How do I open port 25 (or any other port)?

How do I open a port on my Linode? I'm currently trying to open port 25.

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Allowing traffic to either enter or leave your Linode is done with iptables. If you are unfamiliar with iptables then check out the following guide as it covers iptables in detail and will show you how to open or close specific ports:

https://linode.com/docs/security/firewalls/control-network-traffic-with-iptables/#block-or-allow-traffic-by-port-number-to-create-an-iptables-firewall

The following iptables rule should help you get started:

sudo iptables -I INPUT -p tcp -m tcp --dport 25 -j ACCEPT

You can also use ufw as an alternative to using iptables. UFW is covered here:

https://linode.com/docs/security/firewalls/configure-firewall-with-ufw/

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