dhclient

dhclient - Dynamic Host Configuration Protocol Client

dhclient [ -p port ] [ -d ] [ if0 [ ...ifN ] ]

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.

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 arbitrarily 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 dhcpd.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 succeeds, 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.

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 identify all network interfaces, elimininating non-broadcast interfaces if possible, and attempt to configure each interface.

If dhclient 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 the -p flag is specified, the client will transmit responses to servers at a port number that is one less than the one specified - i.e., if you specify -p 68, then the client will listen on port 68 and transmit to port 67. 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.

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

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

ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid, DBDIR/dhclient.leases~.

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

dhclient(8) has been written for the Internet Software Consortium by Ted Lemon <mellon@fugue.com> in cooperation with Vixie Enterprises. To learn more about the Internet Software Consortium, see http://www.vix.com/isc. 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-specific configuration code to these operating systems - instead, the shell script can invoke the native tools to accomplish the same purpose.