Question:
When we send the request, we get an error: "The underlying connection was closed: Unexpected error during transmission." What is the problem?
Answer:
Your client is trying to negotiate TLS 1.0 or TLS 1.1. The Pushwoosh API no longer accepts those protocol versions — only TLS 1.2 and TLS 1.3 are supported (TLS 1.0/1.1 were disabled in March 2020).
Older .NET Framework applications are the usual source of this error, because they negotiate the protocol enabled by default in the framework/OS instead of TLS 1.2.
To fix it:
- Enable TLS 1.2 (and, if available, TLS 1.3) on the machine that sends the request.
- Force the protocol in your code, for example in .NET:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
Useful references:
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