SQL Server "A network-related or instance-specific error" (error 40, error 26): how to fix it

1. Is the service running?

On the server:

Get-Service *SQL*

Status   Name               DisplayName
------   ----               -----------
Stopped  MSSQLSERVER        SQL Server (MSSQLSERVER)
Stopped  SQLBrowser         SQL Server Browser

MSSQLSERVER is the default instance; a named one is MSSQL$SQLEXPRESS. If it’s stopped, start it:

Start-Service MSSQLSERVER

If it won’t start, read the error log, usually C:\Program Files\Microsoft SQL Server\MSSQL16.MSSQLSERVER\MSSQL\Log\ERRORLOG (the folder name depends on the version and instance), and the Windows Application log.

2. Is the instance name right?

A default installation of SQL Server Express is a named instance. So db01 fails with error 26 or 40, and db01\SQLEXPRESS is what you want. To see what’s installed:

Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\SQL'

3. Is TCP/IP enabled?

It’s off by default in Express and Developer editions, which only allow local connections out of the box.

Open SQL Server Configuration Manager → SQL Server Network Configuration → Protocols for [instance], enable TCP/IP, and restart the service. Under the IP addresses tab, the IPAll section shows the port: TCP Port for a fixed one, TCP Dynamic Ports for one chosen at startup.

4. Which port does it listen on?

A default instance uses 1433. Named instances use a dynamic port unless you set one. Ask the server:

Get-NetTCPConnection -State Listen -OwningProcess (Get-Process sqlservr).Id | Select-Object LocalAddress, LocalPort

SQL Server also writes it to its error log at startup, in a line starting with Server is listening on. Over a TCP connection you can ask it too (a local shared memory connection shows NULL here):

SELECT local_net_address, local_tcp_port FROM sys.dm_exec_connections WHERE session_id = @@SPID;

A named instance needs one of these two things:

• The SQL Server Browser service running, and UDP port 1434 open. It tells clients which port the instance uses. This is what error 26 usually points to.
• Or a fixed port in Configuration Manager, and the port in the connection string: db01,1500. With a port, the Browser isn’t needed. This is the better choice on servers.

5. The firewall

Test from the client:

Test-NetConnection -ComputerName db01 -Port 1433

TcpTestSucceeded : False while the service is running means a firewall: Windows Firewall on the server, or a cloud security group. On the server, allow only the networks that need it:

New-NetFirewallRule -DisplayName 'SQL Server' -Direction Inbound -Protocol TCP -LocalPort 1433 -RemoteAddress 10.0.0.0/24 -Action Allow

For a named instance with the Browser, also open UDP 1434. Timeouts and refusals tell different stories: see “Connection timed out” vs “Connection refused”.

6. Remote connections and DNS

• Allow remote connections must be on: in SSMS, server Properties → Connections. It’s on by default.
• The name must resolve: try the IP address. If the IP works and the name doesn’t, it’s DNS: see Temporary failure in name resolution.

7. On Linux and in Docker

There’s no Configuration Manager on Linux, and TCP is on by default. Check the service and the port, and connect with an explicit port:

sudo systemctl status mssql-server
sudo ss -tlnp | grep 1433
sqlcmd -S 127.0.0.1,1433 -U sa

In Docker, publish the port (-p 1433:1433), and connect to the service name between containers. If the container exits right after it starts, read docker logs: an MSSQL_SA_PASSWORD that’s too weak is the usual reason.