Generally Useful Iptables Rules

This section includes a variety of iptables commands that will create rules that are generally useful on most servers.

Allow Loopback Connections

To accept all traffic on your loopback interface, run these commands:

sudo iptables -A INPUT -i lo -j ACCEPT sudo iptables -A OUTPUT -o lo -j ACCEPT

Allow Established and Related Incoming Connections

sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

Allow Established Outgoing Connections

sudo iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED -j ACCEPT

Internal to External

sudo iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

Drop Invalid Packets

sudo iptables -A INPUT -m conntrack --ctstate INVALID -j DROP

Block an IP Address

sudo iptables -A INPUT -s 15.15.15.51 -j DROP

In this example, -s 15.15.15.51 specifies a source IP address of "15.15.15.51". The source IP address can be specified in any firewall rule, including an allow rule.

If you want to reject the connection instead, which will respond to the connection request with a "connection refused" error, replace "DROP" with "REJECT" like this:

sudo iptables -A INPUT -s 15.15.15.51 -j REJECT

Block Connections to a Network Interface
iptables -A INPUT -i eth0 -s 15.15.15.51 -j DROP

This is the same as the previous example, with the addition of -i eth0. The network interface can be specified in any firewall rule, and is a great way to limit the rule to a particular network.

Service: SSH
Allow All Incoming SSH

To allow all incoming SSH connections run these commands:

sudo iptables -A INPUT -p tcp --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

The second command, which allows the outgoing traffic of established SSH connections, is only necessary if the OUTPUT policy is not set to ACCEPT.
Allow Incoming SSH from Specific IP address or subnet

sudo iptables -A INPUT -p tcp -s 15.15.15.0/24 --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

The second command, which allows the outgoing traffic of established SSH connections, is only necessary if the OUTPUT policy is not set to ACCEPT.
Allow Outgoing SSH

sudo iptables -A OUTPUT -p tcp --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A INPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow Incoming Rsync from Specific IP Address or Subnet

sudo iptables -A INPUT -p tcp -s 15.15.15.0/24 --dport 873 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 873 -m conntrack --ctstate ESTABLISHED -j ACCEPT

The second command, which allows the outgoing traffic of established rsync connections, is only necessary if the OUTPUT policy is not set to ACCEPT.

Service: Web Server

Allow All Incoming HTTP

sudo iptables -A INPUT -p tcp --dport 80 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 80 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming HTTPS

sudo iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 443 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming HTTP and HTTPS

sudo iptables -A INPUT -p tcp -m multiport --dports 80,443 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Service: MySQL
Allow MySQL from Specific IP Address or Subnet

sudo iptables -A INPUT -p tcp -s 15.15.15.0/24 --dport 3306 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 3306 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow MySQL to Specific Network Interface

sudo iptables -A INPUT -i eth1 -p tcp --dport 3306 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -o eth1 -p tcp --sport 3306 -m conntrack --ctstate ESTABLISHED -j ACCEPT

PostgreSQL from Specific IP Address or Subnet

sudo iptables -A INPUT -p tcp -s 15.15.15.0/24 --dport 5432 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 5432 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow PostgreSQL to Specific Network Interface

sudo iptables -A INPUT -i eth1 -p tcp --dport 5432 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -o eth1 -p tcp --sport 5432 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Service: Mail
Block Outgoing SMTP Mail

sudo iptables -A OUTPUT -p tcp --dport 25 -j REJECT

Allow All Incoming SMTP

sudo iptables -A INPUT -p tcp --dport 25 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 25 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Note: It is common for SMTP servers to use port 587 for outbound mail.
Allow All Incoming IMAP

sudo iptables -A INPUT -p tcp --dport 143 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 143 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming IMAPS

sudo iptables -A INPUT -p tcp --dport 993 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 993 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming POP3

sudo iptables -A INPUT -p tcp --dport 110 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 110 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming POP3S

sudo iptables -A INPUT -p tcp --dport 995 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 995 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Good luck!