dns: resolve own hostname, lock for concurrency

internal/dns/dns.go

@@ -3,7 +3,9 @@ package dns
 import (
 	"log"
 	"net"
+	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"router7/internal/dhcp4d"
@@ -15,29 +17,61 @@ import (
 
 type Server struct {
 	*dns.Server
-	client      *dns.Client
-	domain      string
-	upstream    string
-	sometimes   *rate.Limiter
-	hostsByName map[string]string
-	hostsByIP   map[string]string
+	client    *dns.Client
+	domain    string
+	upstream  string
+	sometimes *rate.Limiter
+
+	mu           sync.Mutex
+	hostname, ip string
+	hostsByName  map[string]string
+	hostsByIP    map[string]string
 }
 
 func NewServer(addr, domain string) *Server {
+	hostname, _ := os.Hostname()
+	ip, _, _ := net.SplitHostPort(addr)
 	server := &Server{
-		Server:      &dns.Server{Addr: addr, Net: "udp"},
-		client:      &dns.Client{},
-		domain:      domain,
-		upstream:    "8.8.8.8:53",
-		sometimes:   rate.NewLimiter(rate.Every(1*time.Second), 1), // at most once per second
-		hostsByName: make(map[string]string),
-		hostsByIP:   make(map[string]string),
+		Server:    &dns.Server{Addr: addr, Net: "udp"},
+		client:    &dns.Client{},
+		domain:    domain,
+		upstream:  "8.8.8.8:53",
+		sometimes: rate.NewLimiter(rate.Every(1*time.Second), 1), // at most once per second
+		hostname:  hostname,
+		ip:        ip,
 	}
+	server.initHostsLocked()
 	dns.HandleFunc(".", server.handleRequest)
 	return server
 }
 
+func (s *Server) initHostsLocked() {
+	s.hostsByName = make(map[string]string)
+	s.hostsByIP = make(map[string]string)
+	if s.hostname != "" && s.ip != "" {
+		s.hostsByName[s.hostname] = s.ip
+		s.hostsByIP[s.ip] = s.hostname
+	}
+}
+
+func (s *Server) hostByName(n string) (string, bool) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	r, ok := s.hostsByName[n]
+	return r, ok
+}
+
+func (s *Server) hostByIP(n string) (string, bool) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	r, ok := s.hostsByIP[n]
+	return r, ok
+}
+
 func (s *Server) SetLeases(leases []dhcp4d.Lease) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.initHostsLocked()
 	for _, l := range leases {
 		s.hostsByName[l.Hostname] = l.Addr.String()
 		if rev, err := dns.ReverseAddr(l.Addr.String()); err == nil {
@@ -90,7 +124,6 @@ func isLocalInAddrArpa(q string) bool {
 	return local
 }
 
-// TODO: is handleRequest called in more than one goroutine at a time?
 // TODO: require search domains to be present, then use HandleFunc("lan.", internalName)
 func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 	if len(r.Question) == 1 { // TODO: answer all questions we can answer
@@ -100,7 +133,7 @@ func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 			name = strings.TrimSuffix(name, "."+s.domain)
 
 			if !strings.Contains(name, ".") {
-				if host, ok := s.hostsByName[name]; ok {
+				if host, ok := s.hostByName(name); ok {
 					rr, err := dns.NewRR(q.Name + " 3600 IN A " + host)
 					if err != nil {
 						log.Fatal(err)
@@ -115,7 +148,7 @@ func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 		}
 		if q.Qtype == dns.TypePTR && q.Qclass == dns.ClassINET {
 			if isLocalInAddrArpa(q.Name) {
-				if host, ok := s.hostsByIP[q.Name]; ok {
+				if host, ok := s.hostByIP(q.Name); ok {
 					rr, err := dns.NewRR(q.Name + " 3600 IN PTR " + host + "." + s.domain)
 					if err != nil {
 						log.Fatal(err)

internal/dns/dns_test.go

@@ -3,6 +3,7 @@ package dns
 import (
 	"bytes"
 	"net"
+	"os"
 	"router7/internal/dhcp4d"
 	"testing"
 
@@ -74,6 +75,29 @@ func TestDHCP(t *testing.T) {
 	}
 }
 
+func TestHostname(t *testing.T) {
+	hostname, err := os.Hostname()
+	if err != nil {
+		t.Skipf("os.Hostname: %v", err)
+	}
+
+	r := &recorder{}
+	s := NewServer("127.0.0.2:0", "lan")
+	m := new(dns.Msg)
+	m.SetQuestion(hostname+".", dns.TypeA)
+	s.handleRequest(r, m)
+	if got, want := len(r.response.Answer), 1; got != want {
+		t.Fatalf("unexpected number of answers for %v: got %d, want %d", m.Question, got, want)
+	}
+	a := r.response.Answer[0]
+	if _, ok := a.(*dns.A); !ok {
+		t.Fatalf("unexpected response type: got %T, want dns.A", a)
+	}
+	if got, want := a.(*dns.A).A.To4(), (net.IP{127, 0, 0, 2}); !bytes.Equal(got, want) {
+		t.Fatalf("unexpected response IP: got %v, want %v", got, want)
+	}
+}
+
 func TestDHCPReverse(t *testing.T) {
 	for _, test := range []struct {
 		ip       net.IP