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... ] ] [ -i ] [ -I ] [ -D LL|LLT ] [ -p port-number ] [ -d ] [ -e
11 VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file
12 ] [ --no-pid ] [ -cf config-file ] [ -sf script-file ] [ -s server-addr
13 ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -nc ] [ -B ] [ -I dhcp-client-
14 identifier ] [ -H host-name ] [ -F fqdn.fqdn ] [ -V vendor-class-iden‐
15 tifier ] [ -R request-option-list ] [ -timeout timeout ] [ -v ] [
16 --version ] [ if0 [ ...ifN ] ]
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19 The Internet Systems Consortium DHCP Client, dhclient, provides a means
20 for configuring one or more network interfaces using the Dynamic Host
21 Configuration Protocol, BOOTP protocol, or if these protocols fail, by
22 statically assigning an address.
23
25 The DHCP protocol allows a host to contact a central server which main‐
26 tains a list of IP addresses which may be assigned on one or more sub‐
27 nets. A DHCP client may request an address from this pool, and then
28 use it on a temporary basis for communication on network. The DHCP
29 protocol also provides a mechanism whereby a client can learn important
30 details about the network to which it is attached, such as the location
31 of a default router, the location of a name server, and so on.
32 There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At
34 startup the client may be started for one or the other via the -4 or -6
35 options.
36 On startup, dhclient reads the dhclient.conf for configuration instruc‐
38 tions. It then gets a list of all the network interfaces that are con‐
39 figured in the current system. For each interface, it attempts to con‐
40 figure the interface using the DHCP protocol.
41 In order to keep track of leases across system reboots and server
43 restarts, dhclient keeps a list of leases it has been assigned in the
44 dhclient.leases file. On startup, after reading the dhclient.conf
45 file, dhclient reads the dhclient.leases file to refresh its memory
46 about what leases it has been assigned.
47 When a new lease is acquired, it is appended to the end of the
49 dhclient.leases file. In order to prevent the file from becoming arbi‐
50 trarily large, from time to time dhclient creates a new dhclient.leases
51 file from its in-core lease database. The old version of the
52 dhclient.leases file is retained under the name dhclient.leases~ until
53 the next time dhclient rewrites the database.
54 Old leases are kept around in case the DHCP server is unavailable when
56 dhclient is first invoked (generally during the initial system boot
57 process). In that event, old leases from the dhclient.leases file
58 which have not yet expired are tested, and if they are determined to be
59 valid, they are used until either they expire or the DHCP server
60 becomes available.
61 A mobile host which may sometimes need to access a network on which no
63 DHCP server exists may be preloaded with a lease for a fixed address on
64 that network. When all attempts to contact a DHCP server have failed,
65 dhclient will try to validate the static lease, and if it succeeds,
66 will use that lease until it is restarted.
67 A mobile host may also travel to some networks on which DHCP is not
69 available but BOOTP is. In that case, it may be advantageous to
70 arrange with the network administrator for an entry on the BOOTP data‐
71 base, so that the host can boot quickly on that network rather than
72 cycling through the list of old leases.
73
75 The names of the network interfaces that dhclient should attempt to
76 configure may be specified on the command line. If no interface names
77 are specified on the command line dhclient will normally identify all
78 network interfaces, eliminating non-broadcast interfaces if possible,
79 and attempt to configure each interface.
80 It is also possible to specify interfaces by name in the dhclient.conf
82 file. If interfaces are specified in this way, then the client will
83 only configure interfaces that are either specified in the configura‐
84 tion file or on the command line, and will ignore all other interfaces.
85 The client normally prints no output during its startup sequence. It
87 can be made to emit verbose messages displaying the startup sequence
88 events until it has acquired an address by supplying the -v command
89 line argument. In either case, the client logs messages using the sys‐
90 log(3) facility.
91
93 -4 Use the DHCPv4 protocol to obtain an IPv4 address and configura‐
94 tion parameters. This is the default and cannot be combined
95 with -6.
96 -6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are
98 available along with configuration parameters. It cannot be
99 combined with -4. The -S -T -P -N and -D arguments provide more
100 control over aspects of the DHCPv6 processing. Note: it is not
101 recommended to mix queries of different types together or even
102 to share the lease file between them.
103 -1 Try to get a lease once. On failure exit with code 2. In
105 DHCPv6 this sets the maximum duration of the initial exchange to
106 timeout (from dhclient.conf with a default of sixty seconds).
107 -d Force dhclient to run as a foreground process. Normally the
109 DHCP client will run in the foreground until is has configured
110 an interface at which time it will revert to running in the
111 background. This option is useful when running the client under
112 a debugger, or when running it out of inittab on System V sys‐
113 tems. This implies -v.
114 -nw Become a daemon immediately (nowait) rather than waiting until
116 an an IP address has been acquired.
117 -q Be quiet at startup, this is the default.
119 -v Enable verbose log messages.
121 -w Continue running even if no broadcast interfaces were found.
123 Normally DHCP client will exit if it isn't able to identify any
124 network interfaces to configure. On laptop computers and other
125 computers with hot-swappable I/O buses, it is possible that a
126 broadcast interface may be added after system startup. This
127 flag can be used to cause the client not to exit when it doesn't
128 find any such interfaces. The omshell(1) program can then be
129 used to notify the client when a network interface has been
130 added or removed, so that the client can attempt to configure an
131 IP address on that interface.
132 -nc Do not drop capabilities.
135 Normally, if dhclient was compiled with libcap-ng support,
137 dhclient drops most capabilities immediately upon startup.
138 While more secure, this greatly restricts the additional actions
139 that hooks in dhclient-script (8) can take. (For example, any
140 daemons that dhclient-script (8) starts or restarts will inherit
141 the restricted capabilities as well, which may interfere with
142 their correct operation.) Thus, the -nc option can be used to
143 prevent dhclient from dropping capabilities.
144 The -nc option is ignored if dhclient was not compiled with lib‐
146 cap-ng support.
147 -B Set the BOOTP broadcast flag in request packets so servers will
150 always broadcast replies.
151 -I <dhcp-client-identifier>
154 Specify the dhcp-client-identifier option to send to the DHCP
155 server.
156 -H <host-name>
159 Specify the host-name option to send to the DHCP server. The
160 host-name string only contains the client's hostname prefix, to
161 which the server will append the ddns-domainname or domain-name
162 options, if any, to derive the fully qualified domain name of
163 the client. The -H option cannot be used with the -F option.
164 -F <fqdn.fqdn>
167 Specify the fqdn.fqdn option to send to the DHCP server. This
168 option cannot be used with the -H option. The fqdn.fqdn option
169 must specify the complete domain name of the client host, which
170 the server may use for dynamic DNS updates.
171 -V <vendor-class-identifier>
174 Specify the vendor-class-identifier option to send to the DHCP
175 server.
176 -R <option>[,<option>...]
179 Specify the list of options the client is to request from the
180 server. The option list must be a single string consisting of
181 option names separated by at least one command and optional
182 space characters. The default option list is:
183 subnet-mask, broadcast-address, time-offset, routers,
186 domain-search, domain-name, domain-name-servers, host-name,
187 nis-domain, nis-servers, ntp-servers, interface-mtu
188 -R option does not append options to the default request, it over‐
191 rides the default request list. Keep this in mind if you want
192 to request an additional option besides the default request
193 list. You will have to specify all option names for the -R
194 parameter.
195 -timeout <timeout>
198 Specify the time after which dhclient will decide that no DHCP
199 servers can be contacted when no responses have been received.
200 -n Do not configure any interfaces. This is most likely to be use‐
203 ful in combination with the -w flag.
204 -e VAR=value
206 Define additional environment variables for the environment
207 where dhclient-script executes. You may specify multiple -e
208 options on the command line.
209 -r Release the current lease and stop the running DHCP client as
211 previously recorded in the PID file. When shutdown via this
212 method dhclient-script will be executed with the specific reason
213 for calling the script set. The client normally doesn't release
214 the current lease as this is not required by the DHCP protocol
215 but some cable ISPs require their clients to notify the server
216 if they wish to release an assigned IP address.
217 -x Stop the running DHCP client without releasing the current
219 lease. Kills existing dhclient process as previously recorded
220 in the PID file. When shutdown via this method dhclient-script
221 will be executed with the specific reason for calling the script
222 set.
223 -p port-number
225 The UDP port number on which the DHCP client should listen and
226 transmit. If unspecified, dhclient uses the default port of 68.
227 This is mostly useful for debugging purposes. If a different
228 port is specified on which the client should listen and trans‐
229 mit, the client will also use a different destination port - one
230 less than the specified port.
231 -s server-addr
233 Specify the server IP address or fully qualified domain name to
234 use as a destination for DHCP protocol messages before dhclient
235 has acquired an IP address. Normally, dhclient transmits these
236 messages to 255.255.255.255 (the IP limited broadcast address).
237 Overriding this is mostly useful for debugging purposes. This
238 feature is not supported in DHCPv6 (-6) mode.
239 -g relay
241 Set the giaddr field of all packets to the relay IP address sim‐
242 ulating a relay agent. This is for testing pruposes only and
243 should not be expected to work in any consistent or useful way.
244 -i Use a DUID with DHCPv4 clients. If no DUID is available in the
246 lease file one will be constructed and saved. The DUID will be
247 used to contstuct a RFC4361 style client id that will be
248 included in the client's messages. This client id can be over‐
249 ridden by setting a client id in the configuration file. Over‐
250 ridding the client id in this fashion is discouraged.
251 -I Use the standard DDNS scheme from RFCs 4701 & 4702.
253 --version
255 Print version number and exit.
256 Options available for DHCPv6 mode:
258 -S Use Information-request to get only stateless configuration
260 parameters (i.e., without address). This implies -6. It also
261 doesn't rewrite the lease database.
262 -T Ask for IPv6 temporary addresses, one set per -T flag. This
264 implies -6 and also disables the normal address query. See -N
265 to restore it.
266 -P Enable IPv6 prefix delegation. This implies -6 and also dis‐
268 ables the normal address query. See -N to restore it. Note
269 only one requested interface is allowed.
270 -D LL or LLT
272 Override the default when selecting the type of DUID to use. By
273 default, DHCPv6 dhclient creates an identifier based on the
274 link-layer address (DUID-LL) if it is running in stateless mode
275 (with -S, not requesting an address), or it creates an identi‐
276 fier based on the link-layer address plus a timestamp (DUID-LLT)
277 if it is running in stateful mode (without -S, requesting an
278 address). When DHCPv4 is configued to use a DUID using -i
279 option the default is to use a DUID-LLT. -D overrides these
280 default, with a value of either LL or LLT.
281 -N Restore normal address query for IPv6. This implies -6. It is
283 used to restore normal operation after using -T or -P.
284 Modifying default file locations: The following options can be used to
286 modify the locations a client uses for it's files. They can be partic‐
287 ularly useful if, for example, /var/lib/dhclient or /var/run have not
288 been mounted when the DHCP client is started.
289 -cf config-file
291 Path to the client configuration file. If unspecified, the
292 default /etc/dhcp/dhclient.conf is used. See dhclient.conf(5)
293 for a description of this file.
294 -lf lease-file
296 Path to the lease database file. If unspecified, the default
297 /var/lib/dhclient/dhclient.leases is used. See
298 dhclient.leases(5) for a description of this file.
299 -pf pid-file
301 Path to the process ID file. If unspecified, the default
302 /var/run/dhclient.pid is used.
303 --no-pid
305 Option to disable writing pid files. By default the program
306 will write a pid file. If the program is invoked with this
307 option it will not attempt to kill any existing client processes
308 even if invoked with -r or -x.
309 -sf script-file
311 Path to the network configuration script invoked by dhclient
312 when it gets a lease. If unspecified, the default
313 /usr/sbin/dhclient-script is used. See dhclient-script(8) for a
314 description of this file.
315
319 The syntax of the dhclient.conf(5) file is discussed separately.
320
322 The DHCP client provides some ability to control it while it is run‐
323 ning, without stopping it. This capability is provided using OMAPI, an
324 API for manipulating remote objects. OMAPI clients connect to the
325 client using TCP/IP, authenticate, and can then examine the client's
326 current status and make changes to it.
327 Rather than implementing the underlying OMAPI protocol directly, user
329 programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
330 wrapper that handles some of the housekeeping chores that OMAPI does
331 not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
332 and omapi(3). Most things you'd want to do with the client can be done
333 directly using the omshell(1) command, rather than having to write a
334 special program.
335
337 The control object allows you to shut the client down, releasing all
338 leases that it holds and deleting any DNS records it may have added.
339 It also allows you to pause the client - this unconfigures any inter‐
340 faces the client is using. You can then restart it, which causes it to
341 reconfigure those interfaces. You would normally pause the client
342 prior to going into hibernation or sleep on a laptop computer. You
343 would then resume it after the power comes back. This allows PC cards
344 to be shut down while the computer is hibernating or sleeping, and then
345 reinitialized to their previous state once the computer comes out of
346 hibernation or sleep.
347 The control object has one attribute - the state attribute. To shut
349 the client down, set its state attribute to 2. It will automatically
350 do a DHCPRELEASE. To pause it, set its state attribute to 3. To
351 resume it, set its state attribute to 4.
352
354 The following environment variables may be defined to override the
355 builtin defaults for file locations. Note that use of the related com‐
356 mand-line options will ignore the corresponding environment variable
357 settings.
358 PATH_DHCLIENT_CONF
360 The dhclient.conf configuration file.
361 PATH_DHCLIENT_DB
363 The dhclient.leases database.
364 PATH_DHCLIENT_PID
366 The dhclient PID file.
367 PATH_DHCLIENT_SCRIPT
369 The dhclient-script file.
370
372 /usr/sbin/dhclient-script, /etc/dhcp/dhclient.conf,
373 /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
374 /var/lib/dhclient/dhclient.leases~.
375
377 dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
378 dhclient.leases(5), dhcp-eval(5).
379
381 dhclient(8) has been written for Internet Systems Consortium by Ted
382 Lemon in cooperation with Vixie Enterprises. To learn more about
383 Internet Systems Consortium, see https://www.isc.org To learn more
384 about Vixie Enterprises, see http://www.vix.com.
385 This client was substantially modified and enhanced by Elliot Poger for
387 use on Linux while he was working on the MosquitoNet project at Stan‐
388 ford.
389 The current version owes much to Elliot's Linux enhancements, but was
391 substantially reorganized and partially rewritten by Ted Lemon so as to
392 use the same networking framework that the Internet Systems Consortium
393 DHCP server uses. Much system-specific configuration code was moved
394 into a shell script so that as support for more operating systems is
395 added, it will not be necessary to port and maintain system-specific
396 configuration code to these operating systems - instead, the shell
397 script can invoke the native tools to accomplish the same purpose.
398 dhclient(8)