client/dhclient.8

   1 .\"     $Id: dhclient.8,v 1.21 2007/05/08 23:05:20 dhankins Exp $
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   3 .\" Copyright (c) 2004 by Internet Systems Consortium, Inc. ("ISC")
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  18 .\"   Internet Systems Consortium, Inc.
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  27 .TH dhclient 8
  28 .SH NAME
  29 dhclient - Dynamic Host Configuration Protocol Client
  30 .SH SYNOPSIS
  31 .B dhclient
  32 [
  33 .B -4
  34 |
  35 .B -6
  36 ]
  37 [
  38 .B -p
  39 .I port
  40 ]
  41 [
  42 .B -d
  43 ]
  44 [
  45 .B -e
  46 .I VAR=value
  47 ]
  48 [
  49 .B -q
  50 ]
  51 [
  52 .B -1
  53 ]
  54 [
  55 .B -r
  56 ]
  57 [
  58 .B -lf
  59 .I lease-file
  60 ]
  61 [
  62 .B -pf
  63 .I pid-file
  64 ]
  65 [
  66 .B -cf
  67 .I config-file
  68 ]
  69 [
  70 .B -sf
  71 .I script-file
  72 ]
  73 [
  74 .B -s
  75 server
  76 ]
  77 [
  78 .B -g
  79 relay
  80 ]
  81 [
  82 .B -n
  83 ]
  84 [
  85 .B -nw
  86 ]
  87 [
  88 .B -w
  89 ]
  90 [
  91 .I if0
  92 [
  93 .I ...ifN
  94 ]
  95 ]
  96 .SH DESCRIPTION
  97 The Internet Systems Consortium DHCP Client, dhclient, provides a
  98 means for configuring one or more network interfaces using the Dynamic
  99 Host Configuration Protocol, BOOTP protocol, or if these protocols
 100 fail, by statically assigning an address.
 101 .SH OPERATION
 102 .PP
 103 The DHCP protocol allows a host to contact a central server which
 104 maintains a list of IP addresses which may be assigned on one or more
 105 subnets.   A DHCP client may request an address from this pool, and
 106 then use it on a temporary basis for communication on network.   The
 107 DHCP protocol also provides a mechanism whereby a client can learn
 108 important details about the network to which it is attached, such as
 109 the location of a default router, the location of a name server, and
 110 so on.
 111 .PP
 112 If given the -4 command line argument (default), dhclient will use the
 113 DHCPv4 protocol to obtain an IPv4 address and configuration parameters.
 114 .PP
 115 If given the -6 command line argument, dhclient will use the DHCPv6
 116 protocol to obtain whatever PIv6 addresses are available along with
 117 configuration parameters.  Information-request is not yet supported.
 118 .PP
 119 On startup, dhclient reads the
 120 .IR dhclient.conf
 121 for configuration instructions.   It then gets a list of all the
 122 network interfaces that are configured in the current system.   For
 123 each interface, it attempts to configure the interface using the DHCP
 124 protocol.
 125 .PP
 126 In order to keep track of leases across system reboots and server
 127 restarts, dhclient keeps a list of leases it has been assigned in the
 128 dhclient.leases(5) file.   On startup, after reading the dhclient.conf
 129 file, dhclient reads the dhclient.leases file to refresh its memory
 130 about what leases it has been assigned.
 131 .PP
 132 When a new lease is acquired, it is appended to the end of the
 133 dhclient.leases file.   In order to prevent the file from becoming
 134 arbitrarily large, from time to time dhclient creates a new
 135 dhclient.leases file from its in-core lease database.  The old version
 136 of the dhclient.leases file is retained under the name
 137 .IR dhclient.leases~
 138 until the next time dhclient rewrites the database.
 139 .PP
 140 Old leases are kept around in case the DHCP server is unavailable when
 141 dhclient is first invoked (generally during the initial system boot
 142 process).   In that event, old leases from the dhclient.leases file
 143 which have not yet expired are tested, and if they are determined to
 144 be valid, they are used until either they expire or the DHCP server
 145 becomes available.
 146 .PP
 147 A mobile host which may sometimes need to access a network on which no
 148 DHCP server exists may be preloaded with a lease for a fixed
 149 address on that network.   When all attempts to contact a DHCP server
 150 have failed, dhclient will try to validate the static lease, and if it
 151 succeeds, will use that lease until it is restarted.
 152 .PP
 153 A mobile host may also travel to some networks on which DHCP is not
 154 available but BOOTP is.   In that case, it may be advantageous to
 155 arrange with the network administrator for an entry on the BOOTP
 156 database, so that the host can boot quickly on that network rather
 157 than cycling through the list of old leases.
 158 .SH COMMAND LINE
 159 .PP
 160 The names of the network interfaces that dhclient should attempt to
 161 configure may be specified on the command line.  If no interface names
 162 are specified on the command line dhclient will normally identify all
 163 network interfaces, eliminating non-broadcast interfaces if
 164 possible, and attempt to configure each interface.
 165 .PP
 166 It is also possible to specify interfaces by name in the
 167 .B dhclient.conf(5)
 168 file.   If interfaces are specified in this way, then the client will
 169 only configure interfaces that are either specified in the
 170 configuration file or on the command line, and will ignore all other
 171 interfaces.
 172 .PP
 173 If the DHCP client should listen and transmit on a port other than the
 174 standard (port 68), the
 175 .B -p
 176 flag may used.  It should be followed by the udp port number that
 177 dhclient should use.  This is mostly useful for debugging purposes.
 178 If a different port is specified for the client to listen on and
 179 transmit on, the client will also use a different destination port -
 180 one greater than the specified destination port.
 181 .PP
 182 The DHCP client normally transmits any protocol messages it sends
 183 before acquiring an IP address to, 255.255.255.255, the IP limited
 184 broadcast address.   For debugging purposes, it may be useful to have
 185 the server transmit these messages to some other address.   This can
 186 be specified with the
 187 .B -s
 188 flag, followed by the IP address or domain name of the destination.
 189 .PP
 190 For testing purposes, the giaddr field of all packets that the client
 191 sends can be set using the
 192 .B -g
 193 flag, followed by the IP address to send.   This is only useful for testing,
 194 and should not be expected to work in any consistent or useful way.
 195 .PP
 196 The DHCP client will normally run in the foreground until it has
 197 configured an interface, and then will revert to running in the
 198 background.   To run force dhclient to always run as a foreground
 199 process, the
 200 .B -d
 201 flag should be specified.  This is useful when running the client
 202 under a debugger, or when running it out of inittab on System V
 203 systems.
 204 .PP
 205 The dhclient daemon creates its own environment when executing the
 206 dhclient-script to do the grunt work of interface configuration.
 207 To define extra environment variables and their values, use the
 208 .B -e
 209 flag, followed by the environment variable name and value assignment,
 210 just as one would assign a variable in a shell.  Eg:
 211 .B -e
 212 .I IF_METRIC=1
 213 .PP
 214 The client normally prints a startup message and displays the
 215 protocol sequence to the standard error descriptor until it has
 216 acquired an address, and then only logs messages using the
 217 .B syslog (3)
 218 facility.   The
 219 .B -q
 220 flag prevents any messages other than errors from being printed to the
 221 standard error descriptor.
 222 .PP
 223 The client normally doesn't release the current lease as it is not
 224 required by the DHCP protocol.  Some cable ISPs require their clients
 225 to notify the server if they wish to release an assigned IP address.
 226 The
 227 .B -r
 228 flag explicitly releases the current lease, and once the lease has been
 229 released, the client exits.
 230 .PP
 231 The
 232 .B -1
 233 flag will cause dhclient to try once to get a lease.  If it fails, dhclient
 234 exits with exit code two.
 235 .PP
 236 The DHCP client normally gets its configuration information from
 237 .B ETCDIR/dhclient.conf,
 238 its lease database from
 239 .B DBDIR/dhclient.leases,
 240 stores its process ID in a file called
 241 .B RUNDIR/dhclient.pid,
 242 and configures the network interface using
 243 .B CLIENTBINDIR/dhclient-script
 244 To specify different names and/or locations for these files, use the
 245 .B -cf,
 246 .B -lf,
 247 .B -pf
 248 and
 249 .B -sf
 250 flags, respectively, followed by the name of the file.   This can be
 251 particularly useful if, for example,
 252 .B DBDIR
 253 or
 254 .B RUNDIR
 255 has not yet been mounted when the DHCP client is started.
 256 .PP
 257 The DHCP client normally exits if it isn't able to identify any
 258 network interfaces to configure.   On laptop computers and other
 259 computers with hot-swappable I/O buses, it is possible that a
 260 broadcast interface may be added after system startup.   The
 261 .B -w
 262 flag can be used to cause the client not to exit when it doesn't find
 263 any such interfaces.   The
 264 .B omshell (1)
 265 program can then be used to notify the client when a network interface
 266 has been added or removed, so that the client can attempt to configure an IP
 267 address on that interface.
 268 .PP
 269 The DHCP client can be directed not to attempt to configure any interfaces
 270 using the
 271 .B -n
 272 flag.   This is most likely to be useful in combination with the
 273 .B -w
 274 flag.
 275 .PP
 276 The client can also be instructed to become a daemon immediately, rather
 277 than waiting until it has acquired an IP address.   This can be done by
 278 supplying the
 279 .B -nw
 280 flag.
 281 .SH CONFIGURATION
 282 The syntax of the dhclient.conf(5) file is discussed separately.
 283 .SH OMAPI
 284 The DHCP client provides some ability to control it while it is
 285 running, without stopping it.  This capability is provided using OMAPI,
 286 an API for manipulating remote objects.  OMAPI clients connect to the
 287 client using TCP/IP, authenticate, and can then examine the client's
 288 current status and make changes to it.
 289 .PP
 290 Rather than implementing the underlying OMAPI protocol directly, user
 291 programs should use the dhcpctl API or OMAPI itself.   Dhcpctl is a
 292 wrapper that handles some of the housekeeping chores that OMAPI does
 293 not do automatically.   Dhcpctl and OMAPI are documented in \fBdhcpctl(3)\fR
 294 and \fBomapi(3)\fR.   Most things you'd want to do with the client can
 295 be done directly using the \fBomshell(1)\fR command, rather than
 296 having to write a special program.
 297 .SH THE CONTROL OBJECT
 298 The control object allows you to shut the client down, releasing all
 299 leases that it holds and deleting any DNS records it may have added.
 300 It also allows you to pause the client - this unconfigures any
 301 interfaces the client is using.   You can then restart it, which
 302 causes it to reconfigure those interfaces.   You would normally pause
 303 the client prior to going into hibernation or sleep on a laptop
 304 computer.   You would then resume it after the power comes back.
 305 This allows PC cards to be shut down while the computer is hibernating
 306 or sleeping, and then reinitialized to their previous state once the
 307 computer comes out of hibernation or sleep.
 308 .PP
 309 The control object has one attribute - the state attribute.   To shut
 310 the client down, set its state attribute to 2.   It will automatically
 311 do a DHCPRELEASE.   To pause it, set its state attribute to 3.   To
 312 resume it, set its state attribute to 4.
 313 .PP
 314 .SH FILES
 315 .B CLIENTBINDIR/dhclient-script,
 316 .B ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid,
 317 .B DBDIR/dhclient.leases~.
 318 .SH SEE ALSO
 319 dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
 320 dhclient.leases(5), dhcp-eval(5).
 321 .SH AUTHOR
 322 .B dhclient(8)
 323 has been written for Internet Systems Consortium
 324 by Ted Lemon in cooperation with Vixie
 325 Enterprises.  To learn more about Internet Systems Consortium,
 326 see
 327 .B http://www.isc.org
 328 To learn more about Vixie
 329 Enterprises, see
 330 .B http://www.vix.com.
 331 .PP
 332 This client was substantially modified and enhanced by Elliot Poger
 333 for use on Linux while he was working on the MosquitoNet project at
 334 Stanford.
 335 .PP
 336 The current version owes much to Elliot's Linux enhancements, but
 337 was substantially reorganized and partially rewritten by Ted Lemon
 338 so as to use the same networking framework that the Internet Systems
 339 Consortium DHCP server uses.   Much system-specific configuration code
 340 was moved into a shell script so that as support for more operating
 341 systems is added, it will not be necessary to port and maintain
 342 system-specific configuration code to these operating systems - instead,
 343 the shell script can invoke the native tools to accomplish the same
 344 purpose.
 345 .PP