My friend called me the other day and asked me to help him troubleshoot the FTP server which runs in his office LAN. The problem is that the outside FTP user is able to connect the FTP server and pass the authentication, but just can not download the files from the server.
OK. It sounds to me like a typical FTP mode relavent issue. Then we had the following conversation:
Jackie: Which FTP mode do you use? Active or Passive.
Friend: I am not quite sure. But I can tell you that I am using FileZilla.
Jackie: OK, which ports did you open to the Internet on your firewall?
Friend: 20 and 21.
Jackie: en, that’s the default ports. Is the outside FTP user behind a firewall?
Friend: I guess so. As he is working at the client side where should have the firewall protection.
Jackie: Yes, it should be. Now I suspect the FTP data transfer connection must be blocked by the client’s firewall. This is a typical FTP issue. Check the Filezilla and see if there are any passive mode settings.
Friend: Wait a minute. Let me check …… Yeah, there is one.
Jackie: Good. What does it allow you to do? say customize ports number.
Friend: Yes, there is a configuration named ‘Use Custom Port Range’.
Jackie: Nice, here is what you need to do. First, customize the port. For example, make it as 4000. Second, open that port to Internet on your firewall. Then let the user try it again.
Friend: Hold on, let me try ………………… Yes, it is working! Why does this work?
Jackie: Congratulations! Want to know how it works? Check out the next post on my blog