SHOW TABLES - Blocked for external users?

[Jörg Radomski]

Hi,
we are running a MySQL-Server V5.5.31-0 on a LINUX-Server which is included in an university LAN and visible worldwide with a static IP-address. There are internal (within the university IP area/network) and external (outside the university network) database users.

Starting within the last two weeks our external users have problems with accessing our databases. We did no changes at our server recently but we don't know yet if there were any "global" changes within the university network.

Strangely enough only the SHOW TABLES command doesn't work. There's no answer from the SQL-server to the client. The user can connect to a database and run other commands like SELECT, SHOW DATABASES, SHOW FULL TABLES etc. Also the SHOW TABLES command will reach the server since it's logged in the server's logfile, but at the client side the client program hangs and waits forever for a response.

We verified this case with different types of client programs from different external locations. Internally everything still works fine with the same programs.

Actually our guess is that there migth be some new misconfigured firewall or similar mechanisms within the university network which are blocking the response of our SQL-server. But how can it block only one command? Any other ideas?

We are a little bit helpless and are looking forward your ideas. Thanks in advance for your help!

Regards
Joerg Radomski