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dhclient(8)                                                        dhclient(8)



NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient  [  -4  |  -6  ]  [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [ -P...  ] ] [ -p port ] [ -d ] [ -e
       VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-file
       ]  [ -s server ] [ -g relay ] [ -n ] [ -nc ] [ -nw ] [ -w ] [ -B ] [ -I dhcp-client-identifier ] [ -H host-name
       ] [ -F fqdn.fqdn ] [ -V vendor-class-identifier ] [ -R request-option-list ] [ -timeout timeout  ]  [  -v  ]  [
       --version ] [ if0 [ ...ifN ] ]

DESCRIPTION
       The  Internet  Systems  Consortium  DHCP Client, dhclient, provides a means for configuring one or more network
       interfaces using the Dynamic Host Configuration Protocol, BOOTP protocol, or if these protocols fail, by stati-
       cally assigning an address.

OPERATION
       The DHCP protocol allows a host to contact a central server which maintains a list of IP addresses which may be
       assigned on one or more subnets.   A DHCP client may request an address from this pool, and then use  it  on  a
       temporary  basis  for  communication on network.   The DHCP protocol also provides a mechanism whereby a client
       can learn important details about the network to which it is attached,  such  as  the  location  of  a  default
       router, the location of a name server, and so on.

       On  startup,  dhclient reads the dhclient.conf for configuration instructions.   It then gets a list of all the
       network interfaces that are configured in the current system.   For each interface, it  attempts  to  configure
       the interface using the DHCP protocol.

       In  order to keep track of leases across system reboots and server restarts, dhclient keeps a list of leases it
       has been assigned in the dhclient.leases(5) file.   On startup, after reading the dhclient.conf file,  dhclient
       reads the dhclient.leases file to refresh its memory about what leases it has been assigned.

       When  a new lease is acquired, it is appended to the end of the dhclient.leases file.   In order to prevent the
       file from becoming arbitrarily large, from time to time dhclient creates a new dhclient.leases  file  from  its
       in-core   lease   database.   The  old  version  of  the  dhclient.leases  file  is  retained  under  the  name
       dhclient.leases~ until the next time dhclient rewrites the database.

       Old leases are kept around in case the DHCP server is unavailable when dhclient  is  first  invoked  (generally
       during  the  initial system boot process).   In that event, old leases from the dhclient.leases file which have
       not yet expired are tested, and if they are determined to be valid, they are used until either they  expire  or
       the DHCP server becomes available.

       A mobile host which may sometimes need to access a network on which no DHCP server exists may be preloaded with
       a lease for a fixed address on that network.   When all attempts to contact a DHCP server have failed, dhclient
       will try to validate the static lease, and if it succeeds, will use that lease until it is restarted.

       A  mobile host may also travel to some networks on which DHCP is not available but BOOTP is.   In that case, it
       may be advantageous to arrange with the network administrator for an entry on the BOOTP database, so  that  the
       host can boot quickly on that network rather than cycling through the list of old leases.

COMMAND LINE
       The  names  of the network interfaces that dhclient should attempt to configure may be specified on the command
       line.  If no interface names are specified on the command line dhclient  will  normally  identify  all  network
       interfaces, eliminating non-broadcast interfaces if possible, and attempt to configure each interface.

       It  is  also possible to specify interfaces by name in the dhclient.conf(5) file.   If interfaces are specified
       in this way, then the client will only configure interfaces that are either specified in the configuration file
       or on the command line, and will ignore all other interfaces.

OPTIONS
       -4     Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters (default).


       -6     Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parame-
              ters.  The functionality of DHCPv6 mode may be modified with the -S , -T , and -N options.


       -S     Perform an information-only request over DHCPv6 to get stateless configuration parameters.   It  is  not
              recommended to combine this option with the -N , -P , or -T options or to share lease files between dif-
              ferent modes of operation.  Only valid with the -6 option.


       -N     Perform a normal (IA_NA) address query over DHCPv6.  It is not recommended to combine this  option  with
              the  -P  ,  -S , or -T options or to share lease files between different modes of operation.  Only valid
              with the -6 option.


       -T     Perform a temporary (IA_TA) address query over DHCPv6 (disables normal address query).  It is not recom-
              mended to combine this option with the -N , -P , or -S options or to share lease files between different
              modes of operation.  Only valid with the -6 option.


       -P     Enable IPv6 prefix delegation (disables normal address query).  It is not  not  recommended  to  combine
              this  option  with the -N , -S , or -T options or to share lease files between different modes of opera-
              tion.  Only valid with the -6 option.


       -p <port number>
              The UDP port number the DHCP client should listen and transmit on.  If unspecified,  dhclient  uses  the
              default port 68.  This option is mostly useful for debugging purposes.  If a different port is specified
              for the client to listen and transmit on, the client will also use a different destination  port  -  one
              less than the specified port.


       -d     Force dhclient to run as a foreground process.  This is useful when running the client under a debugger,
              or when running it out of inittab on System V systems.


       -e VAR=value
              Define additional environment variables for the environment where  dhclient-script  executes.   You  may
              specify multiplate -e options on the command line.  For example: -e IF_METRIC=1


       -q     Suppress all terminal and log output except error messages.


       -1     Try once to get a lease.  One failure, exit with code 2.


       -r     Tell  dhclient  to  release  the current lease it has from the server.  This is not required by the DHCP
              protocol, but some ISPs require their clients to notify the server if they wish to release  an  assigned
              IP address.


       -lf <lease-file>
              Path to the lease database file.  If unspecified, the default /var/lib/dhclient/dhclient.leases is used.


       -pf <pid-file>
              Path to the process ID file.  If unspecified, the default /var/run/dhclient.pid is used.


       -cf <config-file>
              Path to the client configuration file.  If unspecified, the default /etc/dhcp/dhclient.conf is used.


       -sf <script-file>
              Path to the network configuration script invoked by dhclient when it gets a lease.  If unspecified,  the
              default /sbin/dhclient-script is used.


       -s <server>
              Specifiy  the  server  IP  address or fully qualified domain name to transmit DHCP protocol messages to.
              Normally, dhclient transmits these messages to  255.255.255.255  (the  IP  limited  broadcast  address).
              Overriding this is mostly useful for debugging purposes.


       -g <relay>
              Only  for  debugging.  Set the giaddr field of all packets the client sends to the IP address specified.
              This should not be expected to work in any consistent or useful way.


       -n     Do not configure any interfaces.  Most useful combined with the -w option.


       -nc    Do not drop capabilities.

              Normally, if dhclient was compiled with libcap-ng support, dhclient drops most capabilities  immediately
              upon  startup.  While more secure, this greatly restricts the additional actions that hooks in dhclient-
              script (8) can take.  (For example, any daemons that dhclient-script (8) starts or restarts will inherit
              the  restricted  capabilities as well, which may interfere with their correct operation.)  Thus, the -nc
              option can be used to prevent dhclient from dropping capabilities.

              The -nc option is ignored if dhclient was not compiled with libcap-ng support.


       -nw    Become a daemon process immediately (nowait) rather than waiting until an IP address has been  acquired.


       -w     Keep  running  even  if  no network interfaces are found.  The omshell program can be used to notify the
              client when a network interface has been added or removed so it can attempt to configure an  IP  address
              on that interface.


       -B     Set the BOOTP broadcast flag in request packets so servers will always broadcast replies.


       -I <dhcp-client-identifier>
              Specify the dhcp-client-identifier option to send to the DHCP server.


       -H <host-name>
              Specify  the  host-name  option  to  send  to  the  DHCP server.  The host-name string only contains the
              client's hostname prefix, to which the server will append the ddns-domainname or domain-name options, if
              any,  to derive the fully qualified domain name of the client.  The -H option cannot be used with the -F
              option.


       -F <fqdn.fqdn>
              Specify the fqdn.fqdn option to send to the DHCP server.  This option cannot be used with the -H option.
              The  fqdn.fqdn option must specify the complete domain name of the client host, which the server may use
              for dynamic DNS updates.


       -V <vendor-class-identifier>
              Specify the vendor-class-identifier option to send to the DHCP server.


       -R <option>[,<option>...]
              Specify the list of options the client is to request from the server.  The option list must be a  single
              string  consisting of option names separated by at least one command and optional space characters.  The
              default option list is:


                  subnet-mask, broadcast-address, time-offset, routers,
                  domain-name, domain-name-servers, host-name,
                  nis-domain, nis-servers, ntp-servers, interface-mtu

              The -R option does not append options to the default request, it overrides  the  default  request  list.
              Keep  this  in  mind  if you want to request an additional option besides the default request list.  You
              will have to specify all option names for the -R parameter.


       -timeout <timeout>
              Specify the time after which dhclient will decide  that  no  DHCP  servers  can  be  contacted  when  no
              responses have been received.


       -v     Enable verbose log messages.


       If  the  client  is  killed by a signale (for example at shutdown or reboot), it will not execute the dhclient-
       script (8) at exit.  However, if you shut the client down gracefully with -r or -x it  will  execute  dhclient-
       script (8) at shutdown with the specific reason for calling the script set in the environment table.

CONFIGURATION
       The syntax of the dhclient.conf(5) file is discussed separately.

OMAPI
       The  DHCP client provides some ability to control it while it is running, without stopping it.  This capability
       is provided using OMAPI, an API for manipulating remote objects.  OMAPI clients connect  to  the  client  using
       TCP/IP, authenticate, and can then examine the client's current status and make changes to it.

       Rather  than  implementing  the underlying OMAPI protocol directly, user programs should use the dhcpctl API or
       OMAPI itself.   Dhcpctl is a wrapper that handles some of the housekeeping chores that OMAPI does not do  auto-
       matically.    Dhcpctl  and OMAPI are documented in dhcpctl(3) and omapi(3).   Most things you'd want to do with
       the client can be done directly using the omshell(1) command, rather than having to write a special program.

THE CONTROL OBJECT
       The control object allows you to shut the client down, releasing all leases that it holds and deleting any  DNS
       records  it  may  have  added.   It  also allows you to pause the client - this unconfigures any interfaces the
       client is using.   You can then restart it, which causes it to reconfigure those interfaces.   You  would  nor-
       mally  pause  the client prior to going into hibernation or sleep on a laptop computer.   You would then resume
       it after the power comes back.  This allows PC cards to be shut down  while  the  computer  is  hibernating  or
       sleeping, and then reinitialized to their previous state once the computer comes out of hibernation or sleep.

       The  control object has one attribute - the state attribute.   To shut the client down, set its state attribute
       to 2.   It will automatically do a DHCPRELEASE.   To pause it, set its state attribute to 3.    To  resume  it,
       set its state attribute to 4.


FILES
       /sbin/dhclient-script,   /etc/dhcp/dhclient.conf,   /var/lib/dhclient/dhclient.leases,   /var/run/dhclient.pid,
       /var/lib/dhclient/dhclient.leases~.

SEE ALSO
       dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5), dhcp-eval(5).

AUTHOR
       dhclient(8) has been written for Internet Systems Consortium by Ted Lemon  in  cooperation  with  Vixie  Enter-
       prises.   To  learn  more  about Internet Systems Consortium, see https://www.isc.org To learn more about Vixie
       Enterprises, see http://www.vix.com.

       This client was substantially modified and enhanced by Elliot Poger for use on Linux while he  was  working  on
       the MosquitoNet project at Stanford.

       The  current  version owes much to Elliot's Linux enhancements, but was substantially reorganized and partially
       rewritten by Ted Lemon so as to use the same networking framework that the  Internet  Systems  Consortium  DHCP
       server  uses.    Much  system-specific  configuration code was moved into a shell script so that as support for
       more operating systems is added, it will not be necessary to port and  maintain  system-specific  configuration
       code  to these operating systems - instead, the shell script can invoke the native tools to accomplish the same
       purpose.




                                                                   dhclient(8)