WebRequest: The underlying connection was closed

Question

My app has to download one certain file, here is the url. The app worked fine for years bur after a recent change on provider side I'm getting 'The underlying connection was closed: An unexpected error occurred on a send.'

I have read all related info in net but none of suggested fixes works for me.

Important note: the code works OK with Net Framework 4.6 but I need it to work with 3.5.

Any ideas?

Here is the code:

        var url = new Uri(@"http://www.ezv.admin.ch/pdf_linker.php?doc=edecReceiptResponse_stylesheet_v_3_0");

        var request = (HttpWebRequest)WebRequest.Create(url);
        request.KeepAlive = false;
        request.AllowAutoRedirect = true;
        request.UserAgent = "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36";
        request.ProtocolVersion = HttpVersion.Version10;

update: the stack:

System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Received an unexpected EOF or 0 bytes from the transport stream.
   at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count)
   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.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)
   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)

one more update: checked with Fiddler the app connecting to httpS and compared responses received with net 4.6 (works) with net 3.5 (fails):

4.6 receives two responses, 5670 + 664785 bytes
3.5 also receives two responses, 7 bytes each

Either `www.ezv.admin.ch` closes the connection or you have communication problems (network problems / firewalls). — Cleptus, Apr 24 '17 at 13:46
Show the HTTP request/response from Fiddler or Wireshark too please. — Eugene Komisarenko, Apr 24 '17 at 13:47
**@Eugene Komisarenko**, it looks as there is no responses from the server, the server just closes the connection. Fiddler shows only the request. — sergio, Apr 24 '17 at 13:55
Maybe you are dealing with newer version of security protocol, try this setting http://stackoverflow.com/a/12244894/920557 — Eugene Komisarenko, Apr 24 '17 at 14:11
So if the server keeps sending you to HTTPS instead of HTTP, why not just change the URL to access HTTPS and skip needing to do the redirect in the first place? — mason, Apr 24 '17 at 14:17
**@Eugene Komisarenko**, I have already seen and tried this, no help — sergio, Apr 24 '17 at 14:17
**@mason**, https makes no difference, the resuly is exactly the same. Please see also my last update — sergio, Apr 24 '17 at 14:29
Unfortunately you are dealing with the limitation of the .NET Framework 3.5, see my answer below. — Eugene Komisarenko, Apr 24 '17 at 14:35
**@Eugene Komisarenko**, that sounds reasonable. The only thing is that the provider tends always inform changes and see no TLS mentioned on their site. In what way I can check what protocol 4.6 version uses? — sergio, Apr 24 '17 at 14:53
You can test it using the following app https://www.ssllabs.com/ssltest/analyze.html?d=www.ezv.admin.ch — Eugene Komisarenko, Apr 24 '17 at 16:18

Eugene Komisarenko · Accepted Answer · 2017-04-25T11:00:22.120

As per conducted test here are the protocols supported by www.ezv.admin.ch.

Protocols
TLS 1.2 Yes
TLS 1.1 No
TLS 1.0 No
SSL 3   No
SSL 2   No

The server side upgrade has most likely upgraded the version of the security protocol to be exclusively TLS 1.2.

As per the following article .NET Framework 3.5 does not support this version and the only options you have is to upgrade your client libraries or patch them, see below.

.NET 3.5 or below. TLS 1.2 is not supported (*) and there is no workaround. Upgrade your application to more recent version of the framework.

Here is some more information of how to patch your client to add support.

P.P.S. As Christian Pop from Microsoft mentioned below, there is a recent patch available for .NET 3.5 which is enabling TLS1.2 support.

See:

KB3154518 – Reliability Rollup HR-1605 – NDP 2.0 SP2 – Win7 SP1/Win 2008 R2 SP1 KB3154519 – Reliability Rollup HR-1605 – NDP 2.0 SP2 – Win8 RTM/Win 2012 RTM KB3154520 – Reliability Rollup HR-1605 – NDP 2.0 SP2 – Win8.1RTM/Win 2012 R2 RTM KB3156421 -1605 HotFix Rollup through Windows Update for Windows 10.

OK. Everything is clear now. Many thanks for your help. – sergio Apr 24 '17 at 17:22 — sergio, Apr 24 '17 at 17:22

WebRequest: The underlying connection was closed

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