FTP blocked?

[Nepomuk]

Hi everybody!

I'm facing a problem right now - I'm supposed to write a program, which should download some data from a server and it should be run from computers within networks, which have FTP blocked, so I'll have to use an alternative - which would be HTTP.

Now my question is, what exactly is (might be) blocked, when FTP isn't allowed - is just Port 21 blocked or is it the actual protocol? Or are there various common ways of blocking FTP and if so, how do they work?

My thought was, that I could establish an FTP connection via the local Port 80 (HTTP does use the local port 80 normally, doesn't it?) and so use already available code.

Can anyone help?

Greetings,
Nepomuk

[docdiesel]

Hi,

ftp protocol is using two channels. The first one is port 21 for basic communication. If data is to be transfered (directory listing, files, etc.), this is to be sent via a second channel. Usually the ftp server is opening a channel back to the client (active ftp), but because these days clients usually are placed behind firewalls etc., one may open the 2nd channel from client to server alternatively (passive ftp).

Try passive ftp first before searching for other alternatives. It's just a setting in your ftp client program.

Regards, Bernd

[Nepomuk]

Thanks, but that doesn't solve the problem, because

1. I'm writing the program myself and
2. I have no possibility to check every single firewall (software AND hardware firewalls that is), that the clients may have installed and which may or may not block the Protocol itself (never mind the ports).

I can switch to passive mode of course, but still that might or might not work. So I have to try alternatives or know exactly, that no firewall will stop my ftp client from using port 80 (or any other port).

Greetings,
Nepomuk