Add max upstream connections dns-proxy option

Allows defining a limit to the number of connections that can be
established with the upstream DNS host.

If left unset, there may be situations where connections fail to
establish, which causes the Transport to create an influx of connections
causing upstream to throttle our requests and triggering a runaway
effect resulting in high CPU usage. See https://github.com/cloudflare/cloudflared/issues/91

cmd/cloudflared/app_resolver_service.go

@@ -13,6 +13,7 @@ const (
 
 	LogFieldResolverAddress          = "resolverAddress"
 	LogFieldResolverPort             = "resolverPort"
+	LogFieldResolverMaxUpstreamConns = "resolverMaxUpstreamConns"
 )
 
 // ResolverService is used to wrap the tunneldns package's DNS over HTTP
@@ -57,7 +58,7 @@ func (s *ResolverService) Shutdown() {
 func (s *ResolverService) Run() error {
 	// create a listener
 	l, err := tunneldns.CreateListener(s.resolver.AddressOrDefault(), s.resolver.PortOrDefault(),
-		s.resolver.UpstreamsOrDefault(), s.resolver.BootstrapsOrDefault(), s.log)
+		s.resolver.UpstreamsOrDefault(), s.resolver.BootstrapsOrDefault(), s.resolver.MaxUpstreamConnectionsOrDefault(), s.log)
 	if err != nil {
 		return err
 	}
@@ -74,6 +75,7 @@ func (s *ResolverService) Run() error {
 	resolverLog := s.log.With().
 		Str(LogFieldResolverAddress, s.resolver.AddressOrDefault()).
 		Uint16(LogFieldResolverPort, s.resolver.PortOrDefault()).
+		Int(LogFieldResolverMaxUpstreamConns, s.resolver.MaxUpstreamConnectionsOrDefault()).
 		Logger()
 
 	resolverLog.Info().Msg("Starting resolver")

cmd/cloudflared/config/model.go

@@ -30,6 +30,7 @@ type DNSResolver struct {
 	Port       uint16   `json:"port,omitempty"`
 	Upstreams  []string `json:"upstreams,omitempty"`
 	Bootstraps []string `json:"bootstraps,omitempty"`
+	MaxUpstreamConnections int `json:"max_upstream_connections,omitempty"`
 }
 
 // Root is the base options to configure the service
@@ -59,6 +60,7 @@ func (r *DNSResolver) Hash() string {
 	io.WriteString(h, strings.Join(r.Bootstraps, ","))
 	io.WriteString(h, strings.Join(r.Upstreams, ","))
 	io.WriteString(h, fmt.Sprintf("%d", r.Port))
+	io.WriteString(h, fmt.Sprintf("%d", r.MaxUpstreamConnections))
 	io.WriteString(h, fmt.Sprintf("%v", r.Enabled))
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
@@ -99,3 +101,11 @@ func (r *DNSResolver) BootstrapsOrDefault() []string {
 	}
 	return []string{"https://162.159.36.1/dns-query", "https://162.159.46.1/dns-query", "https://[2606:4700:4700::1111]/dns-query", "https://[2606:4700:4700::1001]/dns-query"}
 }
+
+// MaxUpstreamConnectionsOrDefault return the max upstream connections or returns the default if 0
+func (r *DNSResolver) MaxUpstreamConnectionsOrDefault() int {
+    if r.MaxUpstreamConnections >= 0 {
+        return r.MaxUpstreamConnections
+    }
+    return 0
+}

cmd/cloudflared/tunnel/server.go

@@ -13,7 +13,11 @@ func runDNSProxyServer(c *cli.Context, dnsReadySignal, shutdownC chan struct{},
 	if port <= 0 || port > 65535 {
 		return errors.New("The 'proxy-dns-port' must be a valid port number in <1, 65535> range.")
 	}
-	listener, err := tunneldns.CreateListener(c.String("proxy-dns-address"), uint16(port), c.StringSlice("proxy-dns-upstream"), c.StringSlice("proxy-dns-bootstrap"), log)
+	maxUpstreamConnections := c.Int("proxy-dns-max-upstream-conns")
+	if maxUpstreamConnections < 0 {
+		return errors.New("'proxy-dns-max-upstream-conns' must be 0 or higher")
+	}
+	listener, err := tunneldns.CreateListener(c.String("proxy-dns-address"), uint16(port), c.StringSlice("proxy-dns-upstream"), c.StringSlice("proxy-dns-bootstrap"), maxUpstreamConnections, log)
 	if err != nil {
 		close(dnsReadySignal)
 		listener.Stop()

tunneldns/https_upstream.go

@@ -26,16 +26,17 @@ type UpstreamHTTPS struct {
 	client         *http.Client
 	endpoint       *url.URL
 	bootstraps     []string
+	maxConnections int
 	log            *zerolog.Logger
 }
 
 // NewUpstreamHTTPS creates a new DNS over HTTPS upstream from endpoint
-func NewUpstreamHTTPS(endpoint string, bootstraps []string, log *zerolog.Logger) (Upstream, error) {
+func NewUpstreamHTTPS(endpoint string, bootstraps []string, maxConnections int, log *zerolog.Logger) (Upstream, error) {
 	u, err := url.Parse(endpoint)
 	if err != nil {
 		return nil, err
 	}
-	return &UpstreamHTTPS{client: configureClient(u.Hostname()), endpoint: u, bootstraps: bootstraps, log: log}, nil
+	return &UpstreamHTTPS{client: configureClient(u.Hostname(), maxConnections), endpoint: u, bootstraps: bootstraps, log: log}, nil
 }
 
 // Exchange provides an implementation for the Upstream interface
@@ -122,17 +123,18 @@ func configureBootstrap(bootstrap string) (*url.URL, *http.Client, error) {
 		return nil, nil, fmt.Errorf("bootstrap address of %s must be an IP address", b.Hostname())
 	}
 
-	return b, configureClient(b.Hostname()), nil
+	return b, configureClient(b.Hostname(), 0), nil
 }
 
 // configureClient will configure a HTTPS client for upstream DoH requests
-func configureClient(hostname string) *http.Client {
+func configureClient(hostname string, maxUpstreamConnections int) *http.Client {
 	// Update TLS and HTTP client configuration
 	tlsConfig := &tls.Config{ServerName: hostname}
 	transport := &http.Transport{
 		TLSClientConfig:    tlsConfig,
 		DisableCompression: true,
 		MaxIdleConns:       1,
+		MaxConnsPerHost:    maxUpstreamConnections,
 		Proxy:              http.ProxyFromEnvironment,
 	}
 	_ = http2.ConfigureTransport(transport)

tunneldns/tunnel.go

@@ -68,6 +68,12 @@ func Command(hidden bool) *cli.Command {
 				Value:   cli.NewStringSlice("https://162.159.36.1/dns-query", "https://162.159.46.1/dns-query", "https://[2606:4700:4700::1111]/dns-query", "https://[2606:4700:4700::1001]/dns-query"),
 				EnvVars: []string{"TUNNEL_DNS_BOOTSTRAP"},
 			},
+			&cli.IntFlag{
+				Name:    "max-upstream-conns",
+				Usage:   "Maximum concurrent connections to upstream, unlimited by default",
+				Value:   0,
+				EnvVars: []string{"TUNNEL_DNS_MAX_UPSTREAM_CONNS"},
+			},
 		},
 		ArgsUsage: " ", // can't be the empty string or we get the default output
 		Hidden:    hidden,
@@ -90,8 +96,10 @@ func Run(c *cli.Context) error {
 		uint16(c.Uint("port")),
 		c.StringSlice("upstream"),
 		c.StringSlice("bootstrap"),
+		c.Int("max-upstream-conns"),
 		log,
 	)
+
 	if err != nil {
 		log.Err(err).Msg("Failed to create the listeners")
 		return err
@@ -173,12 +181,12 @@ func (l *Listener) Stop() error {
 }
 
 // CreateListener configures the server and bound sockets
-func CreateListener(address string, port uint16, upstreams []string, bootstraps []string, log *zerolog.Logger) (*Listener, error) {
+func CreateListener(address string, port uint16, upstreams []string, bootstraps []string, maxUpstreamConnections int, log *zerolog.Logger) (*Listener, error) {
 	// Build the list of upstreams
 	upstreamList := make([]Upstream, 0)
 	for _, url := range upstreams {
 		log.Info().Str(LogFieldURL, url).Msg("Adding DNS upstream")
-		upstream, err := NewUpstreamHTTPS(url, bootstraps, log)
+		upstream, err := NewUpstreamHTTPS(url, bootstraps, maxUpstreamConnections, log)
 		if err != nil {
 			return nil, errors.Wrap(err, "failed to create HTTPS upstream")
 		}