client/dhclient.8

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  27 .\" $Id: dhclient.8,v 1.18 2006/02/24 23:16:27 dhankins Exp $
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  29 .TH dhclient 8
  30 .SH NAME
  31 dhclient - Dynamic Host Configuration Protocol Client
  32 .SH SYNOPSIS
  33 .B dhclient
  34 [
  35 .B -p
  36 .I port
  37 ]
  38 [
  39 .B -d
  40 ]
  41 [
  42 .B -e
  43 .I VAR=value
  44 ]
  45 [
  46 .B -q
  47 ]
  48 [
  49 .B -1
  50 ]
  51 [
  52 .B -r
  53 ]
  54 [
  55 .B -lf
  56 .I lease-file
  57 ]
  58 [
  59 .B -pf
  60 .I pid-file
  61 ]
  62 [
  63 .B -cf
  64 .I config-file
  65 ]
  66 [
  67 .B -sf
  68 .I script-file
  69 ]
  70 [
  71 .B -s
  72 server
  73 ]
  74 [
  75 .B -g
  76 relay
  77 ]
  78 [
  79 .B -n
  80 ]
  81 [
  82 .B -nw
  83 ]
  84 [
  85 .B -w
  86 ]
  87 [
  88 .I if0
  89 [
  90 .I ...ifN
  91 ]
  92 ]
  93 .SH DESCRIPTION
  94 The Internet Systems Consortium DHCP Client, dhclient, provides a
  95 means for configuring one or more network interfaces using the Dynamic
  96 Host Configuration Protocol, BOOTP protocol, or if these protocols
  97 fail, by statically assigning an address.
  98 .SH OPERATION
  99 .PP
 100 The DHCP protocol allows a host to contact a central server which
 101 maintains a list of IP addresses which may be assigned on one or more
 102 subnets.   A DHCP client may request an address from this pool, and
 103 then use it on a temporary basis for communication on network.   The
 104 DHCP protocol also provides a mechanism whereby a client can learn
 105 important details about the network to which it is attached, such as
 106 the location of a default router, the location of a name server, and
 107 so on.
 108 .PP
 109 On startup, dhclient reads the
 110 .IR dhclient.conf
 111 for configuration instructions.   It then gets a list of all the
 112 network interfaces that are configured in the current system.   For
 113 each interface, it attempts to configure the interface using the DHCP
 114 protocol.
 115 .PP
 116 In order to keep track of leases across system reboots and server
 117 restarts, dhclient keeps a list of leases it has been assigned in the
 118 dhclient.leases(5) file.   On startup, after reading the dhclient.conf
 119 file, dhclient reads the dhclient.leases file to refresh its memory
 120 about what leases it has been assigned.
 121 .PP
 122 When a new lease is acquired, it is appended to the end of the
 123 dhclient.leases file.   In order to prevent the file from becoming
 124 arbitrarily large, from time to time dhclient creates a new
 125 dhclient.leases file from its in-core lease database.  The old version
 126 of the dhclient.leases file is retained under the name
 127 .IR dhclient.leases~
 128 until the next time dhclient rewrites the database.
 129 .PP
 130 Old leases are kept around in case the DHCP server is unavailable when
 131 dhclient is first invoked (generally during the initial system boot
 132 process).   In that event, old leases from the dhclient.leases file
 133 which have not yet expired are tested, and if they are determined to
 134 be valid, they are used until either they expire or the DHCP server
 135 becomes available.
 136 .PP
 137 A mobile host which may sometimes need to access a network on which no
 138 DHCP server exists may be preloaded with a lease for a fixed
 139 address on that network.   When all attempts to contact a DHCP server
 140 have failed, dhclient will try to validate the static lease, and if it
 141 succeeds, will use that lease until it is restarted.
 142 .PP
 143 A mobile host may also travel to some networks on which DHCP is not
 144 available but BOOTP is.   In that case, it may be advantageous to
 145 arrange with the network administrator for an entry on the BOOTP
 146 database, so that the host can boot quickly on that network rather
 147 than cycling through the list of old leases.
 148 .SH COMMAND LINE
 149 .PP
 150 The names of the network interfaces that dhclient should attempt to
 151 configure may be specified on the command line.  If no interface names
 152 are specified on the command line dhclient will normally identify all
 153 network interfaces, eliminating non-broadcast interfaces if
 154 possible, and attempt to configure each interface.
 155 .PP
 156 It is also possible to specify interfaces by name in the
 157 .B dhclient.conf(5)
 158 file.   If interfaces are specified in this way, then the client will
 159 only configure interfaces that are either specified in the
 160 configuration file or on the command line, and will ignore all other
 161 interfaces.
 162 .PP
 163 If the DHCP client should listen and transmit on a port other than the
 164 standard (port 68), the
 165 .B -p
 166 flag may used.  It should be followed by the udp port number that
 167 dhclient should use.  This is mostly useful for debugging purposes.
 168 If a different port is specified for the client to listen on and
 169 transmit on, the client will also use a different destination port -
 170 one greater than the specified destination port.
 171 .PP
 172 The DHCP client normally transmits any protocol messages it sends
 173 before acquiring an IP address to, 255.255.255.255, the IP limited
 174 broadcast address.   For debugging purposes, it may be useful to have
 175 the server transmit these messages to some other address.   This can
 176 be specified with the
 177 .B -s
 178 flag, followed by the IP address or domain name of the destination.
 179 .PP
 180 For testing purposes, the giaddr field of all packets that the client
 181 sends can be set using the
 182 .B -g
 183 flag, followed by the IP address to send.   This is only useful for testing,
 184 and should not be expected to work in any consistent or useful way.
 185 .PP
 186 The DHCP client will normally run in the foreground until it has
 187 configured an interface, and then will revert to running in the
 188 background.   To run force dhclient to always run as a foreground
 189 process, the
 190 .B -d
 191 flag should be specified.  This is useful when running the client
 192 under a debugger, or when running it out of inittab on System V
 193 systems.
 194 .PP
 195 The dhclient daemon creates its own environment when executing the
 196 dhclient-script to do the grunt work of interface configuration.
 197 To define extra environment variables and their values, use the
 198 .B -e
 199 flag, followed by the environment variable name and value assignment,
 200 just as one would assign a variable in a shell.  Eg:
 201 .B -e
 202 .I IF_METRIC=1
 203 .PP
 204 The client normally prints a startup message and displays the
 205 protocol sequence to the standard error descriptor until it has
 206 acquired an address, and then only logs messages using the
 207 .B syslog (3)
 208 facility.   The
 209 .B -q
 210 flag prevents any messages other than errors from being printed to the
 211 standard error descriptor.
 212 .PP
 213 The client normally doesn't release the current lease as it is not
 214 required by the DHCP protocol.  Some cable ISPs require their clients
 215 to notify the server if they wish to release an assigned IP address.
 216 The
 217 .B -r
 218 flag explicitly releases the current lease, and once the lease has been
 219 released, the client exits.
 220 .PP
 221 The
 222 .B -1
 223 flag cause dhclient to try once to get a lease.  If it fails, dhclient exits
 224 with exit code two.
 225 .PP
 226 The DHCP client normally gets its configuration information from
 227 .B ETCDIR/dhclient.conf,
 228 its lease database from
 229 .B DBDIR/dhclient.leases,
 230 stores its process ID in a file called
 231 .B RUNDIR/dhclient.pid,
 232 and configures the network interface using
 233 .B CLIENTBINDIR/dhclient-script
 234 To specify different names and/or locations for these files, use the
 235 .B -cf,
 236 .B -lf,
 237 .B -pf
 238 and
 239 .B -sf
 240 flags, respectively, followed by the name of the file.   This can be
 241 particularly useful if, for example,
 242 .B DBDIR
 243 or
 244 .B RUNDIR
 245 has not yet been mounted when the DHCP client is started.
 246 .PP
 247 The DHCP client normally exits if it isn't able to identify any
 248 network interfaces to configure.   On laptop computers and other
 249 computers with hot-swappable I/O buses, it is possible that a
 250 broadcast interface may be added after system startup.   The
 251 .B -w
 252 flag can be used to cause the client not to exit when it doesn't find
 253 any such interfaces.   The
 254 .B omshell (1)
 255 program can then be used to notify the client when a network interface
 256 has been added or removed, so that the client can attempt to configure an IP
 257 address on that interface.
 258 .PP
 259 The DHCP client can be directed not to attempt to configure any interfaces
 260 using the
 261 .B -n
 262 flag.   This is most likely to be useful in combination with the
 263 .B -w
 264 flag.
 265 .PP
 266 The client can also be instructed to become a daemon immediately, rather
 267 than waiting until it has acquired an IP address.   This can be done by
 268 supplying the
 269 .B -nw
 270 flag.
 271 .SH CONFIGURATION
 272 The syntax of the dhclient.conf(5) file is discussed separately.
 273 .SH OMAPI
 274 The DHCP client provides some ability to control it while it is
 275 running, without stopping it.  This capability is provided using OMAPI,
 276 an API for manipulating remote objects.  OMAPI clients connect to the
 277 client using TCP/IP, authenticate, and can then examine the client's
 278 current status and make changes to it.
 279 .PP
 280 Rather than implementing the underlying OMAPI protocol directly, user
 281 programs should use the dhcpctl API or OMAPI itself.   Dhcpctl is a
 282 wrapper that handles some of the housekeeping chores that OMAPI does
 283 not do automatically.   Dhcpctl and OMAPI are documented in \fBdhcpctl(3)\fR
 284 and \fBomapi(3)\fR.   Most things you'd want to do with the client can
 285 be done directly using the \fBomshell(1)\fR command, rather than
 286 having to write a special program.
 287 .SH THE CONTROL OBJECT
 288 The control object allows you to shut the client down, releasing all
 289 leases that it holds and deleting any DNS records it may have added.
 290 It also allows you to pause the client - this unconfigures any
 291 interfaces the client is using.   You can then restart it, which
 292 causes it to reconfigure those interfaces.   You would normally pause
 293 the client prior to going into hibernation or sleep on a laptop
 294 computer.   You would then resume it after the power comes back.
 295 This allows PC cards to be shut down while the computer is hibernating
 296 or sleeping, and then reinitialized to their previous state once the
 297 computer comes out of hibernation or sleep.
 298 .PP
 299 The control object has one attribute - the state attribute.   To shut
 300 the client down, set its state attribute to 2.   It will automatically
 301 do a DHCPRELEASE.   To pause it, set its state attribute to 3.   To
 302 resume it, set its state attribute to 4.
 303 .PP
 304 .SH FILES
 305 .B CLIENTBINDIR/dhclient-script,
 306 .B ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid,
 307 .B DBDIR/dhclient.leases~.
 308 .SH SEE ALSO
 309 dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
 310 dhclient.leases(5).
 311 .SH AUTHOR
 312 .B dhclient(8)
 313 has been written for Internet Systems Consortium
 314 by Ted Lemon in cooperation with Vixie
 315 Enterprises.  To learn more about Internet Systems Consortium,
 316 see
 317 .B http://www.isc.org
 318 To learn more about Vixie
 319 Enterprises, see
 320 .B http://www.vix.com.
 321 .PP
 322 This client was substantially modified and enhanced by Elliot Poger
 323 for use on Linux while he was working on the MosquitoNet project at
 324 Stanford.
 325 .PP
 326 The current version owes much to Elliot's Linux enhancements, but
 327 was substantially reorganized and partially rewritten by Ted Lemon
 328 so as to use the same networking framework that the Internet Systems
 329 Consortium DHCP server uses.   Much system-specific configuration code
 330 was moved into a shell script so that as support for more operating
 331 systems is added, it will not be necessary to port and maintain
 332 system-specific configuration code to these operating systems - instead,
 333 the shell script can invoke the native tools to accomplish the same
 334 purpose.
 335 .PP