redis service: Listen on localhost by default. Fixes #100192.

All other database servers in NixOS also use this safe-by-default setting.

nixos/doc/manual/release-notes/rl-2103.xml

@@ -159,6 +159,11 @@
      to <package>nextcloud20</package>.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The setting <xref linkend="opt-services.redis.bind" /> defaults to <literal>127.0.0.1</literal> now, making Redis listen on the loopback interface only, and not all public network interfaces.
+    </para>
+   </listitem>
    <listitem>
     <para>
      NixOS now emits a deprecation warning if systemd's <literal>StartLimitInterval</literal> setting is used in a <literal>serviceConfig</literal> section instead of in a <literal>unitConfig</literal>; that setting is deprecated and now undocumented for the service section by systemd upstream, but still effective and somewhat buggy there, which can be confusing. See <link xlink:href="https://github.com/NixOS/nixpkgs/issues/45785">#45785</link> for details.

nixos/modules/services/databases/redis.nix

@@ -87,9 +87,12 @@ in
 
       bind = mkOption {
         type = with types; nullOr str;
-        default = null; # All interfaces
-        description = "The IP interface to bind to.";
-        example = "127.0.0.1";
+        default = "127.0.0.1";
+        description = ''
+          The IP interface to bind to.
+          <literal>null</literal> means "all interfaces".
+        '';
+        example = "192.0.2.1";
       };
 
       unixSocket = mkOption {