SYNOPSIS


     dhcpcd [-bdeknpqABDEGKLTV] [-c, --script script] [-f, --config file]
            [-h, --hostname hostname] [-i, --vendorclassid vendorclassid]
            [-l, --leasetime seconds] [-m, --metric metric]
            [-o, --option option] [-r, --request address]
            [-s, --inform address[/cidr]] [-t, --timeout seconds]
            [-u, --userclass class] [-v, --vendor code, value]
            [-y, --reboot seconds] [-z, --allowinterfaces pattern]
            [-C, --nohook hook] [-F, --fqdn FQDN] [-I, --clientid clientid]
            [-O, --nooption option] [-Q, --require option]
            [-S, --static value] [-X, --blacklist address[/cidr]]
            [-Z, --denyinterfaces pattern] [interface] [...]
     dhcpcd -k, --release [interface]
     dhcpcd -x, --exit [interface]


DESCRIPTION


     dhcpcd is an implementation of the DHCP client specified in RFC 2131.
     dhcpcd gets the host information (IP address, routes, etc) from a DHCP
     server and configures the network interface of the machine on which it is
     running.  dhcpcd then runs the configuration script which writes DNS
     information to resolvconf(8), if available, otherwise directly to
     /etc/resolv.conf.  If the hostname is currenly blank, (null) or localhost
     then dhcpcd sets the hostname to the one supplied by the DHCP server.
     dhcpcd then daemonises and waits for the lease renewal time to lapse.
     Then it attempts to renew its lease and reconfigure if the new lease
     changes.


     dhcpcd is also an implementation of the BOOTP client specified in RFC
     951.


   Local Link configuration
     If dhcpcd failed to obtain a lease, it probes for a valid IPv4LL address
     (aka ZeroConf, aka APIPA).  Once obtained it restarts the process of
     looking for a DHCP server to get a proper address.


     When using IPv4LL, dhcpcd nearly always succeeds and returns an exit code
     of 0.  In the rare case it fails, it normally means that there is a
     reverse ARP proxy installed which always defeats IPv4LL probing.  To dis-
     able this behaviour, you can use the -L, --noipv4ll option.


   Multiple interfaces
     dhcpcd can be run per interface or as a single instance to manage all
     interfaces.  If a list of interfaces are given on the command line, then
     dhcpcd only works with those interfaces, otherwise dhcpcd discovers
     available interfaces.  If link management is enabled and no interfaces
     are given on the command line, dhcpcd forks to the background right away.


     Interfaces are preferred by carrier, DHCP lease/IPv4LL and then lowest
     metric.  For systems that support route metrics, each route will be
     tagged with the metric, otherwise dhcpcd changes the routes to use the
     interface with the same route and the lowest metric.  See options below
     for controlling what interfaces we allow and deny through the use of pat-
     terns. Also, see the BUGS section if dhcpcd runs on a BSD system.


   Hooking into DHCP events
     dhcpcd runs /libexec/dhcpcd-run-hooks, or the script specified by the -c,
     --script option.  This script runs each script found in
     /libexec/dhcpcd-hooks in a lexical order.  The default installation sup-
     plies the scripts 01-test, 10-mtu, 20-resolv.conf and 30-hostname.  You
     can disable each script by using the -C, --nohook option.  See
     dhcpcd-run-hooks(8) for details on how these scripts work.  dhcpcd cur-
     rently ignores the exit code of the script.


   Fine tuning
     You can fine tune the behaviour of dhcpcd with the following options:


     -b, --background
             Background immediately.  This is useful for startup scripts which
             don't disable link messages for carrier status.


     -c, --script script
             Use this script instead of the default /libexec/dhcpcd-run-hooks.


     -d, --debug
             Echo debug and informational messages to the console.  Subsequent
             debug options stop dhcpcd from daemonising.


     -e, --reconfigure
             dhcpcd will re-apply IP address, routing and run
             dhcpcd-run-hooks(8) for each interface.  This is useful so that a
             3rd party such as PPP or VPN can change the routing table and /
             or DNS, etc and then instruct dhcpcd to put things back after-
             wards.  dhcpcd does not read a new configuration when this hap-
             pens - you should rebind if you need that functionality.


     -f, --config file
             Specify a config to load instead of /etc/dhcpcd.conf.  dhcpcd
             always processes the config file before any command line options.


     -h, --hostname hostname
             By default, dhcpcd sends the current hostname to the DHCP server
             so it can register in DNS.  You can use this option to specify
             the hostname sent, or an empty string to stop any hostname from
             being sent.


     -i, --vendorclassid vendorclassid
             Override the vendorclassid field sent. The default is dhcpcd
             <version>.  If not set then none is sent.


     -k, --release
             This causes an existing dhcpcd process running on the interface
             to release its lease, deconfigure the interface and then exit.
             dhcpcd then waits until this process has exited.


     -l, --leasetime seconds
             Request a specific lease time in seconds.  By default dhcpcd does
             not request any lease time and leaves the it in the hands of the
             DHCP server.


     -m, --metric metric
             Metrics are used to prefer an interface over another one, lowest
             wins.  dhcpcd will supply a default metic of 200 +
             if_nametoindex(3).  An extra 100 will be added for wireless
             interfaces.


     -o, --option option
             Request the DHCP option variable for use in
             /libexec/dhcpcd-run-hooks.


     -n, --rebind
             Notifies an existing dhcpcd process running on the interface to
             rebind it's lease.  dhcpcd will not re-configure itself or use
             any other command line arguments.  dhcpcd will timeout the rebind
             after 30 seconds at which point the lease will be expired and
             dhcpcd will enter the discovery state to obtain a new lease.  Use
             the -t, --timeout option to change this.  If dhcpcd is not run-
             ning, then it starts up as normal.  This option used to be renew,
             but rebind is more accurate as we need to broadcast the request
             instead of unicasting.


     -p, --persistent
             dhcpcd normally deconfigures the interface and configuration when
             it exits.  Sometimes, this isn't desirable if for example you
             have root mounted over NFS.  You can use this option to stop this
             from happening.


     -r, --request [address]
             dhcpcd normally sends a DHCP DISCOVER to find servers to offer an
             address.  dhcpcd then requests the address used.  You can use
             this option to skip the BROADCAST step and just request the
             address.  The downside is if you request an address the DHCP
             server does not know about or the DHCP server is not authorative,
             it will remain silent.  In this situation, we go back to the init
             state and DISCOVER again.  If no address is given then the first
             address currently assigned to the interface is used.


     -s, --inform [address[/cidr]]
             Behaves like -r, --request as above, but sends a DHCP INFORM
             instead of a REQUEST.  This does not get a lease as such, just
             notifies the DHCP server of the address in use.  You should also
             include the optional cidr network number in-case the address is
             not already configured on the interface.  dhcpcd remains running
             and pretends it has an infinite lease.  dhcpcd will not de-con-
             figure the interface when it exits.  If dhcpcd fails to contact a
             DHCP server then it returns a failure instead of falling back on
             IPv4LL.


     -t, --timeout seconds
             Timeout after seconds, instead of the default 30.  A setting of 0
             seconds causes dhcpcd to wait forever to get a lease.


     -u, --userclass class
             Tags the DHCP message with the userclass class.  DHCP servers use
             this give members of the class DHCP options other than the
             default, without having to know things like hardware address or
             hostname.


     -v, --vendor code,value
             Add an enscapulated vendor option.  code should be between 1 and
             254 inclusive.  Examples.


             Set the vendor option 01 with an IP address.
                   dhcpcd -v 01,192.168.0.2 eth0
             Set the vendor option 02 with a hex code.
                   dhcpcd -v 02,01:02:03:04:05 eth0
             Do the above and set a third option with a string and not an IP
             address.
                   dhcpcd -v 01,192.168.0.2 -v 02,01:02:03:04:05 -v
                   03,"192.168.0.2" eth0


     -x, --exit
             This will signal an existing dhcpcd process running on the
             interface to deconfigure the interface and exit.  dhcpcd then
             waits until this process has exited.


     -y, --reboot seconds
             Allow reboot seconds before moving to the discover phase if we
             have an old lease to use.  The default is 10 seconds.  A setting
             if 0 seconds causes dhcpcd to skip the reboot phase and go
             straight into discover.


     -D, --duid
             Generate an RFC 4361 compliant clientid.  This requires persis-
             tent storage and not all DHCP servers work with it so it's not
             enabled by default.  dhcpcd generates the DUID and stores in it
             /etc/dhcpcd.duid This file should not be copied to other hosts.


     -E, --lastlease
             If dhcpcd cannot obtain a lease, then try to use the last lease
             acquired for the interface.  If the -p, --persistent option is
             not given then the lease is used if it hasn't expired.


     -F, --fqdn fqdn
             Requests that the DHCP server updates DNS using FQDN instead of
             just a hostname.  Valid values for fqdn are disable, none, ptr
             and both.  dhcpcd itself never does any DNS updates.  dhcpcd
             encodes the FQDN hostname as specified in RFC1035.


     -I, --clientid clientid
             Send the clientid.  If the string is of the format 01:02:03 then
             it is encoded as hex.  For interfaces whose hardware address is
             longer than 8 bytes, or if the clientid is an empty string then
             dhcpcd sends a default clientid of the hardware family and the
             hardware address.


   Restricting behaviour
     dhcpcd will try to do as much as it can by default.  However, there are
     sometimes situations where you don't want the things to be configured
     exactly how the the DHCP server wants.  Here are some options that deal
     with turning these bits off.


     -q, --quiet
             Quiet dhcpcd on the command line, only warnings and errors will
             be displayed.  The messages are still logged though.


     -z, --allowinterfaces pattern
             When discovering interfaces, the interface name must match
             pattern which is a space or comma separated list of patterns
             passed to fnmatch(3).  If the same interface is matched in -Z,
             --denyinterfaces then it is still denied.


     -A, --noarp
             Don't request or claim the address by ARP.  This also disables
             IPv4LL.


     -B, --nobackground
             Don't run in the background when we acquire a lease.  This is
             mainly useful for running under the control of another process,
             such as a debugger or a network manager.


     -C, --nohook script
             Don't run this hook script.  Matches full name, or prefixed with
             2 numbers optionally ending with .sh.


             So to stop dhcpcd from touching your DNS or MTU settings you
             would do:-
                   dhcpcd -C resolv.conf -C mtu eth0


     -G, --nogateway
             Don't set any default routes.


     -K, --nolink
             Don't receive link messages for carrier status.  You should only
             have to use this with buggy device drivers or running dhcpcd
             through a network manager.


     -L, --noipv4ll
             Don't use IPv4LL (aka APIPA, aka Bonjour, aka ZeroConf).


     -O, --nooption option
             Don't request the specified option.  If no option given, then
             don't request any options other than those to configure the
             interface and routing.


     -Q, --require option
             Requires the option to be present in all DHCP messages, otherwise
             the message is ignored.  To enforce that dhcpcd only responds to
             DHCP servers and not BOOTP servers, you can -Q dhcp_message_type.


     -S, --static value
             Configures a static value.  If you set ip_address then dhcpcd
             will not attempt to obtain a lease and just use the value for the
             address with an infinite lease time.


             Here is an example which configures a static address, routes and
             dns.
                   dhcpcd -S ip_address=192.168.0.10/24 
                   -S routers=192.168.0.1 
                   -S domain_name_servers=192.168.0.1 
                   eth0


     -T, --test
             On receipt of OFFER messages just call /libexec/dhcpcd-run-hooks
             with the reason of TEST which echo's the DHCP variables found in
             the message to the console.  The interface configuration isn't
             touched and neither are any configuration files.


     -V, --variables
             Display a list of option codes and the associated variable for
             use in dhcpcd-run-hooks(8).  Variables are prefixed with new_ and
             old_ unless the option number is -.  Variables without an option
             are part of the DHCP message and cannot be directly requested.


     -X, --blacklist address[/cidr]
             Ignore all packets from address[/cidr].


     -Z, --denyinterfaces pattern
             When discovering interfaces, the interface name must not match
             pattern which is a space or comma separated list of patterns
             passed to fnmatch(3).
You should read the  dhcpcd.conf man page and put your options into /etc/dhcpcd.conf. The default configuration file should work for most people just fine. Here it is, in case you lose it.
# A sample configuration for dhcpcd.
# See dhcpcd.conf(5) for details.


# We normally want to inform the DHCP server of our hostname for DDNS.
hostname


# A list of options we should request from the DHCP server.
option domain_name_servers, domain_name, domain_search, host_name
# Most distros have ntp support.
option ntp_servers


# We should behave nicely on networks and respect their MTU.
# However, a lot of buggy DHCP servers set invalid MTUs so this is not
# enabled by default.
#option interface_mtu


# We provide a hook script to lookup the hostname if not set by the DHCP
# server, but we should not run it by default.
nohook lookup-hostname

一些總結：

1.dhcpcd配置靜態ip。

編寫配置檔案dhcpcd.conf，此檔案安卓環境下預設在/system/etc/dhcpcd/目錄下，也可使用-f引數指定配置檔案路徑。配置靜態ip，內容可如下所示：

interface eth0
static ip_address=x.x.x.x
static subnet_mask=x.x.x.x
static routers=x.x.x.x
static domain_name_servers=x.x.x.x

此問題是是由於對於一個網路介面（eth0），只能有一個dhcpcd的例項執行，開機的時候dhcpcd就執行在後臺了，會在/data/misc/dhcp/目錄下生成一個.pid檔案，如果再次點設定，會通過libnetutiles.so重新啟動dhcpcd，檢測到/data/misc/dhcp/下的pid檔案，就會失敗返回，返回前會把.pid檔案刪除，下次再呼叫，就會成功了。一種解決辦法就是在呼叫前把pid檔案刪除就好了

3.dhcp新增option 60欄位

dhcpcd可配置傳送option 60 欄位，新增“-i vendor_class_id”引數就可以，但是加完後，需要在原始碼中加入驗證的程式碼，不含有相同option 60欄位的offer以及ACK，扔掉，這樣就只響應特定的dhcp伺服器了。