udhcpc in the Busybox

udhcpc [-Cfbnqtvo] [-c CID] [-V VCLS] [-H HOSTNAME] [-i INTERFACE]
[-p pidfile] [-r IP] [-s script] [-O dhcp-option]...



        -V,--vendorclass=CLASSID        Vendor class identifier
        -i,--interface=INTERFACE        Interface to use (default eth0)
        -H,-h,--hostname=HOSTNAME       Client hostname
        -c,--clientid=CLIENTID  Client identifier
        -C,--clientid-none      Suppress default client identifier
        -p,--pidfile=file       Create pidfile
        -r,--request=IP         IP address to request
        -s,--script=file        Run file at DHCP events (default /usr/share/udhcpc/default.script)
        -t,--retries=N          Send up to N request packets
        -T,--timeout=N          Try to get a lease for N seconds (default 3)
        -A,--tryagain=N         Wait N seconds (default 20) after failure
        -O,--request-option=OPT Request DHCP option OPT (cumulative)
        -o,--no-default-options Do not request any options (unless -O is also given)
        -f,--foreground Run in foreground
        -b,--background Background if lease is not immediately obtained
        -S,--syslog     Log to syslog too
        -n,--now        Exit with failure if lease is not immediately obtained
        -q,--quit       Quit after obtaining lease
        -R,--release    Release IP on quit
        -a,--arping     Use arping to validate offered address

1. renew the DHCP lease
kill -SIGUSR1 `cat /var/run/udhcpc.pid`

2. release current release
kill -SIGUSR2 `cat /var/run/udhcpc.pid`

reference from BusyBox 應用 – udhcpc