netfilter Operation: Part Four

netfilter operationPermitting Firewall Traffic

In the previous article, we set up netfilter with a default policy of DENY. Now that we have done that, we want to make sure that management traffic is permitted to the firewall itself. This is done first, because once you have enabled the firewall with a default policy of DENY, you will not be able to manage the firewall remotely until you have configured the firewall rules to permit the management traffic. This traffic is processed against the INPUT chain, because the destination is the netfilter host itself. To allow secure shell (SSH) connections to the firewall, use the following command:

iptables -A INPUT -p tcp -s –dport 22 -j ACCEPT

In this example, you are appending (-A) a rule to the INPUT chain to allow traffic from the network to a destination port of TCP 22. With no other configurations, all other traffic through or to the firewall would be dropped. This will show up in the rule listing as follows:

iptables -L INPUT
Chain INPUT (policy DROP)

Target prot opt source destination
ACCEPT tcp — anywhere tcp dpt:ssh

Although the aforementioned rules will permit the inbound SSH session, there is currently no rule to permit the reply traffic for the SSH session. If you were to change the default policy for the OUTPUT chain to ACCEPT, this would permit the reply packet, but we will instead address this more securely in the next few examples.

If you wanted to allow access to the firewall with a destination of TCP port 80, you could use the same syntax with -A to append the rule, which would put the new rule for port 80 after the rule for port 22. You could also use -I for insert, as in the iptables -I INPUT 1 -p tcp -s -dport 80 -j ACCEPT command. This would insert the new rule in the INPUT chain as rule #1, meaning the rule for port 80 would come before the rule for port 22. Remember, this is still permitting only half the conversation; you still need to permit the outbound reply packets. It is sometimes useful to list the chains with rule numbers using the iptables -L (or –line-numbers) command.

For outbound traffic (i.e. traffic generated by the firewall), you need to create rules in the OUTPUT chain. To enable syslog traffic from the firewall to a remote syslog server (, you would enter the following:

iptables -A OUTPUT -p udb -d –dport 514

This assumes you are using the default UDP syslog port of 514. Because syslog over UDP is a one-way conversation, you will not need to permit any inbound replies for permitted traffic that you allowed inbound in the preceding examples. You could create rules to permit SSH and HTTP specifically, but there is also a way to permit all traffic that is a reply to a permitted session. You can enter:

iptables -A OUTPUT -m state –state RELATED, ESTABLISHED -j ACCEPT

This will instruct netfilter to permit any outbound traffic that is part of an established session (ESTABLISHED). The RELATED keyword is similar, but is for traffic that is part of a different session, but where the session is related to an established session. Some protocols will open additional ports (such as FTP) as part of their normal behavior. For those that netfilter understands, it can see the request for the additional port and permit that new session.

External Links:

Simple Firewall Configuration Using NetFilter/iptables at

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