FTP access stuck on FEAT command most of the time

Question

Most of the times when I try to FTP to my VPS the log looks something like this:

27/06/2011 22:08:06  Stat> Connected.
27/06/2011 22:08:06  Recv> 220 Microsoft FTP Service
27/06/2011 22:08:06  Sent> USER myuser
27/06/2011 22:08:06  Recv> 331 Password required for myuser.
27/06/2011 22:08:06  Sent> PASS ********
27/06/2011 22:08:06  Recv> 230 User logged in.
27/06/2011 22:08:06  Sent> FEAT
27/06/2011 22:08:07  Recv> 211-Extended features supported:
27/06/2011 22:08:26  Stat> Disconnected.

The connection is stuck on the line before the last one for half a minute and then disconnects.

Sometimes it succeeds in completing the login - in these cases right after the "Extended features supported" line indeed appears a feature list. But most of the time it simply gets stuck on that line (and sometimes 2-3 features appear and then it's stuck).

I tried various software, but the problem seems consistent.

Is there a way to workaround this?
How can I diagnose it?
What would be a probable cause?

Answer 1

Many firewalls and NAT routers scan FTP connections for PORT/PASV commands in order to adjust the firewall rules, and sometimes they have bugs.

If possible, use FTPS (FTP with SSL) or even SFTP (SSH file transfer). (Supported by WinSCP and FileZilla clients.)