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Brian Park
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How to Read IP Tables

17 Jan 2014

iptables is an administrative tool for packet filtering and NAT.

In order to see the details of your iptables, use the following commands:

$ iptables-save

The output will look like this:

...
-A INPUT -p tcp -m multiport --dports 22 -j fail2ban-ssh
-A FORWARD -i docker0 -o docker0 -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A fail2ban-ssh -j RETURN
...

Lines starting with -A are important. Each line represents a rule that a packet will pass through. Depending on what the rule says, the packet may be ACCEPTed, DROPped or REJECTed. Since the packet passes through the rules from top to bottom, the ordering of the rules are important.

There are three main chains of rules that a packet goes through:

  • INPUT chain holds rules for incoming packet
  • FORWARD chain holds rules for packets that are forwarded to an IP behind the server
  • OUTPUT chain holds rules for outgoing packet

There can be many more than three chains mentioned above but those three are the most essential ones.

Let's take a look at an example line and see what each option does. -A INPUT -p tcp -m multiport --dports 22 -j fail2ban-ssh

  • -A INPUT: appends a rule to INPUT chain
  • -p tcp: only checks the packet with TCP
  • -m multiport: uses multiport module. iptables can use extended packet matching modules. See man iptables for detailed explanation of what each module does.
  • --dports 22: only checks the packet with the destination port 22 (ssh)
  • -j fail2ban-ssh: jumps to fail2ban-ssh chain. -j option specifies the target of the rule. The target could be a user-defined chain (as in this example) or special built-in targets (e.g. ACCEPT, DROP, REJECT) which determines the fate of the packet.

Let's take another example: -A fail2ban-ssh -j RETURN

  • -A fail2ban-ssh: appends a rule to a user-defined chain, 'fail2ban-ssh'
  • -j RETURN: goes back to the rule that sent the packet to this rule (via -j option)

By following iptables rules like this, you will be able to determine how network packets will behave.

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