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。
下面引用网友的一篇文章：
udhcpc 的配置记录
udhcpc scripts
Here's some debian scripts I wrote for udhcpc 0.9.6. I wanted to make it easy to run local scripts for DHCP changes so I created /etc/udhcpc.d. All the /usr/share/udhcpc/default.* scripts have been merged into /etc/udhcpc.d/S50default. S99local is an example script that patches resolv.conf and calls a DNS update script.
/etc/init.d/udhcpc
/etc/udhcpc.script
/etc/udhcpc.d/S50default
/etc/udhcpc.d/S99local
Last modified: Mon Feb 4 10:23:49 PST 2002
单调的分割线
http://udhcp.busybox.net/README.udhcpc
udhcp client (udhcpc)
--------------------
The udhcp client negotiates a lease with the DHCP server and notifies
a set of scripts when a leases is obtained or lost.
command line options
-------------------
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
