![]() Thus, to support this mode, not only should your server have to have multiple ports available, your firewall should also allow connections to all those ports to pass through! However, passive mode would make your system vulnerable to attacks because clients are supposed to connect to random server ports. So it would be best for you to configure server to support passive mode FTP. ![]() However, since the server is expected to receive a greater number of connection requests compared to a client, then it would be but logical for the server admin to adapt to the situation and open up a selection of ports to satisfy passive mode configurations. Of course, it's possible for the server side to have a firewall too. As a result, an attempt to connect to it will be blocked by the firewall and no connection will be established.Ī passive configuration will not have this problem since the client will be the one initiating the connection. So chances are, that port wouldn't be one of those predefined ports. In an active mode configuration, the server will attempt to connect to a random client-side port. I will use image to explain how the FTP works as an additional source for previous answer. I recently run into this question in my work place so I think I should say something more here. Once the client knows that, it can then successfully create the data channel and continue. ![]() However, it then requests the server (on the command channel) to start listening on a port (at the servers discretion) rather than trying to establish a connection back to the client.Īs part of this, the server also returns to the client the port number it has selected to listen on, so that the client knows how to connect to it. We already know it establishes the command channel in active mode and it does the same here. In passive mode, the client establishes both channels. This can actually be a problem if, for example, the client machine is protected by firewalls and will not allow unauthorised session requests from external parties. In active mode, the client establishes the command channel but the server is responsible for establishing the data channel. As per the RFC, this is only mandated for a subset of commands, such as quitting, aborting the current transfer, and getting the status. This separation of command information and data into separate channels a nifty way of being able to send commands to the server without having to wait for the current data transfer to finish. The command channel is for commands and responses while the data channel is for actually transferring files. Active and passive are the two modes that FTP can run in.įor background, FTP actually uses two channels between client and server, the command and data channels, which are actually separate TCP connections.
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