dhclient(8)						      System Manager's Manual						       dhclient(8)

NAME

       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS

       dhclient  [ -p port ] [ -d ] [ -q ] [ -1 ] [ -r ] [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-file ] [ -s server ]
       [ -g relay ] [ -n ] [ -nw ] [ -w ] [ if0 [ ...ifN ] ]

DESCRIPTION

       The Internet Software Consortium DHCP Client, dhclient, provides a means for configuring one or more network interfaces using  the  Dynamic
       Host Configuration Protocol, BOOTP protocol, or if these protocols fail, by statically assigning an address.

OPERATION

       The  DHCP  protocol  allows  a host to contact a central server which maintains a list of IP addresses which may be assigned on one or more
       subnets.   A DHCP client may request an address from this pool, and then use it on a temporary basis for communication  on  network.    The
       DHCP protocol also provides a mechanism whereby a client can learn important details about the network to which it is attached, such as the
       location of a default router, the location of a name server, and so on.

       On startup, dhclient reads the dhclient.conf for configuration instructions.   It then gets a list of all the network interfaces  that  are
       configured in the current system.   For each interface, it attempts to configure the interface using the DHCP protocol.

       In  order  to  keep  track of leases across system reboots and server restarts, dhclient keeps a list of leases it has been assigned in the
       dhclient.leases(5) file.   On startup, after reading the dhclient.conf file, dhclient reads the dhclient.leases file to refresh its  memory
       about what leases it has been assigned.

       When a new lease is acquired, it is appended to the end of the dhclient.leases file.   In order to prevent the file from becoming arbitrar-
       ily large, from time to time dhclient creates a new dhclient.leases file  from  its  in-core  lease  database.	The  old  version  of  the
       dhclient.leases file is retained under the name dhclient.leases~ until the next time dhclient rewrites the database.

       Old  leases are kept around in case the DHCP server is unavailable when dhclient is first invoked (generally during the initial system boot
       process).   In that event, old leases from the dhclient.leases file which have not yet expired are tested, and if they are determined to be
       valid, they are used until either they expire or the DHCP server becomes available.

       A mobile host which may sometimes need to access a network on which no DHCP server exists may be preloaded with a lease for a fixed address
       on that network.   When all attempts to contact a DHCP server have failed, dhclient will try to validate the static lease, and if  it  suc-
       ceeds, will use that lease until it is restarted.

       A  mobile  host	may  also  travel  to some networks on which DHCP is not available but BOOTP is.   In that case, it may be advantageous to
       arrange with the network administrator for an entry on the BOOTP database, so that the host can boot quickly on that  network  rather  than
       cycling through the list of old leases.

COMMAND LINE

       The  names of the network interfaces that dhclient should attempt to configure may be specified on the command line.  If no interface names
       are specified on the command line dhclient will normally identify all network interfaces, elimininating non-broadcast interfaces if  possi-
       ble, and attempt to configure each interface.

       It  is also possible to specify interfaces by name in the dhclient.conf(5) file.   If interfaces are specified in this way, then the client
       will only configure interfaces that are either specified in the configuration file or on the command line, and will ignore all other inter-
       faces.

       If  the DHCP client should listen and transmit on a port other than the standard (port 68), the -p flag may used.  It should be followed by
       the udp port number that dhclient should use.  This is mostly useful for debugging purposes.  If a different  port  is  specified  for  the
       client  to  listen  on  and transmit on, the client will also use a different destination port - one greater than the specified destination
       port.

       The DHCP client normally transmits any protocol messages it sends before acquiring an IP address to, 255.255.255.255, the IP limited broad-
       cast  address.	 For  debugging purposes, it may be useful to have the server transmit these messages to some other address.   This can be
       specified with the -s flag, followed by the IP address or domain name of the destination.

       For testing purposes, the giaddr field of all packets that the client sends can be set using the -g flag, followed by  the  IP  address	to
       send.   This is only useful for testing, and should not be expected to work in any consistent or useful way.

       The  DHCP  client  will	normally  run in the foreground until it has configured an interface, and then will revert to running in the back-
       ground.	 To run force dhclient to always run as a foreground process, the -d flag should be specified.	This is useful	when  running  the
       client under a debugger, or when running it out of inittab on System V systems.

       The  client  normally prints a startup message and displays the protocol sequence to the standard error descriptor until it has acquired an
       address, and then only logs messages using the syslog (3) facility.   The -q flag prevents  any	messages  other  than  errors  from  being
       printed to the standard error descriptor.

       The client normally doesn't release the current lease as it is not required by the DHCP protocol.  Some cable ISPs require their clients to
       notify the server if they wish to release an assigned IP address.  The -r flag explicitly releases the current lease, and  once	the  lease
       has been released, the client exits.

       The -1 flag cause dhclient to try once to get a lease.  If it fails, dhclient exits with exit code two.

       The DHCP client normally gets its configuration information from /etc/dhclient.conf, its lease database from /var/lib/dhcp/dhclient.leases,
       stores its process ID in a file called /var/run/dhclient.pid, and configures the network interface using /sbin/dhclient-script  To  specify
       different  names  and/or  locations  for  these files, use the -cf, -lf, -pf and -sf flags, respectively, followed by the name of the file.
       This can be particularly useful if, for example, /var/lib/dhcp or /var/run has not yet been mounted when the DHCP client is started.

       The DHCP client normally exits if it isn't able to identify any network interfaces to configure.   On laptop computers and other  computers
       with  hot-swappable  I/O  buses,  it is possible that a broadcast interface may be added after system startup.	The -w flag can be used to
       cause the client not to exit when it doesn't find any such interfaces.	The omshell (8) program can then be used to notify the client when
       a network interface has been added or removed, so that the client can attempt to configure an IP address on that interface.

       The  DHCP client can be directed not to attempt to configure any interfaces using the -n flag.	This is most likely to be useful in combi-
       nation with the -w flag.

       The client can also be instructed to become a daemon immediately, rather than waiting until it has acquired an IP address.    This  can	be
       done by supplying the -nw flag.

CONFIGURATION

       The syntax of the dhclient.conf(8) file is discussed seperately.

OMAPI

       The  DHCP client provides some ability to control it while it is running, without stopping it.  This capability is provided using OMAPI, an
       API for manipulating remote objects.  OMAPI clients connect to the client using TCP/IP, authenticate, and can  then  examine  the  client's
       current status and make changes to it.

       Rather  than  implementing the underlying OMAPI protocol directly, user programs should use the dhcpctl API or OMAPI itself.   Dhcpctl is a
       wrapper that handles some of the housekeeping chores that OMAPI does not do automatically.   Dhcpctl and OMAPI are documented in dhcpctl(3)
       and omapi(3).   Most things you'd want to do with the client can be done directly using the omshell(1) command, rather than having to write
       a special program.

THE CONTROL OBJECT

       The control object allows you to shut the client down, releasing all leases that it holds and deleting any DNS records it may  have  added.
       It  also  allows you to pause the client - this unconfigures any interfaces the client is using.   You can then restart it, which causes it
       to reconfigure those interfaces.   You would normally pause the client prior to going into hibernation or sleep on a laptop computer.   You
       would  then  resume it after the power comes back.  This allows PC cards to be shut down while the computer is hibernating or sleeping, and
       then reinitialized to their previous state once the computer comes out of hibernation or sleep.

       The control object has one attribute - the state attribute.   To shut the client down, set its state attribute to 2.    It  will  automati-
       cally do a DHCPRELEASE.	 To pause it, set its state attribute to 3.   To resume it, set its state attribute to 4.

FILES

       /sbin/dhclient-script, /etc/dhclient.conf, /var/lib/dhcp/dhclient.leases, /var/run/dhclient.pid, /var/lib/dhcp/dhclient.leases~.

SEE ALSO

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

AUTHOR

       dhclient(8)  has been written for the Internet Software Consortium by Ted Lemon in cooperation with Vixie Enterprises.  To learn more about
       the Internet Software Consortium, see http://www.isc.org To learn more about Vixie Enterprises, see http://www.vix.com.

       This client was substantially modified and enhanced by Elliot Poger for use on Linux while he was working on  the  MosquitoNet  project	at
       Stanford.

       The  current version owes much to Elliot's Linux enhancements, but was substantially reorganized and partially rewritten by Ted Lemon so as
       to use the same networking framework that the Internet Software Consortium DHCP server uses.   Much system-specific configuration code  was
       moved into a shell script so that as support for more operating systems is added, it will not be necessary to port and maintain system-spe-
       cific configuration code to these operating systems - instead, the shell script can invoke the native tools to accomplish the same purpose.

																       dhclient(8)