1dhclient(8) System Manager's Manual dhclient(8)
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6 dhclient - Dynamic Host Configuration Protocol Client
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9 dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [
10 -P... ] ] [ -R ] [ -i ] [ -I ] [ -4o6 port ] [ -D LL|LLT ] [ -p port-
11 number ] [ -d ] [ -df duid-lease-file ] [ -e VAR=value ] [ -q ] [ -1 ]
12 [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf con‐
13 fig-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [
14 -nw ] [ -w ] [ -nc ] [ -B ] [ -C dhcp-client-identifier ] [ -H host-
15 name ] [ -F fqdn.fqdn ] [ -V vendor-class-identifier ] [ --request-op‐
16 tions request-option-list ] [ --timeout timeout ] [ --dad-wait-time
17 seconds ] [ --prefix-len-hint length ] [ --decline-wait-time seconds ]
18 [ -v ] [ --version ] [ if0 [ ...ifN ] ]
19
21 The Internet Systems Consortium DHCP Client, dhclient, provides a means
22 for configuring one or more network interfaces using the Dynamic Host
23 Configuration Protocol, BOOTP protocol, or if these protocols fail, by
24 statically assigning an address.
25
27 The DHCP protocol allows a host to contact a central server which main‐
28 tains a list of IP addresses which may be assigned on one or more sub‐
29 nets. A DHCP client may request an address from this pool, and then
30 use it on a temporary basis for communication on network. The DHCP
31 protocol also provides a mechanism whereby a client can learn important
32 details about the network to which it is attached, such as the location
33 of a default router, the location of a name server, and so on.
34 There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At
36 startup the client may be started for one or the other via the -4 or -6
37 options.
38 On startup, dhclient reads the dhclient.conf for configuration instruc‐
40 tions. It then gets a list of all the network interfaces that are con‐
41 figured in the current system. For each interface, it attempts to con‐
42 figure the interface using the DHCP protocol.
43 In order to keep track of leases across system reboots and server
45 restarts, dhclient keeps a list of leases it has been assigned in the
46 dhclient.leases file. On startup, after reading the dhclient.conf
47 file, dhclient reads the dhclient.leases file to refresh its memory
48 about what leases it has been assigned.
49 When a new lease is acquired, it is appended to the end of the
51 dhclient.leases file. In order to prevent the file from becoming arbi‐
52 trarily large, from time to time dhclient creates a new dhclient.leases
53 file from its in-core lease database. The old version of the
54 dhclient.leases file is retained under the name dhclient.leases~ until
55 the next time dhclient rewrites the database.
56 Old leases are kept around in case the DHCP server is unavailable when
58 dhclient is first invoked (generally during the initial system boot
59 process). In that event, old leases from the dhclient.leases file
60 which have not yet expired are tested, and if they are determined to be
61 valid, they are used until either they expire or the DHCP server be‐
62 comes available.
63 A mobile host which may sometimes need to access a network on which no
65 DHCP server exists may be preloaded with a lease for a fixed address on
66 that network. When all attempts to contact a DHCP server have failed,
67 dhclient will try to validate the static lease, and if it succeeds,
68 will use that lease until it is restarted.
69 A mobile host may also travel to some networks on which DHCP is not
71 available but BOOTP is. In that case, it may be advantageous to ar‐
72 range with the network administrator for an entry on the BOOTP data‐
73 base, so that the host can boot quickly on that network rather than cy‐
74 cling through the list of old leases.
75
77 The names of the network interfaces that dhclient should attempt to
78 configure may be specified on the command line. If no interface names
79 are specified on the command line dhclient will normally identify all
80 network interfaces, eliminating non-broadcast interfaces if possible,
81 and attempt to configure each interface.
82 It is also possible to specify interfaces by name in the dhclient.conf
84 file. If interfaces are specified in this way, then the client will
85 only configure interfaces that are either specified in the configura‐
86 tion file or on the command line, and will ignore all other interfaces.
87 The client normally prints no output during its startup sequence. It
89 can be made to emit verbose messages displaying the startup sequence
90 events until it has acquired an address by supplying the -v command
91 line argument. In either case, the client logs messages using the sys‐
92 log(3) facility.
93
95 -4 Use the DHCPv4 protocol to obtain an IPv4 address and configura‐
96 tion parameters. This is the default and cannot be combined
97 with -6.
98 -6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are
100 available along with configuration parameters. It cannot be
101 combined with -4. The -S -T -P -N and -D arguments provide more
102 control over aspects of the DHCPv6 processing. Note: it is not
103 recommended to mix queries of different types together or even
104 to share the lease file between them.
105 -4o6 port
107 Participate in the DHCPv4 over DHCPv6 protocol specified by RFC
108 7341. This associates a DHCPv4 and a DHCPv6 client to allow the
109 v4 client to send v4 requests encapsulated in a v6 packet. Com‐
110 munication between the two clients is done on a pair of UDP
111 sockets bound to ::1 port and port + 1. Both clients must be
112 launched using the same port argument.
113 -1 Try to get a lease once. On failure exit with code 2. In
115 DHCPv6 this sets the maximum duration of the initial exchange to
116 timeout (from dhclient.conf with a default of sixty seconds).
117 -d Force dhclient to run as a foreground process. Normally the
119 DHCP client will run in the foreground until is has configured
120 an interface at which time it will revert to running in the
121 background. This option is useful when running the client under
122 a debugger, or when running it out of inittab on System V sys‐
123 tems. This implies -v.
124 -nw Become a daemon immediately (nowait) rather than waiting until
126 an IP address has been acquired.
127 -q Be quiet at startup, this is the default.
129 -v Enable verbose log messages.
131 -w Continue running even if no broadcast interfaces were found.
133 Normally DHCP client will exit if it isn't able to identify any
134 network interfaces to configure. On laptop computers and other
135 computers with hot-swappable I/O buses, it is possible that a
136 broadcast interface may be added after system startup. This
137 flag can be used to cause the client not to exit when it doesn't
138 find any such interfaces. The omshell(1) program can then be
139 used to notify the client when a network interface has been
140 added or removed, so that the client can attempt to configure an
141 IP address on that interface.
142 -nc Do not drop capabilities.
144 Normally, if dhclient was compiled with libcap-ng support,
146 dhclient drops most capabilities immediately upon startup.
147 While more secure, this greatly restricts the additional actions
148 that hooks in dhclient-script (8) can take. (For example, any
149 daemons that dhclient-script (8) starts or restarts will inherit
150 the restricted capabilities as well, which may interfere with
151 their correct operation.) Thus, the -nc option can be used to
152 prevent dhclient from dropping capabilities.
153 The -nc option is ignored if dhclient was not compiled with lib‐
155 cap-ng support.
156 -n Do not configure any interfaces. This is most likely to be use‐
159 ful in combination with the -w flag.
160 -e VAR=value
162 Define additional environment variables for the environment
163 where dhclient-script executes. You may specify multiple -e op‐
164 tions on the command line.
165 -r Release the current lease and stop the running DHCP client as
167 previously recorded in the PID file. When shutdown via this
168 method dhclient-script will be executed with the specific reason
169 for calling the script set. The client normally doesn't release
170 the current lease as this is not required by the DHCP protocol
171 but some cable ISPs require their clients to notify the server
172 if they wish to release an assigned IP address.
173 -x Stop the running DHCP client without releasing the current
175 lease. Kills existing dhclient process as previously recorded
176 in the PID file. When shutdown via this method dhclient-script
177 will be executed with the specific reason for calling the script
178 set.
179 -p port-number
181 The UDP port number on which the DHCP client should listen and
182 transmit. If unspecified, dhclient uses the default port of 68.
183 This is mostly useful for debugging purposes. If a different
184 port is specified on which the client should listen and trans‐
185 mit, the client will also use a different destination port - one
186 less than the specified port.
187 -s server-addr
189 Specify the server IP address or fully qualified domain name to
190 use as a destination for DHCP protocol messages before dhclient
191 has acquired an IP address. Normally, dhclient transmits these
192 messages to 255.255.255.255 (the IP limited broadcast address).
193 Overriding this is mostly useful for debugging purposes. This
194 feature is not supported in DHCPv6 (-6) mode.
195 -g relay
197 Set the giaddr field of all packets to the relay IP address sim‐
198 ulating a relay agent. This is for testing purposes only and
199 should not be expected to work in any consistent or useful way.
200 -i Use a DUID with DHCPv4 clients. If no DUID is available in the
202 lease file one will be constructed and saved. The DUID will be
203 used to construct a RFC4361 style client id that will be in‐
204 cluded in the client's messages. This client id can be overrid‐
205 den by setting a client id in the configuration file. Overrid‐
206 ing the client id in this fashion is discouraged.
207 -I Use the standard DDNS scheme from RFCs 4701 & 4702.
209 --decline-wait-time seconds
212 Specify the time (in seconds) that an IPv4 client should wait
213 after declining an address before issuing a discover. The de‐
214 fault is 10 seconds as recommended by RFC 2131, Section 3.1.5.
215 A value of zero equates to no wait at all.
216 --version Print version number and exit.
218 Options available for DHCPv6 mode:
220 -S Use Information-request to get only stateless configuration pa‐
222 rameters (i.e., without address). This implies -6. It also
223 doesn't rewrite the lease database.
224 -T Ask for IPv6 temporary addresses, one set per -T flag. This im‐
226 plies -6 and also disables the normal address query. See -N to
227 restore it.
228 -P Enable IPv6 prefix delegation. This implies -6 and also dis‐
230 ables the normal address query. See -N to restore it. Multiple
231 prefixes can be requested with multiple -P flags. Note only one
232 requested interface is allowed.
233 -R Require that responses include all of the items requested by any
235 -N, -T, or -P options. Normally even if the command line in‐
236 cludes a number of these the client will be willing to accept
237 the best lease it can even if the lease doesn't include all of
238 the requested items. This option causes the client to only ac‐
239 cept leases that include all of the requested items.
240 Note well: enabling this may prevent the client from using any
242 leases it receives if the servers aren't configured to supply
243 all of the items.
244 -D LL or LLT
246 Override the default when selecting the type of DUID to use. By
247 default, DHCPv6 dhclient creates an identifier based on the
248 link-layer address (DUID-LL) if it is running in stateless mode
249 (with -S, not requesting an address), or it creates an identi‐
250 fier based on the link-layer address plus a timestamp (DUID-LLT)
251 if it is running in stateful mode (without -S, requesting an ad‐
252 dress). When DHCPv4 is configured to use a DUID using -i option
253 the default is to use a DUID-LLT. -D overrides these default,
254 with a value of either LL or LLT.
255 -N Restore normal address query for IPv6. This implies -6. It is
257 used to restore normal operation after using -T or -P. Multiple
258 addresses can be requested with multiple -N flags.
259 --address-prefix-len length
261 Specify the length of the prefix for IPv6 addresses. This value
262 is passed by dhclient into the client script via the environment
263 variable, ip6_prefixlen, when binding IPv6 addresses. The de‐
264 fault value is 128. Alternatively you may change the default at
265 compile time by setting DHCLIENT_DEFAULT_PREFIX_LEN in in‐
266 cludes/site.h.
267 --dad-wait-time seconds
269 Specify maximum time (in seconds) that the client should wait
270 for the duplicate address detection (DAD) to complete on an in‐
271 terface. This value is propagated to the dhclient script in a
272 dad_wait_time environment variable. If any of the IPv6 addresses
273 on the interface are tentative (DAD is in progress), the script
274 will wait for the specified number of seconds for DAD to com‐
275 plete. If the script ignores this variable the parameter has no
276 effect.
277 --prefix-len-hint length
279 When used in conjunction with -P, it directs the client to use
280 the given length to use a prefix hint of, "::/length", when re‐
281 questing new prefixes.
282 Modifying default file locations: The following options can be used to
284 modify the locations a client uses for its files. They can be particu‐
285 larly useful if, for example, /var/lib/dhclient or /var/run have not
286 been mounted when the DHCP client is started.
287 -cf config-file
289 Path to the client configuration file. If unspecified, the de‐
290 fault /etc/dhcp/dhclient.conf is used. See dhclient.conf(5) for
291 a description of this file.
292 -df duid-lease-file
294 Path to a secondary lease file. If the primary lease file
295 doesn't contain a DUID this file will be searched. The DUID
296 read from the secondary will be written to the primary. This
297 option can be used to allow an IPv4 instance of the client to
298 share a DUID with an IPv6 instance. After starting one of the
299 instances the second can be started with this option pointing to
300 the lease file of the first instance. There is no default. If
301 no file is specified no search is made for a DUID should one not
302 be found in the main lease file.
303 -lf lease-file
305 Path to the lease database file. If unspecified, the default
306 /var/lib/dhclient/dhclient.leases is used. See
307 dhclient.leases(5) for a description of this file.
308 -pf pid-file
310 Path to the process ID file. If unspecified, the default
311 /var/run/dhclient.pid is used.
312 --no-pid
314 Option to disable writing pid files. By default the program
315 will write a pid file. If the program is invoked with this op‐
316 tion it will not attempt to kill any existing client processes
317 even if invoked with -r or -x.
318 -sf script-file
320 Path to the network configuration script invoked by dhclient
321 when it gets a lease. If unspecified, the default
322 /usr/sbin/dhclient-script is used. See dhclient-script(8) for a
323 description of this file.
324 -B Always set the bootp broadcast flag in request packets, so that
326 servers will always broadcast replies. This option is provided
327 as an extension to enable dhclient to work on IBM s390 Linux
328 guests.
329
331 During operations the client may use multiple UDP ports to provide dif‐
332 ferent functions. Which ports are opened depends on both the way you
333 compiled your code and the configuration you supply. The following
334 should provide you an idea of what ports may be in use.
335 Normally a DHCPv4 client will open a raw UDP socket to receive and send
337 most DHCPv4 packets. It also opens a fallback UDP socket for use in
338 sending unicast packets. Normally these will both use the well known
339 port number for BOOTPC.
340 For DHCPv6 the client opens a UDP socket on the well known client port
342 and a fallback UDP socket on a random port for use in sending unicast
343 messages. Unlike DHCPv4 the well known socket doesn't need to be
344 opened in raw mode.
345 If you have included an omapi port statement in your configuration file
347 then the client will open a TCP socket on that port to listen for OMPAI
348 connections. When something connects another port will be used for the
349 established connection.
350 When DDNS is enabled at compile time (see includes/site.h) the client
352 will open both a v4 and a v6 UDP socket on random ports. These ports
353 are not opened unless/until the client first attempts to do an update.
354 If the client is not configured to do updates, the ports will never be
355 opened.
356
358 The syntax of the dhclient.conf(5) file is discussed separately.
359
361 The DHCP client provides some ability to control it while it is run‐
362 ning, without stopping it. This capability is provided using OMAPI, an
363 API for manipulating remote objects. OMAPI clients connect to the
364 client using TCP/IP, authenticate, and can then examine the client's
365 current status and make changes to it.
366 Rather than implementing the underlying OMAPI protocol directly, user
368 programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
369 wrapper that handles some of the housekeeping chores that OMAPI does
370 not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
371 and omapi(3). Most things you'd want to do with the client can be done
372 directly using the omshell(1) command, rather than having to write a
373 special program.
374
376 The control object allows you to shut the client down, releasing all
377 leases that it holds and deleting any DNS records it may have added.
378 It also allows you to pause the client - this unconfigures any inter‐
379 faces the client is using. You can then restart it, which causes it to
380 reconfigure those interfaces. You would normally pause the client
381 prior to going into hibernation or sleep on a laptop computer. You
382 would then resume it after the power comes back. This allows PC cards
383 to be shut down while the computer is hibernating or sleeping, and then
384 reinitialized to their previous state once the computer comes out of
385 hibernation or sleep.
386 The control object has one attribute - the state attribute. To shut
388 the client down, set its state attribute to 2. It will automatically
389 do a DHCPRELEASE. To pause it, set its state attribute to 3. To re‐
390 sume it, set its state attribute to 4.
391
393 The following environment variables may be defined to override the
394 builtin defaults for file locations. Note that use of the related com‐
395 mand-line options will ignore the corresponding environment variable
396 settings.
397 PATH_DHCLIENT_CONF
399 The dhclient.conf configuration file.
400 PATH_DHCLIENT_DB
402 The dhclient.leases database.
403 PATH_DHCLIENT_PID
405 The dhclient PID file.
406 PATH_DHCLIENT_SCRIPT
408 The dhclient-script file.
409
411 /usr/sbin/dhclient-script, /etc/dhcp/dhclient.conf,
412 /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
413 /var/lib/dhclient/dhclient.leases~.
414
416 dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
417 dhclient.leases(5), dhcp-eval(5).
418
420 dhclient(8) To learn more about Internet Systems Consortium, see
421 https://www.isc.org
422 This client was substantially modified and enhanced by Elliot Poger for
424 use on Linux while he was working on the MosquitoNet project at Stan‐
425 ford.
426 The current version owes much to Elliot's Linux enhancements, but was
428 substantially reorganized and partially rewritten by Ted Lemon so as to
429 use the same networking framework that the Internet Systems Consortium
430 DHCP server uses. Much system-specific configuration code was moved
431 into a shell script so that as support for more operating systems is
432 added, it will not be necessary to port and maintain system-specific
433 configuration code to these operating systems - instead, the shell
434 script can invoke the native tools to accomplish the same purpose.
435 dhclient(8)