dhclient(8) - NetBSD Manual Pages

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dhclient(8)                                           dhclient(8)

dhclient - Dynamic Host Configuration Protocol (DHCP) Client
dhclient [ -p port ] [ -d ] [ -D ] [ -q ] [ -1 ] [ -r ] [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-file ] [ -s server ] [ -g relay ] [ -n ] [ -w ] [ 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.
You must have the Berkeley Packet Filter (bpf) configured in your NetBSD kernel. You must have at least one /dev/bpf* file for each broadcast network interface that is attached to your system.
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 configu- ration instructions. It then gets a list of all the net- work 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 1 dhclient(8) dhclient(8) database. Old leases are kept around in case the DHCP server is unavailable when dhclient is first invoked (generally dur- ing 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, elimininating 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 greater than the specified destina- tion port. The DHCP client normally transmits any protocol messages it sends before acquiring an IP address to,, 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. 2 dhclient(8) dhclient(8) 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 client writes a temporary shell script whenever it invokes dhclient-script. This script is normally deleted after the client runs, but it can be helpful when debug- ging the client script to see what the client wrote. The client can be configured not to delete these scripts by specifying the -D flag. 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 mes- sages using the syslog (3) facility. The -q flag pre- vents 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. 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 informa- tion from /etc/dhclient.conf, its lease database from /var/db/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 exam- ple, /var/db 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 iden- tify any network interfaces to configure. On laptop com- puters 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 3 dhclient(8) dhclient(8) 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 config- ure any interfaces using the -n flag. This is most likely to be useful in combination with the -w flag.
The syntax of the dhclient.conf(5) file is discussed seperately.
/sbin/dhclient-script /etc/dhclient.conf, /var/db/dhclient.leases, /var/run/dhclient.pid, /var/db/dhclient.leases~.
dhcpd(8), dhcrelay(8), dhclient.conf(5), dhclient.leases(5), dhclient-script(8)
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 enhance- ments, 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. 4
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