dhcpd.conf(5) dhcpd.conf(5)

NAME dhcpd.conf - dhcpd configuration file

DESCRIPTION The dhcpd.conf file contains configuration information for dhcpd, the Internet Systems Consortium DHCP Server.

   The dhcpd.conf file is a free-form ASCII text file.   It is  parsed  by
   the  recursive-descent  parser built into dhcpd.	  The file may contain
   extra tabs and newlines for formatting purposes.	 Keywords in the  file
   are case-insensitive.   Comments may be placed anywhere within the file
   (except within quotes).	 Comments begin with the # character  and  end
   at the end of the line.

   The  file  essentially  consists	 of a list of statements.   Statements
   fall into two broad categories - parameters and declarations.

   Parameter statements either say how to do something (e.g., how  long  a
   lease  to  offer),  whether to do something (e.g., should dhcpd provide
   addresses to unknown clients), or what parameters  to  provide  to  the
   client (e.g., use gateway

   Declarations  are  used	to  describe  the  topology of the network, to
   describe clients on the network,	 to  provide  addresses	 that  can  be
   assigned	 to  clients,  or to apply a group of parameters to a group of
   declarations.   In any group of parameters and declarations, all param-
   eters  must  be specified before any declarations which depend on those
   parameters may be specified.

   Declarations about network topology include the shared-network and  the
   subnet  declarations.	If  clients  on	 a  subnet  are to be assigned
   addresses dynamically, a range declaration must appear within the  sub-
   net  declaration.    For clients with statically assigned addresses, or
   for installations where only known clients will be  served,  each  such
   client  must have a host declaration.   If parameters are to be applied
   to a group of declarations which are not related strictly on a per-sub-
   net basis, the group declaration can be used.

   For  every  subnet  which will be served, and for every subnet to which
   the dhcp server is connected, there must	 be  one  subnet  declaration,
   which  tells  dhcpd how to recognize that an address is on that subnet.
   A subnet declaration is required for each subnet even if	 no  addresses
   will be dynamically allocated on that subnet.

   Some  installations  have  physical  networks on which more than one IP
   subnet operates.	  For example, if there	 is  a	site-wide  requirement
   that  8-bit subnet masks be used, but a department with a single physi-
   cal ethernet network expands to the point where it has  more  than  254
   nodes,  it may be necessary to run two 8-bit subnets on the same ether-
   net until such time as a new physical network can be added.    In  this
   case,  the  subnet declarations for these two networks must be enclosed
   in a shared-network declaration.

   Note that even when the shared-network declaration is absent, an	 empty
   one  is	created	 by  the  server to contain the subnet (and any scoped
   parameters included in the subnet).  For practical purposes, this means
   that  "stateless"  DHCP	clients,  which are not tied to addresses (and
   therefore subnets) will receive	the  same  configuration  as  stateful

   Some  sites  may	 have  departments which have clients on more than one
   subnet, but it may be desirable to offer those clients a uniform set of
   parameters  which  are  different than what would be offered to clients
   from other departments on the same subnet.   For clients which will  be
   declared	 explicitly  with host declarations, these declarations can be
   enclosed in a group declaration along with  the	parameters  which  are
   common to that department.   For clients whose addresses will be dynam-
   ically assigned, class declarations and conditional declarations may be
   used  to	 group	parameter  assignments based on information the client

   When a client is to be booted, its boot parameters  are	determined  by
   consulting that client's host declaration (if any), and then consulting
   any class declarations matching the client, followed by the pool,  sub-
   net  and shared-network declarations for the IP address assigned to the
   client.	 Each of these declarations itself appears  within  a  lexical
   scope,  and  all	 declarations at less specific lexical scopes are also
   consulted for client option declarations.   Scopes are never considered
   twice,  and  if	parameters  are	 declared  in more than one scope, the
   parameter declared in the most specific scope is the one that is used.

   When dhcpd tries to find a host declaration  for	 a  client,  it	 first
   looks for a host declaration which has a fixed-address declaration that
   lists an IP address that is valid for the subnet or shared  network  on
   which  the  client  is booting.	 If it doesn't find any such entry, it
   tries to find an entry which has no fixed-address declaration.

EXAMPLES A typical dhcpd.conf file will look something like this:

   global parameters...

   subnet netmask {
 subnet-specific parameters...

   subnet netmask {
 subnet-specific parameters...

   subnet netmask {
 subnet-specific parameters...

   group {
 group-specific parameters...
 host zappo.test.isc.org {
   host-specific parameters...
 host beppo.test.isc.org {
   host-specific parameters...
 host harpo.test.isc.org {
   host-specific parameters...

			      Figure 1

   Notice that at the beginning of the file, there's a  place  for	global
   parameters.    These  might  be	things	like the organization's domain
   name, the addresses of the name servers (if  they  are  common  to  the
   entire organization), and so on.	  So, for example:

    option domain-name "isc.org";
    option domain-name-servers ns1.isc.org, ns2.isc.org;

			      Figure 2

   As  you	can see in Figure 2, you can specify host addresses in parame-
   ters using their domain names rather than their numeric	IP  addresses.
   If  a given hostname resolves to more than one IP address (for example,
   if that host has two ethernet interfaces), then	where  possible,  both
   addresses are supplied to the client.

   The  most obvious reason for having subnet-specific parameters as shown
   in Figure 1 is that each subnet, of necessity, has its own router.   So
   for the first subnet, for example, there should be something like:

    option routers;

   Note  that  the	address	 here  is specified numerically.   This is not
   required - if you have a different domain name for  each	 interface  on
   your  router, it's perfectly legitimate to use the domain name for that
   interface instead of the numeric	 address.    However,  in  many	 cases
   there  may  be only one domain name for all of a router's IP addresses,
   and it would not be appropriate to use that name here.

   In Figure 1 there is also a  group  statement,  which  provides	common
   parameters  for	a set of three hosts - zappo, beppo and harpo.	As you
   can see, these hosts are all in the test.isc.org domain,	 so  it	 might
   make  sense  for a group-specific parameter to override the domain name
   supplied to these hosts:

    option domain-name "test.isc.org";

   Also, given the domain they're in, these are  probably  test  machines.
   If we wanted to test the DHCP leasing mechanism, we might set the lease
   timeout somewhat shorter than the default:

    max-lease-time 120;
    default-lease-time 120;

   You may have noticed that while some parameters start with  the	option
   keyword,	 some  do  not.	   Parameters starting with the option keyword
   correspond to actual DHCP options, while parameters that do  not	 start
   with  the option keyword either control the behavior of the DHCP server
   (e.g., how long a lease dhcpd will give out), or specify client parame-
   ters  that  are not optional in the DHCP protocol (for example, server-
   name and filename).

   In Figure 1, each host  had  host-specific  parameters.	  These	 could
   include	such  things  as  the  hostname	 option, the name of a file to
   upload (the filename parameter) and the	address	 of  the  server  from
   which to upload the file (the next-server parameter).   In general, any
   parameter can appear anywhere that parameters are allowed, and will  be
   applied according to the scope in which the parameter appears.

   Imagine	that  you  have	 a site with a lot of NCD X-Terminals.	 These
   terminals come in a variety of models, and you want to specify the boot
   files for each model.   One way to do this would be to have host decla-
   rations for each server and group them by model:

   group {
 filename "Xncd19r";
 next-server ncd-booter;

 host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
 host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
 host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }

   group {
 filename "Xncd19c";
 next-server ncd-booter;

 host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
 host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }

   group {
 filename "XncdHMX";
 next-server ncd-booter;

 host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
 host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
 host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }

ADDRESS POOLS The pool declaration can be used to specify a pool of addresses that will be treated differently than another pool of addresses, even on the same network segment or subnet. For example, you may want to provide a large set of addresses that can be assigned to DHCP clients that are registered to your DHCP server, while providing a smaller set of addresses, possibly with short lease times, that are available for unknown clients. If you have a firewall, you may be able to arrange for addresses from one pool to be allowed access to the Internet, while addresses in another pool are not, thus encouraging users to register their DHCP clients. To do this, you would set up a pair of pool dec- larations:

   subnet netmask {
 option routers;

 # Unknown clients get this pool.
 pool {
   option domain-name-servers bogus.example.com;
   max-lease-time 300;
   allow unknown-clients;

 # Known clients get this pool.
 pool {
   option domain-name-servers ns1.example.com, ns2.example.com;
   max-lease-time 28800;
   deny unknown-clients;

   It is also possible to set up entirely different subnets for known  and
   unknown	clients - address pools exist at the level of shared networks,
   so address ranges within pool declarations can be on different subnets.

   As you can see in the preceding example, pools can  have	 permit	 lists
   that  control  which  clients  are allowed access to the pool and which
   aren't.	Each entry in a pool's permit  list  is	 introduced  with  the
   allow  or  deny keyword.	  If a pool has a permit list, then only those
   clients that match specific entries on the permit list will be eligible
   to  be  assigned	 addresses from the pool.   If a pool has a deny list,
   then only those clients that do not match any entries on the deny  list
   will  be	 eligible.     If both permit and deny lists exist for a pool,
   then only clients that match the permit list and do not match the  deny
   list will be allowed access.

DYNAMIC ADDRESS ALLOCATION Address allocation is actually only done when a client is in the INIT state and has sent a DHCPDISCOVER message. If the client thinks it has a valid lease and sends a DHCPREQUEST to initiate or renew that lease, the server has only three choices - it can ignore the DHCPREQUEST, send a DHCPNAK to tell the client it should stop using the address, or send a DHCPACK, telling the client to go ahead and use the address for a while.

   If  the	server	finds  the  address the client is requesting, and that
   address is available to the client, the server will send a DHCPACK.  If
   the  address  is	 no longer available, or the client isn't permitted to
   have it, the server will send a DHCPNAK.	 If the server	knows  nothing
   about  the address, it will remain silent, unless the address is incor-
   rect for the network segment to which the client has been attached  and
   the server is authoritative for that network segment, in which case the
   server will send a DHCPNAK  even	 though	 it  doesn't  know  about  the

   There  may  be a host declaration matching the client's identification.
   If that host declaration	 contains  a  fixed-address  declaration  that
   lists  an IP address that is valid for the network segment to which the
   client is connected.  In this case,  the	 DHCP  server  will  never  do
   dynamic	address	 allocation.   In this case, the client is required to
   take the address specified in the host  declaration.    If  the	client
   sends  a	 DHCPREQUEST  for  some other address, the server will respond
   with a DHCPNAK.

   When the DHCP server allocates a new address for	 a  client  (remember,
   this  only  happens  if	the  client has sent a DHCPDISCOVER), it first
   looks to see if the client already has a valid lease on an IP  address,
   or  if there is an old IP address the client had before that hasn't yet
   been reassigned.	 In that case, the server will take that  address  and
   check  it  to  see  if the client is still permitted to use it.	If the
   client is no longer permitted to use it, the  lease  is	freed  if  the
   server  thought it was still in use - the fact that the client has sent
   a DHCPDISCOVER proves to the server that the client is no longer	 using
   the lease.

   If no existing lease is found, or if the client is forbidden to receive
   the existing lease, then the server will look in the  list  of  address
   pools  for  the	network	 segment to which the client is attached for a
   lease that is not in use and that the client is permitted to have.   It
   looks through each pool declaration in sequence (all range declarations
   that appear outside of pool declarations are grouped into a single pool
   with  no	 permit	 list).	   If  the permit list for the pool allows the
   client to be allocated an address from that pool, the pool is  examined
   to  see	if  there is an address available.   If so, then the client is
   tentatively assigned  that  address.    Otherwise,  the	next  pool  is
   tested.	 If no addresses are found that can be assigned to the client,
   no response is sent to the client.

   If an address is found that the client is permitted to have,  and  that
   has  never  been	 assigned to any client before, the address is immedi-
   ately allocated to the client.	If the address is available for	 allo-
   cation  but  has	 been  previously  assigned to a different client, the
   server will keep looking in hopes of finding an address that has	 never
   before been assigned to a client.

   The  DHCP  server  generates  the list of available IP addresses from a
   hash table.   This means that the addresses are not sorted in any  par-
   ticular	order, and so it is not possible to predict the order in which
   the DHCP server will allocate IP addresses.   Users  of	previous  ver-
   sions  of  the  ISC  DHCP server may have become accustomed to the DHCP
   server allocating IP addresses in  ascending  order,  but  this	is  no
   longer  possible,  and  there is no way to configure this behavior with
   version 3 of the ISC DHCP server.

IP ADDRESS CONFLICT PREVENTION The DHCP server checks IP addresses to see if they are in use before allocating them to clients. It does this by sending an ICMP Echo request message to the IP address being allocated. If no ICMP Echo reply is received within a second, the address is assumed to be free. This is only done for leases that have been specified in range state- ments, and only when the lease is thought by the DHCP server to be free - i.e., the DHCP server or its failover peer has not listed the lease as in use.

   If  a  response	is  received  to an ICMP Echo request, the DHCP server
   assumes that there is a configuration error - the IP address is in  use
   by  some	 host on the network that is not a DHCP client.	  It marks the
   address as abandoned, and will not assign it to clients.

   If a DHCP client tries to get an IP address, but	 none  are  available,
   but there are abandoned IP addresses, then the DHCP server will attempt
   to reclaim an abandoned IP address.   It marks one IP address as	 free,
   and  then  does	the same ICMP Echo request check described previously.
   If there is no answer to the ICMP Echo request, the address is assigned
   to the client.

   The  DHCP  server  does not cycle through abandoned IP addresses if the
   first IP address it tries to reclaim is free.   Rather, when  the  next
   DHCPDISCOVER comes in from the client, it will attempt a new allocation
   using the same method described here, and will typically try a  new  IP

DHCP FAILOVER This version of the ISC DHCP server supports the DHCP failover protocol as documented in draft-ietf-dhc-failover-07.txt. This is not a final protocol document, and we have not done interoperability testing with other vendors’ implementations of this protocol, so you must not assume that this implementation conforms to the standard. If you wish to use the failover protocol, make sure that both failover peers are running the same version of the ISC DHCP server.

   The failover protocol allows two DHCP servers (and no more than two) to
   share a common address pool.   Each server will have about half of  the
   available  IP  addresses	 in the pool at any given time for allocation.
   If one server fails, the other server will continue to renew leases out
   of the pool, and will allocate new addresses out of the roughly half of
   available addresses that it had	when  communications  with  the	 other
   server were lost.

   It  is possible during a prolonged failure to tell the remaining server
   that the other server is down, in which case the remaining server  will
   (over  time)  reclaim  all the addresses the other server had available
   for allocation, and begin to reuse them.	  This is called  putting  the
   server into the PARTNER-DOWN state.

   You  can put the server into the PARTNER-DOWN state either by using the
   omshell (1) command or by stopping the server, editing  the  last  peer
   state  declaration  in  the lease file, and restarting the server.   If
   you use this last method, be sure to leave the date  and	 time  of  the
   start of the state blank:

   failover peer name state {
   my state partner-down;
   peer state state at date;

   When the other server comes back online, it should automatically detect
   that it has been offline and request a complete update from the	server
   that  was running in the PARTNER-DOWN state, and then both servers will
   resume processing together.

   It is possible to get into a dangerous situation: if you put one server
   into  the PARTNER-DOWN state, and then *that* server goes down, and the
   other server comes back up, the other server will  not  know  that  the
   first  server  was  in  the PARTNER-DOWN state, and may issue addresses
   previously issued by the other server to different  clients,  resulting
   in  IP  address	conflicts.   Before putting a server into PARTNER-DOWN
   state, therefore, make sure that the  other  server  will  not  restart

   The  failover  protocol	defines	 a primary server role and a secondary
   server role.   There are some differences in how primaries  and	secon-
   daries  act, but most of the differences simply have to do with provid-
   ing a way for each peer to behave in the opposite way from  the	other.
   So one server must be configured as primary, and the other must be con-
   figured as secondary, and it doesn't  matter  too  much	which  one  is

FAILOVER STARTUP When a server starts that has not previously communicated with its failover peer, it must establish communications with its failover peer and synchronize with it before it can serve clients. This can happen either because you have just configured your DHCP servers to perform failover for the first time, or because one of your failover servers has failed catastrophically and lost its database.

   The initial recovery process  is	 designed  to  ensure  that  when  one
   failover	 peer  loses  its database and then resynchronizes, any leases
   that the failed server gave out before it failed will be honored.  When
   the  failed  server starts up, it notices that it has no saved failover
   state, and attempts to contact its peer.

   When it has established contact, it asks the peer for a	complete  copy
   its  peer's lease database.  The peer then sends its complete database,
   and sends a message indicating that it is done.	The failed server then
   waits until MCLT has passed, and once MCLT has passed both servers make
   the transition back into normal operation.  This waiting period ensures
   that  any leases the failed server may have given out while out of con-
   tact with its partner will have expired.

   While the failed server is recovering, its partner remains in the part-
   ner-down state, which means that it is serving all clients.  The failed
   server provides no service at all to DHCP clients until it has made the
   transition into normal operation.

   In  the case where both servers detect that they have never before com-
   municated with their partner, they both come up in this recovery	 state
   and  follow  the	 procedure  we have just described.   In this case, no
   service will be provided to DHCP clients until MCLT has expired.

CONFIGURING FAILOVER In order to configure failover, you need to write a peer declaration that configures the failover protocol, and you need to write peer ref- erences in each pool declaration for which you want to do failover. You do not have to do failover for all pools on a given network seg- ment. You must not tell one server it’s doing failover on a particu- lar address pool and tell the other it is not. You must not have any common address pools on which you are not doing failover. A pool dec- laration that utilizes failover would look like this:

   pool {
    failover peer "foo";
    pool specific parameters

   The   server currently  does very  little  sanity checking,  so if  you
   configure it wrong, it will just	 fail in odd ways.  I would  recommend
   therefore  that you either do  failover or don't do failover, but don't
   do any mixed pools.  Also,  use the same master configuration file  for
   both   servers,	and  have  a  separate file  that  contains  the  peer
   declaration and includes the master file.  This will help you to	 avoid
   configuration   mismatches.  As our  implementation evolves,  this will
   become  less of	a  problem.  A	basic  sample dhcpd.conf  file for   a
   primary server might look like this:

   failover peer "foo" {
 address anthrax.rc.vix.com;
 port 519;
 peer address trantor.rc.vix.com;
 peer port 520;
 max-response-delay 60;
 max-unacked-updates 10;
 mclt 3600;
 split 128;
 load balance max seconds 3;

   include "/etc/dhcpd.master";

   The statements in the peer declaration are as follows:

   The primary and secondary statements

 [ primary | secondary ];

 This  determines  whether  the	 server	 is  primary  or secondary, as
 described earlier under DHCP FAILOVER.

   The address statement

 address address;

 The address statement declares the IP address or DNS  name  on	 which
 the  server should listen for connections from its failover peer, and
 also the value to use for the DHCP Failover Protocol  server  identi-
 fier.	 Because  this	value  is used as an identifier, it may not be

   The peer address statement

 peer address address;

 The peer address statement declares the IP address  or	 DNS  name  to
 which	the  server  should  connect  to  reach	 its failover peer for
 failover messages.

   The port statement

 port port-number;

 The port statement declares the TCP port on which the	server	should
 listen	 for  connections from its failover peer.   This statement may
 not currently be omitted, because the failover protocol does not  yet
 have a reserved TCP port number.

   The peer port statement

 peer port port-number;

 The  peer  port  statement  declares the TCP port to which the server
 should connect to reach its  failover	peer  for  failover  messages.
 This  statement may not be omitted because the failover protocol does
 not yet have a reserved TCP port number.   The port  number  declared
 in  the  peer	port  statement	 may  be  the  same as the port number
 declared in the port statement.

   The max-response-delay statement

 max-response-delay seconds;

 The max-response-delay statement tells the DHCP server how many  sec-
 onds  may  pass  without  receiving  a message from its failover peer
 before it assumes that connection has failed.	 This number should be
 small enough that a transient network failure that breaks the connec-
 tion will not result in the servers being out of communication for  a
 long  time,  but large enough that the server isn't constantly making
 and breaking connections.   This parameter must be specified.

   The max-unacked-updates statement

 max-unacked-updates count;

 The max-unacked-updates statement tells the remote  DHCP  server  how
 many BNDUPD messages it can send before it receives a BNDACK from the
 local system.	 We don't have enough operational  experience  to  say
 what a good value for this is, but 10 seems to work.	This parameter
 must be specified.

   The mclt statement

 mclt seconds;

 The mclt statement defines the Maximum Client Lead Time.   It must be
 specified  on the primary, and may not be specified on the secondary.
 This is the length of time for which a lease may be renewed by either
 failover  peer	 without  contacting  the  other.   The longer you set
 this, the longer it will take for the running server  to  recover  IP
 addresses after moving into PARTNER-DOWN state.   The shorter you set
 it, the more load your servers will experience when they are not com-
 municating.	A value of something like 3600 is probably reasonable,
 but again bear in mind that we have no	 real  operational  experience
 with this.

   The split statement

 split index;

 The  split statement specifies the split between the primary and sec-
 ondary for the purposes of load balancing.   Whenever a client	 makes
 a DHCP request, the DHCP server runs a hash on the client identifica-
 tion, resulting in value from 0 to 255.  This is  used	 as  an	 index
 into  a  256 bit field.  If the bit at that index is set, the primary
 is responsible.  If the bit at that index is not set,	the  secondary
 is  responsible.   The split value determines how many of the leading
 bits are set to one.  So, in practice, higher split values will cause
 the  primary  to  serve more clients than the secondary.  Lower split
 values, the converse.	Legal values are between 0 and 255,  of	 which
 the most reasonable is 128.

   The hba statement

 hba colon-separated-hex-list;

 The  hba  statement  specifies the split between the primary and sec-
 ondary as a bitmap rather than a cutoff, which	 theoretically	allows
 for  finer-grained  control.	In practice, there is probably no need
 for such fine-grained control, however.   An example hba statement:

   hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:

 This is equivalent to a split 128;  statement,	 and  identical.   The
 following two examples are also equivalent to a split of 128, but are
 not identical:

   hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:

   hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:

 They are equivalent, because half the bits are set to 0, half are set
 to  1	(0xa and 0x5 are 1010 and 0101 binary respectively) and conse-
 quently this would roughly divide the	clients	 equally  between  the
 servers.  They are not identical, because the actual peers this would
 load balance to each server are different for each example.

 You must only have split or hba defined, never both.  For most cases,
 the  fine-grained  control that hba offers isn't necessary, and split
 should be used.

   The load balance max seconds statement

 load balance max seconds seconds;

 This statement allows you to configure a cutoff after which load bal-
 ancing	 is  disabled.	 The  cutoff is based on the number of seconds
 since the client sent its first DHCPDISCOVER or DHCPREQUEST  message,
 and only works with clients that correctly implement the secs field -
 fortunately most clients do.  We recommend setting this to  something
 like 3 or 5.  The effect of this is that if one of the failover peers
 gets into a state where it is responding to failover messages but not
 responding to some client requests, the other failover peer will take
 over its client load automatically as the clients retry.

   The Failover pool balance statements.

  max-lease-misbalance percentage;
  max-lease-ownership percentage;
  min-balance seconds;
  max-balance seconds;

 This version of the DHCP Server evaluates pool balance on a schedule,
 rather	 than  on demand as leases are allocated.  The latter approach
 proved to be slightly klunky when pool misbalanced reach total	 satu-
 ration...when	any  server  ran out of leases to assign, it also lost
 its ability to notice it had run dry.

 In order to understand pool balance, some elements of	its  operation
 first	need  to  be  defined.	 First,	 there are 'free' and 'backup'
 leases.  Both of these	 are  referred	to  as	'free  state  leases'.
 'free'	 and  'backup'	are  'the free states' for the purpose of this
 document.  The difference is that only the primary may allocate  from
 'free'	 leases	 unless under special circumstances, and only the sec-
 ondary may allocate 'backup' leases.

 When pool balance is performed, the only plausible expectation is  to
 provide  a  50/50  split  of  the  free  state leases between the two
 servers.  This is because no one can predict which server will	 fail,
 regardless  of the relative load placed upon the two servers, so giv-
 ing each server half the leases gives both servers the same amount of
 'failure  endurance'.	 Therefore,  there  is no way to configure any
 different behaviour, outside of  some	very  small  windows  we  will
 describe shortly.

 The  first  thing  calculated	on  any	 pool  balance	run is a value
 referred to as 'lts', or "Leases To Send".  This, simply, is the dif-
 ference  in the count of free and backup leases, divided by two.  For
 the secondary, it is the difference in the backup  and	 free  leases,
 divided  by  two.   The resulting value is signed: if it is positive,
 the local server is expected to hand out leases  to  retain  a	 50/50
 balance.   If	it  is	negative, the remote server would need to send
 leases to balance the pool.  Once the lts  value  reaches  zero,  the
 pool  is perfectly balanced (give or take one lease in the case of an
 odd number of total free state leases).

 The current approach is still	something  of  a  hybrid  of  the  old
 approach,  marked  by the presence of the max-lease-misbalance state-
 ment.	This parameter configures what used to be a 10% fixed value in
 previous  versions:  if lts is less than free+backup * max-lease-mis-
 balance percent, then the server will skip balancing a given pool (it
 won't	bother	moving	any  leases,  even  if some leases "should" be
 moved).  The meaning of this value is also somewhat overloaded,  how-
 ever,	in  that  it also governs the estimation of when to attempt to
 balance the pool (which may then also be skipped over).   The	oldest
 leases	 in  the  free	and backup states are examined.	 The time they
 have resided in their respective queues is used  as  an  estimate  to
 indicate how much time it is probable it would take before the leases
 at the top of the list would be consumed (and thus, how long it would
 take  to  use all leases in that state).  This percentage is directly
 multiplied by this time, and fit into the schedule if it falls within
 the  min-balance  and	max-balance  configured values.	 The scheduled
 pool check time is only moved in a downwards direction, it  is	 never
 increased.  Lastly, if the lts is more than double this number in the
 negative direction, the local server  will  'panic'  and  transmit  a
 Failover  protocol POOLREQ message, in the hopes that the remote sys-
 tem will be woken up into action.

 Once the lts value exceeds  the  max-lease-misbalance	percentage  of
 total	free  state leases as described above, leases are moved to the
 remote server.	 This is done in two passes.

 In the first pass, only leases whose most recent bound	 client	 would
 have been served by the remote server - according to the Load Balance
 Algorithm (see above split and hba configuration  statements)	-  are
 given	away  to  the  peer.  This first pass will happily continue to
 give away leases, decrementing the lts value by one for  each,	 until
 the  lts value has reached the negative of the total number of leases
 multiplied by the max-lease-ownership percentage.  So it  is  through
 this  value that you can permit a small misbalance of the lease pools
 - for the purpose of giving the peer  more  than  a  50/50  share  of
 leases	 in  the hopes that their clients might some day return and be
 allocated by the peer (operating normally).  This process is referred
 to  as	 'MAC  Address	Affinity',  but	 this is somewhat misnamed: it
 applies equally to DHCP Client Identifier options.   Note  also  that
 affinity is applied to leases when they enter the state be moved from
 free to backup if the secondary already has more than its share.

 The second pass is only entered into  if  the	first  pass  fails  to
 reduce	 the lts underneath the total number of free state leases mul-
 tiplied by the max-lease-ownership percentage.	  In  this  pass,  the
 oldest leases are given over to the peer without second thought about
 the Load Balance Algorithm, and this continues until  the  lts	 falls
 under	this  value.   In this way, the local server will also happily
 keep a small percentage of the leases that would normally  load  bal-
 ance to itself.

 So,  the  max-lease-misbalance	 value	acts  as  a  behavioural gate.
 Smaller values will cause more leases to transition states to balance
 the pools over time, higher values will decrease the amount of change
 (but may lead to pool starvation if there's a run on leases).

 The max-lease-ownership value permits a small	(percentage)  skew  in
 the  lease  balance of a percentage of the total number of free state

 Finally, the min-balance and max-balance make certain that  a	sched-
 uled rebalance event happens within a reasonable timeframe (not to be
 thrown off by, for example, a 7 year old free lease).

 Plausible values for the percentages lie between 0  and  100,	inclu-
 sive, but values over 50 are indistinguishable from one another (once
 lts exceeds 50% of the free state leases, one server  must  therefore
 have  100% of the leases in its respective free state).  It is recom-
 mended to select a max-lease-ownership value that is lower  than  the
 value	selected for the max-lease-misbalance value.  max-lease-owner-
 ship defaults to 10, and max-lease-misbalance defaults to 15.

 Plausible values for the min-balance and max-balance times also range
 from  0  to  (2^32)-1	(or the limit of your local time_t value), but
 default to values 60 and 3600 respectively (to place  balance	events
 between 1 minute and 1 hour).

CLIENT CLASSING Clients can be separated into classes, and treated differently depend- ing on what class they are in. This separation can be done either with a conditional statement, or with a match statement within the class declaration. It is possible to specify a limit on the total number of clients within a particular class or subclass that may hold leases at one time, and it is possible to specify automatic subclassing based on the contents of the client packet.

   To  add	clients	 to  classes  based on conditional evaluation, you can
   specify a matching expression in the class statement:

   class "ras-clients" {
 match if substring (option dhcp-client-identifier, 1, 3) = "RAS";

   Note that whether you use matching expressions or  add  statements  (or
   both)  to  classify  clients, you must always write a class declaration
   for any class that you use.   If there will be no match	statement  and
   no  in-scope  statements	 for a class, the declaration should look like

   class "ras-clients" {

SUBCLASSES In addition to classes, it is possible to declare subclasses. A sub- class is a class with the same name as a regular class, but with a spe- cific submatch expression which is hashed for quick matching. This is essentially a speed hack - the main difference between five classes with match expressions and one class with five subclasses is that it will be quicker to find the subclasses. Subclasses work as follows:

   class "allocation-class-1" {
 match pick-first-value (option dhcp-client-identifier, hardware);

   class "allocation-class-2" {
 match pick-first-value (option dhcp-client-identifier, hardware);

   subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
   subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
   subclass "allocation-class-1" 1:0:0:c4:aa:29:44;

   subnet netmask {
 pool {
   allow members of "allocation-class-1";
 pool {
   allow members of "allocation-class-2";

   The data following the class name in the subclass declaration is a con-
   stant value to use in matching the  match  expression  for  the	class.
   When class matching is done, the server will evaluate the match expres-
   sion and then look the result up in the hash table.    If  it  finds  a
   match, the client is considered a member of both the class and the sub-

   Subclasses can be declared with or without scope.   In the above	 exam-
   ple,  the  sole purpose of the subclass is to allow some clients access
   to one address pool, while other clients are given access to the	 other
   pool, so these subclasses are declared without scopes.	If part of the
   purpose of the subclass were to define different parameter  values  for
   some clients, you might want to declare some subclasses with scopes.

   In  the above example, if you had a single client that needed some con-
   figuration parameters, while most didn't, you might write the following
   subclass declaration for that client:

   subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
 option root-path "samsara:/var/diskless/alphapc";
 filename "/tftpboot/netbsd.alphapc-diskless";

   In  this	 example,  we've  used subclassing as a way to control address
   allocation on a per-client basis.  However, it's also possible  to  use
   subclassing  in ways that are not specific to clients - for example, to
   use the value of the vendor-class-identifier option to  determine  what
   values  to  send in the vendor-encapsulated-options option.  An example
   of this is shown under the VENDOR  ENCAPSULATED	OPTIONS	 head  in  the
   dhcp-options(5) manual page.

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION You may specify a limit to the number of clients in a class that can be assigned leases. The effect of this will be to make it difficult for a new client in a class to get an address. Once a class with such a limit has reached its limit, the only way a new client in that class can get a lease is for an existing client to relinquish its lease, either by letting it expire, or by sending a DHCPRELEASE packet. Classes with lease limits are specified as follows:

   class "limited-1" {
 lease limit 4;

   This will produce a class in which a maximum of four members may hold a
   lease at one time.

SPAWNING CLASSES It is possible to declare a spawning class. A spawning class is a class that automatically produces subclasses based on what the client sends. The reason that spawning classes were created was to make it possible to create lease-limited classes on the fly. The envisioned application is a cable-modem environment where the ISP wishes to pro- vide clients at a particular site with more than one IP address, but does not wish to provide such clients with their own subnet, nor give them an unlimited number of IP addresses from the network segment to which they are connected.

   Many cable modem head-end systems can be	 configured  to	 add  a	 Relay
   Agent Information option to DHCP packets when relaying them to the DHCP
   server.	 These systems typically add a circuit ID or remote ID	option
   that  uniquely  identifies  the	customer  site.	  To take advantage of
   this, you can write a class declaration as follows:

   class "customer" {
 spawn with option agent.circuit-id;
 lease limit 4;

   Now whenever a request comes in from a customer site,  the  circuit  ID
   option  will be checked against the class's hash table.	 If a subclass
   is found that matches the circuit ID, the client will be classified  in
   that subclass and treated accordingly.	If no subclass is found match-
   ing the circuit ID, a new  one  will  be	 created  and  logged  in  the
   dhcpd.leases file, and the client will be classified in this new class.
   Once the client has been classified, it will be	treated	 according  to
   the  rules  of the class, including, in this case, being subject to the
   per-site limit of four leases.

   The use of the subclass spawning mechanism is not restricted  to	 relay
   agent  options  - this particular example is given only because it is a
   fairly straightforward one.

COMBINING MATCH, MATCH IF AND SPAWN WITH In some cases, it may be useful to use one expression to assign a client to a particular class, and a second expression to put it into a subclass of that class. This can be done by combining the match if and spawn with statements, or the match if and match statements. For example:

   class "jr-cable-modems" {
 match if option dhcp-vendor-identifier = "jrcm";
 spawn with option agent.circuit-id;
 lease limit 4;

   class "dv-dsl-modems" {
 match if opton dhcp-vendor-identifier = "dvdsl";
 spawn with option agent.circuit-id;
 lease limit 16;

   This allows you to have two classes that both have the same spawn  with
   expression without getting the clients in the two classes confused with
   each other.

DYNAMIC DNS UPDATES The DHCP server has the ability to dynamically update the Domain Name System. Within the configuration files, you can define how you want the Domain Name System to be updated. These updates are RFC 2136 com- pliant so any DNS server supporting RFC 2136 should be able to accept updates from the DHCP server.

   Two DNS update  schemes	are  currently	implemented,  and  another  is
   planned.	   The	two  that  are	currently available are the ad-hoc DNS
   update mode and the interim DHCP-DNS interaction draft update mode.  If
   and  when  the  DHCP-DNS	 interaction draft and the DHCID draft make it
   through the IETF standards process, there will be a third  mode,	 which
   will  be the standard DNS update method.	  The DHCP server must be con-
   figured to use one of the two currently-supported methods, or not to do
   dns  updates.	This can be done with the ddns-update-style configura-
   tion parameter.

THE AD-HOC DNS UPDATE SCHEME The ad-hoc Dynamic DNS update scheme is now deprecated and does not work. In future releases of the ISC DHCP server, this scheme will not likely be available. The interim scheme works, allows for failover, and should now be used. The following description is left here for informational purposes only.

   The ad-hoc Dynamic DNS update scheme implemented in this version of the
   ISC  DHCP  server is a prototype design, which does not have much to do
   with the standard update method that is being standardized in the  IETF
   DHC  working  group, but rather implements some very basic, yet useful,
   update capabilities.   This mode does not work with the failover proto-
   col  because  it	 does not account for the possibility of two different
   DHCP servers updating the same set of DNS records.

   For the ad-hoc DNS update method, the client's FQDN is derived  in  two
   parts.	 First, the hostname is determined.   Then, the domain name is
   determined, and appended to the hostname.

   The DHCP server determines the client's hostname by first looking for a
   ddns-hostname  configuration  option,  and using that if it is present.
   If no such option is present, the server looks for a valid hostname  in
   the  FQDN option sent by the client.  If one is found, it is used; oth-
   erwise, if the client sent a host-name option, that  is	used.	Other-
   wise,  if  there	 is a host declaration that applies to the client, the
   name from that declaration will be used.	 If none of these applies, the
   server will not have a hostname for the client, and will not be able to
   do a DNS update.

   The domain name is determined from  the	ddns-domainname	 configuration
   option.	The default configuration for this option is:

 option server.ddns-domainname = config-option domain-name;

   So  if this configuration option is not configured to a different value
   (over-riding the above default), or if a	 domain-name  option  has  not
   been  configured	 for  the  client's  scope,  then  the server will not
   attempt to perform a DNS update.

   The client's fully-qualified domain name, derived as we have described,
   is  used	 as  the  name	on  which an "A" record will be stored.	 The A
   record will contain the IP address that the client was assigned in  its
   lease.	 If there is already an A record with the same name in the DNS
   server, no update of either the A or PTR records will occur - this pre-
   vents a client from claiming that its hostname is the name of some net-
   work  server.	For  example,  if  you	have   a   fileserver	called
   "fs.sneedville.edu", and the client claims its hostname is "fs", no DNS
   update will be done for that client,  and  an  error  message  will  be

   If  the	A record update succeeds, a PTR record update for the assigned
   IP address will be done, pointing to the A  record.    This  update  is
   unconditional  - it will be done even if another PTR record of the same
   name exists.   Since the IP address  has	 been  assigned	 to  the  DHCP
   server, this should be safe.

   Please note that the current implementation assumes clients only have a
   single network interface.   A client with two network  interfaces  will
   see  unpredictable  behavior.	This  is considered a bug, and will be
   fixed in a later release.   It may be helpful to enable the  one-lease-
   per-client  parameter  so that roaming clients do not trigger this same

   The DHCP protocol normally involves a four-packet exchange - first  the
   client sends a DHCPDISCOVER message, then the server sends a DHCPOFFER,
   then the client sends a DHCPREQUEST, then the server sends  a  DHCPACK.
   In  the	current version of the server, the server will do a DNS update
   after it has received the DHCPREQUEST, and before it has sent the  DHC-
   PACK.	It  only  sends	 the DNS update if it has not sent one for the
   client's address before, in order to minimize the impact	 on  the  DHCP

   When the client's lease expires, the DHCP server (if it is operating at
   the time, or when next it operates) will remove the client's A and  PTR
   records	from  the  DNS database.   If the client releases its lease by
   sending a DHCPRELEASE message, the server will likewise	remove	the  A
   and PTR records.

THE INTERIM DNS UPDATE SCHEME The interim DNS update scheme operates mostly according to several drafts that are being considered by the IETF and are expected to become standards, but are not yet standards, and may not be standardized exactly as currently proposed. These are:


   Because our implementation is slightly different than the standard,  we
   will briefly document the operation of this update style here.

   The  first  point  to understand about this style of DNS update is that
   unlike the ad-hoc style, the DHCP server does  not  necessarily	always
   update  both  the  A  and the PTR records.   The FQDN option includes a
   flag which, when sent by the client, indicates that the	client	wishes
   to  update  its own A record.   In that case, the server can be config-
   ured either to honor the client's intentions or ignore them.   This  is
   done  with  the statement allow client-updates; or the statement ignore
   client-updates;.	  By default, client updates are allowed.

   If the server is configured to allow client updates, then if the client
   sends a fully-qualified domain name in the FQDN option, the server will
   use that name the client sent in the FQDN  option  to  update  the  PTR
   record.	 For example, let us say that the client is a visitor from the
   "radish.org" domain, whose hostname is "jschmoe".   The server  is  for
   the  "example.org"  domain.    The  DHCP	 client	 indicates in the FQDN
   option that its FQDN is "jschmoe.radish.org.".	It also indicates that
   it  wants  to update its own A record.	The DHCP server therefore does
   not attempt to set up an A record for the client, but does set up a PTR
   record  for  the	 IP  address  that  it assigns the client, pointing at
   jschmoe.radish.org.   Once the DHCP client has an IP  address,  it  can
   update its own A record, assuming that the "radish.org" DNS server will
   allow it to do so.

   If the server is configured not to allow	 client	 updates,  or  if  the
   client doesn't want to do its own update, the server will simply choose
   a name for the client from either the fqdn option (if present)  or  the
   hostname	 option (if present).  It will use its own domain name for the
   client, just as in the ad-hoc update scheme.  It will then update  both
   the A and PTR record, using the name that it chose for the client.   If
   the client sends a fully-qualified domain name in the fqdn option,  the
   server  uses only the leftmost part of the domain name - in the example
   above, "jschmoe" instead of "jschmoe.radish.org".

   Further, if the ignore client-updates;  directive  is  used,  then  the
   server  will  in addition send a response in the DHCP packet, using the
   FQDN Option, that implies to the client that it should perform its  own
   updates	if it chooses to do so.	 With deny client-updates;, a response
   is sent which indicates the client may not perform updates.

   Also, if the use-host-decl-names configuration option is enabled,  then
   the  host  declaration's hostname will be used in place of the hostname
   option, and the same rules will apply as described above.

   The other difference between the ad-hoc scheme and the  interim	scheme
   is that with the interim scheme, a method is used that allows more than
   one DHCP server to update the DNS database without accidentally	delet-
   ing  A  records	that shouldn't be deleted nor failing to add A records
   that should be added.   The scheme works as follows:

   When the DHCP server issues a client a new lease,  it  creates  a  text
   string  that  is an MD5 hash over the DHCP client's identification (see
   draft-ietf-dnsext-dhcid-rr-??.txt for details).	 The update adds an  A
   record  with  the name the server chose and a TXT record containing the
   hashed identifier string	 (hashid).    If  this	update	succeeds,  the
   server is done.

   If  the update fails because the A record already exists, then the DHCP
   server attempts to add the A record with the  prerequisite  that	 there
   must be a TXT record in the same name as the new A record, and that TXT
   record's contents must be equal to hashid.   If this  update  succeeds,
   then  the  client  has its A record and PTR record.   If it fails, then
   the name the client has been assigned (or requested)  is	 in  use,  and
   can't  be  used by the client.	At this point the DHCP server gives up
   trying to do a DNS update for the client until the client chooses a new

   The  interim  DNS  update  scheme  is  called  interim for two reasons.
   First, it does not quite follow the drafts.   The current  versions  of
   the  drafts call for a new DHCID RRtype, but this is not yet available.
   The interim DNS update scheme uses a TXT record	instead.    Also,  the
   existing ddns-resolution draft calls for the DHCP server to put a DHCID
   RR on the PTR record, but the interim update method does not  do	 this.
   It is our position that this is not useful, and we are working with the
   author in hopes of removing it from the next version of the  draft,  or
   better understanding why it is considered useful.

   In  addition to these differences, the server also does not update very
   aggressively.  Because each DNS update involves a round trip to the DNS
   server,	there  is a cost associated with doing updates even if they do
   not actually modify the DNS  database.	 So  the  DHCP	server	tracks
   whether	or not it has updated the record in the past (this information
   is stored on the lease) and does not attempt to update records that  it
   thinks it has already updated.

   This  can  lead	to cases where the DHCP server adds a record, and then
   the record is deleted through some  other  mechanism,  but  the	server
   never  again  updates  the  DNS	because	 it thinks the data is already
   there.	In this case the data can be removed from  the	lease  through
   operator	 intervention,	and  once  this has been done, the DNS will be
   updated the next time the client renews.

DYNAMIC DNS UPDATE SECURITY When you set your DNS server up to allow updates from the DHCP server, you may be exposing it to unauthorized updates. To avoid this, you should use TSIG signatures - a method of cryptographically signing updates using a shared secret key. As long as you protect the secrecy of this key, your updates should also be secure. Note, however, that the DHCP protocol itself provides no security, and that clients can therefore provide information to the DHCP server which the DHCP server will then use in its updates, with the constraints described previ- ously.

   The DNS server must be configured to allow updates for  any  zone  that
   the DHCP server will be updating.  For example, let us say that clients
   in  the	sneedville.edu	domain	will  be  assigned  addresses  on  the  subnet.	In  that case, you will need a key declaration
   for the TSIG key you will be using, and also two	 zone  declarations  -
   one  for the zone containing A records that will be updates and one for
   the zone containing PTR records - for ISC BIND, something like this:

 algorithm HMAC-MD5.SIG-ALG.REG.INT;
 secret pRP5FapFoJ95JEL06sv4PQ==;

   zone "example.org" {
    type master;
    file "example.org.db";
    allow-update { key DHCP_UPDATER; };

   zone "17.10.10.in-addr.arpa" {
    type master;
    file "10.10.17.db";
    allow-update { key DHCP_UPDATER; };

   You will also have to configure your DHCP server to do updates to these
   zones.	 To  do	 so,  you  need	 to  add  something  like this to your
   dhcpd.conf file:

 algorithm HMAC-MD5.SIG-ALG.REG.INT;
 secret pRP5FapFoJ95JEL06sv4PQ==;

   zone EXAMPLE.ORG. {

   zone 17.127.10.in-addr.arpa. {

   The primary statement specifies the IP address of the name server whose
   zone information is to be updated.

   Note that the zone declarations have to correspond to authority records
   in your name server - in the above example, there must be an SOA record
   for  "example.org." and for "17.10.10.in-addr.arpa.".   For example, if
   there were a subdomain "foo.example.org"	 with  no  separate  SOA,  you
   could not write a zone declaration for "foo.example.org."  Also keep in
   mind that zone names in your DHCP configuration should end  in  a  ".";
   this  is	 the  preferred syntax.	 If you do not end your zone name in a
   ".", the DHCP server will figure it out.	 Also note that	 in  the  DHCP
   configuration,  zone  names  are not encapsulated in quotes where there
   are in the DNS configuration.

   You should choose your own secret key, of course.  The ISC BIND 8 and 9
   distributions  come  with  a  program for generating secret keys called
   dnssec-keygen.  The version that comes with BIND 9 is likely to produce
   a  substantially more random key, so we recommend you use that one even
   if you are not using BIND 9 as your DNS server.	If you are using  BIND
   9's dnssec-keygen, the above key would be created as follows:

    dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

   If  you	are  using the BIND 8 dnskeygen program, the following command
   will generate a key as seen above:

    dnskeygen -H 128 -u -c -n DHCP_UPDATER

   You may wish to enable logging of DNS updates on your DNS  server.   To
   do so, you might write a logging statement like the following:

   logging {
    channel update_debug {
	 file "/var/log/update-debug.log";
	 severity  debug 3;
	 print-category yes;
	 print-severity yes;
	 print-time	yes;
    channel security_info    {
	 file "/var/log/named-auth.info";
	 severity  info;
	 print-category yes;
	 print-severity yes;
	 print-time	yes;

    category update { update_debug; };
    category security { security_info; };

   You  must  create  the  /var/log/named-auth.info	 and  /var/log/update-
   debug.log files before starting the name server.	  For more information
   on configuring ISC BIND, consult the documentation that accompanies it.

REFERENCE: EVENTS There are three kinds of events that can happen regarding a lease, and it is possible to declare statements that occur when any of these events happen. These events are the commit event, when the server has made a commitment of a certain lease to a client, the release event, when the client has released the server from its commitment, and the expiry event, when the commitment expires.

   To declare a set of statements to execute when an  event	 happens,  you
   must  use the on statement, followed by the name of the event, followed
   by a series of statements to execute when the event  happens,  enclosed
   in  braces.    Events  are used to implement DNS updates, so you should
   not define your own event handlers if you are using  the	 built-in  DNS
   update mechanism.

   The  built-in  version  of the DNS update mechanism is in a text string
   towards the top of server/dhcpd.c.   If you  want  to  use  events  for
   things  other than DNS updates, and you also want DNS updates, you will
   have to start out by copying this code into your	 dhcpd.conf  file  and
   modifying it.

REFERENCE: DECLARATIONS The include statement

include "filename";

   The  include statement is used to read in a named file, and process the
   contents of that file as though it were entered in place of the include

   The shared-network statement

shared-network name {
  [ parameters ]
  [ declarations ]

   The  shared-network  statement  is  used to inform the DHCP server that
   some IP subnets actually share the same physical network.  Any  subnets
   in  a  shared network should be declared within a shared-network state-
   ment.  Parameters specified in the  shared-network  statement  will  be
   used  when  booting clients on those subnets unless parameters provided
   at the subnet or host level override them.  If any subnet in  a	shared
   network has addresses available for dynamic allocation, those addresses
   are collected into a common pool for that shared network	 and  assigned
   to  clients  as needed.	There is no way to distinguish on which subnet
   of a shared network a client should boot.

   Name should be the name of the shared network.	This name is used when
   printing debugging messages, so it should be descriptive for the shared
   network.	  The name  may	 have  the  syntax  of	a  valid  domain  name
   (although  it  will  never be used as such), or it may be any arbitrary
   name, enclosed in quotes.

   The subnet statement

subnet subnet-number netmask netmask {
  [ parameters ]
  [ declarations ]

   The subnet statement is used to provide dhcpd with  enough  information
   to tell whether or not an IP address is on that subnet.	It may also be
   used  to	 provide  subnet-specific  parameters  and  to	specify	  what
   addresses  may be dynamically allocated to clients booting on that sub-
   net.   Such addresses are specified using the range declaration.

   The subnet-number should be an IP address or domain name which resolves
   to  the	subnet	number	of  the	 subnet being described.   The netmask
   should be an IP address or domain name which  resolves  to  the	subnet
   mask  of the subnet being described.   The subnet number, together with
   the netmask, are sufficient to determine whether any given  IP  address
   is on the specified subnet.

   Although	 a  netmask must be given with every subnet declaration, it is
   recommended that if there is any variance in subnet masks at a site,  a
   subnet-mask  option statement be used in each subnet declaration to set
   the desired subnet mask, since any subnet-mask  option  statement  will
   override the subnet mask declared in the subnet statement.

   The subnet6 statement

subnet6 subnet6-number {
  [ parameters ]
  [ declarations ]

   The  subnet6 statement is used to provide dhcpd with enough information
   to tell whether or not an IPv6 address is on that subnet6.  It may also
   be  used	 to  provide  subnet-specific  parameters  and to specify what
   addresses may be dynamically allocated to clients booting on that  sub-

   The  subnet6-number  should be an IPv6 network identifier, specified as

   The range statement

   range [ dynamic-bootp ] low-address [ high-address];

   For any subnet on which addresses will be assigned  dynamically,	 there
   must  be	 at least one range statement.	 The range statement gives the
   lowest and highest IP addresses in a range.   All IP addresses  in  the
   range should be in the subnet in which the range statement is declared.
   The dynamic-bootp flag may be specified if addresses in	the  specified
   range  may  be  dynamically  assigned  to BOOTP clients as well as DHCP
   clients.	  When specifying a single address, high-address can be	 omit-

   The range6 statement

   range6 low-address high-address;
   range6 subnet6-number;
   range6 subnet6-number temporary;
   range6 address temporary;

   For  any	 IPv6 subnet6 on which addresses will be assigned dynamically,
   there must be at least one range6 statement. The range6	statement  can
   either  be  the	lowest	and highest IPv6 addresses in a range6, or use
   CIDR notation, specified as ip6-address/bits. All IP addresses  in  the
   range6  should  be  in  the  subnet6  in	 which the range6 statement is

   The temporay variant makes the prefix (by default on 64 bits) available
   for  temporary  (RFC  4941)  addresses. A new address per prefix in the
   shared network is computed  at  each  request  with  an	IA_TA  option.
   Release and Confirm ignores temporary addresses.

   Any IPv6 addresses given to hosts with fixed-address6 are excluded from
   the range6, as are IPv6 addresses on the server itself.

   The prefix6 statement

   prefix6 low-address high-address / bits;

   The prefix6 is the range6 equivalent for Prefix Delegation (RFC	3633).
   Prefixes	 of  bits  length  are	assigned between low-address and high-

   Any IPv6 prefixes given to static entries  (hosts)  with	 fixed-prefix6
   are excluded from the prefix6.

   This  statement is currently global but it should have a shared-network

   The host statement

host hostname {
  [ parameters ]
  [ declarations ]

   The host declaration provides a scope in which to provide configuration
   information  about a specific client, and also provides a way to assign
   a client a fixed address.  The host declaration provides a way for  the
   DHCP  server  to	 identify  a  DHCP  or BOOTP client, and also a way to
   assign the client a static IP address.

   If it is desirable to be able to boot a DHCP or BOOTP  client  on  more
   than  one  subnet  with	fixed  addresses, more than one address may be
   specified in the fixed-address  declaration,  or	 more  than  one  host
   statement may be specified matching the same client.

   If  client-specific boot parameters must change based on the network to
   which the client is attached, then multiple host declarations should be
   used.   The  host declarations will only match a client if one of their
   fixed-address statements is viable on the subnet	 (or  shared  network)
   where  the  client  is attached.	 Conversely, for a host declaration to
   match a client being allocated a dynamic address, it must not have  any
   fixed-address  statements.   You	 may  therefore need a mixture of host
   declarations for any given client...some	 having	 fixed-address	state-
   ments, others without.

   hostname	 should	 be a name identifying the host.  If a hostname option
   is not specified for the host, hostname is used.

   Host declarations are matched to actual DHCP or BOOTP clients by match-
   ing the dhcp-client-identifier option specified in the host declaration
   to the one supplied by the client, or, if the host declaration  or  the
   client  does  not  provide a dhcp-client-identifier option, by matching
   the hardware parameter in the host declaration to the network  hardware
   address supplied by the client.	 BOOTP clients do not normally provide
   a dhcp-client-identifier, so the hardware address must be used for  all
   clients that may boot using the BOOTP protocol.

   DHCPv6 servers can use the host-identifier option parameter in the host
   declaration, and specify any option with	 a  fixed  value  to  identify

   Please  be  aware  that	only the dhcp-client-identifier option and the
   hardware address can be used to match a host declaration, or the	 host-
   identifier  option  parameter  for DHCPv6 servers.   For example, it is
   not possible to match a host declaration to a host-name option.	  This
   is  because  the host-name option cannot be guaranteed to be unique for
   any given client, whereas both the hardware  address  and  dhcp-client-
   identifier option are at least theoretically guaranteed to be unique to
   a given client.

   The group statement

group {
  [ parameters ]
  [ declarations ]

   The group statement is used simply to apply one or more parameters to a
   group  of  declarations.	   It  can be used to group hosts, shared net-
   works, subnets, or even other groups.

REFERENCE: ALLOW AND DENY The allow and deny statements can be used to control the response of the DHCP server to various sorts of requests. The allow and deny key- words actually have different meanings depending on the context. In a pool context, these keywords can be used to set up access lists for address allocation pools. In other contexts, the keywords simply con- trol general server behavior with respect to clients based on scope. In a non-pool context, the ignore keyword can be used in place of the deny keyword to prevent logging of denied requests.

ALLOW DENY AND IGNORE IN SCOPE The following usages of allow and deny will work in any scope, although it is not recommended that they be used in pool declarations.

   The unknown-clients keyword

allow unknown-clients;
deny unknown-clients;
ignore unknown-clients;

   The unknown-clients flag is used to tell dhcpd whether or not to dynam-
   ically  assign  addresses to unknown clients.   Dynamic address assign-
   ment to unknown clients is allowed by default.  An  unknown  client  is
   simply a client that has no host declaration.

   The  use	 of  this  option  is  now  deprecated.	  If you are trying to
   restrict access on your network to known clients, you should  use  deny
   unknown-clients;	 inside	 of  your address pool, as described under the

   The bootp keyword

allow bootp;
deny bootp;
ignore bootp;

   The bootp flag is used to tell dhcpd whether or not to respond to bootp
   queries.	 Bootp queries are allowed by default.

   This  option  does  not	satisfy	 the requirement of failover peers for
   denying dynamic bootp clients.  The deny dynamic bootp clients;	option
   should be used instead. See the ALLOW AND DENY WITHIN POOL DECLARATIONS
   section of this man page for more details.

   The booting keyword

allow booting;
deny booting;
ignore booting;

   The booting flag is used to tell dhcpd whether or  not  to  respond  to
   queries	from  a particular client.  This keyword only has meaning when
   it appears in a host declaration.   By default, booting is allowed, but
   if it is disabled for a particular client, then that client will not be
   able to get an address from the DHCP server.

   The duplicates keyword

allow duplicates;
deny duplicates;

   Host declarations can match client messages based on  the  DHCP	Client
   Identifier  option  or  based on the client's network hardware type and
   MAC address.   If the MAC address is used, the  host  declaration  will
   match  any  client  with that MAC address - even clients with different
   client identifiers.   This doesn't normally  happen,  but  is  possible
   when  one computer has more than one operating system installed on it -
   for example, Microsoft Windows and NetBSD or Linux.

   The duplicates flag tells the DHCP server that if a request is received
   from  a	client that matches the MAC address of a host declaration, any
   other leases matching that MAC  address	should	be  discarded  by  the
   server,	even  if the UID is not the same.   This is a violation of the
   DHCP protocol, but can prevent clients whose client identifiers	change
   regularly  from	holding	 many  leases  at  the same time.  By default,
   duplicates are allowed.

   The declines keyword

allow declines;
deny declines;
ignore declines;

   The DHCPDECLINE message is used by DHCP clients to  indicate  that  the
   lease the server has offered is not valid.   When the server receives a
   DHCPDECLINE  for	 a  particular	address,  it  normally	abandons  that
   address,	 assuming that some unauthorized system is using it.  Unfortu-
   nately, a malicious or buggy client can,	 using	DHCPDECLINE  messages,
   completely exhaust the DHCP server's allocation pool.   The server will
   reclaim these leases, but while the client is running through the pool,
   it  may	cause serious thrashing in the DNS, and it will also cause the
   DHCP server to forget old DHCP client address allocations.

   The declines flag tells the DHCP server whether or not to honor DHCPDE-
   CLINE messages.	 If it is set to deny or ignore in a particular scope,
   the DHCP server will not respond to DHCPDECLINE messages.

   The client-updates keyword

allow client-updates;
deny client-updates;

   The client-updates flag tells the DHCP server whether or not  to	 honor
   the  client's  intention to do its own update of its A record.  This is
   only relevant when doing interim DNS updates.   See  the	 documentation
   under the heading THE INTERIM DNS UPDATE SCHEME for details.

   The leasequery keyword

allow leasequery;
deny leasequery;

   The leasequery flag tells the DHCP server whether or not to answer DHC-
   PLEASEQUERY packets. The answer to  a  DHCPLEASEQUERY  packet  includes
   information about a specific lease, such as when it was issued and when
   it will expire. By default, the server will not respond to these	 pack-

ALLOW AND DENY WITHIN POOL DECLARATIONS The uses of the allow and deny keywords shown in the previous section work pretty much the same way whether the client is sending a DHCPDIS- COVER or a DHCPREQUEST message - an address will be allocated to the client (either the old address it’s requesting, or a new address) and then that address will be tested to see if it’s okay to let the client have it. If the client requested it, and it’s not okay, the server will send a DHCPNAK message. Otherwise, the server will simply not respond to the client. If it is okay to give the address to the client, the server will send a DHCPACK message.

   The  primary  motivation	 behind	 pool  declarations is to have address
   allocation pools whose allocation policies are  different.    A	client
   may be denied access to one pool, but allowed access to another pool on
   the same network segment.   In order for this to work,  access  control
   has  to be done during address allocation, not after address allocation
   is done.

   When a DHCPREQUEST message is processed, address allocation simply con-
   sists  of looking up the address the client is requesting and seeing if
   it's still available for the client.  If it is, then  the  DHCP	server
   checks  both  the  address  pool permit lists and the relevant in-scope
   allow and deny statements to see if it's okay to give the lease to  the
   client.	 In the case of a DHCPDISCOVER message, the allocation process
   is done as described previously in the ADDRESS ALLOCATION section.

   When declaring permit lists for address allocation pools, the following
   syntaxes are recognized following the allow or deny keywords:


   If  specified, this statement either allows or prevents allocation from
   this pool to any client that has a host declaration (i.e.,  is  known).
   A  client  is known if it has a host declaration in any scope, not just
   the current scope.


   If specified, this statement either allows or prevents allocation  from
   this  pool  to  any  client  that has no host declaration (i.e., is not

members of "class";

   If specified, this statement either allows or prevents allocation  from
   this pool to any client that is a member of the named class.

dynamic bootp clients;

   If  specified, this statement either allows or prevents allocation from
   this pool to any bootp client.

authenticated clients;

   If specified, this statement either allows or prevents allocation  from
   this  pool  to  any  client  that has been authenticated using the DHCP
   authentication protocol.	  This is not yet supported.

unauthenticated clients;

   If specified, this statement either allows or prevents allocation  from
   this  pool to any client that has not been authenticated using the DHCP
   authentication protocol.	  This is not yet supported.

all clients;

   If specified, this statement either allows or prevents allocation  from
   this  pool  to all clients.   This can be used when you want to write a
   pool declaration for some reason, but hold it in reserve, or  when  you
   want  to	 renumber  your	 network  quickly, and thus want the server to
   force all clients that have been allocated addresses from this pool  to
   obtain new addresses immediately when they next renew.

after time;

   If  specified, this statement either allows or prevents allocation from
   this pool after a given date. This can be used when you	want  to  move
   clients	from one pool to another. The server adjusts the regular lease
   time so that the latest expiry time is  at  the	given  time+min-lease-
   time.   A short min-lease-time enforces a step change, whereas a longer
   min-lease-time allows for a gradual  change.   time  is	either	second
   since  epoch,  or  a  UTC  time string e.g.  4 2007/08/24 09:14:32 or a
   string with time zone offset in	seconds	 e.g.  4  2007/08/24  11:14:32

REFERENCE: PARAMETERS The adaptive-lease-time-threshold statement

 adaptive-lease-time-threshold percentage;

 When  the  number  of	allocated leases within a pool rises above the
 percentage given in this statement, the  DHCP	server	decreases  the
 lease	length for new clients within this pool to min-lease-time sec-
 onds. Clients renewing an already valid (long) leases	get  at	 least
 the  remaining	 time  from the current lease. Since the leases expire
 faster, the server may either recover	more  quickly  or  avoid  pool
 exhaustion  entirely.	Once the number of allocated leases drop below
 the threshold, the server reverts back to normal lease times.	 Valid
 percentages are between 1 and 99.

   The always-broadcast statement

 always-broadcast flag;

 The  DHCP  and BOOTP protocols both require DHCP and BOOTP clients to
 set the broadcast bit in the flags field of the BOOTP message header.
 Unfortunately, some DHCP and BOOTP clients do not do this, and there-
 fore may not receive responses	 from  the  DHCP  server.    The  DHCP
 server	 can  be  made to always broadcast its responses to clients by
 setting this flag to 'on' for the  relevant  scope;  relevant	scopes
 would	be inside a conditional statement, as a parameter for a class,
 or as a parameter for a host declaration.   To avoid creating	excess
 broadcast traffic on your network, we recommend that you restrict the
 use of this option to as few clients as possible.   For example,  the
 Microsoft  DHCP  client is known not to have this problem, as are the
 OpenTransport and ISC DHCP clients.

   The always-reply-rfc1048 statement

 always-reply-rfc1048 flag;

 Some BOOTP clients expect RFC1048-style responses, but do not	follow
 RFC1048  when sending their requests.	 You can tell that a client is
 having this problem if it is not getting the options you have config-
 ured  for  it	and  if	 you  see in the server log the message "(non-
 rfc1048)" printed with each BOOTREQUEST that is logged.

 If you want to send rfc1048 options to such a client, you can set the
 always-reply-rfc1048  option  in  that client's host declaration, and
 the DHCP server will respond with an  RFC-1048-style  vendor  options
 field.	   This	 flag  can  be	set  in any scope, and will affect all
 clients covered by that scope.

   The authoritative statement


 not authoritative;

 The DHCP server will normally assume that the configuration  informa-
 tion  about a given network segment is not known to be correct and is
 not authoritative.  This is so that if a naive user installs  a  DHCP
 server	 not fully understanding how to configure it, it does not send
 spurious DHCPNAK messages to clients  that  have  obtained  addresses
 from a legitimate DHCP server on the network.

 Network  administrators  setting  up  authoritative  DHCP servers for
 their networks should always write authoritative; at the top of their
 configuration file to indicate that the DHCP server should send DHCP-
 NAK messages to misconfigured clients.	  If this is not done, clients
 will  be  unable  to  get a correct IP address after changing subnets
 until their old lease has expired, which  could  take	quite  a  long

 Usually,  writing  authoritative; at the top level of the file should
 be sufficient.	  However, if a DHCP server is to be set up so that it
 is aware of some networks for which it is authoritative and some net-
 works for which it is not, it may  be	more  appropriate  to  declare
 authority on a per-network-segment basis.

 Note  that the most specific scope for which the concept of authority
 makes any sense is the physical network segment -  either  a  shared-
 network  statement or a subnet statement that is not contained within
 a shared-network statement.  It is not meaningful to specify that the
 server is authoritative for some subnets within a shared network, but
 not authoritative for others, nor is it meaningful  to	 specify  that
 the  server  is authoritative for some host declarations and not oth-

   The boot-unknown-clients statement

 boot-unknown-clients flag;

 If the boot-unknown-clients statement is present and has a  value  of
 false	or  off,  then	clients for which there is no host declaration
 will not be allowed to obtain IP addresses.   If  this	 statement  is
 not  present  or has a value of true or on, then clients without host
 declarations will be allowed to obtain IP addresses, as long as those
 addresses  are	 not  restricted  by  allow and deny statements within
 their pool declarations.

   The db-time-format statement

 db-time-format [ default | local ] ;

 The DHCP server software  outputs  several  timestamps	 when  writing
 leases	 to  persistent storage.  This configuration parameter selects
 one of two output formats.  The default format prints the day,	 date,
 and  time  in	UTC, while the local format prints the system seconds-
 since-epoch, and helpfully provides the day and time  in  the	system
 timezone  in  a comment.  The time formats are described in detail in
 the dhcpd.leases(5) manpage.

   The ddns-hostname statement

 ddns-hostname name;

 The name parameter should be the hostname that will be used  in  set-
 ting  up  the	client's  A  and PTR records.	If no ddns-hostname is
 specified in scope, then the server will derive the hostname automat-
 ically,  using	 an  algorithm	that  varies for each of the different
 update methods.

   The ddns-domainname statement

 ddns-domainname name;

 The name parameter should be the domain name that will be appended to
 the client's hostname to form a fully-qualified domain-name (FQDN).

   The ddns-rev-domainname statement

 ddns-rev-domainname  name;  The  name	parameter should be the domain
 name that will be appended to the client's  reversed  IP  address  to
 produce a name for use in the client's PTR record.   By default, this
 is "in-addr.arpa.", but the default can be overridden here.

 The reversed IP address to which this	domain	name  is  appended  is
 always	 the  IP  address  of  the  client,  in	 dotted quad notation,
 reversed - for example, if the IP address assigned to the  client  is,  then  the  reversed  IP	address is	  So a
 client with that IP address would, by default, be given a PTR	record

   The ddns-update-style parameter

 ddns-update-style style;

 The  style  parameter	must  be  one of ad-hoc, interim or none.  The
 ddns-update-style statement is only meaningful in the outer  scope  -
 it  is	 evaluated once after reading the dhcpd.conf file, rather than
 each time a client is assigned an IP address, so there is no  way  to
 use different DNS update styles for different clients. The default is

   The ddns-updates statement

  ddns-updates flag;

 The ddns-updates parameter controls whether or not  the  server  will
 attempt  to  do a DNS update when a lease is confirmed.   Set this to
 off if the server should not attempt to do updates within  a  certain
 scope.	 The ddns-updates parameter is on by default.	To disable DNS
 updates in all scopes, it is preferable to use the  ddns-update-style
 statement, setting the style to none.

   The default-lease-time statement

 default-lease-time time;

 Time should be the length in seconds that will be assigned to a lease
 if the client requesting the lease does not ask for a specific	 expi-
 ration	 time.	 This is used for both DHCPv4 and DHCPv6 leases (it is
 also known as the "valid lifetime" in DHCPv6).

   The delayed-ack and max-ack-delay statements

 delayed-ack count; max-ack-delay microseconds;

 Count should be an integer value from zero to 2^16-1, and defaults to
 28.   The  count  represents  how many DHCPv4 replies maximum will be
 queued pending transmission until after a database commit event.   If
 this  number  is reached, a database commit event (commonly resulting
 in fsync() and representing a performance penalty) will be made,  and
 the  reply  packets  will be transmitted in a batch afterwards.  This
 preserves the RFC2131 direction  that	"stable	 storage"  be  updated
 prior	to  replying  to  clients.  Should the DHCPv4 sockets "go dry"
 (select() returns immediately with no read sockets),  the  commit  is
 made and any queued packets are transmitted.

 Similarly, microseconds indicates how many microseconds are permitted
 to pass inbetween queuing a packet pending an fsync,  and  performing
 the  fsync.   Valid  values  range  from 0 to 2^32-1, and defaults to
 250,000 (1/4 of a second).

 Please note  that  as	delayed-ack  is	 currently  experimental,  the
 delayed-ack  feature  is  not	compiled  in  by  default, but must be
 enabled at compile time with './configure --enable-delayed-ack'.

   The do-forward-updates statement

 do-forward-updates flag;

 The do-forward-updates statement instructs  the  DHCP	server	as  to
 whether it should attempt to update a DHCP client's A record when the
 client acquires or renews a lease.   This  statement  has  no	effect
 unless	 DNS  updates  are  enabled  and  ddns-update-style  is set to
 interim.   Forward updates are enabled by default.   If  this	state-
 ment  is  used to disable forward updates, the DHCP server will never
 attempt to update the client's A record, and will only	 ever  attempt
 to update the client's PTR record if the client supplies an FQDN that
 should be placed in the PTR record using the fqdn option.  If forward
 updates  are enabled, the DHCP server will still honor the setting of
 the client-updates flag.

   The dynamic-bootp-lease-cutoff statement

 dynamic-bootp-lease-cutoff date;

 The dynamic-bootp-lease-cutoff statement sets the ending time for all
 leases	 assigned dynamically to BOOTP clients.	 Because BOOTP clients
 do not have any way of renewing leases, and  don't  know  that	 their
 leases	 could expire, by default dhcpd assigns infinite leases to all
 BOOTP clients.	 However, it may make sense in some situations to  set
 a cutoff date for all BOOTP leases - for example, the end of a school
 term, or the time at night when a facility is closed and all machines
 are required to be powered off.

 Date  should be the date on which all assigned BOOTP leases will end.
 The date is specified in the form:


 W is the day of the week expressed as a number from zero (Sunday)  to
 six  (Saturday).  YYYY is the year, including the century.  MM is the
 month expressed as a number from 1 to 12.   DD	 is  the  day  of  the
 month,	 counting from 1.  HH is the hour, from zero to 23.  MM is the
 minute and SS is the second.  The time is always in Coordinated  Uni-
 versal Time (UTC), not local time.

   The dynamic-bootp-lease-length statement

 dynamic-bootp-lease-length length;

 The dynamic-bootp-lease-length statement is used to set the length of
 leases dynamically assigned to BOOTP clients.	 At some sites, it may
 be  possible to assume that a lease is no longer in use if its holder
 has not used BOOTP or DHCP to get its address within a	 certain  time
 period.    The	 period is specified in length as a number of seconds.
 If a client reboots using BOOTP during the timeout period, the	 lease
 duration  is reset to length, so a BOOTP client that boots frequently
 enough will never lose its lease.  Needless to	 say,  this  parameter
 should be adjusted with extreme caution.

   The filename statement

 filename "filename";

 The filename statement can be used to specify the name of the initial
 boot file which is to be loaded by a client.  The filename should  be
 a filename recognizable to whatever file transfer protocol the client
 can be expected to use to load the file.

   The fixed-address declaration

 fixed-address address [, address ... ];

 The fixed-address declaration is used to assign one or more fixed  IP
 addresses  to a client.  It should only appear in a host declaration.
 If more than one address is supplied, then when the client boots,  it
 will be assigned the address that corresponds to the network on which
 it is booting.	 If none of the addresses in the fixed-address	state-
 ment are valid for the network to which the client is connected, that
 client will not match the host	 declaration  containing  that	fixed-
 address  declaration.	 Each address in the fixed-address declaration
 should be either an IP address or a domain name that resolves to  one
 or more IP addresses.

   The fixed-address6 declaration

 fixed-address6 ip6-address ;

 The  fixed-address6  declaration  is  used  to	 assign	 a  fixed IPv6
 addresses to a client.	 It should only appear in a host declaration.

   The get-lease-hostnames statement

 get-lease-hostnames flag;

 The get-lease-hostnames statement is used to tell  dhcpd  whether  or
 not  to  look	up  the domain name corresponding to the IP address of
 each address in the lease pool and use	 that  address	for  the  DHCP
 hostname  option.   If flag is true, then this lookup is done for all
 addresses in the current scope.   By default, or if flag is false, no
 lookups are done.

   The hardware statement

 hardware hardware-type hardware-address;

 In  order  for	 a BOOTP client to be recognized, its network hardware
 address must be declared using a hardware clause in the  host	state-
 ment.	 hardware-type	must be the name of a physical hardware inter-
 face type.   Currently, only the ethernet and	token-ring  types  are
 recognized,  although	support	 for a fddi hardware type (and others)
 would also be desirable.  The hardware-address should	be  a  set  of
 hexadecimal  octets  (numbers from 0 through ff) separated by colons.
 The hardware statement may also be used for DHCP clients.

   The host-identifier option statement

 host-identifier option option-name option-data;

 This identifies a DHCPv6 client in a host statement.  option-name  is
 any  option,  and  option-data	 is  the value for the option that the
 client will send. The option-data must be a constant value.

   The infinite-is-reserved statement

 infinite-is-reserved flag;

 ISC DHCP now supports 'reserved' leases.  See the section on RESERVED
 LEASES	 below.	  If  this  flag  is on, the server will automatically
 reserve leases allocated  to  clients	which  requested  an  infinite
 (0xffffffff) lease-time.

 The default is off.

   The lease-file-name statement

 lease-file-name name;

 Name  should  be  the	name  of  the  DHCP  server's lease file.   By
 default, this is DBDIR/dhcpd.leases.	This statement must appear  in
 the  outer  scope  of	the configuration file - if it appears in some
 other scope, it will have no effect.  Furthermore, it has  no	effect
 if  overridden by the -lf flag or the PATH_DHCPD_DB environment vari-

   The limit-addrs-per-ia statement

 limit-addrs-per-ia number;

 By default, the DHCPv6 server will limit clients to one IAADDR per IA
 option,  meaning  one address.	 If you wish to permit clients to hang
 onto multiple addresses at a time, configure a larger number here.

 Note that there is no present	method	to  configure  the  server  to
 forcibly  configure the client with one IP address per each subnet on
 a shared network.  This is left to future work.

   The dhcpv6-lease-file-name statement

 dhcpv6-lease-file-name name;

 Name is the name of the lease file to use if and only if  the	server
 is  running in DHCPv6 mode.  By default, this is DBDIR/dhcpd6.leases.
 This statement, like lease-file-name, must appear in the outer	 scope
 of the configuration file.  It has no effect if overridden by the -lf
 flag or the PATH_DHCPD6_DB environment	 variable.   If	 dhcpv6-lease-
 file-name  is not specified, but lease-file-name is, the latter value
 will be used.

   The local-port statement

 local-port port;

 This statement causes the DHCP server to listen for DHCP requests  on
 the UDP port specified in port, rather than on port 67.

   The local-address statement

 local-address address;

 This  statement  causes  the  DHCP server to listen for DHCP requests
 sent to the specified address,	 rather	 than  requests	 sent  to  all
 addresses.  Since serving directly attached DHCP clients implies that
 the server must respond to requests sent to the all-ones IP  address,
 this  option  cannot be used if clients are on directly attached net-
 works...it is only realistically  useful  for	a  server  whose  only
 clients are reached via unicasts, such as via DHCP relay agents.

 Note:	 This  statement  is only effective if the server was compiled
 using the USE_SOCKETS #define statement, which is default on a	 small
 number	 of  operating	systems, and must be explicitly chosen at com-
 pile-time for all others.  You can be sure if your server is compiled
 with USE_SOCKETS if you see lines of this format at startup:

  Listening on Socket/eth0

 Note  also  that since this bind()s all DHCP sockets to the specified
 address, that only one address may be supported  in  a	 daemon	 at  a
 given time.

   The log-facility statement

 log-facility facility;

 This statement causes the DHCP server to do all of its logging on the
 specified log facility once the dhcpd.conf file has been  read.    By
 default  the  DHCP server logs to the daemon facility.	  Possible log
 facilities include auth, authpriv,  cron,  daemon,  ftp,  kern,  lpr,
 mail,	mark,  news,  ntp,  security,  syslog,	user, uucp, and local0
 through local7.   Not all of these facilities are  available  on  all
 systems,  and	there  may be other facilities available on other sys-

 In addition to setting this value, you may need to modify  your  sys-
 log.conf file to configure logging of the DHCP server.	  For example,
 you might add a line like this:

      local7.debug /var/log/dhcpd.log

 The syntax of the syslog.conf file may be different on some operating
 systems  -  consult  the  syslog.conf manual page to be sure.	To get
 syslog to start logging to the new file, you must  first  create  the
 file  with correct ownership and permissions (usually, the same owner
 and permissions of your /var/log/messages or  /usr/adm/messages  file
 should	 be  fine) and send a SIGHUP to syslogd.  Some systems support
 log rollover using a shell script  or	program	 called	 newsyslog  or
 logrotate, and you may be able to configure this as well so that your
 log file doesn't grow uncontrollably.

 Because the log-facility setting  is  controlled  by  the  dhcpd.conf
 file,	log  messages  printed	while  parsing	the dhcpd.conf file or
 before parsing it are logged to the default log facility.  To prevent
 this,	see  the  README  file	included with this distribution, which
 describes how to change the default log facility.  When this  parame-
 ter is used, the DHCP server prints its startup message a second time
 after parsing the configuration file, so that the log will be as com-
 plete as possible.

   The max-lease-time statement

 max-lease-time time;

 Time should be the maximum length in seconds that will be assigned to
 a lease.   The only exception to this is  that	 Dynamic  BOOTP	 lease
 lengths,  which  are  not specified by the client, are not limited by
 this maximum.

   The min-lease-time statement

 min-lease-time time;

 Time should be the minimum length in seconds that will be assigned to
 a lease.

   The min-secs statement

 min-secs seconds;

 Seconds  should be the minimum number of seconds since a client began
 trying to acquire a new lease before the DHCP server will respond  to
 its  request.	 The  number  of  seconds  is based on what the client
 reports, and the maximum value that the client can report is 255 sec-
 onds.	 Generally, setting this to one will result in the DHCP server
 not responding to the client's first request, but  always  responding
 to its second request.

 This can be used to set up a secondary DHCP server which never offers
 an address to a client until the primary  server  has	been  given  a
 chance	 to  do	 so.	If the primary server is down, the client will
 bind to the secondary server, but  otherwise  clients	should	always
 bind  to  the primary.	  Note that this does not, by itself, permit a
 primary server and a secondary server to share a pool of dynamically-
 allocatable addresses.

   The next-server statement

 next-server server-name;

 The  next-server statement is used to specify the host address of the
 server from which the initial boot file (specified  in	 the  filename
 statement)  is	 to  be	 loaded.    Server-name should be a numeric IP
 address or a domain name.

   The omapi-port statement

 omapi-port port;

 The omapi-port statement causes the DHCP server to listen  for	 OMAPI
 connections  on  the  specified port.	 This statement is required to
 enable the OMAPI protocol, which is used to examine  and  modify  the
 state of the DHCP server as it is running.

   The one-lease-per-client statement

 one-lease-per-client flag;

 If  this flag is enabled, whenever a client sends a DHCPREQUEST for a
 particular lease, the server will automatically free any other leases
 the  client  holds.	This  presumes	that  when  the client sends a
 DHCPREQUEST, it has forgotten any lease not mentioned in the  DHCPRE-
 QUEST	-  i.e., the client has only a single network interface and it
 does not remember leases it's holding on networks to which it is  not
 currently  attached.	Neither of these assumptions are guaranteed or
 provable, so we urge caution in the use of this statement.

   The pid-file-name statement

 pid-file-name name;

 Name should be the name of the DHCP server's process ID file.	  This
 is  the file in which the DHCP server's process ID is stored when the
 server starts.	  By default, this  is	RUNDIR/dhcpd.pid.    Like  the
 lease-file-name  statement,  this  statement must appear in the outer
 scope of the configuration file.  It has no effect if	overridden  by
 the -pf flag or the PATH_DHCPD_PID environment variable.

 The dhcpv6-pid-file-name statement

    dhcpv6-pid-file-name name;

    Name  is the name of the pid file to use if and only if the server
    is running in DHCPv6 mode.	By default, this is  DBDIR/dhcpd6.pid.
    This statement, like pid-file-name, must appear in the outer scope
    of the configuration file.	It has no effect if overridden by  the
    -pf	  flag	 or  the  PATH_DHCPD6_PID  environment	variable.   If
    dhcpv6-pid-file-name is not specified, but pid-file-name  is,  the
    latter value will be used.

 The ping-check statement

    ping-check flag;

    When  the  DHCP server is considering dynamically allocating an IP
    address to a client, it first sends an ICMP Echo request (a	 ping)
    to	the address being assigned.   It waits for a second, and if no
    ICMP Echo response has been heard, it assigns the address.	 If  a
    response is heard, the lease is abandoned, and the server does not
    respond to the client.

    This ping check introduces a default one-second delay in  respond-
    ing	 to  DHCPDISCOVER  messages,  which  can be a problem for some
    clients.   The default delay of one second may be configured using
    the	 ping-timeout parameter.  The ping-check configuration parame-
    ter can be used to control checking - if its value	is  false,  no
    ping check is done.

 The ping-timeout statement

    ping-timeout seconds;

    If	the DHCP server determined it should send an ICMP echo request
    (a ping) because the ping-check statement  is  true,  ping-timeout
    allows  you	 to  configure how many seconds the DHCP server should
    wait for an ICMP Echo response  to	be  heard,  if	no  ICMP  Echo
    response  has been received before the timeout expires, it assigns
    the address.  If a response is heard, the lease is abandoned,  and
    the	 server	 does  not respond to the client.  If no value is set,
    ping-timeout defaults to 1 second.

 The preferred-lifetime statement

    preferred-lifetime seconds;

    IPv6 addresses have 'valid' and 'preferred' lifetimes.  The	 valid
    lifetime  determines  at what point at lease might be said to have
    expired, and is no longer useable.	A  preferred  lifetime	is  an
    advisory  condition	 to  help applications move off of the address
    and onto currently valid addresses (should there still be any open
    TCP sockets or similar).

    The preferred lifetime defaults to the renew+rebind timers, or 3/4
    the default lease time if none were specified.

 The remote-port statement

    remote-port port;

    This statement causes the DHCP server to transmit  DHCP  responses
    to	DHCP  clients upon the UDP port specified in port, rather than
    on port 68.	 In the event that the UDP response is transmitted  to
    a  DHCP Relay, the server generally uses the local-port configura-
    tion value.	 Should the DHCP  Relay	 happen	 to  be	 addressed  as,	however, the DHCP Server transmits its response to the
    remote-port configuration value.  This is  generally  only	useful
    for	 testing  purposes, and this configuration value should gener-
    ally not be used.

 The server-identifier statement

    server-identifier hostname;

    The server-identifier statement can be used to  define  the	 value
    that  is  sent  in	the  DHCP Server Identifier option for a given
    scope.   The value specified must be an IP address	for  the  DHCP
    server,  and must be reachable by all clients served by a particu-
    lar scope.

    The use of the server-identifier statement is  not	recommended  -
    the	 only  reason  to  use	it  is to force a value other than the
    default value to be sent on	 occasions  where  the	default	 value
    would  be  incorrect.    The default value is the first IP address
    associated with  the  physical  network  interface	on  which  the
    request arrived.

    The	 usual	case where the server-identifier statement needs to be
    sent is when a physical interface has more than  one  IP  address,
    and	 the  one  being sent by default isn't appropriate for some or
    all clients served by that interface.  Another common case is when
    an	alias  is  defined  for	 the purpose of having a consistent IP
    address for the DHCP server, and it is desired  that  the  clients
    use this IP address when contacting the server.

    Supplying a value for the dhcp-server-identifier option is equiva-
    lent to using the server-identifier statement.

 The server-duid statement

    server-duid LLT [ hardware-type timestamp hardware-address ] ;

    server-duid EN enterprise-number enterprise-identifier ;

    server-duid LL [ hardware-type hardware-address ] ;

    The server-duid statement configures the server DUID. You may pick
    either  LLT (link local address plus time), EN (enterprise), or LL
    (link local).

    If you choose LLT or LL, you may specify the exact contents of the
    DUID.   Otherwise the server will generate a DUID of the specified

    If you choose EN, you must include the enterprise number  and  the

    The default server-duid type is LLT.

 The server-name statement

    server-name name ;

    The	 server-name statement can be used to inform the client of the
    name of the server from which it is booting.   Name should be  the
    name that will be provided to the client.

 The site-option-space statement

    site-option-space name ;

    The site-option-space statement can be used to determine from what
    option space site-local options will be taken.   This can be  used
    in	much the same way as the vendor-option-space statement.	 Site-
    local options in DHCP are those options whose  numeric  codes  are
    greater  than  224.	  These options are intended for site-specific
    uses, but are frequently used by vendors of embedded hardware that
    contains  DHCP  clients.   Because site-specific options are allo-
    cated on an ad hoc basis, it is quite possible that	 one  vendor's
    DHCP  client  might use the same option code that another vendor's
    client  uses,  for	different  purposes.	The  site-option-space
    option  can	 be  used  to  assign a different set of site-specific
    options for each such vendor, using	 conditional  evaluation  (see
    dhcp-eval (5) for details).

 The stash-agent-options statement

    stash-agent-options flag;

    If	the  stash-agent-options parameter is true for a given client,
    the server will record the relay agent  information	 options  sent
    during  the	 client's  initial DHCPREQUEST message when the client
    was in the SELECTING state and behave  as  if  those  options  are
    included in all subsequent DHCPREQUEST messages sent in the RENEW-
    ING state.	 This works around a problem with relay agent informa-
    tion options, which is that they usually not appear in DHCPREQUEST
    messages sent by the client in the RENEWING	 state,	 because  such
    messages are unicast directly to the server and not sent through a
    relay agent.

 The update-conflict-detection statement

    update-conflict-detection flag;

    If the update-conflict-detection parameter	is  true,  the	server
    will  perform  standard  DHCID  multiple-client, one-name conflict
    detection.	If the parameter has been set false, the  server  will
    skip this check and instead simply tear down any previous bindings
    to install the new binding without question.  The default is true.

 The update-optimization statement

    update-optimization flag;

    If the update-optimization parameter is false for a given  client,
    the server will attempt a DNS update for that client each time the
    client renews its lease, rather than  only	attempting  an	update
    when it appears to be necessary.   This will allow the DNS to heal
    from database inconsistencies more easily, but the	cost  is  that
    the	 DHCP  server  must  do	 many more DNS updates.	  We recommend
    leaving this option enabled, which is the  default.	  This	option
    only  affects  the	behavior of the interim DNS update scheme, and
    has no effect on the ad-hoc DNS update scheme.   If this parameter
    is	not  specified,	 or  is true, the DHCP server will only update
    when the client information changes, the client gets  a  different
    lease, or the client's lease expires.

 The update-static-leases statement

    update-static-leases flag;

    The	 update-static-leases flag, if enabled, causes the DHCP server
    to do DNS updates for clients even	if  those  clients  are	 being
    assigned  their  IP address using a fixed-address statement - that
    is, the client is being given a static assignment.	 This can only
    work  with	the interim DNS update scheme.	 It is not recommended
    because the DHCP server has no way to tell	that  the  update  has
    been done, and therefore will not delete the record when it is not
    in use.   Also, the server must attempt the update each  time  the
    client  renews  its	 lease, which could have a significant perfor-
    mance impact in environments that place heavy demands on the  DHCP

 The use-host-decl-names statement

    use-host-decl-names flag;

    If	the  use-host-decl-names  parameter  is true in a given scope,
    then for every host declaration within that scope, the  name  pro-
    vided  for	the host declaration will be supplied to the client as
    its hostname.   So, for example,

	group {
	  use-host-decl-names on;

	  host joe {
	    hardware ethernet 08:00:2b:4c:29:32;
	    fixed-address joe.fugue.com;

    is equivalent to

	  host joe {
	    hardware ethernet 08:00:2b:4c:29:32;
	    fixed-address joe.fugue.com;
	    option host-name "joe";

    An option host-name statement within a host declaration will over-
    ride the use of the name in the host declaration.

    It	should	be noted here that most DHCP clients completely ignore
    the host-name option sent by the DHCP server, and there is no  way
    to configure them not to do this.	So you generally have a choice
    of either not having any hostname to  client  IP  address  mapping
    that  the  client  will  recognize,	 or doing DNS updates.	 It is
    beyond the scope of this document to describe  how	to  make  this

 The use-lease-addr-for-default-route statement

    use-lease-addr-for-default-route flag;

    If	the  use-lease-addr-for-default-route  parameter  is true in a
    given scope, then instead of sending the value  specified  in  the
    routers option (or sending no value at all), the IP address of the
    lease being assigned is sent  to  the  client.    This  supposedly
    causes  Win95  machines  to ARP for all IP addresses, which can be
    helpful if your router is configured for proxy ARP.	  The  use  of
    this  feature  is  not recommended, because it won't work for many
    DHCP clients.

 The vendor-option-space statement

    vendor-option-space string;

    The vendor-option-space  parameter	determines  from  what	option
    space  vendor  options  are taken.	 The use of this configuration
    parameter is illustrated in the dhcp-options(5)  manual  page,  in

SETTING PARAMETER VALUES USING EXPRESSIONS Sometimes it’s helpful to be able to set the value of a DHCP server parameter based on some value that the client has sent. To do this, you can use expression evaluation. The dhcp-eval(5) manual page describes how to write expressions. To assign the result of an evalu- ation to an option, define the option as follows:

 my-parameter = expression ;

   For example:

 ddns-hostname = binary-to-ascii (16, 8, "-",
				  substring (hardware, 1, 6));

RESERVED LEASES It’s often useful to allocate a single address to a single client, in approximate perpetuity. Host statements with fixed-address clauses exist to a certain extent to serve this purpose, but because host statements are intended to approximate ‘static configuration’, they suffer from not being referenced in a littany of other Server Services, such as dynamic DNS, failover, ‘on events’ and so forth.

   If a standard dynamic lease, as from any	 range	statement,  is	marked
   'reserved', then the server will only allocate this lease to the client
   it is identified by (be that by client identifier or hardware address).

   In practice, this means that the lease follows the normal state engine,
   enters  ACTIVE  state  when  the	 client is bound to it, expires, or is
   released, and any events or services that would	normally  be  supplied
   during  these  events are processed normally, as with any other dynamic
   lease.  The only difference is that  failover  servers  treat  reserved
   leases  as  special  when  they	enter the FREE or BACKUP states - each
   server applies the lease into the state it may allocate from - and  the
   leases  are  not	 placed	 on the queue for allocation to other clients.
   Instead they may only be 'found' by client  identity.   The  result  is
   that the lease is only offered to the returning client.

   Care  should  probably  be taken to ensure that the client only has one
   lease within a given subnet that it is identified by.

   Leases may be set 'reserved'  either  through  OMAPI,  or  through  the
   ´infinite-is-reserved'  configuration  option (if this is applicable to
   your environment and mixture of clients).

   It should also be noted that leases marked 'reserved'  are  effectively
   treated the same as leases marked 'bootp'.

REFERENCE: OPTION STATEMENTS DHCP option statements are documented in the dhcp-options(5) manual page.

REFERENCE: EXPRESSIONS Expressions used in DHCP option statements and elsewhere are documented in the dhcp-eval(5) manual page.

SEE ALSO dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.

AUTHOR dhcpd.conf(5) was written by Ted Lemon under a contract with Vixie Labs. Funding for this project was provided by Internet Systems Con- sortium. Information about Internet Systems Consortium can be found at https://www.isc.org.