Provided by: dhcp3-server_3.0.6.dfsg-1ubuntu9_i386 bug

NAME

       dhcpd.conf - dhcpd configuration file

DESCRIPTION

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

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

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

       Parameter statements either say how to do something (e.g., how  long  a
       lease  to  offer),  whether to do something (e.g., should dhcpd provide
       addresses to unknown clients), or what parameters  to  provide  to  the
       client (e.g., use gateway 220.177.244.7).

       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 sends.

       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
       lexical scope, and all declarations at less specific lexical scopes are
       also  consulted  for  client  option  declarations.    Scopes are never
       considered twice, and if parameters  are  declared  in  more  than  one
       scope,  the  parameter  declared  in the most specific scope is the one
       that is used.

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

EXAMPLES

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

       global parameters...

       subnet 204.254.239.0 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.10 204.254.239.30;
       }

       subnet 204.254.239.32 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.42 204.254.239.62;
       }

       subnet 204.254.239.64 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.74 204.254.239.94;
       }

       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 204.254.239.1;

       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
       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; }
       }

ADDRESS POOLS

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

       subnet 10.0.0.0 netmask 255.255.255.0 {
         option routers 10.0.0.254;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers bogus.example.com;
           max-lease-time 300;
           range 10.0.0.200 10.0.0.253;
           allow unknown-clients;
         }

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           range 10.0.0.5 10.0.0.199;
           deny unknown-clients;
         }
       }

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

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

DYNAMIC ADDRESS ALLOCATION

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

       If the server finds the address the  client  is  requesting,  and  that
       address is available to the client, the server will send a DHCPACK.  If
       the address is no longer available, or the client  isn’t  permitted  to
       have  it,  the server will send a DHCPNAK.  If the server knows nothing
       about the address,  it  will  remain  silent,  unless  the  address  is
       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.
       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
       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 generates the list of available  IP  addresses  from  a
       hash  table.    This  means  that  the  addresses are not sorted in any
       particular order, and so it is not possible to  predict  the  order  in
       which  the  DHCP server will allocate IP addresses.   Users of previous
       versions of the ISC DHCP server may have become accustomed to the  DHCP
       server  allocating  IP  addresses  in  ascending  order, but this is no
       longer possible, and there is no way to configure  this  behavior  with
       version 3 of the ISC DHCP server.

IP ADDRESS CONFLICT PREVENTION

       The  DHCP  server  checks IP addresses to see if they are in use before
       allocating them to clients.   It does this  by  sending  an  ICMP  Echo
       request  message  to  the IP address being allocated.   If no ICMP Echo
       reply is received within a second, the address is assumed to  be  free.
       This  is  only  done  for  leases  that  have  been  specified in range
       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 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
       address.

DHCP FAILOVER

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

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

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

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

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

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

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

FAILOVER STARTUP

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

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

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

CONFIGURING FAILOVER

       In  order  to  configure failover, you need to write a peer declaration
       that configures the failover protocol,  and  you  need  to  write  peer
       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";
            deny dynamic bootp clients;
            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" {
         primary;
         address anthrax.rc.vix.com;
         port 519;
         peer address trantor.rc.vix.com;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;
       }

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

          [ primary | secondary ];

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

       The address statement

          address address;

          The address statement declares the IP address or DNS name  on  which
          the server should listen for connections from its failover peer, and
          also the  value  to  use  for  the  DHCP  Failover  Protocol  server
          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 connections from its failover peer.   This statement may
          not currently be omitted, because the failover protocol does not yet
          have a reserved TCP port number.

       The peer port statement

          peer port port-number;

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

       The max-response-delay statement

          max-response-delay seconds;

          The max-response-delay statement tells  the  DHCP  server  how  many
          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
          connection 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 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 communicating.   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
          secondary  for  the  purposes 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 primary.

       The hba statement

          hba colon-separated-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 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:
                00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;

          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 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.

CLIENT CLASSING

       Clients  can  be  separated  into  classes,  and  treated   differently
       depending  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
       this:

       class "ras-clients" {
       }

SUBCLASSES

       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 10.0.0.0 netmask 255.255.255.0 {
         pool {
           allow members of "allocation-class-1";
           range 10.0.0.11 10.0.0.50;
         }
         pool {
           allow members of "allocation-class-2";
           range 10.0.0.51 10.0.0.100;
         }
       }

       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 parameters,  while  most  didn’t,  you  might  write  the
       following subclass declaration for that client:

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

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

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION

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

       class "limited-1" {
         lease limit 4;
       }

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

SPAWNING CLASSES

       It is possible to declare a spawning class.   A  spawning  class  is  a
       class  that  automatically produces subclasses based on what the client
       sends.   The reason that spawning classes were created was to  make  it
       possible  to  create lease-limited classes on the fly.   The envisioned
       application is a  cable-modem  environment  where  the  ISP  wishes  to
       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  subclass 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.

COMBINING MATCH, MATCH IF AND SPAWN WITH

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

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

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

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

DYNAMIC DNS UPDATES

       The DHCP server has the ability to dynamically update the  Domain  Name
       System.   Within  the  configuration files, you can define how you want
       the Domain Name System to be  updated.   These  updates  are  RFC  2136
       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 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
       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 DNS UPDATE SCHEME

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

       The ad-hoc Dynamic DNS update scheme implemented in this version of the
       ISC  DHCP  server is a prototype design, which does not have much to do
       with the standard update method that is being standardized in the  IETF
       DHC  working  group, but rather implements some very basic, yet useful,
       update capabilities.   This  mode  does  not  work  with  the  failover
       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 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;
       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 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
       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
       behavior.

       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
       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

       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:

                        draft-ietf-dhc-ddns-resolution-??.txt
                          draft-ietf-dhc-fqdn-option-??.txt
                          draft-ietf-dnsext-dhcid-rr-??.txt

       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
       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 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".

       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
       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 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
       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 associated with doing updates even if they do
       not actually modify the DNS  database.    So  the  DHCP  server  tracks
       whether  or not it has updated the record in the past (this information
       is stored on the lease) and does not attempt to update records that  it
       thinks it has already updated.

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

DYNAMIC DNS UPDATE SECURITY

       When you set your DNS server up to allow updates from the DHCP  server,
       you  may  be  exposing  it to unauthorized updates.  To avoid this, you
       should use TSIG signatures -  a  method  of  cryptographically  signing
       updates using a shared secret key.   As long as you protect the secrecy
       of this key, your updates should also be secure.   Note, however,  that
       the  DHCP  protocol  itself  provides no security, and that clients can
       therefore provide information to the DHCP server which the DHCP  server
       will   then   use  in  its  updates,  with  the  constraints  described
       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
       10.10.17.0/24  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.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. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       zone 17.127.10.in-addr.arpa. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

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

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

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

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

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

            dnskeygen -H 128 -u -c -n DHCP_UPDATER

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

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

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

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

REFERENCE: EVENTS

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

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

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

REFERENCE: DECLARATIONS

       The include statement

        include "filename";

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

       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
       statement.   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
       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 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 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
       omitted.

       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
       statements, 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
       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 protocol.

       Please be aware that only the  dhcp-client-identifier  option  and  the
       hardware  address  can  be  used  to  match  a  host declaration.   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
       networks, subnets, or even other groups.

REFERENCE: ALLOW AND DENY

       The allow and deny statements can be used to control  the  response  of
       the  DHCP  server  to  various  sorts  of requests.  The allow and deny
       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 general server behavior with respect to clients based on scope.
       In a non-pool context, the ignore keyword can be used in place  of  the
       deny keyword to prevent logging of denied requests.

ALLOW DENY AND IGNORE IN SCOPE

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

       The unknown-clients keyword

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

       The unknown-clients flag is used  to  tell  dhcpd  whether  or  not  to
       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.

       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.
       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 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.

ALLOW AND DENY WITHIN POOL DECLARATIONS

       The uses of the allow and deny keywords shown in the  previous  section
       work  pretty  much  the  same  way  whether  the  client  is  sending a
       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 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
       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 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:

        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.

REFERENCE: PARAMETERS

       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 sending 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  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

          authoritative;

          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 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  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 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 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 declarations.

       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
          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
          10.17.92.74,  then  the  reversed  IP address is 74.92.17.10.   So a
          client with that IP address would, by default, be given a PTR record
          of 10.17.92.74.in-addr.arpa.

       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 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
          expiration time.

       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
          statement  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 YYYY/MM/DD HH:MM:SS

          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.  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
          statement  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 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)  separated  by
          colons.    The hardware statement may also be used for DHCP clients.

       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/dhcp3/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.

       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  networks...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
          compile-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
          systems.

          In  addition  to  setting  this  value,  you may need to modify your
          syslog.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
          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  primary  server and a secondary server to share a pool of
          dynamically-allocatable addresses.

       The next-server statement

          next-server server-name;

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

       The omapi-port statement

          omapi-port port;

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

       The one-lease-per-client statement

          one-lease-per-client flag;

          If this flag is enabled, whenever a client sends a DHCPREQUEST for a
          particular  lease,  the  server  will  automatically  free any other
          leases the client holds.   This presumes that when the client  sends
          a  DHCPREQUEST,  it  has  forgotten  any  lease not mentioned in the
          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.    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.

       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 responding
          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  parameter  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 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  configuration
          value.   Should  the DHCP Relay happen to be addressed as 127.0.0.1,
          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 generally 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 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  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
          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 declaration 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 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  the  VENDOR
          ENCAPSULATED OPTIONS section.

SETTING PARAMETER VALUES USING EXPRESSIONS

       Sometimes  it’s  helpful  to  be able to set the value of a DHCP server
       parameter based on some value that the client has sent.   To  do  this,
       you  can  use  expression  evaluation.    The  dhcp-eval(5) manual page
       describes how to write  expressions.    To  assign  the  result  of  an
       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));

REFERENCE: OPTION STATEMENTS

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

REFERENCE: EXPRESSIONS

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

SEE ALSO

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

AUTHOR

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

                                                                 dhcpd.conf(5)