anchor.diff:              iptables -t nat -F POSTROUTING
             '';
         };
  -
     };
  -
   }
  
positive.code:     };
  
      networking.nat.internalIPs = mkOption {
        example = "192.168.1.0/24";
        description =
          ''
            The IP address range for which to perform NAT.  Packets
            coming from these addresses and destined for the external
            interface will be rewritten.
          '';
      };
  
negative.code:           ''
              iptables -t nat -F POSTROUTING
              iptables -t nat -X
            '' 
            + (concatMapStrings (network: 
              ''
              iptables -t nat -A POSTROUTING \
                -s ${network} -o ${cfg.externalInterface} \
                ${if cfg.externalIP == ""
                  then "-j MASQUERADE"
                  else "-j SNAT --to-source ${cfg.externalIP}"}
              ''
            ) cfg.internalIPs) +
            ''
              echo 1 > /proc/sys/net/ipv4/ip_forward
            '';
  
  