Fix location estimation on Windows

Also, switch to a better Portal Test IP.

netenv/location_windows.go

@@ -0,0 +1,61 @@
+package netenv
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"os"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	SIO_RCVALL = syscall.IOC_IN | syscall.IOC_VENDOR | 1
+
+	RCVALL_OFF             = 0
+	RCVALL_ON              = 1
+	RCVALL_SOCKETLEVELONLY = 2
+	RCVALL_IPLEVEL         = 3
+)
+
+func newICMPListener(address string) (net.PacketConn, error) {
+	// This is an attempt to work around the problem described here:
+	// https://github.com/golang/go/issues/38427
+
+	// First, get the correct local interface address, as SIO_RCVALL can't be set on a 0.0.0.0 listeners.
+	dialedConn, err := net.Dial("ip4:icmp", address)
+	if err != nil {
+		return nil, fmt.Errorf("failed to dial: %s", err)
+	}
+	localAddr := dialedConn.LocalAddr()
+	dialedConn.Close()
+
+	// Configure the setup routine in order to extract the socket handle.
+	var socketHandle syscall.Handle
+	cfg := net.ListenConfig{
+		Control: func(network, address string, c syscall.RawConn) error {
+			return c.Control(func(s uintptr) {
+				socketHandle = syscall.Handle(s)
+			})
+		},
+	}
+
+	// Bind to interface.
+	conn, err := cfg.ListenPacket(context.Background(), "ip4:icmp", localAddr.String())
+	if err != nil {
+		return nil, err
+	}
+
+	// Set socket option to receive all packets, such as ICMP error messages.
+	// This is somewhat dirty, as there is guarantee that socketHandle is still valid.
+	// WARNING: The Windows Firewall might just drop the incoming packets you might want to receive.
+	unused := uint32(0) // Documentation states that this is unused, but WSAIoctl fails without it.
+	flag := uint32(RCVALL_IPLEVEL)
+	size := uint32(unsafe.Sizeof(flag))
+	err = syscall.WSAIoctl(socketHandle, SIO_RCVALL, (*byte)(unsafe.Pointer(&flag)), size, nil, 0, &unused, nil, 0)
+	if err != nil {
+		return nil, fmt.Errorf("failed to set socket to listen to all packests: %s", os.NewSyscallError("WSAIoctl", err))
+	}
+
+	return conn, nil
+}