Set Up nftables Firewall on Alpine Linux

This is an original planning guide. It is not a vendor manual, an official standard, or legal advice. Example addresses use documentation ranges. Review every command before you paste it.

What this hour is for

How to Set Up nftables Firewall on Alpine Linux is one lab change on a Alpine Linux host you administer. nftables is one table and one chain. An allow for SSH from the admin prefix is loaded before the policy drops other new connections.

Alpine Linux, a small musl system with apk and OpenRC. It is not Ubuntu and not Debian. The network path on this host is: the interfaces file this host already uses, often /etc/network/interfaces. It is not netplan and not NetworkManager unless you already installed that. The firewall, if this change touches it, is nftables.

Alpine uses OpenRC, not systemd. rc-service and rc-update are the service tools. Do not paste a systemctl command and expect it to work. SELinux is not the Alpine default. Do not disable a confinement tool to make a package start. Do not load the ruleset from the only session. The console stays open. Do not use this host to block a network you do not administer. A new SSH session from the admin prefix still works, and the ruleset names that prefix.

The generator drafts a script. It does not log in. 192.0.2.10/24, gateway 192.0.2.1, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. example.com is a documentation name. They belong in the note. They are not values you leave on a host because a draft included them.

The change you can finish

Apply nothing while you are writing the note. When the lab window opens, it is one change on the host named in the ticket. Read the draft, then the host, then decide.

Rollback is the file, the package version, or the snapshot you saved first. You do not invent a rollback after the only copy is gone.

Write the hostname and confirm you administer it.
Save the current file, package list, or snapshot before any draft is applied.
Generate the draft and delete every line this ticket did not name.
Remove documentation addresses, example.com, and any secret.
Apply one change only when the lab window is open and a console or a second session exists.
Read the result back and compare it with the ticket.
Stop. The next change is a different page.

Mistakes that outlive the command

Set Up nftables Firewall on Alpine Linux goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.

Pasting a draft you did not read.
The wrong hostname.
A secret in the ticket or in git.
Two changes in one window.
No copy saved before the change.
A documentation address left on a host that is leaving the lab.
Disabling a firewall, AppArmor, or SELinux to make a check pass.
Calling the generator output a finished production change.
Do not load the ruleset from the only session. The console stays open. Do not use this host to block a network you do not administer.

Decisions to write down

One decision for Set Up nftables Firewall on Alpine Linux: The hostname matches. The change is only Set Up nftables Firewall. The platform habit was followed: Alpine Linux. A console or a second session exists. No secret is in the note. And the proof is: A new SSH session from the admin prefix still works, and the ruleset names that prefix.

Checks before you call it done

Confirm this before you call the change done: The hostname matches. A rollback copy exists. The draft was edited. Documentation values were removed before any apply. Management still works. And a second person can say what changed. If one of those fails, stop.

# Alpine Linux. Set Up nftables Firewall.
# apk. the interfaces file this host already uses, often /etc/network/interfaces. It is not netplan and not NetworkManager unless you already installed that.
# prompt-for-this is not a password.
# 192.0.2.10/24, gateway 192.0.2.1, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. example.com is a documentation name. They belong in the note. They are not values you leave on a host because a draft included them.

Before you leave the host

How to Set Up nftables Firewall on Alpine Linux ends when the hostname in the prompt matches the hostname in the ticket. If it does not, you are on the wrong host. Stop. A command that succeeded on the wrong host is not progress.

The draft stays a draft until a person has read the names in it. A generator does not log into the host and does not know which disk is empty. You delete a line that names a host, a path, or an account you did not write down.

192.0.2.10/24, gateway 192.0.2.1, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. example.com is a documentation name. They belong in the note. They are not values you leave on a host because a draft included them.

prompt-for-this is a reminder that a secret is missing. It is not a password, a key, or a token you save in the ticket or in git. The real secret stays in the password store.

One change is the window. If two things move and the result is wrong, you will not know which one did it. The next change waits for its own page.

If the same step fails twice, stop for the day. The failure is the result. Adding a forum command beside a half-applied change produces a host nobody can rebuild. Restore the copy you took first.

Tools for this guide

These tools run in the browser on this site. They do not log into your machines.

Alpine Linux nftables Firewall Setup Script Generator

Alpine Linux Server Administration Guide

More in this cluster

The pillar for this page, and the next published pages filed with it.

Companion: Security: Linux Server Hardening Checklist.