dhclient(8)dhclient(8)NAMEdhclient - Dynamic Host Configuration Protocol Client
SYNOPSISdhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [
-P... ] ] [ -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 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.
On startup, dhclient reads the dhclient.conf for configuration instruc‐
tions. 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 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(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 configura‐
tion parameters (default).
-6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are
available along with configuration parameters. The functional‐
ity 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 operation. 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 oper‐
ation. 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.
-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 listen 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 multiplate -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 trans‐
mits these messages to 255.255.255.255 (the IP limited broadcast
address). Overriding this is mostly useful for debugging pur‐
poses.
-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 capabilities 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 lib‐
cap-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 config‐
ure 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-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 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.
FILES
/sbin/dhclient-script, /etc/dhcp/dhclient.conf,
/var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
/var/lib/dhclient/dhclient.leases~.
SEE ALSOdhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
dhclient.leases(5), dhcp-eval(5).
AUTHORdhclient(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 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)