SQL Server connection errors have a different message for each cause. The tests here trigger seven of them against a local SQL Server 2025 instance, show what the client says, what the ERRORLOG records, and how to find the port an instance listens on.
Comparing Session SET Options Between SSMS and the Application
Fast in SSMS, slow in the app? The query may be fine and the connection settings different. Here is how to check, with nothing to set up.
MultiSubnetFailover: Why Clients Time Out After an AG Failover
Diagnose an AG failover timeout with listener DNS, MultiSubnetFailover, RegisterAllProvidersIP, HostRecordTTL, and client-driver checks.
Diagnosing Connection Pool Exhaustion From the SQL Server Side
Diagnose connection pool exhaustion by grouping SQL Server sessions, finding sleeping open transactions, and reading their last command.
Connecting to SQL Server From Linux
Connecting to SQL Server from Linux requires the right Microsoft ODBC packages, a Kerberos plan, and certificate trust that matches the server name.
Diagnosing a Frozen Server With the Dedicated Admin Connection
The dedicated admin connection provides a narrow emergency path to a stalled SQL Server; prepare remote access and keep diagnostics light.
Reading a Network Trace of a SQL Server Connection
Read a network trace of a SQL Server connection by separating TCP, pre-login, TLS, login, and first-query delays before tuning queries.