18.4. Common IPTables Filtering

Preventing remote attackers from accessing a LAN is one of the most important aspects of network security. The integrity of a LAN should be protected from malicious remote users through the use of stringent firewall rules.
However, with a default policy set to block all incoming, outgoing, and forwarded packets, it is impossible for the firewall/gateway and internal LAN users to communicate with each other or with external resources.
To allow users to perform network-related functions and to use networking applications, administrators must open certain ports for communication.
For example, to allow access to port 80 on the firewall, append the following rule:
[root@myServer ~ ] # iptables -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
This allows users to browse websites that communicate using the standard port 80. To allow access to secure websites (for example, https://www.example.com/), you also need to provide access to port 443, as follows:
[root@myServer ~ ] # iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT

Important

When creating an iptables ruleset, order is important.
If a rule specifies that any packets from the 192.168.100.0/24 subnet be dropped, and this is followed by a rule that allows packets from 192.168.100.13 (which is within the dropped subnet), then the second rule is ignored.
The rule to allow packets from 192.168.100.13 must precede the rule that drops the remainder of the subnet.
To insert a rule in a specific location in an existing chain, use the -I option. For example:
[root@myServer ~ ] # iptables -I INPUT 1 -i lo -p all -j ACCEPT
This rule is inserted as the first rule in the INPUT chain to allow local loopback device traffic.
There may be times when you require remote access to the LAN. Secure services, for example SSH, can be used for encrypted remote connection to LAN services.
Administrators with PPP-based resources (such as modem banks or bulk ISP accounts), dial-up access can be used to securely circumvent firewall barriers. Because they are direct connections, modem connections are typically behind a firewall/gateway.
For remote users with broadband connections, however, special cases can be made. You can configure iptables to accept connections from remote SSH clients. For example, the following rules allow remote SSH access:
[root@myServer ~ ] # iptables -A INPUT -p tcp --dport 22 -j ACCEPT
[root@myServer ~ ] # iptables -A OUTPUT -p tcp --sport 22 -j ACCEPT
These rules allow incoming and outbound access for an individual system, such as a single PC directly connected to the Internet or a firewall/gateway. However, they do not allow nodes behind the firewall/gateway to access these services. To allow LAN access to these services, you can use Network Address Translation (NAT) with iptables filtering rules.