Connection to SQL Server Works Sometimes

It turned out that TCP/IP was enabled for the IPv4 address, but not for the IPv6 address, of THESERVER.

Apparently some connection attempts ended up using IPv4 and others used IPv6.

Enabling TCP/IP for both IP versions resolved the issue.

The fact that SSMS worked turned out to be coincidental (the first few attempts presumably used IPv4). Some later attempts to connect through SSMS resulted in the same error message.

To enable TCP/IP for additional IP addresses:

• Start Sql Server Configuration Manager
• Open the node SQL Server Network Configuration
• Left-click Protocols for MYSQLINSTANCE
• In the right-hand pane, right-click TCP/IP
• Click Properties
• Select the IP Addresses tab
• For each listed IP address, ensure Active and Enabled are both Yes.

Ive had the same error just come up which aligned suspiciously with the latest round of Microsoft updates (09/02/2016). I found that SSMS connected without issue while my ASP.NET application returned the "timeout period elapsed while attempting to consume the pre-login handshake acknowledgement" error

The solution for me was to add a connection timeout of 30 seconds into the connection string eg:

ConnectionString="Data Source=xyz;Initial Catalog=xyz;Integrated Security=True;Connection Timeout=30;"

In my situation the only affected connection was one that was using integrated Security and I was impersonating a user before connecting, other connections to the same server using SQL Authentication worked fine!

2 test systems (separate clients and Sql servers) were affected at the same time leading me to suspect a microsoft update!