From a086d60edc50f44bae36b2afee9c0da69c06a153 Mon Sep 17 00:00:00 2001
From: Robin B <robin@broda.me>
Date: Thu, 12 Dec 2019 02:42:55 +0100
Subject: [PATCH] Allow configuration of dns listener host/port (#74)

* Allow configuration of dns listener host/port

* Make DNS listen host/port configuration HUP-able
---
 dns_server.go       | 36 ++++++++++++++++++++++++++++--------
 examples/config.yml |  4 ++++
 main.go             | 13 +++++++------
 3 files changed, 39 insertions(+), 14 deletions(-)

diff --git a/dns_server.go b/dns_server.go
index 705d7b9..3fec6b6 100644
--- a/dns_server.go
+++ b/dns_server.go
@@ -12,6 +12,8 @@ import (
 // This whole thing should be rewritten to use context
 
 var dnsR *dnsRecords
+var dnsServer *dns.Server
+var dnsAddr string
 
 type dnsRecords struct {
 	sync.RWMutex
@@ -106,20 +108,38 @@ func handleDnsRequest(w dns.ResponseWriter, r *dns.Msg) {
 	w.WriteMsg(m)
 }
 
-func dnsMain(hostMap *HostMap) {
-
+func dnsMain(hostMap *HostMap, c *Config) {
 	dnsR = newDnsRecords(hostMap)
 
 	// attach request handler func
 	dns.HandleFunc(".", handleDnsRequest)
 
-	// start server
-	port := 53
-	server := &dns.Server{Addr: ":" + strconv.Itoa(port), Net: "udp"}
-	l.Debugf("Starting DNS responder at %d\n", port)
-	err := server.ListenAndServe()
-	defer server.Shutdown()
+	c.RegisterReloadCallback(reloadDns)
+	startDns(c)
+}
+
+func getDnsServerAddr(c *Config) string {
+	return c.GetString("lighthouse.dns.host", "") + ":" + strconv.Itoa(c.GetInt("lighthouse.dns.port", 53))
+}
+
+func startDns(c *Config) {
+	dnsAddr = getDnsServerAddr(c)
+	dnsServer = &dns.Server{Addr: dnsAddr, Net: "udp"}
+	l.Debugf("Starting DNS responder at %s\n", dnsAddr)
+	err := dnsServer.ListenAndServe()
+	defer dnsServer.Shutdown()
 	if err != nil {
 		l.Errorf("Failed to start server: %s\n ", err.Error())
 	}
 }
+
+func reloadDns(c *Config) {
+	if dnsAddr == getDnsServerAddr(c) {
+		l.Debug("No DNS server config change detected")
+		return
+	}
+
+	l.Debug("Restarting DNS server")
+	dnsServer.Shutdown()
+	go startDns(c)
+}
diff --git a/examples/config.yml b/examples/config.yml
index b25a48e..d48c347 100644
--- a/examples/config.yml
+++ b/examples/config.yml
@@ -27,6 +27,10 @@ lighthouse:
   # serve_dns optionally starts a dns listener that responds to various queries and can even be
   # delegated to for resolution
   #serve_dns: false
+  #dns:
+    # The DNS host defines the IP to bind the dns listener to. This also allows binding to the nebula node IP.
+    #host: 0.0.0.0
+    #port: 53
   # interval is the number of seconds between updates from this node to a lighthouse.
   # during updates, a node sends information about its current IP addresses to each node.
   interval: 60
diff --git a/main.go b/main.go
index 5a99e88..39c10ab 100644
--- a/main.go
+++ b/main.go
@@ -204,7 +204,6 @@ func Main(configPath string, configTest bool, buildVersion string) {
 		lighthouseHosts[i] = ip2int(ip)
 	}
 
-	serveDns := config.GetBool("lighthouse.serve_dns", false)
 	lightHouse := NewLightHouse(
 		amLighthouse,
 		ip2int(tunCidr.IP),
@@ -216,11 +215,6 @@ func Main(configPath string, configTest bool, buildVersion string) {
 		punchBack,
 	)
 
-	if amLighthouse && serveDns {
-		l.Debugln("Starting dns server")
-		go dnsMain(hostMap)
-	}
-
 	//TODO: Move all of this inside functions in lighthouse.go
 	for k, v := range config.GetMap("static_host_map", map[interface{}]interface{}{}) {
 		vpnIp := net.ParseIP(fmt.Sprintf("%v", k))
@@ -264,6 +258,7 @@ func Main(configPath string, configTest bool, buildVersion string) {
 	//handshakeMACKey := config.GetString("handshake_mac.key", "")
 	//handshakeAcceptedMACKeys := config.GetStringSlice("handshake_mac.accepted_keys", []string{})
 
+	serveDns := config.GetBool("lighthouse.serve_dns", false)
 	checkInterval := config.GetInt("timers.connection_alive_interval", 5)
 	pendingDeletionInterval := config.GetInt("timers.pending_deletion_interval", 10)
 	ifConfig := &InterfaceConfig{
@@ -313,6 +308,12 @@ func Main(configPath string, configTest bool, buildVersion string) {
 	attachCommands(ssh, hostMap, handshakeManager.pendingHostMap, lightHouse, ifce)
 	ifce.Run(config.GetInt("tun.routines", 1), udpQueues, buildVersion)
 
+	// Start DNS server last to allow using the nebula IP as lighthouse.dns.host
+	if amLighthouse && serveDns {
+		l.Debugln("Starting dns server")
+		go dnsMain(hostMap, config)
+	}
+
 	// Just sit here and be friendly, main thread.
 	shutdownBlock(ifce)
 }