[client] Fix dns ipv6 upstream (#4257)

Author: lixmal

client/internal/dns/server.go

@@ -586,10 +586,7 @@ func (s *DefaultServer) registerFallback(config HostDNSConfig) {
 			continue
 		}
 
-		ns = fmt.Sprintf("%s:%d", ns, defaultPort)
-		if ip, err := netip.ParseAddr(ns); err == nil && ip.Is6() {
-			ns = fmt.Sprintf("[%s]:%d", ns, defaultPort)
-		}
+		ns = formatAddr(ns, defaultPort)
 
 		handler.upstreamServers = append(handler.upstreamServers, ns)
 	}
@@ -774,7 +771,15 @@ func (s *DefaultServer) updateMux(muxUpdates []handlerWrapper) {
 }
 
 func getNSHostPort(ns nbdns.NameServer) string {
-	return fmt.Sprintf("%s:%d", ns.IP.String(), ns.Port)
+	return formatAddr(ns.IP.String(), ns.Port)
+}
+
+// formatAddr formats a nameserver address with port, handling IPv6 addresses properly
+func formatAddr(address string, port int) string {
+	if ip, err := netip.ParseAddr(address); err == nil && ip.Is6() {
+		return fmt.Sprintf("[%s]:%d", address, port)
+	}
+	return fmt.Sprintf("%s:%d", address, port)
 }
 
 // upstreamCallbacks returns two functions, the first one is used to deactivate

client/internal/dns/server_test.go

@@ -2053,3 +2053,56 @@ func TestLocalResolverPriorityConstants(t *testing.T) {
 	assert.Equal(t, PriorityLocal, localMuxUpdates[0].priority, "Local handler should use PriorityLocal")
 	assert.Equal(t, "local.example.com", localMuxUpdates[0].domain)
 }
+
+func TestFormatAddr(t *testing.T) {
+	tests := []struct {
+		name     string
+		address  string
+		port     int
+		expected string
+	}{
+		{
+			name:     "IPv4 address",
+			address:  "8.8.8.8",
+			port:     53,
+			expected: "8.8.8.8:53",
+		},
+		{
+			name:     "IPv4 address with custom port",
+			address:  "1.1.1.1",
+			port:     5353,
+			expected: "1.1.1.1:5353",
+		},
+		{
+			name:     "IPv6 address",
+			address:  "fd78:94bf:7df8::1",
+			port:     53,
+			expected: "[fd78:94bf:7df8::1]:53",
+		},
+		{
+			name:     "IPv6 address with custom port",
+			address:  "2001:db8::1",
+			port:     5353,
+			expected: "[2001:db8::1]:5353",
+		},
+		{
+			name:     "IPv6 localhost",
+			address:  "::1",
+			port:     53,
+			expected: "[::1]:53",
+		},
+		{
+			name:     "Invalid address treated as hostname",
+			address:  "dns.example.com",
+			port:     53,
+			expected: "dns.example.com:53",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := formatAddr(tt.address, tt.port)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}