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Verziószám: isc-dhclient-4.3.5
Fejlesztő/tulajdonos: Internet Systems Consortium
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man dhclient
dhclient(8) System Manager's Manual dhclient(8)
NAME
dhclient - Dynamic Host Configuration Protocol Client
SYNOPSIS
dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ] -R
] [ -i ] [ -I ] [ -4o6 port ] [ -D LL|LLT ] [ -p port-number ] [ -d ] [ -df duid-
lease-file ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-
file ] [ --no-pid ] [ -cf config-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 configur‐
ing 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.
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 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 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 adminis‐
trator 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-broad‐
cast 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 configuration 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 syslog(3) facility.
OPTIONS
-4 Use the DHCPv4 protocol to obtain an IPv4 address and configuration parame‐
ters. 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
-N and -D arguments provide more control over aspects of the DHCPv6 process‐
ing. Note: it is not recommended to mix queries of different types together
or even to share the lease file between them.
-4o6 port
Participate in the DHCPv4 over DHCPv6 protocol specified by RFC 7341. This
associates a DHCPv4 and a DHCPv6 client to allow the v4 client to send v4
requests encapsulated in a v6 packet. Communication between the two clients
is done on a pair of UDP sockets bound to ::1 port and port + 1. Both clients
must be launched using the same port argument.
-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
systems. 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 con‐
figure. 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 useful in combina‐
tion 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 shut‐
down 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 transmit, 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 des‐
tination 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 simulating a relay
agent. This is for testing purposes only and should not be expected to work
in any consistent or useful way.
-i Use a DUID with DHCPv4 clients. If no DUID is available in the lease file one
will be constructed and saved. The DUID will be used to construct a RFC4361
style client id that will be included in the client's messages. This client
id can be overridden by setting a client id in the configuration file. Over‐
ridding the client id in this fashion is discouraged.
-I Use the standard DDNS scheme from RFCs 4701 & 4702.
--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 data‐
base.
-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 disables the normal
address query. See -N to restore it. Multiple prefixes can be requested with
multiple -P flags. Note only one requested interface is allowed.
-R Require that responses include all of the items requested by any -N, -T, or -P
options. Normally even if the command line includes a number of these the
client will be willing to accept the best lease it can even if the lease
doesn't include all of the requested items. This option causes the client to
only accept leases that include all of the requested items.
Note well: enabling this may prevent the client from using any leases it
receives if the servers aren't configured to supply all of the items.
-D LL or LLT
Override the default when selecting the type of DUID to use. 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). When DHCPv4 is configured to use a DUID using -i option the default
is to use a DUID-LLT. -D overrides these default, with a value of either LL
or LLT.
-N Restore normal address query for IPv6. This implies -6. It is used to restore
normal operation after using -T or -P. Multiple addresses can be requested
with multiple -N flags.
Modifying default file locations: The following options can be used to modify the
locations a client uses for its files. They can be particularly useful if, for exam‐
ple, /var/lib/dhcp or /var/run have not been mounted when the DHCP client is started.
-cf config-file
Path to the client configuration file. If unspecified, the default
/etc/dhcp/dhclient.conf is used. See dhclient.conf(5) for a description of
this file.
-df duid-lease-file
Path to a secondary lease file. If the primary lease file doesn't contain a
DUID this file will be searched. The DUID read from the secondary will be
written to the primary. This option can be used to allow an IPv4 instance of
the client to share a DUID with an IPv6 instance. After starting one of the
instances the second can be started with this option pointing to the lease
file of the first instance. There is no default. If no file is specified no
search is made for a DUID should one not be found in the main lease file.
-lf lease-file
Path to the lease database file. If unspecified, the default
/var/lib/dhcp/dhclient.leases is used. See dhclient.leases(5) for a descrip‐
tion of this file.
-pf pid-file
Path to the process ID file. If unspecified, the default
/var/run/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 /sbin/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 different func‐
tions. 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.
When DDNS is enabled at compile time (see includes/site.h) the client will open both
a v4 and a v6 UDP socket on random ports. These ports are opened even if DDNS is
disabled in the configuration file.
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 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 doc‐
umented 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.
ENVIRONMENT VARIABLES
The following environment variables may be defined to override the builtin defaults
for file locations. Note that use of the related command-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
/sbin/dhclient-script, /etc/dhcp/dhclient.conf, /var/lib/dhcp/dhclient.leases,
/var/run/dhclient.pid, /var/lib/dhcp/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 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-spe‐
cific 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-spe‐
cific configuration code to these operating systems - instead, the shell script can
invoke the native tools to accomplish the same purpose.
dhclient(8)
Súgó kimenet
dhclient --help
Internet Systems Consortium DHCP Client 4.3.5
Copyright 2004-2016 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Unknown command: --help
Usage: dhclient [-4|-6] [-SNTPRI1dvrxi] [-nw] [-p <port>] [-D LL|LLT]
[-s server-addr] [-cf config-file]
[-df duid-file] [-lf lease-file]
[-pf pid-file] [--no-pid] [-e VAR=val]
[-sf script-file] [interface]*
If you think you have received this message due to a bug rather
than a configuration issue please read the section on submitting
bugs on either our web page at www.isc.org or in the README file
before submitting a bug. These pages explain the proper
process and the information we find helpful for debugging..
exiting.
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