post-up - Run command after bringing the interface up.

A quick note on post-up.

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

post-up /etc/network/if-up.d/
post-up /etc/network/if-up.d/
post-up /etc/network/if-up.d/
post-up /etc/network/if-up.d/ 

where post-up brings up a bif firewall , sets funny custom routing for AWMN with my exception :) sets an IPv6 tunnel and an IPv6 firewall

post-up --run a command once an if is up