Add an option to always ignore the system resolver

This makes startup faster when DoH resolvers without a static IP
are used (Google).

dnscrypt-proxy/config.go

@@ -42,6 +42,7 @@ type Config struct {
 	SourceIPv6          bool                    `toml:"ipv6_servers"`
 	MaxClients          uint32                  `toml:"max_clients"`
 	FallbackResolver    string                  `toml:"fallback_resolver"`
+	IgnoreSystemDNS     bool                    `toml:"ignore_system_dns"`
 }
 
 func newConfig() Config {
@@ -61,6 +62,7 @@ func newConfig() Config {
 		SourceIPv6:          false,
 		MaxClients:          100,
 		FallbackResolver:    DefaultFallbackResolver,
+		IgnoreSystemDNS:     false,
 	}
 }
 
@@ -132,6 +134,9 @@ func ConfigLoad(proxy *Proxy, svcFlag *string, config_file string) error {
 		dlog.UseLogFile(*config.LogFile)
 	}
 	proxy.xTransport.fallbackResolver = config.FallbackResolver
+	if len(config.FallbackResolver) > 0 {
+		proxy.xTransport.ignoreSystemDNS = config.IgnoreSystemDNS
+	}
 	proxy.timeout = time.Duration(config.Timeout) * time.Millisecond
 	proxy.maxClients = config.MaxClients
 	proxy.mainProto = "udp"

dnscrypt-proxy/dnscrypt-proxy.toml

@@ -93,6 +93,11 @@ cert_refresh_delay = 30
 fallback_resolver = "9.9.9.9:53"
 
 
+## Never try to use the system DNS settings; unconditionally use the
+## fallback resolver.
+
+ignore_system_dns = false
+
 
 #########################
 #        Filters        #

dnscrypt-proxy/xtransport.go

@@ -30,6 +30,7 @@ type XTransport struct {
 	timeout          time.Duration
 	cachedIPs        CachedIPs
 	fallbackResolver string
+	ignoreSystemDNS  bool
 }
 
 func NewXTransport(timeout time.Duration) *XTransport {
@@ -37,6 +38,7 @@ func NewXTransport(timeout time.Duration) *XTransport {
 		cachedIPs:        CachedIPs{cache: make(map[string]string)},
 		timeout:          timeout,
 		fallbackResolver: DefaultFallbackResolver,
+		ignoreSystemDNS:  false,
 	}
 	dialer := &net.Dialer{Timeout: timeout, KeepAlive: timeout, DualStack: true}
 	transport := &http.Transport{
@@ -87,6 +89,12 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 	if body != nil {
 		req.Body = *body
 	}
+	var err error
+	host := url.Host
+	xTransport.cachedIPs.RLock()
+	cachedIP := xTransport.cachedIPs.cache[host]
+	xTransport.cachedIPs.RUnlock()
+	if !xTransport.ignoreSystemDNS || len(cachedIP) > 0 {
 		start := time.Now()
 		resp, err := client.Do(req)
 		rtt := time.Since(start)
@@ -98,13 +106,13 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 			}
 			return resp, rtt, err
 		}
-	host := url.Host
+		dlog.Debugf("[%s]: [%s]", req.URL, err)
-	xTransport.cachedIPs.RLock()
+	} else {
-	cachedIP := xTransport.cachedIPs.cache[host]
+		dlog.Debug("Ignoring system DNS")
-	xTransport.cachedIPs.RUnlock()
+	}
-	if len(cachedIP) > 0 {
+	if len(cachedIP) > 0 && err != nil {
 		dlog.Debugf("IP for [%s] was cached to [%s], but connection failed: [%s]", host, cachedIP, err)
-		return resp, rtt, err
+		return nil, 0, err
 	}
 	dnsClient := new(dns.Client)
 	msg := new(dns.Msg)
@@ -113,10 +121,10 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 	dlog.Noticef("System DNS configuration not usable yet, exceptionally resolving [%s] using fallback resolver [%s]", host, xTransport.fallbackResolver)
 	in, _, err := dnsClient.Exchange(msg, xTransport.fallbackResolver)
 	if err != nil {
-		return resp, rtt, err
+		return nil, 0, err
 	}
 	if len(in.Answer) <= 0 {
-		return resp, rtt, fmt.Errorf("No IP found for [%s]", host)
+		return nil, 0, fmt.Errorf("No IP found for [%s]", host)
 	}
 	foundIP := in.Answer[0].(*dns.A).A.String()
 	xTransport.cachedIPs.Lock()
@@ -124,9 +132,9 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 	xTransport.cachedIPs.Unlock()
 	dlog.Debugf("[%s] IP address [%s] added to the cache", host, foundIP)
 
-	start = time.Now()
+	start := time.Now()
-	resp, err = client.Do(req)
+	resp, err := client.Do(req)
-	rtt = time.Since(start)
+	rtt := time.Since(start)
 	if err == nil {
 		if resp == nil {
 			err = errors.New("Webserver returned an error")
@@ -134,6 +142,9 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 			err = fmt.Errorf("Webserver returned code %d", resp.StatusCode)
 		}
 	}
+	if err != nil {
+		dlog.Debugf("[%s]: [%s]", req.URL, err)
+	}
 	return resp, rtt, err
 }