IPTABLES (firewall) with commandline

# iptables -t filter -L

show all chains of filtering table [man]

# iptables -t nat -L

show all chains of nat table [man]

# iptables -t filter -F

clear all rules from filtering table [man]

# iptables -t nat -F

clear all rules from table nat [man]

# iptables -t filter -X

delete any chains created by user [man]

# iptables -t filter -A INPUT -p tcp --dport telnet -j ACCEPT

allow telnet connections to input [man]

# iptables -t filter -A OUTPUT -p tcp --dport http -j DROP

block HTTP connections to output [man]

# iptables -t filter -A FORWARD -p tcp --dport pop3 -j ACCEPT

allow POP3 connections to forward chain [man]

# iptables -t filter -A INPUT -j LOG --log-prefix

Logging on input chain [man]

# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

configure a PAT (Port Address Traslation) on eth0 masking outbound packets [man]

# iptables -t nat -A PREROUTING -d 192.168.0.1 -p tcp -m tcp --dport 22 -j DNAT --to-destination 10.0.0.2:22

redirect packets addressed to a host to another host [man]