Martians are simply packets arriving on impossible interfaces. Private
ip-addresses like 192.168.x.x should not appear on the Internet i.e. the
input of the external interface of a firewall, nor should 192.168.x.x
appear as output on the external interface.
Crackers may attempt to forge the source address of their packets, so
they look like packets originating from a private address range (typical
LAN ips in an office). These packets may be routed from Internet to
your LAN if not blocked by the firewall.
Modern operating systems have built in protection. Linux for instance,
has /proc/sys/net/ipv4/conf/eth0 set to 1 for this protection (0 if you
want to turn it off). In this case, you're already protected. If you
turn it off, you've to implement the protection by firewall rules.
Hope this helps :-)
