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Support for Passive FTP

Article Number: KB 1178

Product: Web Hosting

Audience:eNom Reseller, eNom Central, Bulk Register

Problem

Do you support Passive FTP? What range of ports is it supported on? How does active mode FTP work versus passive mode FTP?

Solution

Our web hosting servers support passive mode FTP on the port range 5500-6000. Passive mode is useful if the client computer is behind a firewall that blocks incoming connections. If connecting in passive mode, you should set the FTP client to connect to port 21, as normal.

Overview of Active Mode FTP

In active mode, the FTP client opens an unprivileged port locally (N > 1023), and connects to the server on its command port, 21. The FTP client then starts listening on port N+1. On the command connection, the client sends a PORT command with N+1, the port on which it's listening. The server then opens a connection from port 20, its data port, to port N+1 on the client. Data can then be moved back and forth on this connection.

Example: The client opens a connection from port 4357 to the server's command port, 21. The client then starts listening on port 4358. From the client's port 4357 to the server's port 21, the client sends a PORT command containing the port on which the client is listening, 4358. The server then initiates a connection from port 20, its data port, to the client's port 4358.

Overview of Passive Mode FTP

In passive mode, the FTP client opens two unprivileged ports locally (N > 1023 and N+1). From the first port, N, the FTP client sends a PASV command to the FTP server on its command port, 21. The server then opens a random unprivileged port (between 5500 to 6000, inclusive, on our servers). The server then sends a PORT command, containing the random unprivileged port, from port 21 to port N on the client. The client then connects from N+1 to the random unprivileged port on the server. Data can then be moved back and forth on this connection.

Example: The client opens two unprivileged ports locally (4357 and 4358). From port 4357, the client sends a PASV command to the FTP server on its command port, 21. The server then opens the port 5587. The server sends a PORT command, containing port 5587, from port 21 to port 4357 on the client. The client then connects from 4358 to port 5587 on the server.

More Information

For directions on using FTP on web hosting accounts, please refer to article, Upload Files to Web Hosting Account.

Citation: http://slacksite.com/other/ftp.html.

Last Updated: October, 2015

 

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