udhcpc 后台运行的方法

最近在做项目的时候卡在了dhcp处，当没有网络的时候，板子一直发送dhcp请求，导致程序不往下执行，解决的办法是把它切换到后台运行，可是如何切换到后台呢，有办法，它自带参数可以实现该功能。如下：

udhcpc -b -i eth0 -p /var/run/udhcpc.pid -R

解释一下，-b就是切换到后台指令，-i是指定使用哪个网络接口，双网卡的时候一定要使用它来指定eth0 or eth1。

【最近新添加 -R】

对于 -R参数，其实也很重要，在关闭udhcpc时，可以向dhcpserver发送release取消租约。

The command line options for the udhcp client are:-c, --clientid=CLIENTID         Client identifier
-H, --hostname=HOSTNAME         Client hostname
-h,    Alias for -H
-f, --foreground                Do not fork after getting lease
-b, --background                Fork to background if lease cannot beimmediately negotiated.
-i, --interface=INTERFACE       Interface to use (default: eth0)
-n, --now                       Exit with failure if lease cannot beimmediately negotiated.
-p, --pidfile=file              Store process ID of daemon in file
-q, --quit                      Quit after obtaining lease
-r, --request=IP                IP address to request (default: none)
-s, --script=file               Run file at dhcp events (default:/usr/share/udhcpc/default.script)
-v, --version                   Display versionIf the requested IP address cannot be obtained, the client accepts the
address that the server offers.udhcp client scripts
-------------------When an event occurs, udhcpc calls the action script. The script by
default is /usr/share/udhcpc/default.script but this can be changed via
the command line arguments. The three possible arguments to the script
are:deconfig: This argument is used when udhcpc starts, andwhen a leases is lost. The script should put the interface in anup, but deconfigured state, ie: ifconfig $interface 0.0.0.0.bound: This argument is used when udhcpc moves from anunbound, to a bound state. All of the paramaters are set inenviromental variables, The script should configure the interface,and set any other relavent parameters (default gateway, dns server, etc).renew: This argument is used when a DHCP lease is renewed. All ofthe paramaters are set in enviromental variables. This argument isused when the interface is already configured, so the IP address,will not change, however, the other DHCP paramaters, such as thedefault gateway, subnet mask, and dns server may change.nak: This argument is used with udhcpc receives a NAK message.The script with the deconfig argument will be called directlyafterwards, so no changes to the network interface are neccessary.This hook is provided for purely informational purposes (themessage option may contain a reason for the NAK).The paramaters for enviromental variables are as follows:$HOME  - The set $HOME env or "/"$PATH  - the set $PATH env or "/bin:/usr/bin:/sbin:/usr/sbin"$1  - What action the script should performinterface - The interface this was obtained onip  - The obtained IPsiaddr  - The bootp next server optionsname  - The bootp server name optionboot_file - The bootp boot file optionsubnet  - The assigend subnet masktimezone - Offset in seconds from UTCrouter  - A list of routerstimesvr  - A list of time serversnamesvr  - A list of IEN 116 name serversdns  - A list of DNS serverlogsvr  - A list of MIT-LCS UDP log serverscookiesvr - A list of RFC 865 cookie serverslprsvr  - A list of LPR servershostname - The assigned hostnamebootsize - The length in 512 octect blocks of the bootfiledomain  - The domain name of the networkswapsvr  - The IP address of the client's swap serverrootpath - The path name of the client's root diskipttl  - The TTL to use for this networkmtu  - The MTU to use for this networkbroadcast - The broadcast address for this networkntpsrv  - A list of NTP serverswins  - A list of WINS serverslease  - The lease time, in secondsdhcptype - DHCP message type (safely ignored)serverid - The IP of the servermessage  - Reason for a DHCPNAKtftp  - The TFTP server namebootfile - The bootfile nameadditional options are easily added in options.c.note on udhcpc's random seed
---------------------------udhcpc will seed its random number generator (used for generating xid's)
by reading /dev/urandom. If you have a lot of embedded systems on the same
network, with no entropy, you can either seed /dev/urandom by a method of
your own, or doing the following on startup:ifconfig eth0 > /dev/urandomin order to seed /dev/urandom with some data (mac address) unique to your
system. If reading /dev/urandom fails, udhcpc will fall back to its old
behavior of seeding with time(0).signals accepted by udhcpc
-------------------------udhcpc also responds to SIGUSR1 and SIGUSR2. SIGUSR1 will force a renew state,
and SIGUSR2 will force a release of the current lease, and cause udhcpc to
go into an inactive state (until it is killed, or receives a SIGUSR1). You do
not need to sleep between sending signals, as signals received are processed
sequencially in the order they are received.compile time options
-------------------options.c contains a set of dhcp options for the client:name[10]: The name of the option as it will appear in scriptsflags: The type of option, as well as if it will be requestedby the client (OPTION_REQ)code: The DHCP code for this option

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