[client] Use native windows sock opts to avoid routing loops (#4314)

- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
 - Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var

client/net/dial.go

@@ -0,0 +1,82 @@
+//go:build !ios
+
+package net
+
+import (
+	"fmt"
+	"net"
+	"sync"
+
+	"github.com/pion/transport/v3"
+	log "github.com/sirupsen/logrus"
+)
+
+func DialUDP(network string, laddr, raddr *net.UDPAddr) (transport.UDPConn, error) {
+	if CustomRoutingDisabled() {
+		return net.DialUDP(network, laddr, raddr)
+	}
+
+	dialer := NewDialer()
+	dialer.LocalAddr = laddr
+
+	conn, err := dialer.Dial(network, raddr.String())
+	if err != nil {
+		return nil, fmt.Errorf("dialing UDP %s: %w", raddr.String(), err)
+	}
+
+	switch c := conn.(type) {
+	case *net.UDPConn:
+		// Advanced routing: plain connection
+		return c, nil
+	case *Conn:
+		// Legacy routing: wrapped connection preserves close hooks
+		udpConn, ok := c.Conn.(*net.UDPConn)
+		if !ok {
+			if err := conn.Close(); err != nil {
+				log.Errorf("Failed to close connection: %v", err)
+			}
+			return nil, fmt.Errorf("expected UDP connection, got %T", c.Conn)
+		}
+		return &UDPConn{UDPConn: udpConn, ID: c.ID, seenAddrs: &sync.Map{}}, nil
+	}
+
+	if err := conn.Close(); err != nil {
+		log.Errorf("failed to close connection: %v", err)
+	}
+	return nil, fmt.Errorf("unexpected connection type: %T", conn)
+}
+
+func DialTCP(network string, laddr, raddr *net.TCPAddr) (transport.TCPConn, error) {
+	if CustomRoutingDisabled() {
+		return net.DialTCP(network, laddr, raddr)
+	}
+
+	dialer := NewDialer()
+	dialer.LocalAddr = laddr
+
+	conn, err := dialer.Dial(network, raddr.String())
+	if err != nil {
+		return nil, fmt.Errorf("dialing TCP %s: %w", raddr.String(), err)
+	}
+
+	switch c := conn.(type) {
+	case *net.TCPConn:
+		// Advanced routing: plain connection
+		return c, nil
+	case *Conn:
+		// Legacy routing: wrapped connection preserves close hooks
+		tcpConn, ok := c.Conn.(*net.TCPConn)
+		if !ok {
+			if err := conn.Close(); err != nil {
+				log.Errorf("Failed to close connection: %v", err)
+			}
+			return nil, fmt.Errorf("expected TCP connection, got %T", c.Conn)
+		}
+		return &TCPConn{TCPConn: tcpConn, ID: c.ID}, nil
+	}
+
+	if err := conn.Close(); err != nil {
+		log.Errorf("failed to close connection: %v", err)
+	}
+	return nil, fmt.Errorf("unexpected connection type: %T", conn)
+}