nixpkgs/nixos/doc/manual/configuration/firewall.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-firewall">

<title>Firewall</title>

<para>NixOS has a simple stateful firewall that blocks incoming
connections and other unexpected packets.  The firewall applies to
both IPv4 and IPv6 traffic. It is enabled by default. It can be
disabled as follows:

<programlisting>
<xref linkend="opt-networking.firewall.enable"/> = false;
</programlisting>

If the firewall is enabled, you can open specific TCP ports to the
outside world:

<programlisting>
<xref linkend="opt-networking.firewall.allowedTCPPorts"/> = [ 80 443 ];
</programlisting>

Note that TCP port 22 (ssh) is opened automatically if the SSH daemon
is enabled (<option><xref linkend="opt-services.openssh.enable"/> = true</option>). UDP
ports can be opened through
<xref linkend="opt-networking.firewall.allowedUDPPorts"/>.</para>

<para>To open ranges of TCP ports:

<programlisting>
<xref linkend="opt-networking.firewall.allowedTCPPortRanges"/> = [
  { from = 4000; to = 4007; }
  { from = 8000; to = 8010; }
];
</programlisting>

Similarly, UDP port ranges can be opened through
<xref linkend="opt-networking.firewall.allowedUDPPortRanges"/>.</para>

<para>Also of interest is

<programlisting>
<xref linkend="opt-networking.firewall.allowPing"/> = true;
</programlisting>

to allow the machine to respond to ping requests.  (ICMPv6 pings are
always allowed.)</para>

</section>