This article demonstrates how to configure a basic firewall using iptables. Using the iptables program, you can explicitly grant and deny access to selected services running on your server, as well as to selected IP addresses.
The iptables program enables you to view and modify the Linux kernel's built-in network packet filtering capabilities. You can grant or deny access to specific network services (such as SSH, HTTP, and so on), as well as permit or block specific IP addresses from connecting to the server.
To do this, you define sets of rules, which are grouped together into chains. By default, iptables uses three chains: INPUT (for incoming packets), FORWARD (for forwarding packets), and OUTPUT (for outgoing packets). In this article we will only work with the INPUT chain to selectively block and accept incoming packets to the server.
The iptables program is included in most major Linux distributions by default, including Debian, Ubuntu, CentOS and Fedora.
By default, iptables does not have any rules defined. You can verify this yourself on a new server by typing the following command:
Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination
As you can see, there are no targets and no destinations defined. So let's add some basic rules. At the command prompt, type the following commands:
iptables -A INPUT -i lo -j ACCEPT iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT iptables -A INPUT -p tcp -m tcp --dport 7822 -j ACCEPT iptables -A INPUT -j DROP
In all of these commands, the -A option instructs iptables to append the rule to the end of the specified chain (in this case, the INPUT chain). Let's step through each command:
Now if you type the iptables -L command, you should see the following output:
Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere tcp dpt:7822 DROP all -- anywhere anywhere Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination
To test the configuration, try connecting to the server using SSH. It should allow you to connect. Connections on any other ports, however (such as an HTTP connection on port 80) will be rejected.
The set of rules we defined above is pretty limited. If SSH is the only incoming connection you want to allow, then you're all set. Most likely, though, you will need to add access to services as you configure your server.
However, if we just add a rule using the -A option shown above, it will be the last rule in the chain, right after our DROP rule. Because iptables works through rules in sequence, this means that it will never get to the new rule, because the packet will have already been dropped. Therefore, we need a way to insert new rules into the chain.
The -I option enables us to insert a new rule anywhere in the chain. Let's insert a rule that allows incoming TCP connections on port 80 (HTTP). We want the rule to come just before the DROP rule, which is currently the fourth rule in the chain:
iptables -I INPUT 4 -p tcp -m tcp --dport 80 -j ACCEPT
This inserts our HTTP rule in the fourth line, and pushes the DROP rule down to the fifth line. Now if you type the iptables -L command, you should see the following output:
Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere tcp dpt:7822 ACCEPT tcp -- anywhere anywhere tcp dpt:http DROP all -- anywhere anywhere Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination
To quickly view the line numbers for all of the rules in a chain, type the following command:
iptables -L --line-numbers
The rules above define access by service (SSH, HTTP, etc.). However, you can also set rules that permit or block specific IP addresses.
For example, suppose you find in your server log files that there are repeated SSH login attempts from a particular IP address. To block all subsequent SSH connections from the IP address, type the following command. Replace rulenum with the rule number in the chain, and replace xxx.xxx.xxx.xxx with the IP address to block:
iptables -I INPUT rulenum -s xxx.xxx.xxx.xxx -p tcp -m tcp --dport 7822 -j DROP
To block all traffic from an IP address regardless of the service requested, type the following command:
iptables -I INPUT rulenum -s xxx.xxx.xxx.xxx -j DROP
To delete a rule, use the -D option. You need to know the number of the rule you want to delete (just as you must know the number when you insert a rule). The following command demonstrates how to delete the fifth rule from the INPUT chain:
iptables -D INPUT 5
If you want to delete all of the rules at once, type the following command:
If you reboot the server now, all of the rules you defined will be erased. To maintain rules across system restarts, you must save them. The steps to do this depend on the Linux distribution you are running.
To save the iptables rules on a server running Debian or Ubuntu, follow these steps:
apt-get install iptables-persistent
At the Save current IPv6 rules? prompt, press Tab, and then press Enter.
Steps 2 to 3 only appear once during initial package installation. If you make any subsequent modifications to iptables rules, type the following command to save them:
iptables-save > /etc/iptables/rules.v4
To save the iptables rules on a server running CentOS or Fedora, type the following command:
/sbin/service iptables save
This article is only a brief introduction to some of iptables' capabilities. For more information about iptables, type the following command at the command prompt:
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