Now, I realize that I can probably easily fix this problem by applying the following entries to our Firewall: ?ğTP server's ports > 1023 to remote ports > 1023 (Server sends ACKs (and data) to client's data port) ?ğTP server's ports > 1023 from anywhere (Client initiates data connection to random port specified by server) The client then initiates the connection from port N+1 to port P on the server to transfer data.įrom the server-side firewall's standpoint, to support passive mode FTP the following communication channels need to be opened: 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 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. ![]() When opening an FTP connection, the client opens two random unprivileged ports locally (N > 1023 and N+1). 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. 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 order to resolve the issue of the server initiating the connection to the client a different method for FTP connections was developed. From the client side firewall this appears to be an outside system initiating a connection to an internal client-something that is usually blocked. 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. ![]() The main problem with active mode FTP actually falls on the client side. ?ğTP server's port 20 from ports > 1023 (Client sends ACKs to server's data port) ?ğTP server's port 20 to ports > 1023 (Server initiates data connection to client's data port) ?ğTP server's port 21 to ports > 1023 (Server responds to client's control port) ?ğTP server's port 21 from anywhere (Client initiates connection) The server will then connect back to the client's specified data port from its local data port, which is port 20.įrom the server-side firewall's standpoint, to support active mode FTP the following communication channels need to be opened: Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. In active mode FTP the client connects from a random unprivileged port (N > 1023) to the FTP server's command port, port 21. Of course when using Passive FTP, it gets blocked by our firewall. 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. I am trying to setup an FTP server behind our Cisco 2811 Firewall to allow our clients to transfer files to our server. ![]() Ip nat inside source static public.ip route-map t1-mapĪccess-list 111 permit tcp any host public.ip eq ftpĪccess-list 111 permit tcp any host public.ip eq ftp-dataįrom Dos Prompt trying to connect to FTP Siteġ50 Opening ASCII mode data connection for file list.įrom Dos Prompt trying to connect to FTP Site with (quote PASV)Ģ27 Entering Passive Mode (public.ip,19,137). ![]() Partial configuration from Cisco 2811 router: Any help or direction would be greatly appreciated. Below is some config from my router and the problems when I am trying to connect to my ftp site using dos prompt. I am at loss now, and I am not sure where to start. I did search a lot for did issue, but all I find is exactly what I have: nat the local ip to public, then grant access to ftp port and ftp-data port on the public ip. I guess there are something missing with my firewall configurations, and it just keep blocking the service. However, if I try to list, transfer.data from/to my ftp server, it just hang on me. But If I disconnect the vpn, and try to connect to the ftp server using the public IP, I can login without problem. If I vpn in from the internet, I can connect, transfer data to my ftp server using the local ip without problems. The router has configuration with vpn, firewall. This server has a static local ip, and this local ip is Natted to a public IP on my router. I am trying to setup a ftp server using Windows XP Pro. And I really need your help or direction.
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