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Network-related or instance-specific error - Connection Medium Problem Expand / Collapse
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Posted Thursday, November 19, 2009 6:33 AM
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Hi guys, I know there's hundreds of topic about this error out there, but not a single one seems to apply to my specific situation.

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)


We have three branches, 1 head office and two satellite branches. Our in-house developed application connects to a SQL Server 2005 instance running on a server with a live IP which the branches connect through and a local IP that the head office connects through i.e. app at branches connect through <live IP>\<instance name> and the head office connects through <local IP>\<instance name>. The application is exactly the same across all three branches except for the connection string used...it's exactly the same for both satellite branches though.

Head office has no problems connecting with the application. Branch A has no problems either. Branch B however does. Branch B's network is setup as follows...ADSL connection into a wireless/wired router that each user then gets internet access through. If anyone in Branch B connects to the internet through 3G or their cellphones on their PCs, then the application works. But not through the ADSL setup. However, to make it more interesting...one of the guys made an initial connection to the internet using his cellphone as the modem and successfully logged into the application...he then disconnected the internet from his cellphone and connected through a cable connection to the ADSL router and successfully managed to carry on utilising the application, performing actions that would have performed SQL queries to the server...and all ran successfully. But then he closed the app and opened it again (cable to router connection) and again login failed.

Please any ideas.
Post #821548
Posted Thursday, November 19, 2009 9:20 PM


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A network person I am not, however, this looks like a netwrok issue. Please remember, I consider everything from the network card in the desktop to the network card in the server part of the network. This includes the network cards and their drivers, and the IP stack of the OS.

If both branch offices use the same hardware, I would start by comparing the setups of the both.



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Post #822075
Posted Thursday, November 19, 2009 9:45 PM


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It definitely sounds like a networking issues between the branches. I'd double check the IP / network setup in all locations and be sure you don't have conflicts somewhere.

Also, I'd move away from IP to an FQDN. Then you can manage things with a single connection string and DNS name. Local settings can be made in the router or even client machine to override the settings for the IPs.







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Post #822081
Posted Thursday, November 19, 2009 10:38 PM


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Yes networking issue. It really seems like it could be routing table or dns related. You will need to check your routers, dns and adsl setup.



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Post #822102
Posted Friday, November 20, 2009 6:55 AM
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Thanks for the replies so far.

Compared setups etc, and everything checks out. Apart from one really interesting anomaly.

Company (A) which has no problems. When I try a tracert to the server IP it times out continuously. It pings the server IP however

Company (B) Which has the problems runs a succesful tracert and ping to the IP, yet they cannot connect.

What I cannot get my head around is that when someone connects via a 3G modem it logs him in fine, and then without closing the app, disconnects the 3G and connects back onto the network and then continues to use the app successfully. So it seems the only problem is during authentication. Does the logon procedure run on a different port than the default 1433? Or does something special happen during authentication?
Post #822350
Posted Friday, November 20, 2009 7:18 AM


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If you are using a named instance, you probably aren't connecting to port 1433. You may want to check the firewalls on the client sides as well.



Lynn Pettis

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Post #822375
Posted Friday, November 20, 2009 8:29 AM


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Good note, Ian and Lynn.

If I remember this, for a named instance, you'll connect with UDP 1434 and get the port that the instance is using, then you connect on TCP xxx







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