client/dhclient.8

   1 .\"     dhclient.8
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  27 .\" $Id: dhclient.8,v 1.17 2005/03/17 20:14:55 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 -q
  43 ]
  44 [
  45 .B -1
  46 ]
  47 [
  48 .B -r
  49 ]
  50 [
  51 .B -lf
  52 .I lease-file
  53 ]
  54 [
  55 .B -pf
  56 .I pid-file
  57 ]
  58 [
  59 .B -cf
  60 .I config-file
  61 ]
  62 [
  63 .B -sf
  64 .I script-file
  65 ]
  66 [
  67 .B -s
  68 server
  69 ]
  70 [
  71 .B -g
  72 relay
  73 ]
  74 [
  75 .B -n
  76 ]
  77 [
  78 .B -nw
  79 ]
  80 [
  81 .B -w
  82 ]
  83 [
  84 .I if0
  85 [
  86 .I ...ifN
  87 ]
  88 ]
  89 .SH DESCRIPTION
  90 The Internet Systems Consortium DHCP Client, dhclient, provides a
  91 means for configuring one or more network interfaces using the Dynamic
  92 Host Configuration Protocol, BOOTP protocol, or if these protocols
  93 fail, by statically assigning an address.
  94 .SH OPERATION
  95 .PP
  96 The DHCP protocol allows a host to contact a central server which
  97 maintains a list of IP addresses which may be assigned on one or more
  98 subnets.   A DHCP client may request an address from this pool, and
  99 then use it on a temporary basis for communication on network.   The
 100 DHCP protocol also provides a mechanism whereby a client can learn
 101 important details about the network to which it is attached, such as
 102 the location of a default router, the location of a name server, and
 103 so on.
 104 .PP
 105 On startup, dhclient reads the
 106 .IR dhclient.conf
 107 for configuration instructions.   It then gets a list of all the
 108 network interfaces that are configured in the current system.   For
 109 each interface, it attempts to configure the interface using the DHCP
 110 protocol.
 111 .PP
 112 In order to keep track of leases across system reboots and server
 113 restarts, dhclient keeps a list of leases it has been assigned in the
 114 dhclient.leases(5) file.   On startup, after reading the dhclient.conf
 115 file, dhclient reads the dhclient.leases file to refresh its memory
 116 about what leases it has been assigned.
 117 .PP
 118 When a new lease is acquired, it is appended to the end of the
 119 dhclient.leases file.   In order to prevent the file from becoming
 120 arbitrarily large, from time to time dhclient creates a new
 121 dhclient.leases file from its in-core lease database.  The old version
 122 of the dhclient.leases file is retained under the name
 123 .IR dhclient.leases~
 124 until the next time dhclient rewrites the database.
 125 .PP
 126 Old leases are kept around in case the DHCP server is unavailable when
 127 dhclient is first invoked (generally during the initial system boot
 128 process).   In that event, old leases from the dhclient.leases file
 129 which have not yet expired are tested, and if they are determined to
 130 be valid, they are used until either they expire or the DHCP server
 131 becomes available.
 132 .PP
 133 A mobile host which may sometimes need to access a network on which no
 134 DHCP server exists may be preloaded with a lease for a fixed
 135 address on that network.   When all attempts to contact a DHCP server
 136 have failed, dhclient will try to validate the static lease, and if it
 137 succeeds, will use that lease until it is restarted.
 138 .PP
 139 A mobile host may also travel to some networks on which DHCP is not
 140 available but BOOTP is.   In that case, it may be advantageous to
 141 arrange with the network administrator for an entry on the BOOTP
 142 database, so that the host can boot quickly on that network rather
 143 than cycling through the list of old leases.
 144 .SH COMMAND LINE
 145 .PP
 146 The names of the network interfaces that dhclient should attempt to
 147 configure may be specified on the command line.  If no interface names
 148 are specified on the command line dhclient will normally identify all
 149 network interfaces, eliminating non-broadcast interfaces if
 150 possible, and attempt to configure each interface.
 151 .PP
 152 It is also possible to specify interfaces by name in the
 153 .B dhclient.conf(5)
 154 file.   If interfaces are specified in this way, then the client will
 155 only configure interfaces that are either specified in the
 156 configuration file or on the command line, and will ignore all other
 157 interfaces.
 158 .PP
 159 If the DHCP client should listen and transmit on a port other than the
 160 standard (port 68), the
 161 .B -p
 162 flag may used.  It should be followed by the udp port number that
 163 dhclient should use.  This is mostly useful for debugging purposes.
 164 If a different port is specified for the client to listen on and
 165 transmit on, the client will also use a different destination port -
 166 one greater than the specified destination port.
 167 .PP
 168 The DHCP client normally transmits any protocol messages it sends
 169 before acquiring an IP address to, 255.255.255.255, the IP limited
 170 broadcast address.   For debugging purposes, it may be useful to have
 171 the server transmit these messages to some other address.   This can
 172 be specified with the
 173 .B -s
 174 flag, followed by the IP address or domain name of the destination.
 175 .PP
 176 For testing purposes, the giaddr field of all packets that the client
 177 sends can be set using the
 178 .B -g
 179 flag, followed by the IP address to send.   This is only useful for testing,
 180 and should not be expected to work in any consistent or useful way.
 181 .PP
 182 The DHCP client will normally run in the foreground until it has
 183 configured an interface, and then will revert to running in the
 184 background.   To run force dhclient to always run as a foreground
 185 process, the
 186 .B -d
 187 flag should be specified.  This is useful when running the client
 188 under a debugger, or when running it out of inittab on System V
 189 systems.
 190 .PP
 191 The client normally prints a startup message and displays the
 192 protocol sequence to the standard error descriptor until it has
 193 acquired an address, and then only logs messages using the
 194 .B syslog (3)
 195 facility.   The
 196 .B -q
 197 flag prevents any messages other than errors from being printed to the
 198 standard error descriptor.
 199 .PP
 200 The client normally doesn't release the current lease as it is not
 201 required by the DHCP protocol.  Some cable ISPs require their clients
 202 to notify the server if they wish to release an assigned IP address.
 203 The
 204 .B -r
 205 flag explicitly releases the current lease, and once the lease has been
 206 released, the client exits.
 207 .PP
 208 The
 209 .B -1
 210 flag cause dhclient to try once to get a lease.  If it fails, dhclient exits
 211 with exit code two.
 212 .PP
 213 The DHCP client normally gets its configuration information from
 214 .B ETCDIR/dhclient.conf,
 215 its lease database from
 216 .B DBDIR/dhclient.leases,
 217 stores its process ID in a file called
 218 .B RUNDIR/dhclient.pid,
 219 and configures the network interface using
 220 .B CLIENTBINDIR/dhclient-script
 221 To specify different names and/or locations for these files, use the
 222 .B -cf,
 223 .B -lf,
 224 .B -pf
 225 and
 226 .B -sf
 227 flags, respectively, followed by the name of the file.   This can be
 228 particularly useful if, for example,
 229 .B DBDIR
 230 or
 231 .B RUNDIR
 232 has not yet been mounted when the DHCP client is started.
 233 .PP
 234 The DHCP client normally exits if it isn't able to identify any
 235 network interfaces to configure.   On laptop computers and other
 236 computers with hot-swappable I/O buses, it is possible that a
 237 broadcast interface may be added after system startup.   The
 238 .B -w
 239 flag can be used to cause the client not to exit when it doesn't find
 240 any such interfaces.   The
 241 .B omshell (8)
 242 program can then be used to notify the client when a network interface
 243 has been added or removed, so that the client can attempt to configure an IP
 244 address on that interface.
 245 .PP
 246 The DHCP client can be directed not to attempt to configure any interfaces
 247 using the
 248 .B -n
 249 flag.   This is most likely to be useful in combination with the
 250 .B -w
 251 flag.
 252 .PP
 253 The client can also be instructed to become a daemon immediately, rather
 254 than waiting until it has acquired an IP address.   This can be done by
 255 supplying the
 256 .B -nw
 257 flag.
 258 .SH CONFIGURATION
 259 The syntax of the dhclient.conf(5) file is discussed separately.
 260 .SH OMAPI
 261 The DHCP client provides some ability to control it while it is
 262 running, without stopping it.  This capability is provided using OMAPI,
 263 an API for manipulating remote objects.  OMAPI clients connect to the
 264 client using TCP/IP, authenticate, and can then examine the client's
 265 current status and make changes to it.
 266 .PP
 267 Rather than implementing the underlying OMAPI protocol directly, user
 268 programs should use the dhcpctl API or OMAPI itself.   Dhcpctl is a
 269 wrapper that handles some of the housekeeping chores that OMAPI does
 270 not do automatically.   Dhcpctl and OMAPI are documented in \fBdhcpctl(3)\fR
 271 and \fBomapi(3)\fR.   Most things you'd want to do with the client can
 272 be done directly using the \fBomshell(1)\fR command, rather than
 273 having to write a special program.
 274 .SH THE CONTROL OBJECT
 275 The control object allows you to shut the client down, releasing all
 276 leases that it holds and deleting any DNS records it may have added.
 277 It also allows you to pause the client - this unconfigures any
 278 interfaces the client is using.   You can then restart it, which
 279 causes it to reconfigure those interfaces.   You would normally pause
 280 the client prior to going into hibernation or sleep on a laptop
 281 computer.   You would then resume it after the power comes back.
 282 This allows PC cards to be shut down while the computer is hibernating
 283 or sleeping, and then reinitialized to their previous state once the
 284 computer comes out of hibernation or sleep.
 285 .PP
 286 The control object has one attribute - the state attribute.   To shut
 287 the client down, set its state attribute to 2.   It will automatically
 288 do a DHCPRELEASE.   To pause it, set its state attribute to 3.   To
 289 resume it, set its state attribute to 4.
 290 .PP
 291 .SH FILES
 292 .B CLIENTBINDIR/dhclient-script,
 293 .B ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid,
 294 .B DBDIR/dhclient.leases~.
 295 .SH SEE ALSO
 296 dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
 297 dhclient.leases(5).
 298 .SH AUTHOR
 299 .B dhclient(8)
 300 has been written for Internet Systems Consortium
 301 by Ted Lemon in cooperation with Vixie
 302 Enterprises.  To learn more about Internet Systems Consortium,
 303 see
 304 .B http://www.isc.org
 305 To learn more about Vixie
 306 Enterprises, see
 307 .B http://www.vix.com.
 308 .PP
 309 This client was substantially modified and enhanced by Elliot Poger
 310 for use on Linux while he was working on the MosquitoNet project at
 311 Stanford.
 312 .PP
 313 The current version owes much to Elliot's Linux enhancements, but
 314 was substantially reorganized and partially rewritten by Ted Lemon
 315 so as to use the same networking framework that the Internet Systems
 316 Consortium DHCP server uses.   Much system-specific configuration code
 317 was moved into a shell script so that as support for more operating
 318 systems is added, it will not be necessary to port and maintain
 319 system-specific configuration code to these operating systems - instead,
 320 the shell script can invoke the native tools to accomplish the same
 321 purpose.
 322 .PP