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.
e.g.
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
