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 ] [ --ver‐
   sion ] [ 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 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  in‐
   terface.

   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 computers 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 specific 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 shut‐
          down 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 desti‐
          nation 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 ad‐
          dress.   Normally,  dhclient transmits these messages to 255.255.255.255 (the IP limited broadcast address).  Overriding this is mostly useful for de‐
          bugging 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 discouraged.

   -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 rec‐
          ommended by RFC 2131, Section 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 requested 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 time‐
          stamp (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 either 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 re‐
          quested 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_pre‐
          fixlen,  when  binding  IPv6  addresses.   The default value is 128.  Alternatively you may change the default at compile time by setting DHCLIENT_DE‐
          FAULT_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  ef‐
          fect.

   --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 /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.  Un‐
   like 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 manipulat‐
   ing 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 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.

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 corresponding 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

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