dhclient

dhclient(8) System Manager's Manual dhclient(8)

NAME

   dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS

   dhclient  [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [ -P...  ] ] [ -R ] [ -i ] [ -I ] [ -4o6 port ] [ -D LL|LLT ] [ -p port-number ] [ -d ] [ -df duid-lease-file ] [
   -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf config-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ]
   [ --dad-wait-time seconds ] [ --prefix-len-hint length ] [ --decline-wait-time seconds ] [ -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 statically 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.

   There are two versions of the DHCP protocol DHCPv4 and DHCPv6.  At startup the client may be started for one or the other via the -4 or -6 options.

   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 file.  On startup, after  read‐
   ing 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 con‐
   tact 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  en‐
   try 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 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.

   The  client  normally prints no output during its startup sequence.  It can be made to emit verbose messages displaying the startup sequence events until it has acquired an address by
   supplying the -v command line argument.  In either case, the client logs messages using the syslog(3) facility.

OPTIONS

   -4     Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters.  This is the default and cannot be combined with -6.

   -6     Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters.  It cannot be combined with -4.  The -S -T -P -N and  -D  arguments
          provide  more  control  over  aspects  of the DHCPv6 processing.  Note: it is not recommended to mix queries of different types together or even to share the lease file between
          them.

   -4o6 port
          Participate in the DHCPv4 over DHCPv6 protocol specified by RFC 7341.  This associates a DHCPv4 and a DHCPv6 client to allow the v4 client to send v4 requests encapsulated in a
          v6 packet.  Communication between the two clients is done on a pair of UDP sockets bound to ::1 port and port + 1. Both clients must be launched using the same port argument.

   -1     Try to get a lease once.  On failure exit with code 2.  In DHCPv6 this sets the maximum duration of the initial exchange to timeout (from dhclient.conf with a default of  sixty
          seconds).

   -d     Force dhclient to run as a foreground process.  Normally the DHCP client will run in the foreground until is has configured an interface at which time it will revert to running
          in the background.  This option is useful when running the client under a debugger, or when running it out of inittab on System V systems.  This implies -v.

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

   -q     Be quiet at startup, this is the default.

   -v     Enable verbose log messages.

   -w     Continue  running  even if no broadcast interfaces were found.  Normally DHCP client will exit if it isn't able to identify any network interfaces to configure.  On laptop com‐
          puters and other computers with hot-swappable I/O buses, it is possible that a broadcast interface may be added after system startup.  This flag can be used to cause the client
          not to exit when it doesn't find any such interfaces.  The omshell(1) program can then be used to notify the client when a network interface has been added or removed, so  that
          the client can attempt to configure an IP address on that interface.

   -n     Do not configure any interfaces.  This is most likely to be useful in combination with the -w flag.

   -e VAR=value
          Define additional environment variables for the environment where dhclient-script executes.  You may specify multiple -e options on the command line.

   -r     Release the current lease and stop the running DHCP client as previously recorded in the PID file.  When shutdown via this method dhclient-script will be executed with the spe‐
          cific  reason  for calling the script set.  The client normally doesn't release the current lease as this is not required by the DHCP protocol but some cable ISPs require their
          clients to notify the server if they wish to release an assigned IP address.

   -x     Stop the running DHCP client without releasing the current lease.  Kills existing dhclient process as previously recorded in the  PID  file.   When  shutdown  via  this  method
          dhclient-script will be executed with the specific reason for calling the script set.

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

   -s server-addr
          Specify the server IP address or fully qualified domain name to use as a destination for DHCP protocol messages before dhclient has acquired an IP address.  Normally,  dhclient
          transmits  these  messages  to  255.255.255.255  (the IP limited broadcast address).  Overriding this is mostly useful for debugging purposes.  This feature is not supported in
          DHCPv6 (-6) mode.

   -g relay
          Set the giaddr field of all packets to the relay IP address simulating a relay agent.  This is for testing purposes only and should not be expected to work in any consistent or
          useful way.

   -i     Use a DUID with DHCPv4 clients.  If no DUID is available in the lease file one will be constructed and saved.  The DUID will be used to construct a RFC4361 style client id that
          will be included in the client's messages.  This client id can be overridden by setting a client id in the configuration file.  Overriding the client id in this fashion is dis‐
          couraged.

   -I     Use the standard DDNS scheme from RFCs 4701 & 4702.

   --decline-wait-time seconds
          Specify the time (in seconds) that an IPv4 client should wait after declining an address before issuing a discover.  The default is 10 seconds as recommended by RFC 2131,  Sec‐
          tion 3.1.5.  A value of zero equates to no wait at all.

   --version Print version number and exit.

   Options available for DHCPv6 mode:

   -S     Use Information-request to get only stateless configuration parameters (i.e., without address).  This implies -6.  It also doesn't rewrite the lease database.

   -T     Ask for IPv6 temporary addresses, one set per -T flag.  This implies -6 and also disables the normal address query.  See -N to restore it.

   -P     Enable  IPv6  prefix delegation.  This implies -6 and also disables the normal address query.  See -N to restore it.  Multiple prefixes can be requested with multiple -P flags.
          Note only one requested interface is allowed.

   -R     Require that responses include all of the items requested by any -N, -T, or -P options.  Normally even if the command line includes a number of these the client will be willing
          to accept the best lease it can even if the lease doesn't include all of the requested items.  This option causes the client to only accept leases that include all of  the  re‐
          quested items.

          Note well: enabling this may prevent the client from using any leases it receives if the servers aren't configured to supply all of the items.

   -D LL or LLT
          Override  the  default  when selecting the type of DUID to use.  By default, DHCPv6 dhclient creates an identifier based on the link-layer address (DUID-LL) if it is running in
          stateless mode (with -S, not requesting an address), or it creates an identifier based on the link-layer address plus a timestamp (DUID-LLT) if it is running in  stateful  mode
          (without -S, requesting an address).  When DHCPv4 is configured to use a DUID using -i option the default is to use a DUID-LLT.  -D overrides these default, with a value of ei‐
          ther LL or LLT.

   -N     Restore  normal  address  query  for  IPv6. This implies -6.  It is used to restore normal operation after using -T or -P.  Multiple addresses can be requested with multiple -N
          flags.

   --address-prefix-len length
          Specify the length of the prefix for IPv6 addresses. This value is passed by dhclient into the client script via the environment variable, ip6_prefixlen, when binding IPv6  ad‐
          dresses.  The default value is 128.  Alternatively you may change the default at compile time by setting DHCLIENT_DEFAULT_PREFIX_LEN in includes/site.h.

   --dad-wait-time seconds
          Specify  maximum  time  (in seconds) that the client should wait for the duplicate address detection (DAD) to complete on an interface. This value is propagated to the dhclient
          script in a dad_wait_time environment variable. If any of the IPv6 addresses on the interface are tentative (DAD is in progress), the script will wait for the specified  number
          of seconds for DAD to complete. If the script ignores this variable the parameter has no effect.

   --prefix-len-hint length
          When used in conjunction with -P, it directs the client to use the given length to use a prefix hint of, "::/length", when requesting new prefixes.

   Modifying  default  file  locations:  The  following  options  can  be  used  to  modify  the  locations a client uses for its files.  They can be particularly useful if, for example,
   /var/lib/dhcp or /var/run have not been mounted when the DHCP client is started.

   -cf config-file
          Path to the client configuration file.  If unspecified, the default /etc/dhcp/dhclient.conf is used.  See dhclient.conf(5) for a description of this file.

   -df duid-lease-file
          Path to a secondary lease file.  If the primary lease file doesn't contain a DUID this file will be searched.  The DUID read from the secondary will be written to the  primary.
          This  option can be used to allow an IPv4 instance of the client to share a DUID with an IPv6 instance.  After starting one of the instances the second can be started with this
          option pointing to the lease file of the first instance.  There is no default.  If no file is specified no search is made for a DUID should one not be found in the  main  lease
          file.

   -lf lease-file
          Path to the lease database file.  If unspecified, the default /var/lib/dhcp/dhclient.leases is used.  See dhclient.leases(5) for a description of this file.

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

   --no-pid
          Option  to disable writing pid files.  By default the program will write a pid file.  If the program is invoked with this option it will not attempt to kill any existing client
          processes even if invoked with -r or -x.

   -sf script-file
          Path to the network configuration script invoked by dhclient when it gets a lease.  If unspecified, the default /usr/sbin/dhclient-script is used.  See dhclient-script(8) for a
          description of this file.

PORTS

   During operations the client may use multiple UDP ports to provide different functions.  Which ports are opened depends on both the way you compiled your code  and  the  configuration
   you supply.  The following should provide you an idea of what ports may be in use.

   Normally  a  DHCPv4 client will open a raw UDP socket to receive and send most DHCPv4 packets.  It also opens a fallback UDP socket for use in sending unicast packets.  Normally these
   will both use the well known port number for BOOTPC.

   For DHCPv6 the client opens a UDP socket on the well known client port and a fallback UDP socket on a random port for use in sending unicast messages.  Unlike DHCPv4  the  well  known
   socket doesn't need to be opened in raw mode.

   If  you  have included an omapi port statement in your configuration file then the client will open a TCP socket on that port to listen for OMPAI connections.  When something connects
   another port will be used for the established connection.

   When DDNS is enabled at compile time (see includes/site.h) the client will open both a v4 and a v6 UDP socket on random ports.  These ports are  not  opened  unless/until  the  client
   first attempts to do an update.  If the client is not configured to do updates, the ports will never be opened.

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 automatically.  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 normally 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 sleep‐
   ing, 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.

ENVIRONMENT VARIABLES

   The following environment variables may be defined to override the builtin defaults for file locations.  Note that use of the related command-line options will ignore the  correspond‐
   ing environment variable settings.

   PATH_DHCLIENT_CONF
          The dhclient.conf configuration file.

   PATH_DHCLIENT_DB
          The dhclient.leases database.

   PATH_DHCLIENT_PID
          The dhclient PID file.

   PATH_DHCLIENT_SCRIPT
          The dhclient-script file.

FILES

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