Brief: FTP Connection Problem

[aa.]

ftp protocol is using port 21 and 20

A. FTP-COMMAND/CONTROL:
ftp-client: port=R (random) connect to ftp-server: port=21

B. There are two modes for FTP-DATA: active mode and passive mode

1. Active mode:
ftp-server: port=20 connect to ftp-client: port=R+1

problem at client behind firewall:
connection to port R+1 above will be blocked
no available solution

problem at server+firewall
outbond connection request may be limited to certain ports only
solution: server port 20 MUST be allowed to connect TO any/arbitrary port

2. Passive mode:
ftp-client: port=R+1 connect to ftp-server: port=X (also random)

problem at client behind firewall:
outbond connection request may be limited to certain ports only
solution: client MUST be allowed to connect FROM and TO any/arbitrary port

problem at server+firewall
connection to port X above will be blocked
no available solution