MySQL Bugs: #40530: Agent cannot connect to DB with hostname=localhost

Bug #40530	Agent cannot connect to DB with hostname=localhost
Submitted:	5 Nov 2008 15:01	Modified:	21 Nov 2008 17:31
Reporter:	Andrii Nikitin
Status:	Closed
Category:	Monitoring: Agent	Severity:	S3 (Non-critical)
Version:	1.x, 2.0.0.7092	OS:	Any (UNIX)
Assigned to:	Kay Roepke	Target Version:	2.0 RC-02
Tags:	kay_merlin_immediate
Triage:	Needs Triage: D2 (Serious)

Description:
I believe that agent uses same connection rules which described here:
http://dev.mysql.com/doc/refman/5.0/en/connecting.html

"
On Unix, MySQL programs treat the hostname localhost specially, in a way that is likely
different from what you expect compared to other network-based programs. For connections
to localhost, MySQL programs attempt to connect to the local server by using a Unix
socket file
"

So, if 'localhost' is specified in configuration agent fails with error:

(critical) the MySQL server could not be reached at socket '(null)', we will check in 10
seconds

That is probably useful feature but it should be mentioned in documentation also.

How to repeat:
Configure in agent ini file hostname=localhost (maybe it is necessary to not have socket
file in default location).

It gets fixed with either changing localhost to 127.0.0.1 or adding
socket='path_to_socket' parameter

Suggested fix:
Document this feature in agent installation guide and troubleshooting.

Due to the overwhelming number of support cases related to this behavior we should really
consider fixing it or making it easy for a customer to work around.

The agent will now regard 'localhost' as '127.0.0.1', differing to mysqld's usage of
localhost.

A log message will be printed if this is detected:
2008-11-12 15:06:22: (message) agent-inst-config.c:153: [mysqld].hostname was
'localhost', setting to '127.0.0.1' to prevent connection over UNIX socket. If you want
to connect via the socket, specify [mysqld].socket directly.

tested OK in 2.0.0.7100

An entry was added to the 2.0 changelog:

The Agent could not connect to a database with the hostname set to localhost. Doing so
resulted in the error:

(critical) the MySQL server could not be reached at socket '(null)', we will check in 10
seconds