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DHCLIENT(8)	       DragonFly System Manager's Manual	   DHCLIENT(8)

NAME

dhclient -- Dynamic Host Configuration Protocol (DHCP) client

SYNOPSIS

dhclient [-dquwx] [-c file] [-l file] interface

DESCRIPTION

The Dynamic Host Configuration Protocol (DHCP) allows hosts on a TCP/IP network to configure one or more network interfaces based on information collected from a DHCP server. DHCP is often used, for example, by cable modem and DSL network providers to automate network configuration for their customers. Information typically provided via DHCP includes address and subnet mask for the interface, default route, and domain name server. To have DragonFly configure an interface using DHCP (or its predecessor, BOOTP) the dhclient utility is used. dhclient is run on the command line with the name of the interface to be configured. If a dhclient is already running for the interface, the old process will be killed and replaced by the new invocation. The options are as follows: -c file Specify an alternate location to /etc/dhclient.conf for the con- figuration file. -d Forces dhclient to always run as a foreground process. By default, dhclient runs in the foreground until it has configured the interface, and then will revert to running in the back- ground. -l file Specify an alternate location to /var/db/dhclient.leases.<IFNAME> for the leases file. -q Forces dhclient to be less verbose on startup. -u Forces dhclient to reject leases with unknown options in them. The default behaviour is to accept such lease offers. -w dhclient stays around permanently and will attempt to reconnect even if the interface does not exist, is down, or goes down. This is the default when interfaces are configured DHCP in /etc/dhclient.conf -x Kill any running dhclient on the specified interface. The DHCP protocol allows a host to contact a central server which main- tains a list of IP addresses which may be assigned on one or more sub- nets. A DHCP client may request an address from this pool, and then use it on a temporary basis for communication on the network. The DHCP pro- tocol 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 /etc/dhclient.conf for configuration instruc- tions. It then attempts to configure the network interface interface with DHCP. 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 /var/db/dhclient.leases.<IFNAME> file. IFNAME represents the network interface of the DHCP client (e.g. em0), one for each interface. On startup, after reading the dhclient.conf(5) file, dhclient reads the leases file to refresh its memory about what leases it has been assigned. 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.<IFNAME> 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, it 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. dhclient requires at least one /dev/bpf* file for each broadcast network interface. See bpf(4) for more information.

FILES

/etc/dhclient.conf DHCP client configuration file. /var/db/dhclient.leases.<IFNAME> Database of acquired leases. /var/run/dhclient.<IFNAME>.pid Pidfile for running instance. While active, the pidfile is flock()'d by dhclient which can be tested by a program using flock() or by a script using the lockf(1) utility, via a non-blocking lock attempt.

SEE ALSO

bpf(4), dhclient.conf(5), dhclient.leases(5), dhclient-script(8), dhcp(8), dhcpd(8) (net/isc-dhcp42-server), dhcrelay(8) (net/isc-dhcp42-relay)

STANDARDS

R. Droms, Interoperation Between DHCP and BOOTP, RFC 1534, October 1993. R. Droms, Dynamic Host Configuration Protocol, RFC 2131, March 1997. S. Alexander and R. Droms, DHCP Options and BOOTP Vendor Extensions, RFC 2132, March 1997. T. Lemon and S. Cheshire, Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4), RFC 3396, November 2002. T. Lemon, S. Cheshire, and B. Volz, The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4, RFC 3442, December 2002.

HISTORY

dhclient was imported into DragonFly 2.1.

AUTHORS

dhclient was written by Ted Lemon <mellon@fugue.com> and Elliot Poger <elliot@poger.com>. The current implementation was reworked by Henning Brauer <henning@openbsd.org>. DragonFly 5.5 July 23, 2013 DragonFly 5.5 dhclient(8) dhclient(8)

NAME

dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS

dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ] [ -p port-number ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf con- fig-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -v ] [ --version ] [ if0 [ ...ifN ] ]

DESCRIPTION

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.

OPERATION

The DHCP protocol allows a host to contact a central server which main- tains a list of IP addresses which may be assigned on one or more sub- nets. 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 instruc- tions. It then gets a list of all the network interfaces that are con- figured in the current system. For each interface, it attempts to con- figure 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 arbi- trarily 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 data- base, 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, 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 configura- tion 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 sys- log(3) facility.

OPTIONS

-4 Use the DHCPv4 protocol to obtain an IPv4 address and configura- tion 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 and -N 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 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 sys- tems. This implies -v. -nw Become a daemon immediately (nowait) rather than waiting until 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 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. This 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. -n Do not configure any interfaces. This is most likely to be use- ful in combination with the -w flag. -e VAR=value Define additional environment variables for the environment where dhclient-script 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 will be executed with the specific reason for calling the script set. The client normally doesn't 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 will be executed with the specific reason for calling the script set. -p port-number 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 trans- mit, 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 sim- ulating 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 doesn't 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 dis- ables the normal address query. See -N to restore it. Note only one requested interface is allowed. -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 particu- larly 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 description of 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 CLIENT- BINDIR/dhclient-script is used. See dhclient-script(8) for a description of this file.

PORTS

During operations the client may use multiple UDP ports to provide dif- ferent functions. Which ports are opened depends on both the way you compiled your code and the configuration you supply. The following should provide you an idea of what ports may be in use. Normally a DHCPv4 client will open a raw UDP socket to receive and send most DHCPv4 packets. It also opens a fallback UDP socket for use in sending unicast packets. Normally these will both use the well known port number for BOOTPC. For DHCPv6 the client opens a UDP socket on the well known client port and a fallback UDP socket on a random port for use in sending unicast messages. Unlike DHCPv4 the well known socket doesn't need to be opened in raw mode. If you have included an omapi port statement in your configuration file then the client will open a TCP socket on that port to listen for OMPAI connections. When something connects another port will be used for the established connection.

CONFIGURATION

The syntax of the dhclient.conf(5) file is discussed separately.

OMAPI

The DHCP client provides some ability to control it while it is run- ning, 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 inter- faces 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.

ENVIRONMENT VARIABLES

The following environment variables may be defined to override the builtin defaults for file locations. Note that use of the related com- mand-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.

FILES

CLIENTBINDIR/dhclient-script, ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid, DBDIR/dhclient.leases~.

SEE ALSO

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

AUTHOR

dhclient(8) To learn more about Internet Systems Consortium, see https://www.isc.org This client was substantially modified and enhanced by Elliot Poger for use on Linux while he was working on the MosquitoNet project at Stan- ford. 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. dhclient(8)

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