The underlying connection was closed: An unexpected error occurred on a send

操作系统是Windows Server 2003 x64 SP2，使用Framework 4.0，在使用WebClient访问某些特定的HTTPS站点时，会引发异常：

Unhandled Exception: System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Authentication failed because the remote party has closed the transport stream. at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result) at System.Net.TlsStream.Write(Byte[] buffer, Int32 offset, Int32 size) at System.Net.PooledStream.Write(Byte[] buffer, Int32 offset, Int32 size) at System.Net.ConnectStream.WriteHeaders(Boolean async) --- End of inner exception stack trace ---

The underlying connection was closed: An unexpected error occured on a send.
基础连接已关闭，发送时发生错误

从信息上看是ssl认证失败，此错误并不是所有https站点都出现，另外报错的站点，用浏览器或是wget一类的工具访问都正常。挑了一个报异常的网站，查看一下连接信息（用的firefox）：




使用了比较新的TLS1.2，开始怀疑操作系统不支持，于是开始翻微软网站，找到一个页面：
https://support.microsoft.com/en-us/help/948963/an-update-is-available-to-add-support-for-the-tls-rsa-with-aes-128-cbc-sha-aes128-sha-and-tls-rsa-with-aes-256-cbc-sha-aes256-sha-aes-cipher-suites-in-windows-server-2003

确认了Server 2003确实不支持，需要打个hotfix，之后再尝试就一切正常了。有遇到相同问题的同学可以参考一下。