How to use iptables to set up a firewall
Table of Contents
- Firewall programs that use iptables
- System permissions
Your operating system will likely to have iptables installed. Run
iptables --version to check it out. If it’s not installed, you can install it with your system package manager (APT, pacman, yum, etc.).
Firewall programs that use iptables
firewalld and other firewall programs use
iptables as their backend. Take into account that these programs can overwrite your
iptables rules, so if you don’t use them, you may want to uninstall them.
In order to use
iptables you need to run the commands as root or use
Listing firewall rules
iptables -L. You’ll see three main sections:
- Chain INPUT: this section applies to network packets that go from outside to your system.
- Chain OUTPUT: here you’ll see rules for packets that go from your system to outside.
- Chain FORWARD: this applies to packets forwarded for your system.
You can display only one chain by adding chain name at the end of the command:
iptables -L OUTPUT
# iptables -L 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
Firewall actions: ACCEPT, DROP, REJECT
When you create a firewall rule, you decide if a packet that matches that rule will be accepted (pass through), rejected (don’t pass and report the sender) or dropped (don’t pass and don’t report the sender).
Each network packet is analyzed to find if it matches a firewall rule in order, from top to bottom. When you run
iptables -L you’ll see the order in which the rules are checked. You can add the
--line-numbers parameter to also display rules position numbers.
Add a rule
-I <chain> [<rule number>] <rule>,
--insert <chain> [<rule number>] <rule>: insert a rule before another rule (add a rule number) or at the beginning if no rule number is defined.
-A <chain> <rule>,
--append <chain> <rule>: append, insert the rule at the end.
--jump <ACTION>: define what to do with the packet if it matches the rule (
--source <address>: source IP (or range). For example,
--destination <address>: destination IP (or range).
--protocol <protocol>: specify a network protocol (
--destination-port <port>: define a destination port. You must specify a protocol before.
--source-port <port>: define a source port. You must specify a protocol before.
These are some examples:
# Block access to port 80 iptables -A INPUT -p tcp --dport 80 -j DROP
# Allow access from one IP address iptables -A INPUT -s 22.214.171.124 -j ACCEPT
Modify the default policy
When you list firewall rules, next to the chain name you’ll find the default policy for that chain: that means when a network packet does not match any rule, will be accepted, rejected or dropped.
To modify this policy, run
iptables --policy <CHAIN> <ACTION>.
<ACTION> must be either
# Remember to add `INPUT` rules before to not block yourself. iptables --policy INPUT DROP
Delete a rule
-D <chain> <rule>, or
-D <chain> <rule number>.
- List rules and their numbers:
# iptables -L --line-numbers Chain INPUT (policy DROP) num target prot opt source destination 1 ACCEPT all -- 126.96.36.199 anywhere ...
- Remove rule number 1
# iptables -D INPUT 1
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