lib/connections: Try TCP punchthrough (fixes #4259) (#5753)

lib/dialer/internal.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"time"
 
+	"github.com/syncthing/syncthing/lib/util"
 	"golang.org/x/net/proxy"
 )
 
@@ -60,6 +61,11 @@ func socksDialerFunction(u *url.URL, forward proxy.Dialer) (proxy.Dialer, error)
 	return proxy.SOCKS5("tcp", u.Host, auth, forward)
 }
 
+// Sort available addresses, preferring unspecified address.
+func tcpAddrLess(i interface{}, j interface{}) bool {
+	return util.AddressUnspecifiedLess(i.(*net.TCPAddr), j.(*net.TCPAddr))
+}
+
 // dialerConn is needed because proxy dialed connections have RemoteAddr() pointing at the proxy,
 // which then screws up various things such as IsLAN checks, and "let's populate the relay invitation address from
 // existing connection" shenanigans.