feat: DNS fallback server support

Fixes #7

Author: agaffney

internal/config/config.go

@@ -41,8 +41,9 @@ type LoggingConfig struct {
 }
 
 type DnsConfig struct {
-	ListenAddress string `yaml:"address" envconfig:"DNS_LISTEN_ADDRESS"`
-	ListenPort    uint   `yaml:"port" envconfig:"DNS_LISTEN_PORT"`
+	ListenAddress   string   `yaml:"address" envconfig:"DNS_LISTEN_ADDRESS"`
+	ListenPort      uint     `yaml:"port" envconfig:"DNS_LISTEN_PORT"`
+	FallbackServers []string `yaml:"fallbackServers" envconfig:"DNS_FALLBACK_SERVERS"`
 }
 
 type DebugConfig struct {
@@ -78,6 +79,8 @@ var globalConfig = &Config{
 	Dns: DnsConfig{
 		ListenAddress: "",
 		ListenPort:    8053,
+		// hdns.io
+		FallbackServers: []string{"103.196.38.38", "103.196.38.39", "103.196.38.40"},
 	},
 	Debug: DebugConfig{
 		ListenAddress: "localhost",

internal/dns/dns.go

@@ -2,6 +2,7 @@ package dns
 
 import (
 	"fmt"
+	"math/rand"
 	"net"
 	"strings"
 
@@ -84,9 +85,53 @@ func handleQuery(w dns.ResponseWriter, r *dns.Msg) {
 		return
 	}
 
+	// Query fallback servers
+	fallbackResp, err := queryFallbackServer(r)
+	if err != nil {
+		// Send failure response
+		m.SetRcode(r, dns.RcodeServerFailure)
+		if err := w.WriteMsg(m); err != nil {
+			logger.Errorf("failed to write response: %s", err)
+		}
+		logger.Errorf("failed to query fallback server: %s", err)
+		return
+	} else {
+		// Copy relevant data from fallback response into our response
+		m.SetRcode(r, fallbackResp.MsgHdr.Rcode)
+		m.RecursionDesired = r.RecursionDesired
+		m.RecursionAvailable = fallbackResp.RecursionAvailable
+		if fallbackResp.Ns != nil {
+			m.Ns = append(m.Ns, fallbackResp.Ns...)
+		}
+		if fallbackResp.Answer != nil {
+			m.Answer = append(m.Answer, fallbackResp.Answer...)
+		}
+		if fallbackResp.Extra != nil {
+			m.Extra = append(m.Extra, fallbackResp.Extra...)
+		}
+		// Send response
+		if err := w.WriteMsg(m); err != nil {
+			logger.Errorf("failed to write response: %s", err)
+		}
+		return
+	}
+
 	// Return NXDOMAIN if we have no information about the requested domain or any of its parents
 	m.SetRcode(r, dns.RcodeNameError)
 	if err := w.WriteMsg(m); err != nil {
 		logger.Errorf("failed to write response: %s", err)
 	}
 }
+
+func queryFallbackServer(req *dns.Msg) (*dns.Msg, error) {
+	// Pick random fallback server
+	cfg := config.GetConfig()
+	fallbackServer := cfg.Dns.FallbackServers[rand.Intn(len(cfg.Dns.FallbackServers))]
+	// Query chosen server
+	m := new(dns.Msg)
+	m.Id = dns.Id()
+	m.RecursionDesired = req.RecursionDesired
+	m.Question = append(m.Question, req.Question...)
+	in, err := dns.Exchange(m, fmt.Sprintf("%s:53", fallbackServer))
+	return in, err
+}