MDS on Linux sends SPN with instance name instead of port

[David-Engel]

Describe the bug

MDS' managed SNI implementation is sending the SPN in the format MSSQLSVC/SERVERNAME.FQDN:INSTANCENAME instead of MSSQLSVC/SERVERNAME.FQDN:PORT when making TCP connections to named instances.

To reproduce

Connect to a named instance via TCP, managed SNI, and Kerberos auth. Using a network capture, you can see MDS gets a ticket using the named instance instead of the port number.

Expected behavior

MDS should use the MSSQLSVC/SERVERNAME.FQDN:PORT format for consistency with other drivers and aligning with the docs:

User-specified SPNs are supported in client drivers. However, if an SPN isn't provided, it is generated automatically based on the type of a client connection. For a TCP connection, an SPN in the format MSSQLSvc/FQDN:[<port>] is used for both the named and default instances.

For named pipes and shared memory connections, an SPN in the format MSSQLSvc/<FQDN>:<instancename> is used for a named instance and MSSQLSvc/<FQDN> is used for the default instance.

Further technical details

Microsoft.Data.SqlClient version: latest 5.1.1
.NET target: .NET (non Framework)
SQL Server version: All
Operating system: Linux (or managed SNI on Windows)

[David-Engel]

Filing this from an internal issue.