Why does StreamSocket::ConnectAsync set up two TCP connections when proxy is configured in system?

Question

Hello, Dear all,
Per my test, the proxy for HTTPS will be selected as the actual used one for the out-going packets if we do the following test in an corporate network environment. Two connections will be set up: one is with the proxy, the other one is with the target address.
Some info about my env:
1. the configured proxy could only be accessed in the corporate network;
2. the target host has both private and public IPs;
3. I don't check whether there is proxy configured in code.
4. the issue could only occurr in the private network; If I do the test out of the corporate network, the behavior is correct because no proxy is configured. But I am not sure whether the issue would occur if there would be a usable proxy configured.
The repro steps are:
1. Create a Windows Store App project with VS2013. The testing code is pretty simple as the following:
using namespace Windows::Networking::Sockets;
StreamSocket^ streamSock = ref new StreamSocket();
IAsyncAction ^sconn = streamSock->ConnectAsync(
ref new Windows::Networking::HostName("10.1.2.3"), "56789",
SocketProtectionLevel::PlainSocket);
The target address and port (10.1.2.3:56789) could be any meaningful or bad one. We only need to observe the packet sent out from the client side in the following steps.
2. Configure the proxy in Internet Options -> Connections -> LAN Settings -> Advanced -> Secure. Give a good proxy/port to it;
2.1. Run the program and capturing the packet from Wireshark or other tools.
   ==> You will see that a HTTP request with header "CONNECT" is sent to the proxy. That's to say, the client will connect to the proxy.
   ==> You will also see that the client side also sent out a TCP SYN packet to the remote target address (10.1.2.3:56789).
3. Disable or remove the proxy setting;
3.1. Run the program and capturing the packet from Wireshark or other tools.
   ==> You will see that there would be no HTTP request sent out.
I am using Win8.1/VS2013 and the system is up-to-date... I am not sure the behavior is designed as this, or it is a bug of the StreamSocket?
BTW: I didn't use Windows::Networking::Connectivity::NetworkInformation::GetProxyConfigurationAsync on purpose.
For the classical WinSock, we could only see the TCP SYN packet in step 2. That's to say, the behaviors between StreamSocket and WinSocket are different.
Thank you all.

Answer 1

A workaround to avoid proxy connection requests from being sent out is to use the
ConnectAsync(Hostname, String, SocketProtectionLevel,
NetworkAdapter) method with a specific adapter. The proxy-related logic is disabled when a specific network adapter is selected.
You can retrieve the main active network adapter by calling GetInternetConnectionProfile() and then referencing the NetworkAdapter property of the returned information:
ConnectionProfile^ connectionProfile = NetworkInformation::GetInternetConnectionProfile();
NetworkAdapter^ adapter = connectionProfile->NetworkAdapter;
You could also enumerate all possible adapters and try to connect on each of them, one by one:
for each (HostName^ localHostInfo in NetworkInformation::GetHostNames())
if (localHostInfo->IPInformation != nullptr)
NetworkAdapter^ adapter = localHostInfo->IPInformation->NetworkAdapter;
// Attempt to connect with this adapter.