Implement the IPv6 block plugin

dnscrypt-proxy/config.go

@@ -17,6 +17,7 @@ type Config struct {
 	ForceTCP         bool                    `toml:"force_tcp"`
 	Timeout          int                     `toml:"timeout_ms"`
 	CertRefreshDelay int                     `toml:"cert_refresh_delay"`
+	BlockIPv6        bool                    `toml:"block_ipv6"`
 	ServersConfig    map[string]ServerConfig `toml:"servers"`
 }
 
@@ -56,6 +57,7 @@ func ConfigLoad(proxy *Proxy, config_file string) error {
 	}
 	proxy.listenAddresses = config.ListenAddresses
 	proxy.daemonize = config.Daemonize
+	proxy.pluginBlockIPv6 = config.BlockIPv6
 	if len(config.ServerNames) == 0 {
 		for serverName := range config.ServersConfig {
 			config.ServerNames = append(config.ServerNames, serverName)

dnscrypt-proxy/dnscrypt-proxy.toml

@@ -40,6 +40,13 @@ timeout = 2500
 cert_refresh_delay = 30
 
 
+############## Filters ##############
+
+## Immediately respond to IPv6-related queries with an empty response
+## This makes things faster when there is no IPv6 connectivity
+
+block_ipv6 = false
+
 
 ############## Servers ##############
 

dnscrypt-proxy/dnsutils.go

@@ -18,6 +18,15 @@ func TruncatedResponse(packet []byte) ([]byte, error) {
 	return dstMsg.Pack()
 }
 
+func EmptyResponseFromMessage(srcMsg *dns.Msg) (*dns.Msg, error) {
+	dstMsg := srcMsg
+	dstMsg.Response = true
+	dstMsg.Answer = make([]dns.RR, 0)
+	dstMsg.Ns = make([]dns.RR, 0)
+	dstMsg.Extra = make([]dns.RR, 0)
+	return dstMsg, nil
+}
+
 func HasTCFlag(packet []byte) bool {
 	return packet[2]&2 == 2
 }

dnscrypt-proxy/main.go

@@ -23,6 +23,7 @@ type Proxy struct {
 	listenAddresses       []string
 	daemonize             bool
 	registeredServers     []RegisteredServer
+	pluginBlockIPv6       bool
 }
 
 func main() {
@@ -163,14 +164,24 @@ func (proxy *Proxy) processIncomingQuery(serverInfo *ServerInfo, serverProto str
 	if clientAddr == nil {
 		clientProto = "tcp"
 	}
-	pluginsState := NewPluginsState(clientProto)
+	pluginsState := NewPluginsState(proxy, clientProto)
 	query, _ = pluginsState.ApplyQueryPlugins(query)
+	var response []byte
+	var err error
+	if pluginsState.action != PluginsActionForward {
+		if pluginsState.synthResponse != nil {
+			response, err = pluginsState.synthResponse.PackBuffer(response)
+			if err != nil {
+				return
+			}
+		}
+	}
+	if len(response) == 0 {
 		encryptedQuery, clientNonce, err := proxy.Encrypt(serverInfo, query, serverProto)
 		if err != nil {
 			return
 		}
 		serverInfo.noticeBegin(proxy)
-	var response []byte
 		if serverProto == "udp" {
 			response, err = proxy.exchangeWithUDPServer(serverInfo, encryptedQuery, clientNonce)
 		} else {
@@ -180,6 +191,7 @@ func (proxy *Proxy) processIncomingQuery(serverInfo *ServerInfo, serverProto str
 			serverInfo.noticeFailure(proxy)
 			return
 		}
+	}
 	if clientAddr != nil {
 		if len(response) > MaxDNSUDPPacketSize {
 			response, err = TruncatedResponse(response)

dnscrypt-proxy/plugins.go

@@ -11,6 +11,7 @@ const (
 	PluginsActionForward = 1
 	PluginsActionDrop    = 2
 	PluginsActionReject  = 3
+	PluginsActionSynth   = 4
 )
 
 type PluginsState struct {
@@ -21,6 +22,7 @@ type PluginsState struct {
 	proto                  string
 	queryPlugins           *[]Plugin
 	responsePlugins        *[]Plugin
+	synthResponse          *dns.Msg
 }
 
 type Plugin interface {
@@ -29,9 +31,15 @@ type Plugin interface {
 	Eval(pluginsState *PluginsState, msg *dns.Msg) error
 }
 
-func NewPluginsState(proto string) PluginsState {
-	queryPlugins := &[]Plugin{Plugin(new(PluginGetSetPayloadSize))}
+func NewPluginsState(proxy *Proxy, proto string) PluginsState {
+	queryPlugins := &[]Plugin{}
+	if proxy.pluginBlockIPv6 {
+		*queryPlugins = append(*queryPlugins, Plugin(new(PluginBlockIPv6)))
+	}
+	*queryPlugins = append(*queryPlugins, Plugin(new(PluginGetSetPayloadSize)))
+
 	responsePlugins := &[]Plugin{}
+
 	return PluginsState{action: PluginsActionForward, maxPayloadSize: MaxDNSUDPPacketSize - ResponseOverhead,
 		queryPlugins: queryPlugins, responsePlugins: responsePlugins, proto: proto}
 }
@@ -47,6 +55,9 @@ func (pluginsState *PluginsState) ApplyQueryPlugins(packet []byte) ([]byte, erro
 			pluginsState.action = PluginsActionDrop
 			return packet, ret
 		}
+		if pluginsState.action != PluginsActionForward {
+			break
+		}
 	}
 	packet2, err := msg.PackBuffer(packet)
 	if err != nil {
@@ -55,6 +66,8 @@ func (pluginsState *PluginsState) ApplyQueryPlugins(packet []byte) ([]byte, erro
 	return packet2, nil
 }
 
+// -------- get_set_payload_size plugin --------
+
 type PluginGetSetPayloadSize struct{}
 
 func (plugin *PluginGetSetPayloadSize) Name() string {
@@ -86,3 +99,33 @@ func (plugin *PluginGetSetPayloadSize) Eval(pluginsState *PluginsState, msg *dns
 	}
 	return nil
 }
+
+// -------- block_ipv6 plugin --------
+
+type PluginBlockIPv6 struct{}
+
+func (plugin *PluginBlockIPv6) Name() string {
+	return "block_ipv6"
+}
+
+func (plugin *PluginBlockIPv6) Description() string {
+	return "Immediately return a synthetic response to AAAA queries"
+}
+
+func (plugin *PluginBlockIPv6) Eval(pluginsState *PluginsState, msg *dns.Msg) error {
+	questions := msg.Question
+	if len(questions) != 1 {
+		return nil
+	}
+	question := questions[0]
+	if question.Qclass != dns.ClassINET || question.Qtype != dns.TypeAAAA {
+		return nil
+	}
+	synth, err := EmptyResponseFromMessage(msg)
+	if err != nil {
+		return err
+	}
+	pluginsState.synthResponse = synth
+	pluginsState.action = PluginsActionSynth
+	return nil
+}