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I am running FileZilla FTP Server with passive mode enabled, and due to firewall constraints I have to limit the passive mode port range to only 6 ports.

How would FileZilla handle any situation where more than 6 concurrent FTP connections are active and want to upload a file?

Would it queue the connections and prioritise them in a first in, first out manner?

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This isn't how TCP works. There is not a one-to-one relationship between ports and active connections. TCP sockets are uniquely identified by the combination of the following attributes:

  • source IP
  • source port
  • destination IP
  • destination port

Think about a web server. All requests to a web server hit either port 80 or 443, and they are able to serve thousands of concurrent requests without issue.

So, aside from the fact that you are even using FTP to begin with (which you shouldn't be using), you have nothing to worry about.

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I see. I was under the impression that the FTP server only opened a passive data channel upon request (via the PASV command), served a single data transfer, then closed the channel immediately. Why else would it use a range of arbitrary ports instead of a single dedicated port if this was not the case? – Phil K Dec 13 '12 at 14:06
I'm not sure this is the case. A web server accepts the request, forks (or just passes the connection to another waiting process or thread) and goes back to listening to pick up the next connection. This is how it can handle numerous connections on the same port. An FTP server only listens on one of the passive ports when a transfer is requested. A port is chosen from the range, it listens and accepts the connection. I believe you may be limiting simultaneous data transfers to the number of passive ports in the range. – USD Matt Dec 14 '12 at 16:42

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