File Transport Protocol (FTP) is an old and complex multi-port protocol that presents a distinct set of challenges to an Load Balancer Add-On environment. To understand the nature of these challenges, you must first understand some key things about how FTP works.
With most other server client relationships, the client machine opens up a connection to the server on a particular port and the server then responds to the client on that port. When an FTP client connects to an FTP server it opens a connection to the FTP control port 21. Then the client tells the FTP server whether to establish an active or passive connection. The type of connection chosen by the client determines how the server responds and on what ports transactions will occur.
The two types of data connections are:
- Active Connections
When an active connection is established, the server opens a data connection to the client from port 20 to a high range port on the client machine. All data from the server is then passed over this connection.
- Passive Connections
When a passive connection is established, the client asks the FTP server to establish a passive connection port, which can be on any port higher than 10,000. The server then binds to this high-numbered port for this particular session and relays that port number back to the client. The client then opens the newly bound port for the data connection. Each data request the client makes results in a separate data connection. Most modern FTP clients attempt to establish a passive connection when requesting data from servers.
The client determines the type of connection, not the server. This means to effectively cluster FTP, you must configure the LVS routers to handle both active and passive connections.
The FTP client-server relationship can potentially open a large number of ports that the Piranha Configuration Tool and IPVS do not know about.