[Wlug] Client could not connect using FTP in a network which the server using ipchains.

Friends, can you help me here, I've got a problem, I 'm implementing ipchains in my gateway server, so, all the thing I've done were masquerading all the clients connection, so, port 20,21,22,23,25,110,80,53 (ftp data,ftp, ssh,telnet,mail, http, and dns are masqueraded. My clients got no problem using the internet for http, mail access, but they get problem on ftp, they could not connect, the error was " ftp: connect :Unknown error number" can you please tell me whic other port that I should to masquerade in order to enable ftp access from my client, Thanks Aramico