middleware/etcd: reverse addresses (#162)

* middleware/etcd: reverse addresses

Implement reverse (PTR) addresses. Update the documentation on how to
configure test. Added tests as well.

Fixes: #157 #159

* Cleanup readme a little

6 files changed, 79 insertions, 5 deletions

diff --git a/middleware/etcd/README.md b/middleware/etcd/README.md
index c3717ae58..a424244c6 100644
--- a/middleware/etcd/README.md
+++ b/middleware/etcd/README.md
@@ -72,3 +72,48 @@ This is the default SkyDNS setup, with everying specified in full:
     proxy . 8.8.8.8:53 8.8.4.4:53
 }
 ~~~
+
+### Reverse zones
+
+Reverse zones are supported. You need to make CoreDNS aware of the fact that you are also
+authoritative for the reverse. For instance if you want to add the reverse for 10.0.0.0/24, you'll
+need to add the zone `10.in-addr.arpa` to the list of zones (the fun starts with reverse IPv6 zones
+in the ip6.arpa domain). Showing a snippet of a Corefile:
+
+~~~
+    etcd skydns.local 10.in-addr.arpa {
+        stubzones
+    ...
+~~~
+
+Next you'll need to populate the zone with reverse records, here we add a reverse for
+10.0.0.127 pointing to reverse.skydns.local.
+
+~~~
+% curl -XPUT http://127.0.0.1:4001/v2/keys/skydns/arpa/in-addr/10/0/0/127 \
+    -d value='{"host":"reverse.skydns.local."}'
+~~~
+
+Querying with dig:
+
+~~~
+% dig @localhost -x 10.0.0.127 +short
+reverse.atoom.net.
+~~~
+
+Or with *debug* queries enabled:
+
+~~~
+% dig @localhost -p 1053 o-o.debug.127.0.0.10.in-addr.arpa. PTR
+
+;; OPT PSEUDOSECTION:
+; EDNS: version: 0, flags:; udp: 4096
+;; QUESTION SECTION:
+;o-o.debug.127.0.0.10.in-addr.arpa. IN  PTR
+
+;; ANSWER SECTION:
+127.0.0.10.in-addr.arpa. 300    IN      PTR     reverse.atoom.net.
+
+;; ADDITIONAL SECTION:
+127.0.0.10.in-addr.arpa. 300    CH      TXT     "reverse.atoom.net.:0(10,0,,false)[0,]"
+~~~
diff --git a/middleware/etcd/handler.go b/middleware/etcd/handler.go
index d27b274cd..a270e1983 100644
--- a/middleware/etcd/handler.go
+++ b/middleware/etcd/handler.go
@@ -58,6 +58,8 @@ func (e Etcd) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (i
 		records, debug, err = e.TXT(zone, state)
 	case "CNAME":
 		records, debug, err = e.CNAME(zone, state)
+	case "PTR":
+		records, debug, err = e.PTR(zone, state)
 	case "MX":
 		records, extra, debug, err = e.MX(zone, state)
 	case "SRV":
diff --git a/middleware/etcd/lookup.go b/middleware/etcd/lookup.go
index 1223f85a4..5d7d89ce3 100644
--- a/middleware/etcd/lookup.go
+++ b/middleware/etcd/lookup.go
@@ -305,6 +305,21 @@ func (e Etcd) CNAME(zone string, state middleware.State) (records []dns.RR, debu
 	return records, debug, nil
 }
 
+// PTR returns the PTR records, only services that have a domain name as host are included.
+func (e Etcd) PTR(zone string, state middleware.State) (records []dns.RR, debug []msg.Service, err error) {
+	services, debug, err := e.records(state, true)
+	if err != nil {
+		return nil, debug, err
+	}
+
+	for _, serv := range services {
+		if ip := net.ParseIP(serv.Host); ip == nil {
+			records = append(records, serv.NewPTR(state.QName(), serv.Host))
+		}
+	}
+	return records, debug, nil
+}
+
 func (e Etcd) TXT(zone string, state middleware.State) (records []dns.RR, debug []msg.Service, err error) {
 	services, debug, err := e.records(state, false)
 	if err != nil {
diff --git a/middleware/etcd/lookup_test.go b/middleware/etcd/lookup_test.go
index ffc8c737e..14b9d0c98 100644
--- a/middleware/etcd/lookup_test.go
+++ b/middleware/etcd/lookup_test.go
@@ -15,20 +15,22 @@ var services = []*msg.Service{
 	{Host: "10.0.0.1", Port: 8080, Key: "a.server1.prod.region1.skydns.test."},
 	{Host: "10.0.0.2", Port: 8080, Key: "b.server1.prod.region1.skydns.test."},
 	{Host: "::1", Port: 8080, Key: "b.server6.prod.region1.skydns.test."},
-	// Unresolvable internal name
+	// Unresolvable internal name.
 	{Host: "unresolvable.skydns.test", Key: "cname.prod.region1.skydns.test."},
-	// priority
+	// Priority.
 	{Host: "priority.server1", Priority: 333, Port: 8080, Key: "priority.skydns.test."},
-	// Subdomain
+	// Subdomain.
 	{Host: "sub.server1", Port: 0, Key: "a.sub.region1.skydns.test."},
 	{Host: "sub.server2", Port: 80, Key: "b.sub.region1.skydns.test."},
 	{Host: "10.0.0.1", Port: 8080, Key: "c.sub.region1.skydns.test."},
-	// Cname loop
+	// Cname loop.
 	{Host: "a.cname.skydns.test", Key: "b.cname.skydns.test."},
 	{Host: "b.cname.skydns.test", Key: "a.cname.skydns.test."},
 	// Nameservers.
 	{Host: "10.0.0.2", Key: "a.ns.dns.skydns.test."},
 	{Host: "10.0.0.3", Key: "b.ns.dns.skydns.test."},
+	// Reverse.
+	{Host: "reverse.example.com", Key: "1.0.0.10.in-addr.arpa."}, // 10.0.0.1
 }
 
 var dnsTestCases = []test.Case{
@@ -198,4 +200,9 @@ var dnsTestCases = []test.Case{
 		Qname: "skydns_extra.test.", Qtype: dns.TypeSOA,
 		Answer: []dns.RR{test.SOA("skydns_extra.test. 300 IN SOA ns.dns.skydns_extra.test. hostmaster.skydns_extra.test. 1460498836 14400 3600 604800 60")},
 	},
+	// Reverse lookup
+	{
+		Qname: "1.0.0.10.in-addr.arpa.", Qtype: dns.TypePTR,
+		Answer: []dns.RR{test.PTR("1.0.0.10.in-addr.arpa. 300 PTR reverse.example.com.")},
+	},
 }
diff --git a/middleware/etcd/msg/service.go b/middleware/etcd/msg/service.go
index 76d74a463..059b3263b 100644
--- a/middleware/etcd/msg/service.go
+++ b/middleware/etcd/msg/service.go
@@ -102,6 +102,11 @@ func (s *Service) NewTXT(name string) *dns.TXT {
 	return &dns.TXT{Hdr: dns.RR_Header{Name: name, Rrtype: dns.TypeTXT, Class: dns.ClassINET, Ttl: s.Ttl}, Txt: split255(s.Text)}
 }
 
+// NewPTR returns a new PTR record based on the Service.
+func (s *Service) NewPTR(name string, target string) *dns.PTR {
+	return &dns.PTR{Hdr: dns.RR_Header{Name: name, Rrtype: dns.TypePTR, Class: dns.ClassINET, Ttl: s.Ttl}, Ptr: dns.Fqdn(target)}
+}
+
 // NewNS returns a new NS record based on the Service.
 func (s *Service) NewNS(name string) *dns.NS {
 	host := targetStrip(dns.Fqdn(s.Host), s.TargetStrip)
diff --git a/middleware/etcd/setup_test.go b/middleware/etcd/setup_test.go
index 1810e8e70..b1a7b9615 100644
--- a/middleware/etcd/setup_test.go
+++ b/middleware/etcd/setup_test.go
@@ -37,7 +37,7 @@ func init() {
 		PathPrefix: "skydns",
 		Ctx:        context.Background(),
 		Inflight:   &singleflight.Group{},
-		Zones:      []string{"skydns.test.", "skydns_extra.test."},
+		Zones:      []string{"skydns.test.", "skydns_extra.test.", "in-addr.arpa."},
 		Client:     etcdc.NewKeysAPI(cli),
 	}
 }