dhclient - Dynamic Host Configuration Protocol Client
The Control Object
dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ] [ -D LL|LLT ] [ -p port ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf config-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -v ] [ --version ] [ if0 [ ...ifN ] ]
The Internet Systems 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.
There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At startup the client may be started for one or the other via the -4 or -6 options.
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 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 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.
The client normally prints no output during its startup sequence. It can be made to emit verbose messages displaying the startup sequence events until it has acquired an address by supplying the -v command line argument. In either case, the client logs messages using the syslog(3) facility.
-4 Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters. This is the default and cannot be combined with -6. -6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters. It cannot be combined with -4. The -S -T -P -N and -D arguments provide more control over aspects of the DHCPv6 processing. Note: it is not recommended to mix queries of different types together or even to share the lease file between them. -1 Try to get a lease once. On failure exit with code 2. In DHCPv6 this sets the maximum duration of the initial exchange to timeout (from dhclient.conf(5) with a default of sixty seconds). -d Force dhclient to run as a foreground process. Normally the DHCP client will run in the foreground until is has configured an interface at which time it will revert to running in the background. This option is useful when running the client under a debugger, or when running it out of inittab on System V systems. This implies -v. -nw Become a daemon immediately (nowait) rather than waiting until an an IP address has been acquired. -q Be quiet at startup, this is the default. -v Enable verbose log messages. -w Continue running even if no broadcast interfaces were found. Normally DHCP client will exit if it isnt 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. This flag can be used to cause the client not to exit when it doesnt 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. -n Do not configure any interfaces. This is most likely to be useful in combination with the -w flag. -e VAR=val Define additional environment variables for the environment where dhclient-script(8) executes. You may specify multiple -e options on the command line. -r Release the current lease and stop the running DHCP client as previously recorded in the PID file. When shutdown via this method dhclient-script(8) will be executed with the specific reason for calling the script set. The client normally doesnt release the current lease as this is not required by the DHCP protocol but some cable ISPs require their clients to notify the server if they wish to release an assigned IP address. -x Stop the running DHCP client without releasing the current lease. Kills existing dhclient process as previously recorded in the PID file. When shutdown via this method dhclient-script(8) will be executed with the specific reason for calling the script set. -p port The UDP port number on which the DHCP client should listen and transmit. If unspecified, dhclient uses the default port of 68. This is mostly useful for debugging purposes. If a different port is specified on which the client should listen and transmit, the client will also use a different destination port - one less than the specified port. -s server-addr Specify the server IP address or fully qualified domain name to use as a destination for DHCP protocol messages before dhclient has acquired an IP address. Normally, dhclient transmits these messages to 255.255.255.255 (the IP limited broadcast address). Overriding this is mostly useful for debugging purposes. This feature is not supported in DHCPv6 (-6) mode. -g relay Set the giaddr field of all packets to the relay IP address simulating a relay agent. This is for testing pruposes only and should not be expected to work in any consistent or useful way. --version Print version number and exit. Options available for DHCPv6 mode: -S Use Information-request to get only stateless configuration parameters (i.e., without address). This implies -6. It also doesnt rewrite the lease database. -T Ask for IPv6 temporary addresses, one set per -T flag. This implies -6 and also disables the normal address query. See -N to restore it. -P Enable IPv6 prefix delegation. This implies -6 and also disables the normal address query. See -N to restore it. Note only one requested interface is allowed. -D LL or LLT Override the default when selecting the type of DUID to use. By default, DHCPv6 dhclient creates an identifier based on the link-layer address (DUID-LL) if it is running in stateless mode (with -S, not requesting an address), or it creates an identifier based on the link-layer address plus a timestamp (DUID-LLT) if it is running in stateful mode (without -S, requesting an address). -D overrides this default, with a value of either LL or LLT. -N Restore normal address query for IPv6. This implies -6. It is used to restore normal operation after using -T or -P. Modifying default file locations: The following options can be used to modify the locations a client uses for its files. They can be particularly useful if, for example, DBDIR or RUNDIR have not been mounted when the DHCP client is started. -cf config-file Path to the client configuration file. If unspecified, the default ETCDIR/dhclient.conf is used. See dhclient.conf(5) for a description of this file. -lf lease-file Path to the lease database file. If unspecified, the default DBDIR/dhclient.leases is used. See dhclient.leases(5) for a descriptionof this file. -pf pid-file Path to the process ID file. If unspecified, the default RUNDIR/dhclient.pid is used. --no-pid Option to disable writing pid files. By default the program will write a pid file. If the program is invoked with this option it will not attempt to kill any existing client processes even if invoked with -r or -x. -sf script-file Path to the network configuration script invoked by dhclient when it gets a lease. If unspecified, the default CLIENTBINDIR/dhclient-script is used. See dhclient-script(8) for a description of this file.
The syntax of the dhclient.conf(5) file is discussed separately.
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 clients 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 youd 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 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 automatically do a DHCPRELEASE. To pause it, set its state attribute to 3. To resume it, set its state attribute to 4.
The following environment variables may be defined to override the builtin defaults for file locations. Note that use of the related command-line options will ignore the corresponding environment variable settings.
PATH_DHCLIENT_CONF The dhclient.conf configuration file. PATH_DHCLIENT_DB The dhclient.leases database. PATH_DHCLIENT_PID The dhclient PID file. PATH_DHCLIENT_SCRIPT The dhclient-script file.
CLIENTBINDIR/dhclient-script, ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid, DBDIR/dhclient.leases~.
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5), dhcp-eval(5).
dhclient(8) has been written for Internet Systems Consortium by Ted Lemon in cooperation with Vixie Enterprises. To learn more about Internet Systems Consortium, see https://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 Elliots 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.