Iptables setup masquerading for Linux firewall

Posted on in Categories last updated July 15, 2006

Q. How do I setup masquerading for my Linux iptables firewall? What rules I need to apply or write?

A. Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains. However, to setup masquerading you need to use special target called MASQUERADE.
This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynamically assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT target.

You need to type following rule (or add to your script):

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

Also set net.ipv4.ip_forward to 1:

net.ipv4.ip_forward=1

See this previous article for more step by step instructions.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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