最近在做專案的時候卡在了dhcp處，當沒有網路的時候，板子一直髮送dhcp請求，導致程式不往下執行，解決的辦法是把它切換到後臺執行，可是如何切換到後臺呢，有辦法，它自帶引數可以實現該功能。如下：

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

解釋一下，-b就是切換到後臺指令，-i是指定使用哪個網路介面，雙網絡卡的時候一定要使用它來指定eth0  or  eth1。

【最近新新增 -R】

對於 -R引數，其實也很重要，在關閉udhcpc時，可以向dhcpserver傳送release取消租約。

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 be
                                immediately negotiated.
-i, --interface=INTERFACE       Interface to use (default: eth0)
-n, --now                       Exit with failure if lease cannot be
                                immediately 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 version

 

If 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, and
 when a leases is lost. The script should put the interface in an
 up, but deconfigured state, ie: ifconfig $interface 0.0.0.0.
 
 bound: This argument is used when udhcpc moves from an
 unbound, to a bound state. All of the paramaters are set in
 enviromental 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 of
 the paramaters are set in enviromental variables. This argument is
 used when the interface is already configured, so the IP address,
 will not change, however, the other DHCP paramaters, such as the
 default 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 directly
 afterwards, so no changes to the network interface are neccessary.
 This hook is provided for purely informational purposes (the
 message 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 perform
 interface - The interface this was obtained on
 ip  - The obtained IP
 siaddr  - The bootp next server option
 sname  - The bootp server name option
 boot_file - The bootp boot file option
 subnet  - The assigend subnet mask
 timezone - Offset in seconds from UTC
 router  - A list of routers
 timesvr  - A list of time servers
 namesvr  - A list of IEN 116 name servers
 dns  - A list of DNS server
 logsvr  - A list of MIT-LCS UDP log servers
 cookiesvr - A list of RFC 865 cookie servers
 lprsvr  - A list of LPR servers
 hostname - The assigned hostname
 bootsize - The length in 512 octect blocks of the bootfile
 domain  - The domain name of the network
 swapsvr  - The IP address of the client's swap server
 rootpath - The path name of the client's root disk
 ipttl  - The TTL to use for this network
 mtu  - The MTU to use for this network
 broadcast - The broadcast address for this network
 ntpsrv  - A list of NTP servers
 wins  - A list of WINS servers
 lease  - The lease time, in seconds
 dhcptype - DHCP message type (safely ignored)
 serverid - The IP of the server
 message  - Reason for a DHCPNAK
 tftp  - The TFTP server name
 bootfile - The bootfile name

additional 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/urandom

in 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 scripts
 
 flags: The type of option, as well as if it will be requested
 by the client (OPTION_REQ)

 code: The DHCP code for this option