anchor.diff:            destination = mkOption {
               type = types.str;
               example = "10.0.0.1:80";
  -            description = "Forward connection to destination ip:port; to specify a port range, use ip:start-end";
  +            description = "Forward connection to destination ip:port (or [ipv6]:port); to specify a port range, use ip:start-end";
             };
   
             proto = mkOption {
  
positive.code:         kernel.sysctl = {
            "net.ipv4.conf.all.forwarding" = mkOverride 99 true;
            "net.ipv4.conf.default.forwarding" = mkOverride 99 true;
          };
        };
  
  
negative.code:       description =
          ''
            The public IP address to which packets from the local
            network are to be rewritten.  If this is left empty, the
            IP address associated with the external interface will be
            used.
          '';
      };
  
      networking.nat.externalIPv6 = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "2001:dc0:2001:11::175";
        description =
          ''
            The public IPv6 address to which packets from the local
            network are to be rewritten.  If this is left empty, the
            IP address associated with the external interface will be
            used.
  