add support for peer auto-discovery in wgsd

This patch allows clients to auto-discover other mesh peers:
 - add a TXT record to default SRV answer containing the allowed ip and
public key: clients no longer have to have other peers pre-configured,
all necessary configuration is contained in the SRV record. Only a
connection to the registry is necessary to be allowed to connect to any
other peer
 - allow clients to request SRV records by allowed ip in addition to
public key: a client wanted to communicate with a specific ip can now
discover the associated peer w/o knowing its public key before-hand

Limitations:
 - the exported allowed ip config is only the 1st subnet configured
 - for 2 peers to communicate, both must setup the wireguard association

wgsd.go

@@ -35,12 +35,13 @@ type wgctrlClient interface {
 }
 
 const (
-	keyLen             = 56 // the number of characters in a base32-encoded Wireguard public key
-	spPrefix           = "_wireguard._udp."
-	spSubPrefix        = "." + spPrefix
-	serviceInstanceLen = keyLen + len(spSubPrefix)
+	keyLen      = 56 // the number of characters in a base32-encoded Wireguard public key
+	spPrefix    = "_wireguard._udp."
+	spSubPrefix = "." + spPrefix
 )
 
+var emptySubnet = net.IPNet{IP: net.IPv4zero, Mask: net.IPv4Mask(0, 0, 0, 0)}
+
 func (p *WGSD) ServeDNS(ctx context.Context, w dns.ResponseWriter,
 	r *dns.Msg) (int, error) {
 	// request.Request is a convenience struct we wrap around the msg and
@@ -94,32 +95,61 @@ func (p *WGSD) ServeDNS(ctx context.Context, w dns.ResponseWriter,
 		}
 		w.WriteMsg(m) // nolint: errcheck
 		return dns.RcodeSuccess, nil
-	case len(name) == serviceInstanceLen && qtype == dns.TypeSRV:
-		pubKey := name[:keyLen]
+	case qtype == dns.TypeSRV && strings.HasSuffix(name, spSubPrefix):
+		name = name[:len(name)-len(spSubPrefix)]
+		logger.Debugf("received query for: %s type: %s", name,
+			dns.TypeToString[qtype])
 		for _, peer := range device.Peers {
-			if strings.EqualFold(
-				base32.StdEncoding.EncodeToString(peer.PublicKey[:]), pubKey) {
-				endpoint := peer.Endpoint
-				hostRR := getHostRR(pubKey, p.zone, endpoint)
-				if hostRR == nil {
-					return nxDomain(p.zone, w, r)
-				}
-				m.Extra = append(m.Extra, hostRR)
-				m.Answer = append(m.Answer, &dns.SRV{
-					Hdr: dns.RR_Header{
-						Name:   state.Name(),
-						Rrtype: dns.TypeSRV,
-						Class:  dns.ClassINET,
-						Ttl:    0,
-					},
-					Priority: 0,
-					Weight:   0,
-					Port:     uint16(endpoint.Port),
-					Target:   pubKey + spSubPrefix + p.zone,
-				})
-				w.WriteMsg(m) // nolint: errcheck
-				return dns.RcodeSuccess, nil
+			pubKey := base32.StdEncoding.EncodeToString(peer.PublicKey[:])
+			allowedIPs := &emptySubnet
+			if len(peer.AllowedIPs) >= 1 {
+				allowedIPs = &peer.AllowedIPs[0]
 			}
+			if len(name) == keyLen {
+				// check by keyname
+				if !strings.EqualFold(pubKey, name[:keyLen]) {
+					continue
+				}
+			} else {
+				// check by ip
+				ip := net.ParseIP(name)
+				if ip == nil || !allowedIPs.Contains(ip) {
+					continue
+				}
+			}
+			endpoint := peer.Endpoint
+			hostRR := getHostRR(pubKey, p.zone, endpoint)
+			if hostRR == nil {
+				return nxDomain(p.zone, w, r)
+			}
+			m.Extra = append(m.Extra, hostRR)
+			pubKey = strings.ToLower(pubKey)
+			m.Extra = append(m.Extra, &dns.TXT{
+				Hdr: dns.RR_Header{
+					Name:   state.Name(),
+					Rrtype: dns.TypeTXT,
+					Class:  dns.ClassINET,
+					Ttl:    0,
+				},
+				Txt: []string{
+					"allowedip=" + allowedIPs.String(),
+					"pubkey=" + strings.TrimRight(pubKey, "="),
+				},
+			})
+			m.Answer = append(m.Answer, &dns.SRV{
+				Hdr: dns.RR_Header{
+					Name:   state.Name(),
+					Rrtype: dns.TypeSRV,
+					Class:  dns.ClassINET,
+					Ttl:    0,
+				},
+				Priority: 0,
+				Weight:   0,
+				Port:     uint16(endpoint.Port),
+				Target:   pubKey + spSubPrefix + p.zone,
+			})
+			w.WriteMsg(m) // nolint: errcheck
+			return dns.RcodeSuccess, nil
 		}
 		return nxDomain(p.zone, w, r)
 	case len(name) == len(spSubPrefix)+keyLen && (qtype == dns.TypeA ||

wgsd_test.go

@@ -28,22 +28,26 @@ func (m *mockClient) Device(d string) (*wgtypes.Device, error) {
 func TestWGSD(t *testing.T) {
 	key1 := [32]byte{}
 	key1[0] = 1
+	_, allowedip1, _ := net.ParseCIDR("2.2.2.2/32")
 	peer1 := wgtypes.Peer{
 		Endpoint: &net.UDPAddr{
 			IP:   net.ParseIP("1.1.1.1"),
 			Port: 1,
 		},
-		PublicKey: key1,
+		AllowedIPs: []net.IPNet{*allowedip1},
+		PublicKey:  key1,
 	}
 	peer1b32 := strings.ToLower(base32.StdEncoding.EncodeToString(peer1.PublicKey[:]))
 	key2 := [32]byte{}
 	key2[0] = 2
+	_, allowedip2, _ := net.ParseCIDR("::3/128")
 	peer2 := wgtypes.Peer{
 		Endpoint: &net.UDPAddr{
 			IP:   net.ParseIP("::2"),
 			Port: 2,
 		},
-		PublicKey: key2,
+		AllowedIPs: []net.IPNet{*allowedip2},
+		PublicKey:  key2,
 	}
 	peer2b32 := strings.ToLower(base32.StdEncoding.EncodeToString(peer2.PublicKey[:]))
 	p := &WGSD{
@@ -74,6 +78,7 @@ func TestWGSD(t *testing.T) {
 			},
 			Extra: []dns.RR{
 				test.A(fmt.Sprintf("%s._wireguard._udp.example.com. 0 IN A %s", peer1b32, peer1.Endpoint.IP.String())),
+				test.TXT(fmt.Sprintf("%s._wireguard._udp.example.com. 0 IN TXT allowedip=%s pubkey=%s", peer1b32, peer1.AllowedIPs[0].String(), strings.TrimRight(peer1b32, "="))),
 			},
 		},
 		{
@@ -85,6 +90,7 @@ func TestWGSD(t *testing.T) {
 			},
 			Extra: []dns.RR{
 				test.AAAA(fmt.Sprintf("%s._wireguard._udp.example.com. 0 IN AAAA %s", peer2b32, peer2.Endpoint.IP.String())),
+				test.TXT(fmt.Sprintf("%s._wireguard._udp.example.com. 0 IN TXT allowedip=%s pubkey=%s", peer2b32, peer2.AllowedIPs[0].String(), strings.TrimRight(peer2b32, "="))),
 			},
 		},
 		{