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dhcpd.conf(5)		      File Formats Manual		 dhcpd.conf(5)

       dhcpd.conf - dhcpd configuration file

       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.

       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; }

       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-

       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.

       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

       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.

       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

       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

       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.

       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

       Dynamic	BOOTP  leases  are not compatible with failover, and, as such,
       you need to disallow BOOTP in pools that you are using failover for.

       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 647;
	 peer address trantor.rc.vix.com;
	 peer port 847;
	 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.

       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.  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).

       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" {

       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.

       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.

       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.

       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

       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.

       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  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  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.

       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-

       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:

       key DHCP_UPDATER {
	 algorithm hmac-md5;
	 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:

       key DHCP_UPDATER {
	 algorithm hmac-md5;
	 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.

       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.

       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 or pxe-client-id	options	 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  or
       pxe-client-id  options,	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 and pxe-client-id
       options and the hardware address can be used to match a	host  declara-
       tion, 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.

       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.

       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-

       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

       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 ignore-client-uids statement

	 ignore-client-uids flag;

	 If the ignore-client-uids statement is present and  has  a  value  of
	 true or on, the UID for clients will not be recorded.	If this state-
	 ment is not present or has a value of false or off, then client  UIDs
	 will be recorded.

       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 /var/lib/dhcpd/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

       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
	 /var/lib/dhcpd/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 environ-
	 ment 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.  If no next-server statement applies to a
	 given client, the address is used.

       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 /var/run/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
	    /var/lib/dhcpd/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

       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));

       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'.

       DHCP  option  statements	 are  documented in the dhcp-options(5) manual

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

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

       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