Manpage iptables http://unixhelp.ed.ac.uk/CGI/man-cgi?iptables+8
-o, --out-interface [!] name
Name of an interface via which a packet is going to be sent (for
packets entering the FORWARD, OUTPUT and POSTROUTING chains).
When the "!" argument is used before the interface name, the
sense is inverted. If the interface name ends in a "+", then
any interface which begins with this name will match. If this
option is omitted, any interface name will match.
P.S.: Match extensions for ports are:
This module matches a set of source or destination ports. Up to 15
ports can be specified. It can only be used in conjunction with -p tcp
or -p udp.
Match if the source port is one of the given ports. The flag
--sports is a convenient alias for this option.
Match if the destination port is one of the given ports. The
flag --dports is a convenient alias for this option.
Match if the both the source and destination ports are equal to
each other and to one of the given ports.