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You may want to customize the behavior of the default DHCP client (udhcpc from busybox), which is called by /sbin/ifup by having "dhcp" in /etc/network/interfaces.

The default behavior is driven by the script /usr/share/udhcpc/default.script

Entries in /etc/network/interfaces for dhcp interfaces will drive the udhcpc command line. For example:

auto eth0
iface eth0 inet dhcp
        hostname myhostname

Will set these parameters on the command line:

-i eth0 -x hostname:myhostname

The hostname will send the DHCP option to the server to tell the server the name of this client.

The documentation resides Here

Its default configuration may be overwritten by /etc/udhcpc/udhcpc.conf

Authorized key:value pairs are:

key default value possible values
NO_GATEWAY - <list of iface names>
IF_METRIC - <metric value>
IF_PEER_DNS yes <anything but yes>
RESOLV_CONF /etc/resolv.conf no ; NO ; -
NO_DNS - <list of iface names>

Example /etc/udhcpc/udhcpc.conf:

RESOLV_CONF="no" # Prevents overwriting of /etc/resolv.conf

Custom scripts can be added as /etc/udhcpc/pre-* and /etc/udhcpc/post-* to be run before/after deconfig/renew/bound DHCP events - they must be marked as executable by root, e.g. chmod 744

As an example, /etc/udhcpc/post-bound/mtu could contain, to change the interface MTU from the default (1500) to 1492, which is useful if on ADSL (which uses 8 bytes):

r=$(/sbin/ip route | grep ^default | head -n 1)
# Needs iproute2 package, rather than busybox's "ip", to change mtu
/sbin/ip route replace $r mtu 1492

hack alert I needed to restart my firewall (which replaces the iptables script from Alpine) when the client binds to a new IP, so I added the following line in the function bound(), right after 'resolvconf':
/etc/init.d/iptables reload

The reload drops all firewall rules, re-acquires the Internal and external IPs, and re-writes the rules. I'm sure there is a better way.