参见man dhclient

演示示例

获得IP等：dhclient eth0

释放IP：  dhclient -r eth0

NAME

dhclient是一个DHCP协议客户端，它使用DHCP协议或者BOOTP协议或在这两个协议都不可用时使用静态地址来配置一个或多个网络接口

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 ] [ --no-pid ] [ -cf config-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -v ] [ --version ] [  if0

[ ...ifN ] ]

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 dhclient.conf for configuration instructions.   It then gets a list of all the network interfaces that are con‐

figured 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 and -N 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.

-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(5) 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 Sys‐

tem V systems.  This implies -v.

-nw    Become a daemon immediately (nowait) rather than waiting until an 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=val

Define additional environment variables for the environment where dhclient-script(8) 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(8)

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(8) will be executed with the specific reason for calling the script set.

-p port

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 destina‐

tion 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 pruposes only and should not be expected  to

work in any consistent or useful way.

--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.  Note only one  requested  interface

is allowed.

-N     Restore normal address query for IPv6. This implies -6.  It is used to restore normal operation after using -T or -P.

Modifying default file locations: The following options can be used to modify the locations a client uses for it's files.  They can be particularly useful if,

for example, DBDIR or RUNDIR have not been mounted when the DHCP client is started.

-cf config-file

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

-lf lease-file

Path to the lease database file.  If unspecified, the default DBDIR/dhclient.leases is used.  See dhclient.leases(5) for a descriptionof this file.

-pf pid-file

Path to the process ID file.  If unspecified, the default RUNDIR/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 CLIENTBINDIR/dhclient-script is used.

See dhclient-script(8) for a description of this file.

阅读(10077) | 评论(0) | 转发(0) |