dns: serve reverse lookup queries for all RFC 1918 reserved networks

internal/dns/dns.go

@@ -2,6 +2,7 @@ package dns
 
 import (
 	"log"
+	"net"
 	"strings"
 	"time"
 
@@ -45,6 +46,50 @@ func (s *Server) SetLeases(leases []dhcp4d.Lease) {
 	}
 }
 
+func mustParseCIDR(s string) *net.IPNet {
+	_, ipnet, err := net.ParseCIDR(s)
+	if err != nil {
+		panic(err)
+	}
+	return ipnet
+}
+
+var (
+	localNets = []*net.IPNet{
+		// reversed: https://tools.ietf.org/html/rfc1918#section-3
+		mustParseCIDR("10.0.0.0/8"),
+		mustParseCIDR("172.16.0.0/12"),
+		mustParseCIDR("192.168.0.0/16"),
+	}
+)
+
+func reverse(ss []string) {
+	last := len(ss) - 1
+	for i := 0; i < len(ss)/2; i++ {
+		ss[i], ss[last-i] = ss[last-i], ss[i]
+	}
+}
+
+func isLocalInAddrArpa(q string) bool {
+	if !strings.HasSuffix(q, ".in-addr.arpa.") {
+		return false
+	}
+	parts := strings.Split(strings.TrimSuffix(q, ".in-addr.arpa."), ".")
+	reverse(parts)
+	ip := net.ParseIP(strings.Join(parts, "."))
+	if ip == nil {
+		return false
+	}
+	var local bool
+	for _, l := range localNets {
+		if l.Contains(ip) {
+			local = true
+			break
+		}
+	}
+	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) {
@@ -69,7 +114,7 @@ func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 			}
 		}
 		if q.Qtype == dns.TypePTR && q.Qclass == dns.ClassINET {
-			if strings.HasSuffix(q.Name, "168.192.in-addr.arpa.") {
+			if isLocalInAddrArpa(q.Name) {
 				if host, ok := s.hostsByIP[q.Name]; ok {
 					rr, err := dns.NewRR(q.Name + " 3600 IN PTR " + host + "." + s.domain)
 					if err != nil {

internal/dns/dns_test.go

@@ -75,16 +75,41 @@ func TestDHCP(t *testing.T) {
 }
 
 func TestDHCPReverse(t *testing.T) {
+	for _, test := range []struct {
+		ip       net.IP
+		question string
+	}{
+		{
+			ip:       net.IP{192, 168, 42, 23},
+			question: "23.42.168.192.in-addr.arpa.",
+		},
+
+		{
+			ip:       net.IP{10, 0, 0, 2},
+			question: "2.0.0.10.in-addr.arpa.",
+		},
+
+		{
+			ip:       net.IP{172, 16, 0, 1}, // 172.16/12 HostMin
+			question: "1.0.16.172.in-addr.arpa.",
+		},
+
+		{
+			ip:       net.IP{172, 31, 255, 254}, // 172.16/12 HostMax
+			question: "254.255.31.172.in-addr.arpa.",
+		},
+	} {
+		t.Run(test.question, func(t *testing.T) {
 			r := &recorder{}
 			s := NewServer("localhost:0", "lan")
 			s.SetLeases([]dhcp4d.Lease{
 				{
 					Hostname: "xps",
-			Addr:     net.IP{192, 168, 42, 23},
+					Addr:     test.ip,
 				},
 			})
 			m := new(dns.Msg)
-	m.SetQuestion("23.42.168.192.in-addr.arpa.", dns.TypePTR)
+			m.SetQuestion(test.question, dns.TypePTR)
 			s.handleRequest(r, m)
 			if got, want := len(r.response.Answer), 1; got != want {
 				t.Fatalf("unexpected number of answers: got %d, want %d", got, want)
@@ -96,6 +121,8 @@ func TestDHCPReverse(t *testing.T) {
 			if got, want := a.(*dns.PTR).Ptr, "xps.lan."; got != want {
 				t.Fatalf("unexpected response record: got %q, want %q", got, want)
 			}
+		})
+	}
 }
 
 // TODO: multiple questions