Add an option to disable IPv6, this option is available to client only

doh-client/client.go

@@ -131,13 +131,14 @@ func (c *Client) newHTTPClient() error {
 	if c.httpTransport != nil {
 		c.httpTransport.CloseIdleConnections()
 	}
+	dialer := &net.Dialer{
+		Timeout:   time.Duration(c.conf.Timeout) * time.Second,
+		KeepAlive: 30 * time.Second,
+		DualStack: true,
+		Resolver:  c.bootstrapResolver,
+	}
 	c.httpTransport = &http.Transport{
-		DialContext: (&net.Dialer{
+		DialContext:           dialer.DialContext,
-			Timeout:   time.Duration(c.conf.Timeout) * time.Second,
-			KeepAlive: 30 * time.Second,
-			DualStack: true,
-			Resolver:  c.bootstrapResolver,
-		}).DialContext,
 		ExpectContinueTimeout: 1 * time.Second,
 		IdleConnTimeout:       90 * time.Second,
 		MaxIdleConns:          100,
@@ -146,6 +147,14 @@ func (c *Client) newHTTPClient() error {
 		ResponseHeaderTimeout: time.Duration(c.conf.Timeout) * time.Second,
 		TLSHandshakeTimeout:   time.Duration(c.conf.Timeout) * time.Second,
 	}
+	if c.conf.NoIPv6 {
+		c.httpTransport.DialContext = func(ctx context.Context, network, address string) (net.Conn, error) {
+			if strings.HasPrefix(network, "tcp") {
+				network = "tcp4"
+			}
+			return dialer.DialContext(ctx, network, address)
+		}
+	}
 	err := http2.ConfigureTransport(c.httpTransport)
 	if err != nil {
 		return err

doh-client/config.go

@@ -37,6 +37,7 @@ type config struct {
 	Timeout        uint     `toml:"timeout"`
 	NoCookies      bool     `toml:"no_cookies"`
 	NoECS          bool     `toml:"no_ecs"`
+	NoIPv6         bool     `toml:"no_ipv6"`
 	Verbose        bool     `toml:"verbose"`
 }
 

doh-client/doh-client.conf

@@ -65,5 +65,13 @@ no_cookies = false
 # the same configuration as most public DNS servers.
 no_ecs = false
 
+# Disable IPv6 when querying upstream
+#
+# Only enable this if you really have trouble connecting.
+# Doh-client uses both IPv4 and IPv6 by default and should not have problems
+# with an IPv4-only environment.
+# Note that DNS listening and bootstrapping is not controlled by this option.
+no_ipv6 = false
+
 # Enable logging
 verbose = false