post-up is a command that may be used to run programs after an interface is up and it may be put in the interfaces file ( /etc/network/interfaces )
post is useful in bringing up firewalls, setup tunnels and custom routing and it is usually put at the bottom of the interface stanza.
auto eth3 iface eth3 inet static address 192.0.2.154 netmask 255.255.255.240 network 192.0.2.144 broadcast 192.0.2.159 post-up /etc/network/if-up.d/bif.sh post-up /etc/network/if-up.d/all-routes-except-10.sh post-up /etc/network/if-up.d/sixxs-ipv6.sh post-up /etc/network/if-up.d/ipv6fw.sh
where post-up brings up a bif firewall , sets funny custom routing for AWMN with my 10.0.0.0/8 exception :) sets an IPv6 tunnel and an IPv6 firewall
post-up --run a command once an if is up