fix(handler/socks5): RFC 1928 compliance — method negotiation, BND.ADDR, UDP ordering, and safe type assertions

- selector.go: Select() now only returns methods the client actually offered,
  defaulting to MethodNoAcceptable (0xFF) when no mutually-supported method
  exists. Prevents protocol desync when authenticator is set but client
  doesn't offer UserPass. (#1, #2)

- connect.go: CONNECT success reply now carries the outbound connection's
  actual local address as BND.ADDR per RFC 1928 §6, instead of 0.0.0.0:0. (#4)

- udp.go: UDP ASSOCIATE now verifies the upstream chain is reachable before
  sending Success to the client; on failure sends a proper Failure reply.
  Also adds safe type assertions for net.TCPAddr and net.UDPAddr to prevent
  panics on wrapped connections, and returns errors on DNS resolution failure
  instead of silently dropping datagrams. (#5, #1, #2, #3)

handler/socks/v5/connect.go

@@ -82,7 +82,10 @@ func (h *socks5Handler) handleConnect(ctx context.Context, conn net.Conn, networ
 	ro.SrcAddr = cc.LocalAddr().String()
 	ro.DstAddr = cc.RemoteAddr().String()
 
-	resp := gosocks5.NewReply(gosocks5.Succeeded, nil)
+	// Report the actual bound address (BND.ADDR) per RFC 1928 §6.
+	bndAddr := gosocks5.Addr{}
+	bndAddr.ParseFrom(cc.LocalAddr().String())
+	resp := gosocks5.NewReply(gosocks5.Succeeded, &bndAddr)
 	log.Trace(resp)
 	if err := resp.Write(conn); err != nil {
 		log.Error(err)

handler/socks/v5/selector.go

@@ -27,25 +27,42 @@ func (selector *serverSelector) Methods() []uint8 {
 
 func (s *serverSelector) Select(methods ...uint8) (method uint8) {
 	s.logger.Debugf("%d %d %v", gosocks5.Ver5, len(methods), methods)
-	method = gosocks5.MethodNoAuth
+
+	// Determine which methods the client offered.
+	var hasNoAuth, hasUserPass, hasTLS, hasTLSAuth bool
 	for _, m := range methods {
-		if m == socks.MethodTLS && !s.noTLS && s.TLSConfig != nil {
-			method = m
-			break
+		switch m {
+		case gosocks5.MethodNoAuth:
+			hasNoAuth = true
+		case gosocks5.MethodUserPass:
+			hasUserPass = true
+		case socks.MethodTLS:
+			hasTLS = true
+		case socks.MethodTLSAuth:
+			hasTLSAuth = true
 		}
 	}
 
-	// when Authenticator is set, auth is mandatory
+	// When Authenticator is set, authentication is mandatory.
 	if s.Authenticator != nil {
-		if method == gosocks5.MethodNoAuth {
-			method = gosocks5.MethodUserPass
+		if hasTLSAuth && !s.noTLS && s.TLSConfig != nil {
+			return socks.MethodTLSAuth
 		}
-		if method == socks.MethodTLS && !s.noTLS {
-			method = socks.MethodTLSAuth
+		if hasUserPass {
+			return gosocks5.MethodUserPass
 		}
+		return gosocks5.MethodNoAcceptable
 	}
 
-	return
+	// No authenticator — select the best unauthenticated method.
+	if hasTLS && !s.noTLS && s.TLSConfig != nil {
+		return socks.MethodTLS
+	}
+	if hasNoAuth {
+		return gosocks5.MethodNoAuth
+	}
+
+	return gosocks5.MethodNoAcceptable
 }
 
 func (s *serverSelector) OnSelected(method uint8, conn net.Conn) (string, net.Conn, error) {

handler/socks/v5/udp.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"net"
 	"strconv"
@@ -68,6 +69,30 @@ func (h *socks5Handler) handleUDP(ctx context.Context, conn net.Conn, network st
 	})
 	ro.SrcAddr = cc.LocalAddr().String()
 
+	// Verify the upstream chain is reachable before replying Success,
+	// per RFC 1928 §7.
+	var buf bytes.Buffer
+	c, err := h.options.Router.Dial(ictx.ContextWithBuffer(ctx, &buf), network, "") // UDP association
+	ro.Route = buf.String()
+	if err != nil {
+		log.Error(err)
+		reply := gosocks5.NewReply(gosocks5.Failure, nil)
+		log.Trace(reply)
+		reply.Write(conn)
+		return err
+	}
+	defer c.Close()
+
+	pc, ok := c.(net.PacketConn)
+	if !ok {
+		err := errors.New("socks5: wrong connection type")
+		log.Error(err)
+		reply := gosocks5.NewReply(gosocks5.Failure, nil)
+		log.Trace(reply)
+		reply.Write(conn)
+		return err
+	}
+
 	saddr := gosocks5.Addr{}
 	saddr.ParseFrom(cc.LocalAddr().String())
 
@@ -84,23 +109,6 @@ func (h *socks5Handler) handleUDP(ctx context.Context, conn net.Conn, network st
 	}
 
 	log.Debugf("bind on %s OK", cc.LocalAddr())
-
-	// obtain a udp connection
-	var buf bytes.Buffer
-	c, err := h.options.Router.Dial(ictx.ContextWithBuffer(ctx, &buf), network, "") // UDP association
-	ro.Route = buf.String()
-	if err != nil {
-		log.Error(err)
-		return err
-	}
-	defer c.Close()
-
-	pc, ok := c.(net.PacketConn)
-	if !ok {
-		err := errors.New("socks5: wrong connection type")
-		log.Error(err)
-		return err
-	}
 	pc = metrics.WrapPacketConn(ro.Service, pc)
 
 	{
@@ -132,10 +140,16 @@ func (h *socks5Handler) handleUDP(ctx context.Context, conn net.Conn, network st
 		}
 	}
 
+	tcpAddr, ok := conn.RemoteAddr().(*net.TCPAddr)
+	if !ok {
+		err := fmt.Errorf("socks5 udp: unexpected remote address type %T", conn.RemoteAddr())
+		log.Error(err)
+		return err
+	}
 	pc1 := socks.UDPConn(
 		&filteredPacketConn{
 			PacketConn: cc,
-			filterIP:   conn.RemoteAddr().(*net.TCPAddr).IP,
+			filterIP:   tcpAddr.IP,
 		},
 		h.md.udpBufferSize)
 
@@ -179,7 +193,11 @@ func (f *filteredPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error
 			return
 		}
 
-		if udpAddr := addr.(*net.UDPAddr); udpAddr.IP.Equal(f.filterIP) {
+		udpAddr, ok := addr.(*net.UDPAddr)
+		if !ok {
+			continue
+		}
+		if udpAddr.IP.Equal(f.filterIP) {
 			return
 		}
 	}
@@ -228,7 +246,10 @@ func (c *domainResolvePacketConn) WriteTo(b []byte, addr net.Addr) (int, error)
 		if c.log != nil {
 			c.log.Warnf("socks5 udp: dropping datagram to %s (DNS resolution failed)", host)
 		}
-		return 0, nil
+		if err == nil {
+			err = fmt.Errorf("socks5 udp: DNS resolution failed for %s", host)
+		}
+		return 0, err
 	}
 
 	// Pick the first IPv4 address from the result set, falling back to