dhclient(8)dhclient(8)NAMEdhclient - Dynamic Host Configuration Protocol Client
SYNOPSISdhclient [ -p port ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r ] [ -lf
lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-file ] [
-s server ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -x ] [ -I dhcp-client-
identifier ] [ -H host-name | -F fqdn.fqdn ] [ -V vendor-class-identi‐
fier ] [ -R request option list ] [ -T timeout ] [ 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.
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 desti‐
nation port - one greater than the specified destination port.
The DHCP client normally transmits any protocol messages it sends
before acquiring an IP address to, 255.255.255.255, 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.
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 con‐
figured an interface, and then will revert to running in the back‐
ground. 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 dhclient daemon creates its own environment when executing the
dhclient-script to do the grunt work of interface configuration. To
define extra environment variables and their values, use the -e flag,
followed by the environment variable name and value assignment, just as
one would assign a variable in a shell. Eg: -e IF_METRIC=1
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 messages using the syslog (3) facility.
The -q flag prevents 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, and once the lease
has been released, the client exits.
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 information from
/etc/dhclient.conf, its lease database from
/var/lib/dhcpd/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, fol‐
lowed by the name of the file. This can be particularly useful if,
for example, /var/lib/dhcpd 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 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. The -w 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 net‐
work 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 configure any inter‐
faces using the -n flag. This is most likely to be useful in combina‐
tion with the -w flag.
The client can also be instructed to become a daemon immediately,
rather than waiting until it has acquired an IP address. This can be
done by supplying the -nw flag.
The -x argument enables extended option information to be created in
the -s dhclient-script environment, which would allow applications run‐
ning in that environment to handle options they do not know about in
advance - this is a Red Hat extension to support dhcdbd and NetworkMan‐
ager.
The -I <id> argument allows you to specify the dhcp-client-identifier
string, <id>, to be sent to the dhcp server on the command line. It is
equivalent to the top level dhclient.conf statement:
send dhcp-client-identifier "<id>";
The -I <id> command line option will override any top level
dhclient.conf
'send dhcp-client-identifier' statement, but more specific per-inter‐
face
'interface "X" { send dhcp-client-identifier...; }' statements in
dhclient.conf will override the -I <id> command line option for inter‐
face "X". This option is provided as a Red Hat extension to enable
dhclient to work on IBM zSeries z/OS Linux guests.
The -B option instructs dhclient to set the bootp broadcast flag in
request packets, so that servers will always broadcast replies. This is
equivalent to specifying the 'bootp-broadcast-always' option in
dhclient.conf, and has the same effect as specifying 'always-broadcast'
in the server's dhcpd.conf. This option is provided as a Red Hat
extension to enable dhclient to work on IBM zSeries z/OS Linux guests.
The -H <host-name> option allows you to specify the DHCP host-name
option to send to the server on the dhclient command line. It is equiv‐
alent to the top level dhclient.conf statement:
send host-name "<host-name>";
The -H <host-name> option will override any top level dhclient.conf
'send host-name' statement, but more specific per-interface
'interface "X" { send host-name...;' statements in dhclient.conf will
override the -H <host-name> command line option for interface "X". The
host-name option only specifies the client's host name 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 host.
The -H <host-name> option cannot be used with the -F <fqdn.fqdn>
option. Only one -H <host-name> option may be specified. The -H
<host-name> option is provided as a Red Hat extension to simplify con‐
figuration of clients of DHCP servers that require the host-name option
to be sent (eg. some modern cable modems), and for dynamic DNS updates
(DDNS).
The -F <fqdn.fqdn> option allows you to specify the DHCP fqdn.fqdn
option to send to the server on the dhclient command line. It is equiv‐
alent to the top level dhclient.conf statement:
send fqdn.fqdn "<domain-name>";
The -F <fqdn.fqdn> option will override any top level dhclient.conf
'send fqdn.fqdn' statement, but more specific per-interface
'interface "X" { send fqdn.fqdn...;' statements in dhclient.conf will
override the -F <fqdn.fqdn> command line option for interface "X".
This option cannot be used with the -H <host-name> option. The DHCP
fqdn.fqdn option must specify the complete domain name of the client
host, which the server may use for dynamic DNS updates. Only one -F
<fqdn.fqdn> option may be specified. The -F <fqdn.fqdn> option is pro‐
vided as a Red Hat extension to simplify configuration of DDNS.
The -T <timeout> option allows you to specify the time after which the
dhclient will decide that no DHCP servers can be contacted when no
responses have been received. It is equivalent to the
timeout <integer>;
dhclient.conf statement, and will override any such statements in
dhclient.conf.
This option is provided as a Red Hat extension.
The -V <vendor-class-identifier> option allows you to specify the DHCP
vendor-class-identifier option to send to the server on the dhclient
command line. It is equivalent to the top level dhclient.conf state‐
ment:
send vendor-class-identifier "<vendor-class-identifier>";
The -V <vendor-class-identifier> option will override any top level
dhclient.conf
'send vendor-class-identifier' statement, but more specific per-inter‐
face
'interface "X" { send vendor-class-identifier...;' statements in
dhclient.conf will override the -V <vendor-class-identifier> command
line option for interface "X". The -V <vendor-class-identifier> option
is provided as a Red Hat extension to simplify configuration of clients
of DHCP servers that require the vendor-class-identifier option to be
sent.
The -R <request option list> option allows you to specify the list of
options the client is to request from the server on the dhclient com‐
mand line. The option list must be a single string, consisting of
option names separated by at least one comma and optional space charac‐
ters. 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
You can specify a different list of options to request with the -R
<option list> argument. This is equivalent to the dhclient.conf state‐
ment:
request <option list> ;
The -R argument is provided as a Red Hat extension to ISC dhclient to
facilitate requesting a list of options from the server different to
the default.
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/dhclient.conf,
/var/lib/dhcpd/dhclient.leases, /var/run/dhclient.pid,
/var/lib/dhcpd/dhclient.leases~.
SEE ALSOdhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
dhclient.leases(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 http://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)
