On NixOS, our general firewall rules apply with a slight deviation: access is limited by default and can be enabled on a per-case basis.

You are free to open any port you like on the frontend network (ethfe) which will be accessible to the outside world. The server-to-server network is only accessible in a limited way from the outside and freely to the machines in the same project.

Adding custom configuration

To add firewall rules, you can place configuration files in /etc/local/firewall/*. Upon the next config activation all files placed there will be concatenated and placed in a late stage of the firewall configuration.

The files are shell scripts and are intended to be very simple. We check that all lines are either:

  • comments (starting with #)

  • invocations of an iptables command (iptables, ip6tables, ip46tables)

Here’s an example:

# Enable port 1234 to be accessed on the frontend network via
# IPv4 and IPv6
ip46tables -A nixos-fw -i ethfe -p tcp --dport 1234 -j nixos-fw-accept


NixOS has a few special firewall chains that support good re-use of reject/logging and others. You should definitely use the nixos-fw chain instead of the regular INPUT chain to avoid unpredictable behaviour.

After making changes to the firewall configuration, either wait for the agent to apply it or run sudo fc-manage --build.

How to verify

Service users may list currently active firewall rules with sudo iptables -L, e.g.:

iptables -L -nv    # show IPv4 firewall rules w/o DNS resolution
ip6tables -L -nv   # show IPv6 firewall rules w/o DNS resolution

If the intended rules do not show up, check the system journal for possible syntax errors in /etc/local/firewall and re-run fc-manage --build.