Here are some convenient iptables rules.

This first list is for not allowing anything in, accept packets that come back from outgoing connections, complicated related traffic like FTP, everything from the localhost, ICMP (ping and stuff) and SSH. It also sets the default policy to DROP. This you would use on a machine connected directly to the internet. 

iptables -A INPUT --match state --state RELATED,ESTABLISHED -j ACCEPT --match comment --comment "Accept traffic from outgoing connections and stuff like FTP."
iptables -A INPUT -p icmp -j ACCEPT --match comment --comment "Allow ICMP"
iptables -A INPUT -p tcp --dport 22 -j ACCEPT --match comment --comment "Allow SSH"
iptables -A INPUT --in-interface lo -j ACCEPT --match comment --comment "Allow everything on the localhost"
iptables -P INPUT DROP
Here are some rules to allow certain MAC addresses to access everything. Simplifies things on a LAN (even though it’s not attacker-proof, it keeps unwanted people out of my SMB and stuff): 
iptables -A INPUT --match mac --mac-source xx:xx:xx:xx:xx:xx --match comment --comment "Allow everything from [computer]" -j ACCEPT

Besides computers you want to grant full access to a machine, don’t forget to include the MACs of the router and the machine’s own ethernet interface.

When the machine acts as a masquerading SNAT server, use this to forward ports to LAN hosts (be sure to have the –to after the -j): 

iptables -t nat -A PREROUTING --in-interface eth0 -p tcp --dport 80 -j DNAT --to 10.0.0.1:22 --match comment --comment "forwards incoming port 80 to port 22 on 10.0.0.1"

To allow everything for a samba server: 

iptables -A INPUT -p tcp --dport 139 -m comment --comment "Allow Netbios-ssn" -j ACCEPT 
iptables -A INPUT -p tcp --dport 445 -m comment --comment "Allow microsoft-ds" -j ACCEPT 
iptables -A INPUT -p udp --dport 137 -m comment --comment "Allow netbios-ns" -j ACCEPT 
iptables -A INPUT -p udp --dport 138 -m comment --comment "Allow netbios-dgm" -j ACCEPT