udhcpc

One of the utilities bundled in BusyBox is the udhcpc DHCP client.  This is a tool according to the unix principle of: do one thing and do it well.  The udhcpc command handles the DHCP protocol as described in RFC 2131, but it doesn't actually configure the network based on the replies.

However, the documentation available from busybox is not very comprehensive.  There is a man-page available which can be found by googling udhcpc - very small DHCP client.

The operation of udhcpc is simple and unix-y.  When it receives a reply from a DHCP-server, it calls a script with one parameter, which is one of:

• deconfig: remove configuration (when lease is lost or udhcpc starts)
• bound: moving from unbound to bound state (receives configuration)
• renew: lease is renewed
• nak: nak received from server
• leasefail: (not documented in the man-page): run if there is no reply after configured timeouts and retries

lots of other configuration parameters available as environment variables.  The example scripts included with busybox implements these by configuring udhcpd to call a script which calls: simple.$1 (and of course there are 4 scripts: simple.deconfig, simple.bound, simple.renew, simple.nak).  Simple and easy!

I have basically used the sample scripts that came with BusyBox (examples/udhcpc/*), but converted them to using the ip(8)-command instead of ifconfig(8). I also added logging to syslog with the following function:

log() {
  logger -p daemon.info -t dhcp $*
}