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-
16 options request-option-list ] [ --timeout timeout ] [ --dad-wait-time
17 seconds ] [ --prefix-len-hint length ] [ --decline-wait-time seconds ]
18 [ -v ] [ --version ] [ if0 [ ...ifN ] ]
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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
62 becomes 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
72 arrange 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
74 cycling 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
164 options 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
204 included in the client's messages. This client id can be over‐
205 ridden by setting a client id in the configuration file. Over‐
206 riding 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
213 wait after declining an address before issuing a dis‐
214 cover. The default is 10 seconds as recommended by RFC
215 2131, Section 3.1.5. A value of zero equates to no wait
216 at all.
217 --version Print version number and exit.
219 Options available for DHCPv6 mode:
221 -S Use Information-request to get only stateless configura‐
223 tion parameters (i.e., without address). This implies
224 -6. It also doesn't rewrite the lease database.
225 -T Ask for IPv6 temporary addresses, one set per -T flag.
227 This implies -6 and also disables the normal address
228 query. See -N to restore it.
229 -P Enable IPv6 prefix delegation. This implies -6 and also
231 disables the normal address query. See -N to restore it.
232 Multiple prefixes can be requested with multiple -P
233 flags. Note only one requested interface is allowed.
234 -R Require that responses include all of the items requested
236 by any -N, -T, or -P options. Normally even if the com‐
237 mand line includes a number of these the client will be
238 willing to accept the best lease it can even if the lease
239 doesn't include all of the requested items. This option
240 causes the client to only accept leases that include all
241 of the requested items.
242 Note well: enabling this may prevent the client from
244 using any leases it receives if the servers aren't con‐
245 figured to supply all of the items.
246 -D LL or LLT
248 Override the default when selecting the type of DUID to
249 use. By default, DHCPv6 dhclient creates an identifier
250 based on the link-layer address (DUID-LL) if it is run‐
251 ning in stateless mode (with -S, not requesting an
252 address), or it creates an identifier based on the link-
253 layer address plus a timestamp (DUID-LLT) if it is run‐
254 ning in stateful mode (without -S, requesting an
255 address). When DHCPv4 is configured to use a DUID using
256 -i option the default is to use a DUID-LLT. -D overrides
257 these default, with a value of either LL or LLT.
258 -N Restore normal address query for IPv6. This implies -6.
260 It is used to restore normal operation after using -T or
261 -P. Multiple addresses can be requested with multiple -N
262 flags.
263 --address-prefix-len length
265 Specify the length of the prefix for IPv6 addresses. This
266 value is passed by dhclient into the client script via
267 the environment variable, ip6_prefixlen, when binding
268 IPv6 addresses. The default value is 128. Alternatively
269 you may change the default at compile time by setting
270 DHCLIENT_DEFAULT_PREFIX_LEN in includes/site.h.
271 --dad-wait-time seconds
273 Specify maximum time (in seconds) that the client should
274 wait for the duplicate address detection (DAD) to com‐
275 plete on an interface. This value is propagated to the
276 dhclient script in a dad_wait_time environment variable.
277 If any of the IPv6 addresses on the interface are tenta‐
278 tive (DAD is in progress), the script will wait for the
279 specified number of seconds for DAD to complete. If the
280 script ignores this variable the parameter has no effect.
281 --prefix-len-hint length
283 When used in conjunction with -P, it directs the client
284 to use the given length to use a prefix hint of,
285 "::/length", when requesting new prefixes.
286 Modifying default file locations: The following options can be
288 used to modify the locations a client uses for its files. They
289 can be particularly useful if, for example, /var/lib/dhclient or
290 /var/run have not been mounted when the DHCP client is started.
291 -cf config-file
293 Path to the client configuration file. If unspecified,
294 the default /etc/dhcp/dhclient.conf is used. See
295 dhclient.conf(5) for a description of this file.
296 -df duid-lease-file
298 Path to a secondary lease file. If the primary lease
299 file doesn't contain a DUID this file will be searched.
300 The DUID read from the secondary will be written to the
301 primary. This option can be used to allow an IPv4
302 instance of the client to share a DUID with an IPv6
303 instance. After starting one of the instances the second
304 can be started with this option pointing to the lease
305 file of the first instance. There is no default. If no
306 file is specified no search is made for a DUID should one
307 not be found in the main lease file.
308 -lf lease-file
310 Path to the lease database file. If unspecified, the
311 default /var/lib/dhclient/dhclient.leases is used. See
312 dhclient.leases(5) for a description of this file.
313 -pf pid-file
315 Path to the process ID file. If unspecified, the default
316 /var/run/dhclient.pid is used.
317 --no-pid
319 Option to disable writing pid files. By default the pro‐
320 gram will write a pid file. If the program is invoked
321 with this option it will not attempt to kill any existing
322 client processes even if invoked with -r or -x.
323 -sf script-file
325 Path to the network configuration script invoked by
326 dhclient when it gets a lease. If unspecified, the
327 default /usr/sbin/dhclient-script is used. See dhclient-
328 script(8) for a description of this file.
329 -B Always set the bootp broadcast flag in request packets,
331 so that servers will always broadcast replies. This
332 option is provided as an extension to enable dhclient to
333 work on IBM s390 Linux guests.
334
336 During operations the client may use multiple UDP ports to pro‐
337 vide different functions. Which ports are opened depends on
338 both the way you compiled your code and the configuration you
339 supply. The following should provide you an idea of what ports
340 may be in use.
341 Normally a DHCPv4 client will open a raw UDP socket to receive
343 and send most DHCPv4 packets. It also opens a fallback UDP
344 socket for use in sending unicast packets. Normally these will
345 both use the well known port number for BOOTPC.
346 For DHCPv6 the client opens a UDP socket on the well known
348 client port and a fallback UDP socket on a random port for use
349 in sending unicast messages. Unlike DHCPv4 the well known
350 socket doesn't need to be opened in raw mode.
351 If you have included an omapi port statement in your configura‐
353 tion file then the client will open a TCP socket on that port to
354 listen for OMPAI connections. When something connects another
355 port will be used for the established connection.
356 When DDNS is enabled at compile time (see includes/site.h) the
358 client will open both a v4 and a v6 UDP socket on random ports.
359 These ports are not opened unless/until the client first
360 attempts to do an update. If the client is not configured to do
361 updates, the ports will never be opened.
362
364 The syntax of the dhclient.conf(5) file is discussed separately.
365
367 The DHCP client provides some ability to control it while it is
368 running, without stopping it. This capability is provided using
369 OMAPI, an API for manipulating remote objects. OMAPI clients
370 connect to the client using TCP/IP, authenticate, and can then
371 examine the client's current status and make changes to it.
372 Rather than implementing the underlying OMAPI protocol directly,
374 user programs should use the dhcpctl API or OMAPI itself.
375 Dhcpctl is a wrapper that handles some of the housekeeping
376 chores that OMAPI does not do automatically. Dhcpctl and OMAPI
377 are documented in dhcpctl(3) and omapi(3). Most things you'd
378 want to do with the client can be done directly using the
379 omshell(1) command, rather than having to write a special pro‐
380 gram.
381
383 The control object allows you to shut the client down, releasing
384 all leases that it holds and deleting any DNS records it may
385 have added. It also allows you to pause the client - this
386 unconfigures any interfaces the client is using. You can then
387 restart it, which causes it to reconfigure those interfaces.
388 You would normally pause the client prior to going into hiberna‐
389 tion or sleep on a laptop computer. You would then resume it
390 after the power comes back. This allows PC cards to be shut
391 down while the computer is hibernating or sleeping, and then
392 reinitialized to their previous state once the computer comes
393 out of hibernation or sleep.
394 The control object has one attribute - the state attribute. To
396 shut the client down, set its state attribute to 2. It will
397 automatically do a DHCPRELEASE. To pause it, set its state
398 attribute to 3. To resume it, set its state attribute to 4.
399
401 The following environment variables may be defined to override
402 the builtin defaults for file locations. Note that use of the
403 related command-line options will ignore the corresponding envi‐
404 ronment variable settings.
405 PATH_DHCLIENT_CONF
407 The dhclient.conf configuration file.
408 PATH_DHCLIENT_DB
410 The dhclient.leases database.
411 PATH_DHCLIENT_PID
413 The dhclient PID file.
414 PATH_DHCLIENT_SCRIPT
416 The dhclient-script file.
417
419 /usr/sbin/dhclient-script, /etc/dhcp/dhclient.conf,
420 /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
421 /var/lib/dhclient/dhclient.leases~.
422
424 dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
425 dhclient.leases(5), dhcp-eval(5).
426
428 dhclient(8) To learn more about Internet Systems Consortium, see
429 https://www.isc.org
430 This client was substantially modified and enhanced by Elliot
432 Poger for use on Linux while he was working on the MosquitoNet
433 project at Stanford.
434 The current version owes much to Elliot's Linux enhancements,
436 but was substantially reorganized and partially rewritten by Ted
437 Lemon so as to use the same networking framework that the Inter‐
438 net Systems Consortium DHCP server uses. Much system-specific
439 configuration code was moved into a shell script so that as sup‐
440 port for more operating systems is added, it will not be neces‐
441 sary to port and maintain system-specific configuration code to
442 these operating systems - instead, the shell script can invoke
443 the native tools to accomplish the same purpose.
444 dhclient(8)