anchor.diff:    cfg = config.networking.nat;
   
     dest = if cfg.externalIP == null then "-j MASQUERADE" else "-j SNAT --to-source ${cfg.externalIP}";
  +
  +  externalInterfaceFilter = param:
  +    optionalString (cfg.externalInterface != null) "${param} ${cfg.externalInterface}";
   
     flushNat = ''
       iptables -w -t nat -D PREROUTING -j nixos-nat-pre 2>/dev/null|| true
  
positive.code:     # NAT the marked packets.
      ${optionalString (cfg.internalInterfaces != []) ''
        iptables -w -t nat -A nixos-nat-post -m mark --mark 1 \
          -o ${cfg.externalInterface} ${dest}
      ''}
  
      # NAT packets coming from the internal IPs.
      ${concatMapStrings (range: ''
        iptables -w -t nat -A nixos-nat-post \
          -s '${range}' -o ${cfg.externalInterface} ${dest}
      '') cfg.internalIPs}
  
      # NAT from external ports to internal ports.
      ${concatMapStrings (fwd: ''
        iptables -w -t nat -A nixos-nat-pre \
          -i ${cfg.externalInterface} -p tcp \
          --dport ${builtins.toString fwd.sourcePort} \
          -j DNAT --to-destination ${fwd.destination}
      '') cfg.forwardPorts}
  
negative.code:     };
  
      networking.nat.externalInterface = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "eth1";
        description =
          ''
  