Why Does My TCP Server Show 'Unable to Read Data from Transport Connection,' and How Can I Fix It?

Troubleshooting "Unable to Read Data from Transport Connection" in TCP Servers

TCP servers sometimes encounter the error "Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host" when clients attempt to connect. This article explores common causes and provides a solution.

The error, often appearing around line 96 (in the provided server code example) during sr.ReadLine(), frequently stems from transport-level security misconfigurations.

TLS Handshake and Security Protocol Compatibility

A mismatch in TLS protocol versions between the client and server during the SSL/TLS handshake is a primary cause of this error. The System.Net.ServicePointManager.SecurityProtocol property is key to resolving this.

While .NET typically auto-negotiates TLS versions, inconsistencies between client and server capabilities can lead to handshake failure and the error message.

Solution: Explicitly Setting Security Protocols

To ensure compatibility, explicitly define the supported TLS versions using the SecurityProtocol property. Add this line before establishing the connection:

System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

This code snippet enables support for TLS 1.0, 1.1, and 1.2. Matching TLS versions on both client and server guarantees a successful handshake, allowing the server to read data from the client without interruption.

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