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 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 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 normally identify all network interfaces, eliminating non-broadcast interfaces if possible, 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 interfaces.
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 less than the specified port.
The DHCP client normally transmits any protocol messages it sends before acquiring an IP address to, 255.255.255.255, the IP limited broadcast 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 background. 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 dhclient daemon creates its own environment when executing the dhclient-script to do the grunt work of interface configuration. To define extra environment variables and their values, use the -e flag, followed by the environment variable name and value assignment, just as one would assign a variable in a shell. Eg: -e IF_METRIC=1
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.
If the client is killed by a signal (for example at shutdown or reboot) it won't execute the dhclient-script (8) at exit. However if you shut the client down gracefully with -r or -x it will execute dhclient-script (8) at shutdown with the specific reason for calling the script set.
The -1 flag will 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/dhcp3/dhclient.conf, its lease database from /var/lib/dhcp3/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/dhcp3 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 (1) 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 combination 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.
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 has one attribute - the state attribute. To shut the client down, set its state attribute to 2. It will automatically do a DHCPRELEASE. To pause it, set its state attribute to 3. To resume it, set its state attribute to 4.
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 Systems 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.