Overriding udchcp flags used by ifup in BusyBox

Having the same problem, did not want to recompile busybox and wanted to use those flags: "-t 0 -b" to let udhcpc try forever in background, but could not avoid flag "-n" that is passed by default by ifup.

As a super-hack (but it worked for me) I used the following options for udhcpc_opts in /etc/network/interfaces:

udhcpc_opts -t 0 -T 10 -A 20 -S &

The final "&" did the trick, as it launchs udhcpc as a background task and is almost the same of the "-b" flag, but works also if the "-n" is specified in the command line.

Note, that it has to be added to iface option to work, eg:

iface eth0 inet dhcp
    udhcpc_opts -t 0 -T 10 -A 20 -S &

Behavior of udhcpc -n -b has been corrected to behave like udhcpc -b in a fix for busybox bug #11691.

Thus, starting with busybox-1.31 (yet to be released as of now), udhcpc_opts -b should just work as expected.