[DBNETLIB][ConnectionOpen (Connect()).]SQL Server does not exist or access denied Error

Had a totally weird thing ahppen to a sql server today.

I have 2 linked servers and for some unknown reason all queries to the linked server were being met with that error. I removed the linked server, re-added, tested i could connect to the port with telnet - all good.

Check remote connections enabled, checked tcpip enabled. Evrything seemed fine.

One of our machines rebooted with some critical update from Microsoft so wondered if something MS had pushed out may have caused the issue.

Anyhow, we finally fixed the issue but i am still 100% sure exactly what is the problem so maybe someone may be able to shed some light on things.

The way got around it was to remove the liked server and re-add it - but instead of adding it as a SQL Server, it was added using and OLEDB sql data source. After doing that it all sprung back into life.

Now to me that sounds like an issue with SQL Native Client on either the client or server. I wonder if microsoft has done something to some drivers somewhere along the line and pushed it out in a critical update???

Another strange this is i could not add a system datasource to point to the other server either - unless i specified the default port 1433! Totally whacky. The server i was trying to link was a named instance of sql2005. No idea why specifying the port it should have been using anyway worked - but it did!

Hope this sorts you out if you are having this problem. Had me stumped for about 3 hours.

Id be interested to hear your comments if you have an ideas of input. Its gonna bug me until i found out exactly whats happened as it was working fine the night before.