fix(net): defer DNS resolution to final dialer to preserve domain in proxy chain

Commit 0522ae8 added inline DNS resolution (PreferGo: true) in Resolve(),
resolving domain names to IPs before routing. This caused the SOCKS5
connector to send CONNECT with ATYP=IPv4 instead of ATYP=domain, breaking
chains where the backend (e.g., ssh -D) cannot reach the gost-resolved IP.

Revert the DNS fallback in Resolve() — return the address unchanged so
the original hostname flows through the proxy chain. Instead, add
net.Resolver{PreferGo: true} to the TCP dialer in internal/net/dialer,
which resolves DNS at the point of actual TCP connection — still pure
Go (no cgo thread exhaustion) but after the proxy chain routing is done.

Also rename resovle.go → resolve.go (fix typo).

Fixes go-gost/gost#877

internal/net/dialer/dialer.go

@@ -146,6 +146,7 @@ func (d *Dialer) dialOnce(ctx context.Context, network, addr, ifceName string, i
 		return nil, fmt.Errorf("dial: unsupported network %s", network)
 	}
 	netd := net.Dialer{
+		Resolver:  &net.Resolver{PreferGo: true},
 		LocalAddr: ifAddr,
 		Control: func(network, address string, c syscall.RawConn) error {
 			return c.Control(func(fd uintptr) {

internal/net/resolve.go

@@ -13,8 +13,10 @@ import (
 
 // Resolve resolves addr to a concrete IP address. If hosts is non-nil, it is
 // consulted first. If r is non-nil and no host mapping matched, the resolver
-// is used. If neither is available, Go's native pure-Go resolver is used as a
-// fallback to avoid cgo-based DNS lookups that would block OS threads.
+// is used. If neither is available, the address is returned unchanged so that
+// the original hostname is preserved for downstream connectors (e.g., SOCKS5
+// ATYP=domain). DNS resolution is deferred to the final TCP dialer, which uses
+// a pure-Go resolver to avoid cgo thread exhaustion.
 func Resolve(ctx context.Context, network, addr string, r resolver.Resolver, hosts hosts.HostMapper, log logger.Logger) (string, error) {
 	if addr == "" {
 		return addr, nil
@@ -53,22 +55,9 @@ func Resolve(ctx context.Context, network, addr string, r resolver.Resolver, hos
 		return net.JoinHostPort(ips[0].String(), port), nil
 	}
 
-	// A literal IP address needs no resolution.
-	if net.ParseIP(host) != nil {
+	// Return the address unchanged so that the original hostname is
+	// preserved through the proxy chain. DNS resolution happens in the
+	// final TCP dialer (dialer.go) which uses net.Resolver{PreferGo: true}
+	// to avoid cgo-based lookups that block OS threads.
 	return addr, nil
 }
-
-	// Fallback to Go's native (pure-Go) resolver when no custom resolver or
-	// host mapper is configured. This avoids cgo-based DNS lookups in
-	// downstream net.Dialer.DialContext which would block OS threads and
-	// cause thread exhaustion under high connection concurrency.
-	lr := &net.Resolver{PreferGo: true}
-	ips, err := lr.LookupIP(ctx, "ip", host)
-	if err != nil {
-		return "", err
-	}
-	if len(ips) == 0 {
-		return "", fmt.Errorf("resolver: domain %s does not exist", host)
-	}
-	return net.JoinHostPort(ips[0].String(), port), nil
-}

internal/net/resolve_test.go

@@ -115,11 +115,10 @@ func TestResolve_ResolverEmptyResult(t *testing.T) {
 }
 
 func TestResolve_NoResolverNoHosts(t *testing.T) {
-	// When no resolver and no host mapper are configured, the function now
-	// resolves via Go's native DNS resolver (PreferGo: true). This test
-	// uses an IP address which is short-circuited at the top of Resolve,
-	// avoiding the need for actual DNS resolution in unit tests.
-	// DNS fallback behavior is verified by the e2e tests.
+	// When no resolver and no host mapper are configured, the address is
+	// returned unchanged so that the original hostname is preserved
+	// through the proxy chain (e.g., SOCKS5 ATYP=domain).
+	// DNS resolution is deferred to the final TCP dialer.
 	ctx := context.Background()
 	addr, err := Resolve(ctx, "tcp", "1.2.3.4:8080", nil, nil, nil)
 	if err != nil {
@@ -128,6 +127,13 @@ func TestResolve_NoResolverNoHosts(t *testing.T) {
 	if addr != "1.2.3.4:8080" {
 		t.Errorf("expected 1.2.3.4:8080, got %s", addr)
 	}
+	addr, err = Resolve(ctx, "tcp", "example.com:8080", nil, nil, nil)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if addr != "example.com:8080" {
+		t.Errorf("expected example.com:8080, got %s", addr)
+	}
 }
 
 func TestResolve_HostsNoMatchUsesResolver(t *testing.T) {