Charles Steinkuehler's LEAF/LRP Website




       dhcpd - Dynamic Host Configuration Protocol Server


       dhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -cf config-file ]
       [ -lf lease-file ] [ if0 [ ...ifN ] ]


       The  Internet  Software  Consortium  DHCP  Server,  dhcpd,
       implements  the Dynamic Host Configuration Protocol (DHCP)
       and the Internet Bootstrap Protocol (BOOTP).  DHCP  allows
       hosts  on  a  TCP/IP network to request and be assigned IP
       addresses, and also to discover information about the net­
       work  to  which they are attached.  BOOTP provides similar
       functionality, with certain restrictions.


       The DHCP protocol allows a host which is  unknown  to  the
       network  administrator  to be automatically assigned a new
       IP address out of a pool of IP addresses for its  network.
       In order for this to work, the network administrator allo­
       cates address pools in each subnet and  enters  them  into
       the dhcpd.conf(5) file.

       On  startup,  dhcpd reads the dhcpd.conf file and stores a
       list of available addresses  on  each  subnet  in  memory.
       When a client requests an address using the DHCP protocol,
       dhcpd  allocates  an  address  for  it.   Each  client  is
       assigned  a  lease,  which expires after an amount of time
       chosen by the administrator (by default, one day).  Before
       leases  expire,  the  clients to which leases are assigned
       are expected to renew them in order to continue to use the
       addresses.   Once a lease has expired, the client to which
       that lease was assigned is no longer permitted to use  the
       leased IP address.

       In order to keep track of leases across system reboots and
       server restarts, dhcpd keeps  a  list  of  leases  it  has
       assigned  in  the  dhcpd.leases(5)  file.    Before  dhcpd
       grants a lease to a host, it records  the  lease  in  this
       file  and  makes  sure  that  the contents of the file are
       flushed to disk.   This ensures that even in the event  of
       a  system  crash, dhcpd will not forget about a lease that
       it  has  assigned.    On  startup,   after   reading   the
       dhcpd.conf  file,  dhcpd  reads  the  dhcpd.leases file to
       refresh its memory about what leases have been assigned.

       New leases are appended to the  end  of  the  dhcpd.leases
       file.    In  order to prevent the file from becoming arbi­
       trarily large, from time  to  time  dhcpd  creates  a  new
       dhcpd.leases  file  from its in-core lease database.  Once
       this file has been  written  to  disk,  the  old  file  is
       renamed   dhcpd.leases~,  and  the  new  file  is  renamed
       dhcpd.leases.   If the system crashes  in  the  middle  of
       this  process,  whichever  dhcpd.leases  file remains will
       contain all the lease information, so there is no need for
       a special crash recovery process.

       BOOTP  support  is  also  provided by this server.  Unlike
       DHCP, the BOOTP protocol does not provide a  protocol  for
       recovering dynamically-assigned addresses once they are no
       longer needed.    It  is  still  possible  to  dynamically
       assign addresses to BOOTP clients, but some administrative
       process  for  reclaiming  addresses  is   required.     By
       default,  leases  are granted to BOOTP clients in perpetu­
       ity, although the network administrator may set an earlier
       cutoff  date or a shorter lease length for BOOTP leases if
       that makes sense.

       BOOTP clients may also be served in the old standard  way,
       which is to simply provide a declaration in the dhcpd.conf
       file for  each  BOOTP  client,  permanently  assigning  an
       address to each client.

       Whenever  changes  are  made to the dhcpd.conf file, dhcpd
       must be restarted.   To  restart  dhcpd,  send  a  SIGTERM
       (signal    15)    to   the   process   ID   contained   in
       RUNDIR/dhcpd.pid, and then re-invoke dhcpd.   Because  the
       DHCP  server  database  is  not  as lightweight as a BOOTP
       database, dhcpd does not automatically restart itself when
       it sees a change to the dhcpd.conf file.

       Note:  We get a lot of complaints about this.   We realize
       that it would be nice if one could send a  SIGHUP  to  the
       server  and  have  it  reload  the database.   This is not
       technically impossible, but it would require a great  deal
       of work, our resources are extremely limited, and they can
       be better spent  elsewhere.    So  please  don't  complain
       about  this  on the mailing list unless you're prepared to
       fund a project to implement this feature, or  prepared  to
       do it yourself.


       The  names of the network interfaces on which dhcpd should
       listen for broadcasts may  be  specified  on  the  command
       line.   This  should  be  done  on  systems where dhcpd is
       unable to identify non-broadcast  interfaces,  but  should
       not  be  required on other systems.  If no interface names
       are specified on the command line dhcpd will identify  all
       network  interfaces which are up, elimininating non-broad­
       cast interfaces if possible, and listen  for  DHCP  broad­
       casts on each interface.

       If  dhcpd  should listen on a port other than the standard
       (port 67), the -p flag may used.  It should be followed by
       the udp port number on which dhcpd should listen.  This is
       mostly useful for debugging purposes.  If the -p  flag  is
       specified,  the  server will transmit responses to clients
       at a port number that is one greater than the  one  speci­
       fied  -  i.e.,  if you specify -p 67, then the server will
       listen on port 67 and transmit to port 68.  Datagrams that
       must  go  through relay agents are sent to the port number
       specified with the -p flag - if you wish to use  alternate
       port  numbers, you must configure any relay agents you are
       using to use the same alternate port numbers.

       To run dhcpd as a foreground process, rather than allowing
       it  to  run  as  a  daemon  in the background, the -f flag
       should be specified.  This is useful  when  running  dhcpd
       under  a  debugger,  or  when running it out of inittab on
       System V systems.

       To have dhcpd log to the standard error descriptor,  spec­
       ify  the  -d  flag.  This can be useful for debugging, and
       also at sites where a complete log of  all  dhcp  activity
       must be kept but syslogd is not reliable or otherwise can­
       not be used.   Normally, dhcpd will log all  output  using
       the  syslog(3)  function  with  the  log  facility  set to

       Dhcpd can be made to use an alternate  configuration  file
       with the -cf flag, or an alternate lease file with the -lf
       flag.   Because of the importance of using the same  lease
       database  at  all  times when running dhcpd in production,
       these options should be used only for testing lease  files
       or database files in a non-production environment.

       When starting dhcpd up from a system startup script (e.g.,
       /etc/rc), it may not be desirable to print out the  entire
       copyright  message  on  startup.    To avoid printing this
       message, the -q flag may be specified.


       The syntax of the dhcpd.conf(5) file is  discussed  seper­
       ately.   This section should be used as an overview of the
       configuration process, and the dhcpd.conf(5) documentation
       should be consulted for detailed reference information.


       dhcpd needs to know the subnet numbers and netmasks of all
       subnets for which it will be providing service.   In addi­
       tion,  in order to dynamically allocate addresses, it must
       be assigned one or more ranges of addresses on each subnet
       which  it can in turn assign to client hosts as they boot.
       Thus, a very simple configuration providing  DHCP  support
       might look like this:

            subnet netmask {

       Multiple address ranges may be specified like this:

            subnet netmask {

       If  a  subnet will only be provided with BOOTP service and
       no dynamic address assignment, the  range  clause  can  be
       left out entirely, but the subnet statement must appear.

Lease Lengths

       DHCP  leases  can  be assigned almost any length from zero
       seconds to infinity.   What lease length makes  sense  for
       any given subnet, or for any given installation, will vary
       depending on the kinds of hosts being served.

       For example, in an office environment  where  systems  are
       added from time to time and removed from time to time, but
       move relatively infrequently, it might make sense to allow
       lease times of a month of more.   In a final test environ­
       ment on a manufacturing floor, it may make more  sense  to
       assign  a maximum lease length of 30 minutes - enough time
       to go through a simple test procedure on a network  appli­
       ance before packaging it up for delivery.

       It  is  possible to specify two lease lengths: the default
       length that will be assigned if a client doesn't  ask  for
       any  particular  lease length, and a maximum lease length.
       These are specified as clauses to the subnet command:

            subnet netmask {
              default-lease-time 600;
              max-lease-time 7200;

       This particular subnet  declaration  specifies  a  default
       lease  time  of  600  seconds (ten minutes), and a maximum
       lease time of 7200 seconds  (two  hours).    Other  common
       values  would  be  86400  (one day), 604800 (one week) and
       2592000 (30 days).

       Each subnet need not have the same lease--in the  case  of
       an  office  environment  and  a  manufacturing environment
       served by the same DHCP server, it  might  make  sense  to
       have widely disparate values for default and maximum lease
       times on each subnet.

BOOTP Support

       Each BOOTP client  must  be  explicitly  declared  in  the
       dhcpd.conf  file.    A  very basic client declaration will
       specify the client network  interface's  hardware  address
       and  the  IP  address  to  assign to that client.   If the
       client needs to be able to  load  a  boot  file  from  the
       server,  that  file's  name  must be specified.   A simple
       bootp client declaration might look like this:

            host haagen {
              hardware ethernet 08:00:2b:4c:59:23;
              filename "/tftpboot/haagen.boot";


       DHCP (and also BOOTP with Vendor  Extensions)  provides  a
       mechanism  whereby  the server can provide the client with
       information about how to configure its  network  interface
       (e.g.,  subnet  mask),  and also how the client can access
       various network services (e.g., DNS, IP  routers,  and  so

       These options can be specified on a per-subnet basis, and,
       for BOOTP clients, also on a per-client  basis.    In  the
       event  that  a  BOOTP client declaration specifies options
       that are also specified in  its  subnet  declaration,  the
       options  specified  in  the client declaration take prece­
       dence.   An reasonably complete DHCP  configuration  might
       look something like this:

            subnet netmask {
              default-lease-time 600 max-lease-time 7200;
              option subnet-mask;
              option broadcast-address;
              option routers;
              option domain-name-servers,;
              option domain-name "isc.org";

       A  bootp host on that subnet that needs to be in a differ­
       ent domain and  use  a  different  name  server  might  be
       declared as follows:

            host haagen {
              hardware ethernet 08:00:2b:4c:59:23;
              filename "/tftpboot/haagen.boot";
              option domain-name-servers;
              option domain-name "vix.com";

       A  more complete description of the dhcpd.conf file syntax
       is provided in dhcpd.conf(5).


       ETCDIR/dhcpd.conf,  DBDIR/dhcpd.leases,  RUNDIR/dhcpd.pid,


       dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)


       dhcpd(8) was written by Ted Lemon <mellon@vix.com> under a
       contract with Vixie Labs.   Funding for this  project  was
       provided  by  the Internet Software Corporation.  Informa­
       tion about the Internet Software Consortium can  be  found
       at http://www.isc.org/isc.

Man(1) output converted with man2html