fix(handler/socks5): resolve UDP domains via configured resolver to stop DNS leak

SOCKS5 UDP datagrams carrying ATYP=DOMAINNAME were force-resolved via
net.ResolveUDPAddr in udpConn.ReadFrom, which always used the system
resolver and ignored any configured handler resolver (resolver=1.1.1.1),
leaking the query locally. The premature domain→IP conversion also
stripped the hostname before it reached the relay chain, so the exit
node could not resolve DNS itself.

udpConn.ReadFrom now returns a domainAddr for domain targets, letting
the name flow untouched through udp.Relay into the upstream WriteTo.
Chain-backed PacketConns (udpTunConn, udpRelayConn) encode it as
ATYP=Domain and forward to the exit. Direct (no-chain) associations
yield a raw *net.UDPConn that cannot consume a domainAddr, so they are
wrapped with resolvePacketConn, which resolves via hostMapper →
configured resolver → system DNS.

handler/socks/v5/udp.go

@@ -93,6 +93,21 @@ func (h *socks5Handler) handleUDP(ctx context.Context, conn net.Conn, network st
 		return err
 	}
 
+	// A direct (no-chain) UDP association yields a raw *net.UDPConn, which
+	// cannot consume the domainAddr returned by udpConn.ReadFrom for
+	// ATYP=DOMAINNAME datagrams. Wrap it so domains are resolved through the
+	// configured resolver (hostMapper → resolver → system DNS) instead of the
+	// previous hardcoded net.ResolveUDPAddr in the SOCKS5 decode path.
+	// Chain-backed PacketConns (udpTunConn, udpRelayConn, ...) encode domains
+	// as ATYP=Domain themselves and are left untouched.
+	if _, isDirect := pc.(*net.UDPConn); isDirect {
+		pc = &resolvePacketConn{
+			PacketConn:  pc,
+			resolver:    h.options.Router.Options().Resolver,
+			hostMapper:  h.options.Router.Options().HostMapper,
+		}
+	}
+
 	saddr := gosocks5.Addr{}
 	saddr.ParseFrom(cc.LocalAddr().String())
 
@@ -203,6 +218,55 @@ func (f *filteredPacketConn) ReadFrom(p []byte) (n int, addr net.Addr, err error
 	}
 }
 
+// resolvePacketConn wraps the upstream net.PacketConn for a direct (no-chain)
+// UDP association and resolves domain addresses to IPs in WriteTo calls.
+//
+// udpConn.ReadFrom now returns a domainAddr for ATYP=DOMAINNAME datagrams so
+// the domain survives to the chain; a raw *net.UDPConn cannot consume a
+// domainAddr (WriteTo needs *net.UDPAddr), so direct connections are wrapped
+// here. Resolution order is hostMapper → configured resolver → system DNS, so a
+// configured resolver (e.g. resolver=1.1.1.1) is honored instead of leaking
+// through the system resolver.
+type resolvePacketConn struct {
+	net.PacketConn
+	resolver   resolver.Resolver
+	hostMapper hosts.HostMapper
+}
+
+func (c *resolvePacketConn) WriteTo(b []byte, addr net.Addr) (int, error) {
+	host, portStr, err := net.SplitHostPort(addr.String())
+	if err != nil {
+		return c.PacketConn.WriteTo(b, addr)
+	}
+	if net.ParseIP(host) != nil {
+		return c.PacketConn.WriteTo(b, addr)
+	}
+
+	var ips []net.IP
+	if c.hostMapper != nil {
+		ips, _ = c.hostMapper.Lookup(context.Background(), "ip", host)
+	}
+	if len(ips) == 0 && c.resolver != nil {
+		ips, _ = c.resolver.Resolve(context.Background(), "ip", host)
+	}
+	if len(ips) == 0 {
+		ips, _ = net.LookupIP(host)
+	}
+	if len(ips) == 0 {
+		return 0, fmt.Errorf("socks5 udp: cannot resolve %s", host)
+	}
+
+	ip := ips[0]
+	for _, candidate := range ips {
+		if candidate.To4() != nil {
+			ip = candidate
+			break
+		}
+	}
+	port, _ := strconv.Atoi(portStr)
+	return c.PacketConn.WriteTo(b, &net.UDPAddr{IP: ip, Port: port})
+}
+
 // domainResolvePacketConn wraps a net.PacketConn and resolves domain
 // addresses to IPs in WriteTo calls. This guarantees that SOCKS5 UDP
 // response datagrams never carry ATYP=Domain (0x03), which some clients

internal/util/socks/conn.go

@@ -3,6 +3,7 @@ package socks
 import (
 	"bytes"
 	"net"
+	"strconv"
 
 	"github.com/go-gost/core/common/bufpool"
 	"github.com/go-gost/gosocks5"
@@ -106,6 +107,20 @@ func UDPConn(c net.PacketConn, bufferSize int) net.PacketConn {
 	}
 }
 
+// domainAddr is a net.Addr that preserves a domain name without resolving it.
+// Returned by udpConn.ReadFrom when a SOCKS5 UDP datagram carries
+// ATYP=DOMAINNAME, so that downstream consumers (e.g. relay-chain WriteTo) can
+// forward the domain verbatim instead of forcing a local DNS lookup that would
+// leak the query through the system resolver and bypass any configured resolver.
+type domainAddr struct {
+	network string
+	host    string
+	port    int
+}
+
+func (a *domainAddr) Network() string { return a.network }
+func (a *domainAddr) String() string  { return net.JoinHostPort(a.host, strconv.Itoa(a.port)) }
+
 // ReadFrom reads an UDP datagram.
 // NOTE: for server side,
 // the returned addr is the target address the client want to relay to.
@@ -128,7 +143,11 @@ func (c *udpConn) ReadFrom(b []byte) (n int, addr net.Addr, err error) {
 	}
 	n = copy(b, buf[hlen:n])
 
+	if net.ParseIP(socksAddr.Host) != nil {
 		addr, err = net.ResolveUDPAddr("udp", socksAddr.String())
+	} else {
+		addr = &domainAddr{network: "udp", host: socksAddr.Host, port: int(socksAddr.Port)}
+	}
 	return
 }