Notes from the Lab

Perfmon may show incorrect (lower) connections number for POP3 and IMAP4

June 23rd, 2006

There is an interesting Exchange performance counter bug.

When a client connects to the secure POP3/IMAP4 port and doesn’t do an SSL handshake (for instance, if you just do telnet to the port), Perfmon does not register the connection, i.e. Connections Current counter does not change. But when that connection is dropped, Perfmon registers that event, i.e. Connections Current counter value decreases by one. That causes the counter to show incorrect (lower or ridiculously large if drops below 0) connections number.

At the same time, connections number limit is not affected. For instance, if the limit is set to 100 and the server has 100 connections, the next connection attempt will fail even if the Connections Current counter shows 98 (for instance).

How to reproduce the problem

On the Exchange server watch the following performance counter: MSExchangePOP3(1)\Connections Current.
Initial Connections Current = 0

1. On a client machine, run telnet 110. Connections Current = 1.
2. On a client machine, run telnet 995. Connections Current = 1.
3. On a client machine, close the second telnet window. Connections Current = 0.
4. On a client machine, close the first telnet window. Connections Current = 4294967295.

This test was performed on Windows 2003/Exchange 2003, but I’m pretty sure Exchange 2000 has the same bug. Also, I suspect it affects OWA.

Entry Filed under: Exchange