lib/dialer: Bring back address faking connection (fixes #6289) (#6363)

lib/dialer/internal.go

@@ -7,6 +7,7 @@
 package dialer
 
 import (
+	"net"
 	"net/http"
 	"net/url"
 	"os"
@@ -58,3 +59,36 @@ func socksDialerFunction(u *url.URL, forward proxy.Dialer) (proxy.Dialer, error)
 
 	return proxy.SOCKS5("tcp", u.Host, auth, forward)
 }
+
+// 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.
+type dialerConn struct {
+	net.Conn
+	addr net.Addr
+}
+
+func (c dialerConn) RemoteAddr() net.Addr {
+	return c.addr
+}
+
+func newDialerAddr(network, addr string) net.Addr {
+	netAddr, err := net.ResolveIPAddr(network, addr)
+	if err == nil {
+		return netAddr
+	}
+	return fallbackAddr{network, addr}
+}
+
+type fallbackAddr struct {
+	network string
+	addr    string
+}
+
+func (a fallbackAddr) Network() string {
+	return a.network
+}
+
+func (a fallbackAddr) String() string {
+	return a.addr
+}