iface eth0 inet static : define um endereço IP estático para eth0
iface eth0 inet manual : para criar uma interface de rede sem um endereço IP. Normalmente usado por interfaces que são membros de ponte ou agregação, ou tem um dispositivo de VLAN configurado neles
Para mais informações, leia:
Dê uma olhada no arquivo /usr/share/doc/ifupdown/examples/network-interfaces.gz , você pode saber mais sobre manual e alguns casos para usar:
# Set up an interface to read all the traffic on the network. This
# configuration can be useful to setup Network Intrusion Detection
# sensors in 'stealth'-type configuration. This prevents the NIDS
# system to be a direct target in a hostile network since they have
# no IP address on the network. Notice, however, that there have been
# known bugs over time in sensors part of NIDS (for example see
# DSA-297 related to Snort) and remote buffer overflows might even be
# triggered by network packet processing.
#
# auto eth0
# iface eth0 inet manual
# up ifconfig $IFACE 0.0.0.0 up
# up ip link set $IFACE promisc on
# down ip link set $IFACE promisc off
# down ifconfig $IFACE down
# Set up an interface which will not be allocated an IP address by
# ifupdown but will be configured through external programs. This
# can be useful to setup interfaces configured through other programs,
# like, for example, PPPOE scripts.
#
# auto eth0
# iface eth0 inet manual
# up ifconfig $IFACE 0.0.0.0 up
# up /usr/local/bin/myconfigscript
# down ifconfig $IFACE down