iptables cheat sheet

Here are a few handy commands for using iptables. They are tailored for an OpenVZ container with a venet network interface but can easily be adapted to use your interface by replacing venet0 with your network interface.

For setting up iptables in an openVZ this blog post.

Remove existing rules

You can easily delete all existing rules in iptables. Be careful using this command, there is no going back unless you have backed up your rules.

iptables -F

Backup and restore

Backup to file

Rules can easily be saved to an external file for backups or outputting for version control. This will save the rules to /etc/iptables.rules.

iptables-save -c > /etc/iptables.rules

Restore from file

Saved settings can be restored with the following command:

iptables-restore > /etc/iptables.rules

Change the default policy

The default policy can be changed to specify what should happen to traffic which doesn’t have a rule to explicitly define what to do. You can specify to ACCEPT, REJECT or DROP for INPUT, FORWARD and OUTPUT.

Change OUTPUT to ACCEPT

iptables -P OUTPUT ACCEPT

Change INPUT to DROP

iptables -P INPUT DROP

Apply the catchall rule

You can add a rule to the bottom of the rule book to choose what to do with traffic which doesn’t match any other rule. A common use would be to add a DROP as the last rule to drop any traffic which isn’t explicitly allowed by an earlier rule. You can specify to ACCEPT, REJECT or DROP for INPUT, FORWARD and OUTPUT as well as an interface.

ACCEPT INPUT on interface lo

iptables -A INPUT -i lo -j ACCEPT

BLOCK INPUT on interface venet0

iptables -A INPUT -i venet0 -j DROP

List active rules

You can list the active rules with -L, and -v for information on packets affected.

iptables -L -v

Enable established connections rule

Already established connections will not be affected by adding this inbound rule. Traffic affected by other outbound rules will also be honoured. If you add this rule, you won’t likely need to specify inbound rules for many outbound rules.

iptables -A INPUT -i venet0 -m state --state RELATED,ESTABLISHED -j ACCEPT

Adding new rules

New rules can be added to control how traffic passes through an interface. If you have not used the Established connections rule then you will also need to add an outgoing rule. If you do use the Established connections rule then you will only need the top command in each of the below headings.

Add rule for port 80 – such as Apache

iptables -A INPUT -i venet0 -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -o venet0 -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT

Add rule for port 22 – SSH outbound connections

iptables -A OUTPUT -o venet0 -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -A INPUT -i venet0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT

Add rule for 53 – DNS outbound

iptables -A OUTPUT -o venet0 -p udp -o eth0 --dport 53 -j ACCEPT
iptables -A INPUT -i venet0-p udp -i eth0 --sport 53 -j ACCEPT

Add rule for port 22 – SSH inbound connections

iptables -A INPUT -i venet0 -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -o venet0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT

Add rule for outgoing http/ https

iptables -A OUTPUT -o venet0 -p tcp -m multiport --dport 80,443 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -A INPUT -i venet0 -p tcp -m multiport --sports 80,443 -m state --state ESTABLISHED -j ACCEPT

Add rule for ping from remote to local

iptables -A INPUT -i venet0 -p icmp --icmp-type echo-request -j ACCEPT
iptables -A OUTPUT -o ventet -p icmp --icmp-type echo-reply -j ACCEPT

 Add rule for ping from local to remote

iptables -A OUTPUT -o venet0 -p icmp --icmp-type echo-request -j ACCEPT
iptables -A INPUT -i venet0 -p icmp --icmp-type echo-reply -j ACCEPT

Delete rule

The easiest way to remove a rule is to delete it by it’s rule position in the list. To find out the rules position run iptables -L with the line-number argument. The below command is to delete an INPUT command however you can easily change INPUT to OUTPUT as required.

iptables -L INPUT --line-numbers

Then run the delete command for the relevent direction. This will delete the 7th inbound rule.

iptables -D INPUT 7

Example deleting rule 1 for INPUT:

# iptables -L INPUT --line-numbers
Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh state NEW,ESTABLISHED
2    ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
3    ACCEPT     icmp --  anywhere             anywhere             icmp echo-reply
4    ACCEPT     icmp --  anywhere             anywhere             icmp echo-request

# iptables -D INPUT 1

# iptables -L INPUT --line-numbers
Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
2    ACCEPT     icmp --  anywhere             anywhere             icmp echo-reply
3    ACCEPT     icmp --  anywhere             anywhere             icmp echo-request

 

Let me know in the comments if you think anything is missing.