Error: (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

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)

How to fix SQL Network Interfaces, error: 26?

First of all, you get this error message only if you are trying to connect to a SQL Server named instance. For default instance, you never see this. Why? Because even if we failed at this stage (i.e. error locating server/instance specified), we will continue to try connect using default values, e.g defaul TCP port 1433, default pipe name for Named Pipes. You may see other error message due to failure later, but not this error message.

Every time client makes a connection to SQL Server named instance, we will send a SSRP UDP packet to the server machine UDP port 1434. We need this step to know configuration information of the SQL instance, e.g., protocols enabled, TCP port, pipe name etc. Without these information, client does know how to connect the server and it fails with this specified error message.

In a word, the reason that we get this error message is the client stack could not receive SSRP response UDP packet from SQL Browser. It's easy to isolate the issue. Here are the steps:
1) Make sure your server name is correct, e.g., no typo on the name.
2) Make sure your instance name is correct and there is actually such an instance on your target machine. [Update: Some application converts \\ to \. If you are not sure about your application, please try both Server\Instance and Server\\Instance in your connection string]
3) Make sure the server machine is reachable, e.g, DNS can be resolve correctly, you are able to ping the server (not always true).
4) Make sure SQL Browser service is running on the server.
5) If firewall is enabled on the server, you need to put sqlbrowser.exe and/or UDP port 1434 into exception.

Once you are done the steps, you should not see this error message anymore. You may still fail to connect your SQL server, but error message should be different and you have a different issue now. [Update: If it still fails, you may replace server\instance with tcp:server\instance and/or np:server\instance and see if it succeeds with either TCP or NP protocol. That way, you can isolate the issue a little bit. ]