Hi,

I actually have this same problem. I am trying to setup an FTP server behind our Cisco 2811 Firewall to allow our clients to transfer files to our server. The problem with using Active FTP is that if the client who is trying to connect to the FTP server is also behind a firewall, the connection will be blocked by their firewall. Of course when using Passive FTP, it gets blocked by our firewall.

Active FTP

In active mode FTP the client connects from a random unprivileged port (N > 1023) to the FTP server's command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20.

From the server-side firewall's standpoint, to support active mode FTP the following communication channels need to be opened:

? FTP server's port 21 from anywhere (Client initiates connection)

? FTP server's port 21 to ports > 1023 (Server responds to client's control port)

? FTP server's port 20 to ports > 1023 (Server initiates data connection to client's data port)

? FTP server's port 20 from ports > 1023 (Client sends ACKs to server's data port)

The main problem with active mode FTP actually falls on the client side. The FTP client doesn't make the actual connection to the data port of the server--it simply tells the server what port it is listening on and the server connects back to the specified port on the client. From the client side firewall this appears to be an outside system initiating a connection to an internal client--something that is usually blocked.

Passive FTP

In order to resolve the issue of the server initiating the connection to the client a different method for FTP connections was developed. This was known as passive mode, or PASV, after the command used by the client to tell the server it is in passive mode.

In passive mode FTP the client initiates both connections to the server, solving the problem of firewalls filtering the incoming data port connection to the client from the server. When opening an FTP connection, the client opens two random unprivileged ports locally (N > 1023 and N+1). The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command. The result of this is that the server then opens a random unprivileged port (P > 1023) and sends the PORT P command back to the client. The client then initiates the connection from port N+1 to port P on the server to transfer data.

From the server-side firewall's standpoint, to support passive mode FTP the following communication channels need to be opened:

? FTP server's port 21 from anywhere (Client initiates connection)

? FTP server's port 21 to ports > 1023 (Server responds to client's control port)

? FTP server's ports > 1023 from anywhere (Client initiates data connection to random port specified by server)

? FTP server's ports > 1023 to remote ports > 1023 (Server sends ACKs (and data) to client's data port)

Now, I realize that I can probably easily fix this problem by applying the following entries to our Firewall:

Permit tcp any host (External IP) eq ftp

Permit tcp any host (External IP) gt 1023

But the problem is this opens up thousands of ports to everyone. Is there way to avoid having to do this with the IOS Firewall?