jammy (8) dhclient.8.gz

Provided by: isc-dhcp-client_4.4.1-2.3ubuntu2_amd64 bug


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 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 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 
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 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 recommended
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 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 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 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 addresses.  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 usedin 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.  
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 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~.


SEE ALSO


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


AUTHOR


dhclient(8) To learn more about Internet Systems Consortium, see 
https://www.isc.org

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)


/*F*******************************************************
* MORE STUFF 
********************************************************/

Manually force lease generation with dhclient


You can also use the dhclient command to generate a new DHCP lease manually.
Here are the commands:

dhclient -r to release it

dhclient (no option) to lease a new one

dhclient -r eth0 for specific NIC

Note:
use -v for verbose output

Remember, if the client’s IP address is 169.254.x.x, it could not lease an
IP address from the DHCP server.