anchor.diff:      # NAT the marked packets.
       ${optionalString (cfg.internalInterfaces != []) ''
         iptables -w -t nat -A nixos-nat-post -m mark --mark 1 \
  -        ${externalInterfaceFilter "-o"} ${dest}
  +        -o ${cfg.externalInterface} ${dest}
       ''}
   
       # NAT packets coming from the internal IPs.
       ${concatMapStrings (range: ''
         iptables -w -t nat -A nixos-nat-post \
  -        -s '${range}' \! -d '${range}'
  -        ${externalInterfaceFilter "-o"} ${dest}
  +        -s '${range}' -o ${cfg.externalInterface} ${dest}
       '') cfg.internalIPs}
   
       # NAT from external ports to internal ports.
       ${concatMapStrings (fwd: ''
         iptables -w -t nat -A nixos-nat-pre \
  -        ${externalInterfaceFilter "-i"} -p tcp \
  +        -i ${cfg.externalInterface} -p tcp \
           --dport ${builtins.toString fwd.sourcePort} \
           -j DNAT --to-destination ${fwd.destination}
       '') cfg.forwardPorts}
  
positive.code:   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
  
negative.code:     };
  
      networking.nat.externalInterface = mkOption {
        type = types.str;
        example = "eth1";
        description =
          ''
  