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dhclient(8) dhclient(8)
NAME
dhclient - Dynamic Host Configuration Protocol Client
SYNOPSIS
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 ] [ -cf config-file ] [ -sf script-file ] [ -s
server ] [ -g relay ] [ -n ] [ -nc ] [ -nw ] [ -w ] [ -B ] [ -I dhcp-client-identifier ] [ -H host-name ] [ -F fqdn.fqdn
] [ -V vendor-class-identifier ] [ -R request-option-list ] [ -timeout timeout ] [ -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 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 data-
base. 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 avail-
able.
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 vali-
date 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.
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(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.
OPTIONS
-4 Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters (default).
-6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters. The
functionality of DHCPv6 mode may be modified with the -S , -T , and -N options.
-S Perform an information-only request over DHCPv6 to get stateless configuration parameters. It is not recommended
to combine this option with the -N , -P , or -T options or to share lease files between different modes of opera-
tion. Only valid with the -6 option.
-N Perform a normal (IA_NA) address query over DHCPv6. It is not recommended to combine this option with the -P , -S
, or -T options or to share lease files between different modes of operation. Only valid with the -6 option.
-T Perform a temporary (IA_TA) address query over DHCPv6 (disables normal address query). It is not recommended to
combine this option with the -N , -P , or -S options or to share lease files between different modes of operation.
Only valid with the -6 option.
-P Enable IPv6 prefix delegation (disables normal address query). It is not not recommended to combine this option
with the -N , -S , or -T options or to share lease files between different modes of operation. Only valid with
the -6 option.
-D 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".
-p <port number>
The UDP port number the DHCP client should listen and transmit on. If unspecified, dhclient uses the default port
68. This option is mostly useful for debugging purposes. If a different port is specified for the client to lis-
ten and transmit on, the client will also use a different destination port - one less than the specified port.
-d Force dhclient to run as a foreground process. This is useful when running the client under a debugger, or when
running it out of inittab on System V systems.
-e VAR=value
Define additional environment variables for the environment where dhclient-script executes. You may specify mul-
tiplate -e options on the command line. For example: -e IF_METRIC=1
-q Suppress all terminal and log output except error messages.
-1 Try once to get a lease. One failure, exit with code 2.
-r Tell dhclient to release the current lease it has from the server. This is not required by the DHCP protocol, but
some ISPs require their clients to notify the server if they wish to release an assigned IP address.
-lf <lease-file>
Path to the lease database file. If unspecified, the default /var/lib/dhclient/dhclient.leases is used.
-pf <pid-file>
Path to the process ID file. If unspecified, the default /var/run/dhclient.pid is used.
-cf <config-file>
Path to the client configuration file. If unspecified, the default /etc/dhcp/dhclient.conf is used.
-sf <script-file>
Path to the network configuration script invoked by dhclient when it gets a lease. If unspecified, the default
/sbin/dhclient-script is used.
-s <server>
Specifiy the server IP address or fully qualified domain name to transmit DHCP protocol messages to. Normally,
dhclient transmits these messages to 255.255.255.255 (the IP limited broadcast address). Overriding this is
mostly useful for debugging purposes.
-g <relay>
Only for debugging. Set the giaddr field of all packets the client sends to the IP address specified. This
should not be expected to work in any consistent or useful way.
-n Do not configure any interfaces. Most useful combined with the -w option.
-nc Do not drop capabilities.
Normally, if dhclient was compiled with libcap-ng support, dhclient drops most capabilities immediately upon
startup. While more secure, this greatly restricts the additional actions that hooks in dhclient-script (8) can
take. (For example, any daemons that dhclient-script (8) starts or restarts will inherit the restricted capabili-
ties as well, which may interfere with their correct operation.) Thus, the -nc option can be used to prevent
dhclient from dropping capabilities.
The -nc option is ignored if dhclient was not compiled with libcap-ng support.
-nw Become a daemon process immediately (nowait) rather than waiting until an IP address has been acquired.
-w Keep running even if no network interfaces are found. The omshell program can be used to notify the client when a
network interface has been added or removed so it can attempt to configure an IP address on that interface.
-B Set the BOOTP broadcast flag in request packets so servers will always broadcast replies.
-I <dhcp-client-identifier>
Specify the dhcp-client-identifier option to send to the DHCP server.
-H <host-name>
Specify the host-name option to send to the DHCP server. The host-name string only contains the client's hostname
prefix, to which the server will append the ddns-domainname or domain-name options, if any, to derive the fully
qualified domain name of the client. The -H option cannot be used with the -F option.
-F <fqdn.fqdn>
Specify the fqdn.fqdn option to send to the DHCP server. This option cannot be used with the -H option. The
fqdn.fqdn option must specify the complete domain name of the client host, which the server may use for dynamic
DNS updates.
-V <vendor-class-identifier>
Specify the vendor-class-identifier option to send to the DHCP server.
-R <option>[,<option>...]
Specify the list of options the client is to request from the server. The option list must be a single string
consisting of option names separated by at least one command and optional space characters. The default option
list is:
subnet-mask, broadcast-address, time-offset, routers,
domain-search, domain-name, domain-name-servers, host-name,
nis-domain, nis-servers, ntp-servers, interface-mtu
The -R option does not append options to the default request, it overrides the default request list. Keep this in
mind if you want to request an additional option besides the default request list. You will have to specify all
option names for the -R parameter.
-timeout <timeout>
Specify the time after which dhclient will decide that no DHCP servers can be contacted when no responses have
been received.
-v Enable verbose log messages.
If the client is killed by a signale (for example at shutdown or reboot), it will not 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 in the environment table.
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 running, without stopping it. This capability is pro-
vided using OMAPI, an API for manipulating remote objects. OMAPI clients connect to the client using TCP/IP, authenti-
cate, 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 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.
FILES
/sbin/dhclient-script, /etc/dhcp/dhclient.conf, /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
/var/lib/dhclient/dhclient.leases~.
SEE ALSO
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5), dhcp-eval(5).
AUTHOR
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 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)
