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RedHat 9 (Linux i386) - man page for dhcpd.conf (redhat section 5)

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 Software
       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 for-
       matting 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
       parameters 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  subnet  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-subnet
       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 physical 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  ethernet  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.

       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 dynamically 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,  subnet  and shared-network declarations for the IP
       address assigned to the client.	 Each of these declarations itself appears within a lexi-
       cal  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 decla-
       ration  which  has a fixed-address parameter which matches the subnet or shared network on
       which the client is booting.   If it doesn't find any such entry, it then tries to find an
       entry which has no fixed-address parameter.

       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 parameters 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

	    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 parameters
       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 declarations 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 dif-
       ferently 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 declarations:

       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 decla-
       rations 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 while.

       If the server finds the address the client is requesting, and that address is available to
       the client, the server will send a DHCPACK.  If the address is no longer available, or the
       client  isn't  permitted  to have it, the server will send a DHCPNAK.  If the server knows
       nothing about the address, it will remain silent, unless the address is incorrect 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 address.

       There may be a host declaration matching the client's identification, and that host decla-
       ration contains a fixed-address declaration 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  is  requesting  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 permit-
       ted 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 immediately allocated to  the  client.    If
       the  address  is  available for allocation 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  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  statements,  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  aban-
       doned  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 address.

       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 automatically.

       The  failover  protocol defines a primary server role and a secondary server role.   There
       are some differences in how primaries and secondaries act, but  most  of  the  differences
       simply  have  to  do with providing 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 configured
       as secondary, and it doesn't matter too much which one is which.

       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 contact with its partner
       will have expired.

       While  the  failed  server  is  recovering, its partner remains in the partner-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 communicated 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

       In order to configure failover, you need to write a peer declaration that  configures  the
       failover  protocol,  and  you  need  to write peer references 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	segment.     You  must	not  tell  one server it's doing failover on a particular
       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 declaration that utilizes failover would look
       like this:

       pool {
	    failover peer "foo";
	    pool specific parameters

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

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

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

	 [ primary | secondary ];

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

       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  identifier.  Because this value is used as an identifier, it
	 may not be omitted.

       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 connec-
	 tions from its failover peer.	 This statement may not currently be omitted, because the
	 failover protocol does not yet have a reserved TCP port number.

       The peer port statement

	 peer port port-number;

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

       The max-response-delay statement

	 max-response-delay seconds;

	 The max-response-delay statement tells the DHCP server how many seconds 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 con-
	 nection 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 DHCP server how many BNDUPD messages it  can
	 send  before  it receives a BNDACK from the failover peer.   We don't have enough opera-
	 tional 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 communicating.	  A  value  of	something
	 like  3600  is  probably  reasonable, but again bear in mind that we have no real opera-
	 tional experience with this.

       The split statement

	 split index;

	 The split statement specifies the split between the primary and secondary for	the  pur-
	 poses	of load balancing.   Whenever a client makes a DHCP request, the DHCP server runs
	 a hash on the client identification.	If the hash comes out  to  less  than  the  split
	 value,  the  primary  answers.   If it comes out to equal to or more than the split, the
	 secondary answers.   The only meaningful value is 128, and can only be configured on the

       The hba statement

	 hba colon-seperated-hex-list;

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

	   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.	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.  As such, the use of hba is deprecated.

       The load balance max seconds statement

	 load balance max seconds seconds;

	 This statement allows you to configure a cutoff after which load balancing is	disabled.
	 The  cutoff  is  based on the number of seconds since the client sent its first DHCPDIS-
	 COVER 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.

       Clients can be seperated into classes, and treated differently  depending  on  what  class
       they  are in.   This seperation 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 this:
       class "ras-clients" {

       In addition to classes, it is possible to declare subclasses.   A subclass is a class with
       the same name as a regular class, but with a specific 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 constant value to use
       in matching the match expression for the class.	When class matching is done,  the  server
       will  evaluate the match expression 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 subclass.

       Subclasses can be declared with or without scope.   In the above example, 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  configuration  parame-
       ters,  while  most  didn't,  you  might	write the following subclass declaration for that

       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 spe-
       cific 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

       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	automati-
       cally  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 provide
       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 sub-
       class is found matching 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 example:

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

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

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

       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 compliant 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  interac-
       tion  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 configured to use one of the two
       currently-supported methods, or not to do dns updates.	This can be done with  the  ddns-
       update-style configuration 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 protocol
       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  host-

       The DHCP server determines the client's hostname by first looking for a ddns-hostname con-
       figuration 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; otherwise, if the client sent a host-name option, that is used.  Otherwise, 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 based strictly on the server configuration, not on what the
       client sends.   First, if there is a ddns-domainname configuration  option,  it	is  used.
       Second,	if there is a domain-name option configured, that is used.  Otherwise, the server
       will not do the 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 prevents a
       client from claiming that its hostname is the name of some network 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 logged.

       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 behav-

       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  DHCPRE-
       QUEST, 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
       DHCPACK.    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 server.

       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 docu-
       ment 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 configured 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

       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, possibly using the
       hostname supplied by the client ("jschmoe" in the previous example).   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".

       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 deleting 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

       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 name.

       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 associ-
       ated 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  con-
       straints described previously.

       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 decla-
       ration 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.SIG-ALG.REG.INT;
	 secret pRP5FapFoJ95JEL06sv4PQ==;

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

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

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

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

       zone EXAMPLE.ORG. {

       zone 17.127.10.in-addr.arpa. {

       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 subdoman "foo.example.org" with
       no seperate 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 pre-
       ferred 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 doc-
       umentation 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 com-
       mit 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 exe-
       cute 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 shared-network statement

	shared-network name {
	  [ parameters ]
	  [ declarations ]

       The  shared-network statement is used to inform the DHCP server that some IP subnets actu-
       ally share the same physical network.  Any subnets in a shared network should be  declared
       within  a  shared-network statement.  Parameters specified in the shared-network statement
       will be used when booting clients on those subnets unless parameters provided at the  sub-
       net  or	host level override them.  If any subnet in a shared network has addresses avail-
       able 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 syn-
       tax 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 subnet.	 Such addresses are specified using the range declaration.

       The subnet-number should be an IP address or domain name which resolves to the subnet num-
       ber  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 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 state-
       ment 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 speci-
       fying a single address, high-address can be omitted.

       The host statement

	host hostname {
	  [ parameters ]
	  [ declarations ]

       There must be at least one host statement for every BOOTP client that  is  to  be  served.
       host  statements  may  also  be	specified for DHCP clients, although this is not required
       unless booting is only enabled for known hosts.

       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 parameter, or
       more than one host statement may be specified.

       If client-specific boot parameters must change based on the network to which the client is
       attached, then multiple host statements should be used.

       If  a  client  is to be booted using a fixed address if it's possible, but should be allo-
       cated a dynamic address otherwise, then a host  statement  must	be  specified  without	a
       fixed-address  clause.	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 matching the dhcp-client-
       identifier option specified in the host declaration to the one supplied by the client, or,
       if the host declaration or the client does not provide a dhcp-client-identifier option, by
       matching  the  hardware	parameter in the host declaration to the network hardware address
       supplied by the client.	 BOOTP clients do not normally provide a  dhcp-client-identifier,
       so  the hardware address must be used for all clients that may boot using the BOOTP proto-

       The group statement

	group {
	  [ parameters ]
	  [ declarations ]

       The group statement is used simply to apply one or more parameters to a group of  declara-
       tions.	It can be used to group hosts, shared networks, 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 keywords 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  control  gen-
       eral  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

       The  following  usages of allow and deny will work in any scope, although it is not recom-
       mended 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  dynamically  assign
       addresses  to  unknown clients.	 Dynamic address assignment 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 heading ALLOW AND DENY WITHIN POOL DECLARAIONS.

       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.

       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 partic-
       ular 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 and address from the DHCP server.  The duplicates keyword

	allow duplicates;
	deny duplicates;

       Host declarations can match client messages based on the DHCP Client Identifer  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  dif-
       ferent  client  identifiers.   This doesn't normally happen, but is possible when one com-
       puter 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.
       Unfortunately,  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 DHCPDECLINE 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  inten-
       tion  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

       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  DHCPDISCOVER  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 mes-
       sage.   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

       When  a DHCPREQUEST message is processed, address allocation simply consists 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 previ-
       ously in the ADDRESS ALLOCATION section.

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

	known clients;

       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.

	unknown clients;

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

	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.

       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 therefore 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  send-
	 ing  their  requests.	  You  can tell that a client is having this problem if it is not
	 getting the options you have configured for it and if you see in the server log the mes-
	 sage "(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 information  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 legit-
	 imate 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 DHCPNAK 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 time.

	 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 networks 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 spec-
	 ify 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 authorita-
	 tive for some host declarations and not others.

       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  declara-

       The ddns-hostname statement

	 ddns-hostname name;

	 The name parameter should be the hostname that will be used in setting up the client's A
	 and PTR records.   If no ddns-hostname is specified  in  scope,  then	the  server  will
	 derive  the  hostname automatically, using an algorithm that varies for each of the dif-
	 ferent 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 host-
	 name 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

	 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  of

       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 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  state-
	 ment, 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 expiration time.

       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  assignes  infi-
	 nite  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 speci-
	 fied 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 Universal 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.  Need-
	 less 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 statement

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

	 The fixed-address statement 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 sup-
	 plied, then when the client boots, it will be assigned the address which corresponds  to
	 the  network  on  which  it  is  booting.  If none of the addresses in the fixed-address
	 statement are on the network on which the client is booting, that client will not  match
	 the  host  declaration  containing that fixed-address statement.  Each address should be
	 either an IP address or a domain name which resolves to one or more IP addresses.

       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 statement.  hardware-type must be the name
	 of a physical hardware interface 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)  seperated by colons.	 The hardware statement may also be used for DHCP

       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/dhcp/dhcpd.leases.	This statement must appear in the outer scope of the con-
	 figuration file - if it appears in some other scope, it will have no effect.

       The local-port statement

	 local-port port;

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

       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 systems.

	 In addition to setting this value, you may need to modify your syslog.conf file to  con-
	 figure 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  parameter	is  used,
	 the DHCP server prints its startup message a second time after parsing the configuration
	 file, so that the log will be as complete 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  seconds.	  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 pri-
	 mary 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	parameter
	 applies to a given client, the DHCP server's IP 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 DHCPREQUEST - 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.   Nei-
	 ther 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.

       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

	 This ping check introduces a one-second delay in responding  to  DHCPDISCOVER	messages,
	 which can be a problem for some clients.   The ping-check configuration parameter can be
	 used to control checking - if its value is false, no ping check is done.

       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 particular 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  appro-
	 priate 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 equivalent to using the
	 server-identifier statement.

       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 128.   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 allocated 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 RENEWING state.   This works around a
	 problem with relay agent information 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-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 performance impact in environments that
	 place heavy demands on the DHCP server.

       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 dec-
	 laration within that scope, the name provided 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 override 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 determination.

       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 con-
	 figured 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 the VENDOR ENCAPSULATED OPTIONS section.

       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 evaluation to an option, define the option as follows:

	 my-parameter = expression ;

       For example:

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

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

       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, RFC2131.

       dhcpd.conf(5)  was  written  by	Ted Lemon under a contract with Vixie Labs.   Funding for
       this project was provided by the Internet  Software  Consortium.   Information  about  the
       Internet Software Consortium can be found at http://www.isc.org.


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