Telnet 1433 on SQL Server Failing

Question

I work with a number of SQL servers. Some are clusters and some are standalone. Some of the standalone servers run multiple instances and some do not. What I've found is that any SQL server I have running will not respond to a telnet on 1433 unless they are a standalone server with a single instance running. I've disabled the firewall, verified port configuration in the SQL configuration tool, verified remote connectivity is enabled, and checking netstat shows the server is listening on the proper port.

I have no issues connecting to any of the servers with the SQL Management Studio. Why is telneting to these servers on 1433 fail? Is there some configuration I'm missing?

Solution

SQL Server doesn't have to listen on port 1433. In fact, if you're using a named instance, it doesn't by default, it listens on a dynamic port and the connection with SSMS (or any external connection really) is negotiated through the SQL Server Browser Service. If you have more than one instance on a box, only one can listen on 1433 anyway.

I'm going to guess that the servers that you can telnet to 1433 on run default instances, or have 1433 hardcoded in the SQL Server Configuration Management console where "dynamic" port selection would be the default.

Comments

[Untalented]

Netstat shows the server is listening to 1433. I've verified dynamic ports are set to 0 and the port it's set to use is hardcoded to 1433. I did this on a server running two instances and not a cluster just for testing purposes. I still was unable to telnet to it.

[Untalented]

Yes, all local firewalls are off and there is no firewall between the endpoints.

[hachi]

Some versions of MS SQL act weird if you haven't set a static port on all interfaces (all IPs). Verify that on IP Adresses tab in TCP/IP properties for the instance.