.

   --min-hostgroup <numhosts>; --max-hostgroup <numhosts> (Adjust parallel
   scan group sizes)
      Nmap has the ability to port scan or version scan multiple hosts
      in parallel. Nmap does this by dividing the target IP space into
      groups and then scanning one group at a time. In general, larger
      groups are more efficient. The downside is that host results
      can’t be provided until the whole group is finished. So if Nmap
      started out with a group size of 50, the user would not receive
      any reports (except for the updates offered in verbose mode)
      until the first 50 hosts are completed.

      By default, Nmap takes a compromise approach to this conflict.
      It starts out with a group size as low as five so the first
      results come quickly and then increases the groupsize to as high
      as 1024. The exact default numbers depend on the options given.
      For efficiency reasons, Nmap uses larger group sizes for UDP or
      few-port TCP scans.

      When a maximum group size is specified with --max-hostgroup,
      Nmap will never exceed that size. Specify a minimum size with
      --min-hostgroup and Nmap will try to keep group sizes above that
      level. Nmap may have to use smaller groups than you specify if
      there are not enough target hosts left on a given interface to
      fulfill the specified minimum. Both may be set to keep the group
      size within a specific range, though this is rarely desired.

      The primary use of these options is to specify a large minimum
      group size so that the full scan runs more quickly. A common
      choice is 256 to scan a network in Class C sized chunks. For a
      scan with many ports, exceeding that number is unlikely to help
      much. For scans of just a few port numbers, host group sizes of
      2048 or more may be helpful.

   --min-parallelism <numprobes>; --max-parallelism <numprobes> (Adjust
   probe parallelization)
      These options control the total number of probes that may be
      outstanding for a host group. They are used for port scanning
      and host discovery. By default, Nmap calculates an ever-changing
      ideal parallelism based on network performance. If packets are
      being dropped, Nmap slows down and allows fewer outstanding
      probes. The ideal probe number slowly rises as the network
      proves itself worthy. These options place minimum or maximum
      bounds on that variable. By default, the ideal parallelism can
      drop to 1 if the network proves unreliable and rise to several
      hundred in perfect conditions.

      The most common usage is to set --min-parallelism to a number
      higher than one to speed up scans of poorly performing hosts or
      networks. This is a risky option to play with, as setting it too
      high may affect accuracy. Setting this also reduces Nmap’s
      ability to control parallelism dynamically based on network
      conditions. A value of ten might be reasonable, though I only
      adjust this value as a last resort.

      The --max-parallelism option is sometimes set to one to prevent
      Nmap from sending more than one probe at a time to hosts. This
      can be useful in combination with --scan-delay (discussed
      later), although the latter usually serves the purpose well
      enough by itself.

   --min-rtt-timeout <time>, --max-rtt-timeout <time>,
   --initial-rtt-timeout <time> (Adjust probe timeouts)
      Nmap maintains a running timeout value for determining how long
      it will wait for a probe response before giving up or
      retransmitting the probe. This is calculated based on the
      response times of previous probes. If the network latency shows
      itself to be significant and variable, this timeout can grow to
      several seconds. It also starts at a conservative (high) level
      and may stay that way for a while when Nmap scans unresponsive
      hosts.

      Specifying a lower --max-rtt-timeout and --initial-rtt-timeout
      than the defaults can cut scan times significantly. This is
      particularly true for pingless (-P0) scans, and those against
      heavily filtered networks. Don’t get too aggressive though. The
      scan can end up taking longer if you specify such a low value
      that many probes are timing out and retransmitting while the
      response is in transit.

      If all the hosts are on a local network, 100 milliseconds is a
      reasonable aggressive --max-rtt-timeout value. If routing is
      involved, ping a host on the network first with the ICMP ping
      utility, or with a custom packet crafter such as hping2 that is
      more likely to get through a firewall. Look at the maximum round
      trip time out of ten packets or so. You might want to double
      that for the --initial-rtt-timeout and triple or quadruple it
      for the --max-rtt-timeout. I generally do not set the maximum
      rtt below 100ms, no matter what the ping times are. Nor do I
      exceed 1000ms.

      --min-rtt-timeout is a rarely used option that could be useful
      when a network is so unreliable that even Nmap’s default is too
      aggressive. Since Nmap only reduces the timeout down to the
      minimum when the network seems to be reliable, this need is
      unusual and should be reported as a bug to the nmap-dev mailing
      list.

   --max-retries <numtries> (Specify the maximum number of port scan probe
   retransmissions)
      When Nmap receives no response to a port scan probe, it could
      mean the port is filtered. Or maybe the probe or response was
      simply lost on the network. It is also possible that the target
      host has rate limiting enabled that temporarily blocked the
      response. So Nmap tries again by retransmitting the initial
      probe. If Nmap detects poor network reliability, it may try many
      more times before giving up on a port. While this benefits
      accuracy, it also lengthen scan times. When performance is
      critical, scans may be sped up by limiting the number of
      retransmissions allowed. You can even specify --max-retries 0 to
      prevent any retransmissions, though that is rarely recommended.

      The default (with no -T template) is to allow ten
      retransmissions. If a network seems reliable and the target
      hosts aren’t rate limiting, Nmap usually only does one
      retransmission. So most target scans aren’t even affected by
      dropping --max-retries to a low value such as three. Such values
      can substantially speed scans of slow (rate limited) hosts. You
      usually lose some information when Nmap gives up on ports early,
      though that may be preferable to letting the --host-timeout
      expire and losing all information about the target.

   --host-timeout <time> (Give up on slow target hosts)
      Some hosts simply take a long time to scan. This may be due to
      poorly performing or unreliable networking hardware or software,
      packet rate limiting, or a restrictive firewall. The slowest few
      percent of the scanned hosts can eat up a majority of the scan
      time. Sometimes it is best to cut your losses and skip those
      hosts initially. Specify --host-timeout with the maximum amount
      of time you are willing to wait. I often specify 30m to ensure
      that Nmap doesn’t waste more than half an hour on a single host.
      Note that Nmap may be scanning other hosts at the same time
      during that half an hour as well, so it isn’t a complete loss. A
      host that times out is skipped. No port table, OS detection, or
      version detection results are printed for that host.

   --scan-delay <time>; --max-scan-delay <time> (Adjust delay between
   probes)
      This option causes Nmap to wait at least the given amount of
      time between each probe it sends to a given host. This is
      particularly useful in the case of rate limiting. Solaris
      machines (among many others) will usually respond to UDP scan
      probe packets with only one ICMP message per second. Any more
      than that sent by Nmap will be wasteful. A --scan-delay of 1s
      will keep Nmap at that slow rate. Nmap tries to detect rate
      limiting and adjust the scan delay accordingly, but it doesn’t
      hurt to specify it explicitly if you already know what rate
      works best.

      When Nmap adjusts the scan delay upward to cope with rate
      limiting, the scan slows down dramatically. The --max-scan-delay
      option specifies the largest delay that Nmap will allow. Setting
      this value too low can lead to wasteful packet retransmissions
      and possible missed ports when the target implements strict rate
      limiting.

      Another use of --scan-delay is to evade threshold based
      intrusion detection and prevention systems (IDS/IPS).

   --defeat-rst-ratelimit
      Many hosts have long used rate limiting to reduce the number of
      ICMP error messages (such as port-unreachable errors) they send.
      Some systems now apply similar rate limits to the RST (reset)
      packets they generate. This can slow Nmap down dramatically as
      it adjusts its timing to reflect those rate limits. You can tell
      Nmap to ignore those rate limits (for port scans such as SYN
      scan which don’t treat nonresponsive ports as open) by
      specifying --defeat-rst-ratelimit.

      Using this option can reduce accuracy, as some ports will appear
      nonresponse because Nmap didn’t wait long enough for a
      rate-limited RST response. With a SYN scan, the non-response
      results in the port being labeled filtered rather than the
      closed state we see when RST packets are received. This optional
      is useful when you only care about open ports, and
      distinguishing between closed and filtered ports isn’t worth the
      extra time.

   -T <Paranoid|Sneaky|Polite|Normal|Aggressive|Insane> (Set a timing
   template)
      While the fine grained timing controls discussed in the previous
      section are powerful and effective, some people find them
      confusing. Moreover, choosing the appropriate values can
      sometimes take more time than the scan you are trying to
      optimize. So Nmap offers a simpler approach, with six timing
      templates. You can specify them with the -T option and their
      number (0 - 5) or their name. The template names are paranoid
      (0), sneaky (1), polite (2), normal (3), aggressive (4), and
      insane (5). The first two are for IDS evasion. Polite mode slows
      down the scan to use less bandwidth and target machine
      resources. Normal mode is the default and so -T3 does nothing.
      Aggressive mode speeds scans up by making the assumption that
      you are on a reasonably fast and reliable network. Finally
      Insane mode assumes that you are on an extraordinarily fast
      network or are willing to sacrifice some accuracy for speed.

      These templates allow the user to specify how aggressive they
      wish to be, while leaving Nmap to pick the exact timing values.
      The templates also make some minor speed adjustments for which
      fine grained control options do not currently exist. For
      example, -T4 prohibits the dynamic scan delay from exceeding
      10ms for TCP ports and -T5 caps that value at 5 milliseconds.
      Templates can be used in combination with fine grained controls,
      and the fine-grained controls will you specify will take
      precedence over the timing template default for that parameter.
      I recommend using -T4 when scanning reasonably modern and
      reliable networks. Keep that option even when you add fine
      grained controls so that you benefit from those extra minor
      optimizations that it enables.

      If you are on a decent broadband or ethernet connection, I would
      recommend always using -T4. Some people love -T5 though it is
      too aggressive for my taste. People sometimes specify -T2
      because they think it is less likely to crash hosts or because
      they consider themselves to be polite in general. They often
      don’t realize just how slow -T Polite really is. Their scan may
      take ten times longer than a default scan. Machine crashes and
      bandwidth problems are rare with the default timing options
      (-T3) and so I normally recommend that for cautious scanners.
      Omitting version detection is far more effective than playing
      with timing values at reducing these problems.

      While -T0 and -T1 may be useful for avoiding IDS alerts, they
      will take an extraordinarily long time to scan thousands of
      machines or ports. For such a long scan, you may prefer to set
      the exact timing values you need rather than rely on the canned
      -T0 and -T1 values.

      The main effects of T0 are serializing the scan so only one port
      is scanned at a time, and waiting five minutes between sending
      each probe.  T1 and T2 are similar but they only wait 15 seconds
      and 0.4 seconds, respectively, between probes.  T3 is Nmap’s
      default behavior, which includes parallelization.	 T4 does the
      equivalent of --max-rtt-timeout 1250 --initial-rtt-timeout 500
      --max-retries 6 and sets the maximum TCP scan delay to 10
      milliseconds.  T5 does the equivalent of --max-rtt-timeout 300
      --min-rtt-timeout 50 --initial-rtt-timeout 250 --max-retries 2
      --host-timeout 15m as well as setting the maximum TCP scan delay
      to 5ms.

FIREWALL/IDS EVASION AND SPOOFING Many Internet pioneers envisioned a global open network with a universal IP address space allowing virtual connections between any two nodes. This allows hosts to act as true peers, serving and retrieving information from each other. People could access all of their home systems from work, changing the climate control settings or unlocking the doors for early guests. This vision of universal connectivity has been stifled by address space shortages and security concerns. In the early 1990s, organizations began deploying firewalls for the express purpose of reducing connectivity. Huge networks were cordoned off from the unfiltered Internet by application proxies, network address translation, and packet filters. The unrestricted flow of information gave way to tight regulation of approved communication channels and the content that passes over them.

   Network obstructions such as firewalls can make mapping a network
   exceedingly difficult. It will not get any easier, as stifling casual
   reconnaissance is often a key goal of implementing the devices.
   Nevertheless, Nmap offers many features to help understand these
   complex networks, and to verify that filters are working as intended.
   It even supports mechanisms for bypassing poorly implemented defenses.
   One of the best methods of understanding your network security posture
   is to try to defeat it. Place yourself in the mindset of an attacker,
   and deploy techniques from this section against your networks. Launch
   an FTP bounce scan, Idle scan, fragmentation attack, or try to tunnel
   through one of your own proxies.

   In addition to restricting network activity, companies are increasingly
   monitoring traffic with intrusion detection systems (IDS). All of the
   major IDSs ship with rules designed to detect Nmap scans because scans
   are sometimes a precursor to attacks. Many of these products have
   recently morphed into intrusion prevention systems (IPS) that actively
   block traffic deemed malicious. Unfortunately for network
   administrators and IDS vendors, reliably detecting bad intentions by
   analyzing packet data is a tough problem. Attackers with patience,
   skill, and the help of certain Nmap options can usually pass by IDSs
   undetected. Meanwhile, administrators must cope with large numbers of
   false positive results where innocent activity is misdiagnosed and
   alerted on or blocked.

   Occasionally people suggest that Nmap should not offer features for
   evading firewall rules or sneaking past IDSs. They argue that these
   features are just as likely to be misused by attackers as used by
   administrators to enhance security. The problem with this logic is that
   these methods would still be used by attackers, who would just find
   other tools or patch the functionality into Nmap. Meanwhile,
   administrators would find it that much harder to do their jobs.
   Deploying only modern, patched FTP servers is a far more powerful
   defense than trying to prevent the distribution of tools implementing
   the FTP bounce attack.

   There is no magic bullet (or Nmap option) for detecting and subverting
   firewalls and IDS systems. It takes skill and experience. A tutorial is
   beyond the scope of this reference guide, which only lists the relevant
   options and describes what they do.

   -f (fragment packets); --mtu (using the specified MTU)
      The -f option causes the requested scan (including ping scans)
      to use tiny fragmented IP packets. The idea is to split up the
      TCP header over several packets to make it harder for packet
      filters, intrusion detection systems, and other annoyances to
      detect what you are doing. Be careful with this! Some programs
      have trouble handling these tiny packets. The old-school sniffer
      named Sniffit segmentation faulted immediately upon receiving
      the first fragment. Specify this option once, and Nmap splits
      the packets into 8 bytes or less after the IP header. So a
      20-byte TCP header would be split into 3 packets. Two with eight
      bytes of the TCP header, and one with the final four. Of course
      each fragment also has an IP header. Specify -f again to use 16
      bytes per fragment (reducing the number of fragments). Or you
      can specify your own offset size with the --mtu option. Don’t
      also specify -f if you use --mtu. The offset must be a multiple
      of 8. While fragmented packets won’t get by packet filters and
      firewalls that queue all IP fragments, such as the
      CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some
      networks can’t afford the performance hit this causes and thus
      leave it disabled. Others can’t enable this because fragments
      may take different routes into their networks. Some source
      systems defragment outgoing packets in the kernel. Linux with
      the iptables connection tracking module is one such example. Do
      a scan while a sniffer such as Ethereal is running to ensure
      that sent packets are fragmented. If your host OS is causing
      problems, try the --send-eth option to bypass the IP layer and
      send raw ethernet frames.

   -D <decoy1 [,decoy2][,ME],...> (Cloak a scan with decoys)
      Causes a decoy scan to be performed, which makes it appear to
      the remote host that the host(s) you specify as decoys are
      scanning the target network too. Thus their IDS might report
      5-10 port scans from unique IP addresses, but they won’t know
      which IP was scanning them and which were innocent decoys. While
      this can be defeated through router path tracing,
      response-dropping, and other active mechanisms, it is generally
      an effective technique for hiding your IP address.

      Separate each decoy host with commas, and you can optionally use
      ME as one of the decoys to represent the position for your real
      IP address. If you put ME in the 6th position or later, some
      common port scan detectors (such as Solar Designer’s excellent
      scanlogd) are unlikely to show your IP address at all. If you
      don’t use ME, nmap will put you in a random position.

      Note that the hosts you use as decoys should be up or you might
      accidentally SYN flood your targets. Also it will be pretty easy
      to determine which host is scanning if only one is actually up
      on the network. You might want to use IP addresses instead of
      names (so the decoy networks don’t see you in their nameserver
      logs).

      Decoys are used both in the initial ping scan (using ICMP, SYN,
      ACK, or whatever) and during the actual port scanning phase.
      Decoys are also used during remote OS detection (-O). Decoys do
      not work with version detection or TCP connect scan.

      It is worth noting that using too many decoys may slow your scan
      and potentially even make it less accurate. Also, some ISPs will
      filter out your spoofed packets, but many do not restrict
      spoofed IP packets at all.

   -S <IP_Address> (Spoof source address)
      In some circumstances, Nmap may not be able to determine your
      source address ( Nmap will tell you if this is the case). In
      this situation, use -S with the IP address of the interface you
      wish to send packets through.

      Another possible use of this flag is to spoof the scan to make
      the targets think that someone else is scanning them. Imagine a
      company being repeatedly port scanned by a competitor! The -e
      option and -P0 are generally required for this sort of usage.
      Note that you usually won’t receive reply packets back (they
      will be addressed to the IP you are spoofing), so Nmap won’t
      produce useful reports.

   -e <interface> (Use specified interface)
      Tells Nmap what interface to send and receive packets on. Nmap
      should be able to detect this automatically, but it will tell
      you if it cannot.

   --source-port <portnumber>; -g <portnumber> (Spoof source port number)
      One surprisingly common misconfiguration is to trust traffic
      based only on the source port number. It is easy to understand
      how this comes about. An administrator will set up a shiny new
      firewall, only to be flooded with complains from ungrateful
      users whose applications stopped working. In particular, DNS may
      be broken because the UDP DNS replies from external servers can
      no longer enter the network. FTP is another common example. In
      active FTP transfers, the remote server tries to establish a
      connection back to the client to transfer the requested file.

      Secure solutions to these problems exist, often in the form of
      application-level proxies or protocol-parsing firewall modules.
      Unfortunately there are also easier, insecure solutions. Noting
      that DNS replies come from port 53 and active ftp from port 20,
      many admins have fallen into the trap of simply allowing
      incoming traffic from those ports. They often assume that no
      attacker would notice and exploit such firewall holes. In other
      cases, admins consider this a short-term stop-gap measure until
      they can implement a more secure solution. Then they forget the
      security upgrade.

      Overworked network administrators are not the only ones to fall
      into this trap. Numerous products have shipped with these
      insecure rules. Even Microsoft has been guilty. The IPsec
      filters that shipped with Windows 2000 and Windows XP contain an
      implicit rule that allows all TCP or UDP traffic from port 88
      (Kerberos). In another well-known case, versions of the Zone
      Alarm personal firewall up to 2.1.25 allowed any incoming UDP
      packets with the source port 53 (DNS) or 67 (DHCP).

      Nmap offers the -g and --source-port options (they are
      equivalent) to exploit these weaknesses. Simply provide a port
      number and Nmap will send packets from that port where possible.
      Nmap must use different port numbers for certain OS detection
      tests to work properly, and DNS requests ignore the
      --source-port flag because Nmap relies on system libraries to
      handle those. Most TCP scans, including SYN scan, support the
      option completely, as does UDP scan.

   --data-length <number> (Append random data to sent packets)
      Normally Nmap sends minimalist packets containing only a header.
      So its TCP packets are generally 40 bytes and ICMP echo requests
      are just 28. This option tells Nmap to append the given number
      of random bytes to most of the packets it sends. OS detection
      (-O) packets are not affected because accuracy there requires
      probe consistency, but most pinging and portscan packets support
      this. It slows things down a little, but can make a scan
      slightly less conspicuous.

   --ip-options <S|R [route]|L [route]|T|U ... >; --ip-options <hex
   string> (Send packets with specified ip options)
      The [6]IP protocol offers several options which may be placed in
      packet headers. Unlike the ubiquitous TCP options, IP options
      are rarely seen due to practicality and security concerns. In
      fact, many Internet routers block the most dangerous options
      such as source routing. Yet options can still be useful in some
      cases for determining and manipulating the network route to
      target machines. For example, you may be able to use the record
      route option to determine a path to a target even when more
      traditional traceroute-style approaches fail. Or if your packets
      are being dropped by a certain firewall, you may be able to
      specify a different route with the strict or loose source
      routing options.

      The most powerful way to specify IP options is to simply pass in
      values as the argument to --ip-options. Precede each hex number
      with \x then the two digits. You may repeat certain characters
      by following them with an asterisk and then the number of times
      you wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is
      a hex string containing 36 NUL bytes.

      Nmap also offers a shortcut mechanism for specifying options.
      Simply pass the letter R, T, or U to request record-route,
      record-timestamp, or both options together, respectively. Loose
      or strict source routing may be specified with an L or S
      followed by a space and then a space-separated list of IP
      addresses.

      If you wish to see the options in packets sent and received,
      specify --packet-trace. For more information and examples of
      using IP options with Nmap, see
      http://seclists.org/nmap-dev/2006/q3/0052.html.

   --ttl <value> (Set IP time-to-live field)
      Sets the IPv4 time-to-live field in sent packets to the given
      value.

   --randomize-hosts (Randomize target host order)
      Tells Nmap to shuffle each group of up to 8096 hosts before it
      scans them. This can make the scans less obvious to various
      network monitoring systems, especially when you combine it with
      slow timing options. If you want to randomize over larger group
      sizes, increase PING_GROUP_SZ in nmap.h and recompile. An
      alternative solution is to generate the target IP list with a
      list scan (-sL -n -oN filename), randomize it with a Perl
      script, then provide the whole list to Nmap with -iL.

   --spoof-mac <mac address, prefix, or vendor name> (Spoof MAC address)
      Asks Nmap to use the given MAC address for all of the raw
      ethernet frames it sends. This option implies --send-eth to
      ensure that Nmap actually sends ethernet-level packets. The MAC
      given can take several formats. If it is simply the string “0”,
      Nmap chooses a completely random MAC for the session. If the
      given string is an even number of hex digits (with the pairs
      optionally separated by a colon), Nmap will use those as the
      MAC. If less than 12 hex digits are provided, Nmap fills in the
      remainder of the 6 bytes with random values. If the argument
      isn’t a 0 or hex string, Nmap looks through nmap-mac-prefixes to
      find a vendor name containing the given string (it is case
      insensitive). If a match is found, Nmap uses the vendor’s OUI
      (3-byte prefix) and fills out the remaining 3 bytes randomly.
      Valid --spoof-mac argument examples are Apple, 0,
      01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco.

   --badsum (Send packets with bogus TCP/UDP checksums)
      Asks Nmap to use an invalid TCP or UDP checksum for packets sent
      to target hosts. Since virtually all host IP stacks properly
      drop these packets, any responses received are likely coming
      from a firewall or IDS that didn’t bother to verify the
      checksum. For more details on this technique, see
      http://www.phrack.org/phrack/60/p60-0x0c.txt

OUTPUT Any security tools is only as useful as the output it generates. Complex tests and algorithms are of little value if they aren’t presented in an organized and comprehensible fashion. Given the number of ways Nmap is used by people and other software, no single format can please everyone. So Nmap offers several formats, including the interactive mode for humans to read directly and XML for easy parsing by software.

   In addition to offering different output formats, Nmap provides options
   for controlling the verbosity of output as well as debugging messages.
   Output types may be sent to standard output or to named files, which
   Nmap can append to or clobber. Output files may also be used to resume
   aborted scans.

   Nmap makes output available in five different formats. The default is
   called interactive output, and it is sent to standard output (stdout).
   There is also normal output, which is similar to interactive except
   that it displays less runtime information and warnings since it is
   expected to be analyzed after the scan completes rather than
   interactively.

   XML output is one of the most important output types, as it can be
   converted to HTML, easily parsed by programs such as Nmap graphical
   user interfaces, or imported into databases.

   The two remaining output types are the simple grepable output which
   includes most information for a target host on a single line, and
   sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.

   While interactive output is the default and has no associated
   command-line options, the other four format options use the same
   syntax. They take one argument, which is the filename that results
   should be stored in. Multiple formats may be specified, but each format
   may only be specified once. For example, you may wish to save normal
   output for your own review while saving XML of the same scan for
   programmatic analysis. You might do this with the options -oX
   myscan.xml -oN myscan.nmap. While this chapter uses the simple names
   like myscan.xml for brevity, more descriptive names are generally
   recommended. The names chosen are a matter of personal preference,
   though I use long ones that incorporate the scan date and a word or two
   describing the scan, placed in a directory named after the company I’m
   scanning.

   While these options save results to files, Nmap still prints
   interactive output to stdout as usual. For example, the command nmap
   -oX myscan.xml target prints XML to myscan.xml and fills standard
   output with the same interactive results it would have printed if -oX
   wasn’t specified at all. You can change this by passing a hyphen
   character as the argument to one of the format types. This causes Nmap
   to deactivate interactive output, and instead print results in the
   format you specified to the standard output stream. So the command nmap
   -oX - target will send only XML output to stdout. Serious errors may
   still be printed to the normal error stream, stderr.

   Unlike some Nmap arguments, the space between the logfile option flag
   (such as -oX) and the filename or hyphen is mandatory. If you omit the
   flags and give arguments such as -oG- or -oXscan.xml, a backwards
   compatibility feature of Nmap will cause the creation of normal format
   output files named G- and Xscan.xml respectively.

   Nmap also offers options to control scan verbosity and to append to
   output files rather than clobbering them. All of these options are
   described below.

   Nmap Output Formats

   -oN <filespec> (Normal output)
      Requests that normal output be directed to the given filename.
      As discussed above, this differs slightly from interactive
      output.

   -oX <filespec> (XML output)
      Requests that XML output be directed to the given filename. Nmap
      includes a document type definition (DTD) which allows XML
      parsers to validate Nmap XML output. While it is primarily
      intended for programmatic use, it can also help humans interpret
      Nmap XML output. The DTD defines the legal elements of the
      format, and often enumerates the attributes and values they can
      take on. The latest version is always available from
      http://insecure.org/nmap/data/nmap.dtd.

      XML offers a stable format that is easily parsed by software.
      Free XML parsers are available for all major computer languages,
      including C/C++, Perl, Python, and Java. People have even
      written bindings for most of these languages to handle Nmap
      output and execution specifically. Examples are [7]Nmap::Scanner
      and [8]Nmap::Parser in Perl CPAN. In almost all cases that a
      non-trivial application interfaces with Nmap, XML is the
      preferred format.

      The XML output references an XSL stylesheet which can be used to
      format the results as HTML. The easiest way to use this is
      simply to load the XML output in a web browser such as Firefox
      or IE. By default, this will only work on the machine you ran
      Nmap on (or a similarly configured one) due to the hard-coded
      nmap.xsl filesystem path. Use the --webxml or --stylesheet
      options to create portable XML files that render as HTML on any
      web-connected machine.

   -oS <filespec> (ScRipT KIdd|3 oUTpuT)
      Script kiddie output is like interactive output, except that it
      is post-processed to better suit the l33t HaXXorZ who previously
      looked down on Nmap due to its consistent capitalization and
      spelling. Humor impaired people should note that this option is
      making fun of the script kiddies before flaming me for
      supposedly “helping them”.

   -oG <filespec> (Grepable output)
      This output format is covered last because it is deprecated. The
      XML output format is far more powerful, and is nearly as
      convenient for experienced users. XML is a standard for which
      dozens of excellent parsers are available, while grepable output
      is my own simple hack. XML is extensible to support new Nmap
      features as they are released, while I often must omit those
      features from grepable output for lack of a place to put them.

      Nevertheless, grepable output is still quite popular. It is a
      simple format that lists each host on one line and can be
      trivially searched and parsed with standard UNIX tools such as
      grep, awk, cut, sed, diff, and Perl. Even I usually use it for
      one-off tests done at the command line. Finding all the hosts
      with the ssh port open or that are running Solaris takes only a
      simple grep to identify the hosts, piped to an awk or cut
      command to print the desired fields.

      Grepable output consists of comments (lines starting with a
      pound (#)) and target lines. A target line includes a
      combination of 6 labeled fields, separated by tabs and followed
      with a colon. The fields are Host, Ports, Protocols, Ignored
      State, OS, Seq Index, IPID, and Status.

      The most important of these fields is generally Ports, which
      gives details on each interesting port. It is a comma separated
      list of port entries. Each port entry represents one interesting
      port, and takes the form of seven slash (/) separated subfields.
      Those subfields are: Port number, State, Protocol, Owner,
      Service, SunRPC info, and Version info.

      As with XML output, this man page does not allow for documenting
      the entire format. A more detailed look at the Nmap grepable
      output format is available from
      http://www.unspecific.com/nmap-oG-output.

   -oA <basename> (Output to all formats)

      As a convenience, you may specify -oA basename to store scan
      results in normal, XML, and grepable formats at once. They are
      stored in basename.nmap, basename.xml, and basename.gnmap,
      respectively. As with most programs, you can prefix the
      filenames with a directory path, such as ~/nmaplogs/foocorp/ on
      UNIX or c:\hacking\sco on Windows.

   Verbosity and debugging options

   -v (Increase verbosity level)
      Increases the verbosity level, causing Nmap to print more
      information about the scan in progress. Open ports are shown as
      they are found and completion time estimates are provided when
      Nmap thinks a scan will take more than a few minutes. Use it
      twice for even greater verbosity. Using it more than twice has
      no effect.

      Most changes only affect interactive output, and some also
      affect normal and script kiddie output. The other output types
      are meant to be processed by machines, so Nmap can give
      substantial detail by default in those formats without fatiguing
      a human user. However, there are a few changes in other modes
      where output size can be reduced substantially by omitting some
      detail. For example, a comment line in the grepable output that
      provides a list of all ports scanned is only printed in verbose
      mode because it can be quite long.

   -d [level] (Increase or set debugging level)
      When even verbose mode doesn’t provide sufficient data for you,
      debugging is available to flood you with much more! As with the
      verbosity option (-v), debugging is enabled with a command-line
      flag (-d) and the debug level can be increased by specifying it
      multiple times. Alternatively, you can set a debug level by
      giving an argument to -d. For example, -d9 sets level nine. That
      is the highest effective level and will produce thousands of
      lines unless you run a very simple scan with very few ports and
      targets.

      Debugging output is useful when a bug is suspected in Nmap, or
      if you are simply confused as to what Nmap is doing and why. As
      this feature is mostly intended for developers, debug lines
      aren’t always self-explanatory. You may get something like:
      Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==>
      srtt: 14987 rttvar: 14987 to: 100000. If you don’t understand a
      line, your only recourses are to ignore it, look it up in the
      source code, or request help from the development list
      (nmap-dev). Some lines are self explanatory, but the messages
      become more obscure as the debug level is increased.

   --packet-trace (Trace packets and data sent and received)
      Causes Nmap to print a summary of every packet sent or received.
      This is often used for debugging, but is also a valuable way for
      new users to understand exactly what Nmap is doing under the
      covers. To avoid printing thousands of lines, you may want to
      specify a limited number of ports to scan, such as -p20-30. If
      you only care about the goings on of the version detection
      subsystem, use --version-trace instead.

   --open (Show only open (or possibly open) ports)
      Sometimes you only care about ports you can actually connect to
      (open ones), and don’t want results cluttered with closed,
      filtered, and closed|filtered ports. Output customization is
      normally done after the scan using tools such as grep, awk, and
      Perl, but this feature was added due to overwhelming requests.
      Specify --open to only see open, open|filtered, and unfiltered
      ports. These three ports are treated just as they normally are,
      which means that open|filtered and unfiltered may be condensed
      into counts if there are an overwhelming number of them.

   --iflist (List interfaces and routes)
      Prints the interface list and system routes as detected by Nmap.
      This is useful for debugging routing problems or device
      mischaracterization (such as Nmap treating a PPP connection as
      Ethernet).

   --log-errors (Log errors/warnings to normal mode output file)
      Warnings and errors printed by Nmap usually go only to the
      screen (interactive output), leaving any specified normal-fomat
      output files uncluttered. But when you do want to see those
      messages in the normal output file you specified, add this
      option. It is useful when you aren’t watching the interactive
      output or are trying to debug a problem. The messages will also
      still appear in interactive mode. This will not work for most
      errors related to bad command-line arguments, as Nmap may not
      have initialized its output files yet. In addition, some Nmap
      error/warning messages use a different system that does not yet
      support this option. An alternative to using this option is
      redirecting interactive output (including the standard error
      stream) to a file. While most UNIX shells make that approach
      easy, it can be difficult on Windows.

   Miscellaneous output options

   --append-output (Append to rather than clobber output files)
      When you specify a filename to an output format flag such as -oX
      or -oN, that file is overwritten by default. If you prefer to
      keep the existing content of the file and append the new
      results, specify the --append-output option. All output
      filenames specified in that Nmap execution will then be appended
      to rather than clobbered. This doesn’t work well for XML (-oX)
      scan data as the resultant file generally won’t parse properly
      until you fix it up by hand.

   --resume <filename> (Resume aborted scan)
      Some extensive Nmap runs take a very long time -- on the order
      of days. Such scans don’t always run to completion. Restrictions
      may prevent Nmap from being run during working hours, the
      network could go down, the machine Nmap is running on might
      suffer a planned or unplanned reboot, or Nmap itself could
      crash. The admin running Nmap could cancel it for any other
      reason as well, by pressing ctrl-C. Restarting the whole scan
      from the beginning may be undesirable. Fortunately, if normal
      (-oN) or grepable (-oG) logs were kept, the user can ask Nmap to
      resume scanning with the target it was working on when execution
      ceased. Simply specify the --resume option and pass the
      normal/grepable output file as its argument. No other arguments
      are permitted, as Nmap parses the output file to use the same
      ones specified previously. Simply call Nmap as nmap --resume
      logfilename. Nmap will append new results to the data files
      specified in the previous execution. Resumption does not support
      the XML output format because combining the two runs into one
      valid XML file would be difficult.

   --stylesheet <path or URL> (Set XSL stylesheet to transform XML output)
      Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
      translating XML output to HTML. The XML output includes an
      xml-stylesheet directive which points to nmap.xml where it was
      initially installed by Nmap (or in the current working directory
      on Windows). Simply load Nmap’s XML output in a modern web
      browser and it should retrieve nmap.xsl from the filesystem and
      use it to render results. If you wish to use a different
      stylesheet, specify it as the argument to --stylesheet. You must
      pass the full pathname or URL. One common invocation is
      --stylesheet http://insecure.org/nmap/data/nmap.xsl. This tells
      a browser to load the latest version of the stylesheet from
      Insecure.Org. The --webxml option does the same thing with less
      typing and memorization. Loading the XSL from Insecure.Org makes
      it easier to view results on a machine that doesn’t have Nmap
      (and thus nmap.xsl) installed. So the URL is often more useful,
      but the local filesystem location of nmap.xsl is used by default
      for privacy reasons.

   --webxml (Load stylesheet from Insecure.Org)
      This convenience option is simply an alias for --stylesheet
      http://insecure.org/nmap/data/nmap.xsl.

   --no_stylesheet (Omit XSL stylesheet declaration from XML)
      Specify this option to prevent Nmap from associating any XSL
      stylesheet with its XML output. The xml-stylesheet directive is
      omitted.

MISCELLANEOUS OPTIONS This section describes some important (and not-so-important) options that don’t really fit anywhere else.

   -6 (Enable IPv6 scanning)
      Since 2002, Nmap has offered IPv6 support for its most popular
      features. In particular, ping scanning (TCP-only), connect
      scanning, and version detection all support IPv6. The command
      syntax is the same as usual except that you also add the -6
      option. Of course, you must use IPv6 syntax if you specify an
      address rather than a hostname. An address might look like
      3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
      recommended. The output looks the same as usual, with the IPv6
      address on the “interesting ports” line being the only IPv6 give
      away.

      While IPv6 hasn’t exactly taken the world by storm, it gets
      significant use in some (usually Asian) countries and most
      modern operating systems support it. To use Nmap with IPv6, both
      the source and target of your scan must be configured for IPv6.
      If your ISP (like most of them) does not allocate IPv6 addresses
      to you, free tunnel brokers are widely available and work fine
      with Nmap. One of the better ones is run by BT Exact at
      https://tb.ipv6.btexact.com/. I have also used one that
      Hurricane Electric provides at http://ipv6tb.he.net/. 6to4
      tunnels are another popular, free approach.

   -A (Aggressive scan options)
      This option enables additional advanced and aggressive options.
      I haven’t decided exactly which it stands for yet. Presently
      this enables OS Detection (-O) and version scanning (-sV). More
      features may be added in the future. The point is to enable a
      comprehensive set of scan options without people having to
      remember a large set of flags. This option only enables
      features, and not timing options (such as -T4) or verbosity
      options (-v) that you might want as well.

   --datadir <directoryname> (Specify custom Nmap data file location)
      Nmap obtains some special data at runtime in files named
      nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
      nmap-mac-prefixes, and nmap-os-fingerprints. Nmap first searches
      these files in the directory specified with the --datadir option
      (if any). Any files not found there, are searched for in the
      directory specified by the NMAPDIR environmental variable. Next
      comes ~/.nmap for real and effective UIDs (POSIX systems only)
      or location of the Nmap executable (Win32 only), and then a
      compiled-in location such as /usr/local/share/nmap or
      /usr/share/nmap

   --send-eth (Use raw ethernet sending)
      Asks Nmap to send packets at the raw ethernet (data link) layer
      rather than the higher IP (network) layer. By default, Nmap
      chooses the one which is generally best for the platform it is
      running on. Raw sockets (IP layer) are generally most efficient
      for UNIX machines, while ethernet frames are required for
      Windows operation since Microsoft disabled raw socket support.
      Nmap still uses raw IP packets on UNIX despite this option when
      there is no other choice (such as non-ethernet connections).

   --send-ip (Send at raw IP level)
      Asks Nmap to send packets via raw IP sockets rather than sending
      lower level ethernet frames. It is the complement to the
      --send-eth option discussed previously.

   --privileged (Assume that the user is fully privileged)
      Tells Nmap to simply assume that it is privileged enough to
      perform raw socket sends, packet sniffing, and similar
      operations that usually require root privileges on UNIX systems.
      By default Nmap quits if such operations are requested but
      geteuid() is not zero.  --privileged is useful with Linux kernel
      capabilities and similar systems that may be configured to allow
      unprivileged users to perform raw-packet scans. Be sure to
      provide this option flag before any flags for options that
      require privileges (SYN scan, OS detection, etc.). The
      NMAP_PRIVILEGED variable may be set as an equivalent alternative
      to --privileged.

   --unprivileged (Assume that the user lacks raw socket privileges)
      This option is the opposite of --privileged. It tells Nmap to
      treat the user as lacking network raw socket and sniffing
      privileges. This is useful for testing, debugging, or when the
      raw network functionality of your operating system is somehow
      broken.

   --release-memory (Release memory before quitting)
      This option is only useful for memory-leak debugging. It causes
      Nmap to release allocated memory just before it quits so that
      actual memory leaks are easier to spot. Normally Nmap skips this
      as the OS does this anyway upon process termination.

   --interactive (Start in interactive mode)
      Starts Nmap in interactive mode, which offers an interactive
      Nmap prompt allowing easy launching of multiple scans (either
      synchronously or in the background). This is useful for people
      who scan from multi-user systems as they often want to test
      their security without letting everyone else on the system know
      exactly which systems they are scanning. Use --interactive to
      activate this mode and then type h for help. This option is
      rarely used because proper shells are usually more familiar and
      feature-complete. This option includes a bang (!) operator for
      executing shell commands, which is one of many reasons not to
      install Nmap setuid root.

   -V; --version (Print version number)
      Prints the Nmap version number and exits.

   -h; --help (Print help summary page)
      Prints a short help screen with the most common command flags.
      Running Nmap without any arguments does the same thing.

RUNTIME INTERACTION During the execution of nmap, all key presses are captured. This allows you to interact with the program without aborting and restarting it. Certain special keys will change options, while any other keys will print out a status message telling you about the scan. The convention is that lowercase letters increase the amount of printing, and uppercase letters decrease the printing. You may also press ‘?’ for help.

   v / V  Increase / Decrease the Verbosity

   d / D  Increase / Decrease the Debugging Level

   p / P  Turn on / off Packet Tracing

   ?      Print a runtime interaction help screen

   Anything else
      Print out a status message like this:

      Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing
      Service Scan

      Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15
      remaining)

EXAMPLES Here are some Nmap usage examples, from the simple and routine to a little more complex and esoteric. Some actual IP addresses and domain names are used to make things more concrete. In their place you should substitute addresses/names from your own network.. While I don’t think port scanning other networks is or should be illegal, some network administrators don’t appreciate unsolicited scanning of their networks and may complain. Getting permission first is the best approach.

   For testing purposes, you have permission to scan the host
   scanme.nmap.org. This permission only includes scanning via Nmap and
   not testing exploits or denial of service attacks. To conserve
   bandwidth, please do not initiate more than a dozen scans against that
   host per day. If this free scanning target service is abused, it will
   be taken down and Nmap will report Failed to resolve given hostname/IP:
   scanme.nmap.org. These permissions also apply to the hosts
   scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do
   not currently exist.

   nmap -v scanme.nmap.org

   This option scans all reserved TCP ports on the machine scanme.nmap.org
   -v option enables verbose mode.

   nmap -sS -O scanme.nmap.org/24

   Launches a stealth SYN scan against each machine that is up out of the
   255 machines on “class C” network where Scanme resides. It also tries
   to determine what operating system is running on each host that is up
   and running. This requires root privileges because of the SYN scan and
   OS detection.

   nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127

   Launches host enumeration and a TCP scan at the first half of each of
   the 255 possible 8 bit subnets in the 198.116 class B address space.
   This tests whether the systems run sshd, DNS, pop3d, imapd, or port
   4564. For any of these ports found open, version detection is used to
   determine what application is running.

   nmap -v -iR 100000 -P0 -p 80

   Asks Nmap to choose 100,000 hosts at random and scan them for web
   servers (port 80). Host enumeration is disabled with -P0 since first
   sending a couple probes to determine whether a host is up is wasteful
   when you are only probing one port on each target host anyway.

   nmap -P0 -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap
   216.163.128.20/20

   This scans 4096 IPs for any webservers (without pinging them) and saves
   the output in grepable and XML formats.

BUGS Like its author, Nmap isn’t perfect. But you can help make it better by sending bug reports or even writing patches. If Nmap doesn’t behave the way you expect, first upgrade to the latest version available from http://insecure.org/nmap/. If the problem persists, do some research to determine whether it has already been discovered and addressed. Try Googling the error message or browsing the Nmap-dev archives at http://seclists.org/. Read this full munual page as well. If nothing comes of this, mail a bug report to nmap-dev@insecure.org. Please include everything you have learned about the problem, as well as what version of Nmap you are running and what operating system version it is running on. Problem reports and Nmap usage questions sent to nmap-dev@insecure.org are far more likely to be answered than those sent to Fyodor directly.

   Code patches to fix bugs are even better than bug reports. Basic
   instructions for creating patch files with your changes are available
   at http://insecure.org/nmap/data/HACKING. Patches may be sent to
   nmap-dev (recommended) or to Fyodor directly.

AUTHOR Fyodor fyodor@insecure.org (http://insecure.org)

   Hundreds of people have made valuable contributions to Nmap over the
   years. These are detailed in the CHANGELOG file which is distributed
   with Nmap and also available from
   http://insecure.org/nmap/changelog.html.

LEGAL NOTICES Nmap Copyright and Licensing The Nmap Security Scanner is (C) 1996-2005 Insecure.Com LLC. Nmap is also a registered trademark of Insecure.Com LLC. This program is free software; you may redistribute and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; Version 2. This guarantees your right to use, modify, and redistribute this software under certain conditions. If you wish to embed Nmap technology into proprietary software, we may be willing to sell alternative licenses (contact sales@insecure.com). Many security scanner vendors already license Nmap technology such as host discovery, port scanning, OS detection, and service/version detection.

   Note that the GPL places important restrictions on “derived works”, yet
   it does not provide a detailed definition of that term. To avoid
   misunderstandings, we consider an application to constitute a
   “derivative work” for the purpose of this license if it does any of the
   following:

   ·  Integrates source code from Nmap

   ·  Reads or includes Nmap copyrighted data files, such as
  nmap-os-fingerprints or nmap-service-probes.

   ·  Executes Nmap and parses the results (as opposed to typical shell or
  execution-menu apps, which simply display raw Nmap output and so are
  not derivative works.)

   ·  Integrates/includes/aggregates Nmap into a proprietary executable
  installer, such as those produced by InstallShield.

   ·  Links to a library or executes a program that does any of the above.

   The term “Nmap” should be taken to also include any portions or derived
   works of Nmap. This list is not exclusive, but is just meant to clarify
   our interpretation of derived works with some common examples. These
   restrictions only apply when you actually redistribute Nmap. For
   example, nothing stops you from writing and selling a proprietary
   front-end to Nmap. Just distribute it by itself, and point people to
   http://insecure.org/nmap/ to download Nmap.

   We don’t consider these to be added restrictions on top of the GPL, but
   just a clarification of how we interpret “derived works” as it applies
   to our GPL-licensed Nmap product. This is similar to the way Linus
   Torvalds has announced his interpretation of how “derived works”
   applies to Linux kernel modules. Our interpretation refers only to Nmap
   - we don’t speak for any other GPL products.

   If you have any questions about the GPL licensing restrictions on using
   Nmap in non-GPL works, we would be happy to help. As mentioned above,
   we also offer alternative license to integrate Nmap into proprietary
   applications and appliances. These contracts have been sold to many
   security vendors, and generally include a perpetual license as well as
   providing for priority support and updates as well as helping to fund
   the continued development of Nmap technology. Please email
   <sales@insecure.com> for further information.

   As a special exception to the GPL terms, Insecure.Com LLC grants
   permission to link the code of this program with any version of the
   OpenSSL library which is distributed under a license identical to that
   listed in the included Copying.OpenSSL file, and distribute linked
   combinations including the two. You must obey the GNU GPL in all
   respects for all of the code used other than OpenSSL. If you modify
   this file, you may extend this exception to your version of the file,
   but you are not obligated to do so.

   If you received these files with a written license agreement or
   contract stating terms other than the terms above, then that
   alternative license agreement takes precedence over these comments.

Creative Commons license for this Nmap guide This Nmap Reference Guide is (C) 2005 Insecure.Com LLC. It is hereby placed under version 2.5 of the [9]Creative Commons Attribution License. This allows you redistribute and modify the work as you desire, as long as you credit the original source. Alternatively, you may choose to treat this document as falling under the same license as Nmap itself (discussed previously).

Source code availability and community contributions Source is provided to this software because we believe users have a right to know exactly what a program is going to do before they run it. This also allows you to audit the software for security holes (none have been found so far).

   Source code also allows you to port Nmap to new platforms, fix bugs,
   and add new features. You are highly encouraged to send your changes to
   <fyodor@insecure.org> for possible incorporation into the main
   distribution. By sending these changes to Fyodor or one of the
   Insecure.Org development mailing lists, it is assumed that you are
   offering Fyodor and Insecure.Com LLC the unlimited, non-exclusive right
   to reuse, modify, and relicense the code. Nmap will always be available
   Open Source, but this is important because the inability to relicense
   code has caused devastating problems for other Free Software projects
   (such as KDE and NASM). We also occasionally relicense the code to
   third parties as discussed above. If you wish to specify special
   license conditions of your contributions, just say so when you send
   them.

No Warranty This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details at http://www.gnu.org/copyleft/gpl.html, or in the COPYING file included with Nmap.

   It should also be noted that Nmap has occasionally been known to crash
   poorly written applications, TCP/IP stacks, and even operating systems.
   While this is extremely rare, it is important to keep in mind.  Nmap
   should never be run against mission critical systems unless you are
   prepared to suffer downtime. We acknowledge here that Nmap may crash
   your systems or networks and we disclaim all liability for any damage
   or problems Nmap could cause.

Inappropriate Usage Because of the slight risk of crashes and because a few black hats like to use Nmap for reconnaissance prior to attacking systems, there are administrators who become upset and may complain when their system is scanned. Thus, it is often advisable to request permission before doing even a light scan of a network.

   Nmap should never be installed with special privileges (e.g. suid root)
   for security reasons.

Third-Party Software This product includes software developed by the [10]Apache Software Foundation. A modified version of the [11]Libpcap portable packet capture library is distributed along with nmap. The Windows version of Nmap utilized the libpcap-derived [12]WinPcap library instead. Regular expression support is provided by the [13]PCRE library, which is open source software, written by Philip Hazel. Certain raw networking functions use the [14]Libdnet networking library, which was written by Dug Song. A modified version is distributed with Nmap. Nmap can optionally link with the [15]OpenSSL cryptography toolkit for SSL version detection support. All of the third-party software described in this paragraph is freely redistributable under BSD-style software licenses.

US Export Control Classification US Export Control: Insecure.Com LLC believes that Nmap falls under US ECCN (export control classification number) 5D992. This category is called “Information Security software not controlled by 5D002”. The only restriction of this classification is AT (anti-terrorism), which applies to almost all goods and denies export to a handful of rogue nations such as Iran and North Korea. Thus exporting Nmap does not require any special license, permit, or other governmental authorization.

REFERENCES 1. RFC 1122 http://www.rfc-editor.org/rfc/rfc1122.txt

2. RFC 792
   http://www.rfc-editor.org/rfc/rfc792.txt

3. UDP
   http://www.rfc-editor.org/rfc/rfc768.txt

4. TCP RFC
   http://www.rfc-editor.org/rfc/rfc793.txt

5. RFC 959
   http://www.rfc-editor.org/rfc/rfc959.txt

6. IP protocol
   http://www.ietf.org/rfc/rfc0791.txt

7. Nmap::Scanner
   http://sourceforge.net/projects/nmap-scanner/

8. Nmap::Parser
   http://www.nmapparser.com

9. Creative Commons Attribution License
   http://creativecommons.org/licenses/by/2.5/

   10. Apache Software Foundation
   http://www.apache.org

   11. Libpcap portable packet capture library
   http://www.tcpdump.org

   12. WinPcap library
   http://www.winpcap.org

   13. PCRE library
   http://www.pcre.org

   14. Libdnet
   http://libdnet.sourceforge.net

   15. OpenSSL cryptography toolkit
   http://www.openssl.org



			  12/07/2006			       NMAP(1)