The client cannot reach a listening instance: name resolution, ports, protocols, SQL Server Browser and firewalls.
53
A network-related or instance-specific error (server not found)The client could not reach a listening SQL Server instance.
10061
TCP Provider: target machine actively refused itThe machine was reached but nothing accepted TCP connections on that port.
40615
Cannot open server requested by the login (Azure firewall)Azure SQL blocked the client IP address at the firewall.
26
Error locating server or instance specified (SQL Network Interfaces)The client could not resolve the named instance (SQL Server Browser, UDP 1434, wrong name).
40
Could not open a connection to SQL Server (Named Pipes Provider)The Named Pipes provider could not open the instance pipe; the Windows code after it says why.
233
No process is on the other end of the pipe (client error)The server closed the Shared Memory or Named Pipes connection during login.
10054
An existing connection was forcibly closed by the remote host (client error)The TCP connection was reset during the pre-login or TLS handshake.
10060
A connection attempt failed because the connected party did not properly respondThe TCP connection got no answer at all, usually a firewall dropping packets or a wrong address.
258
The wait operation timed out / execution timeout expired (client error)The client stopped waiting for a connection or a query (connection or command timeout).
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Other categories
- Constraint & Integrity 5
- Logins & Permissions 19
- Syntax & Object Resolution 22
- Data Type & Conversion 15
- Concurrency & Locking 2
- Transactions 5
- Log & Storage Space 3
- Backup & Restore 15
- Database State & Recovery 13
- Corruption & Consistency 8
- Memory 6
- Query Processing & Performance 7
- High Availability 8
- SQL Server Agent 4
- Linked Servers 7