Resolve CNAME record like Route 53 alias record

README.md

@@ -26,7 +26,11 @@ amazondns ZONE [ADDRESS] {
 
 ## Examples
 
-Create your Route 53 private hostead zone with `sub.example.org` and attach your VPC. Then add A record for `test.sub.example.org` into the zone.
+Setup Route 53 as below.
+
+* Create your Route 53 private hostead zone with `sub.example.org` and attach your VPC.
+* Add A record as `test.sub.example.org` into the zone.
+* Add CNAME record as `lb.sub.example.org` for your ELB into the zone.
 
 Next, boot EC2 instance and deploy CoreDNS binary, and configure CoreDNS config file as below.
 
@@ -60,14 +64,14 @@ The `test.sub.example.org` is resolved with *AUTHORITY SECTION* and *ADDITIONAL
 ; EDNS: version: 0, flags:; udp: 4096
 ; COOKIE: 23246de45b4a3601 (echoed)
 ;; QUESTION SECTION:
-;test.sub.example.org.		IN	A
+;test.sub.example.org.      IN  A
 
 ;; ANSWER SECTION:
-test.sub.example.org.	60	IN	A	10.0.0.10
+test.sub.example.org.   60  IN  A  10.0.0.10
 
 ;; AUTHORITY SECTION:
-sub.example.org.	60	IN	NS	ns1.sub.example.org.
-sub.example.org.	60	IN	NS	ns2.sub.example.org.
+sub.example.org.        60  IN  NS  ns1.sub.example.org.
+sub.example.org.        60  IN  NS  ns2.sub.example.org.
 
 ;; ADDITIONAL SECTION:
 ns1.sub.example.org.    60  IN  A   192.168.0.1
@@ -95,11 +99,11 @@ Also it can return NS record(s) for subdomain as below.
 ; EDNS: version: 0, flags:; udp: 4096
 ; COOKIE: c1c3332966dba8fd (echoed)
 ;; QUESTION SECTION:
-;sub.example.org.      IN  NS
+;sub.example.org.          IN  NS
 
 ;; ANSWER SECTION:
-sub.example.org.   60  IN  NS  ns1.sub.example.org.
-sub.example.org.   60  IN  NS  ns2.sub.example.org.
+sub.example.org.       60  IN  NS  ns1.sub.example.org.
+sub.example.org.       60  IN  NS  ns2.sub.example.org.
 
 ;; ADDITIONAL SECTION:
 ns1.sub.example.org.   60  IN  A   192.168.0.1
@@ -111,3 +115,39 @@ ns2.sub.example.org.   60  IN  A   192.168.0.2
 ;; MSG SIZE  rcvd: 125
 ```
 
+And it works like Route 53 alias record when answering CNAME record.
+Your CNAME record will be removed and A/AAAA records of the CNAME record will be replaces.
+
+```bash
+> dig @localhost lb.sub.example.org
+
+; <<>> DiG 9.11.1 <<>> @localhost test.sub.example.org +norecurse
+; (1 server found)
+;; global options: +cmd
+;; Got answer:
+;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63630
+;; flags: qr aa ra; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 3
+
+;; OPT PSEUDOSECTION:
+; EDNS: version: 0, flags:; udp: 4096
+; COOKIE: 89a840e16b4d3fc7 (echoed)
+;; QUESTION SECTION:
+;lb.sub.example.org.       IN  A
+
+;; ANSWER SECTION:
+lb.sub.example.org.    54  IN  A   10.0.0.16
+lb.sub.example.org.    54  IN  A   10.0.0.132
+
+;; AUTHORITY SECTION:
+sub.example.org.       60  IN  NS  ns1.sub.example.org.
+sub.example.org.       60  IN  NS  ns2.sub.example.org.
+
+;; ADDITIONAL SECTION:
+ns1.sub.example.org.   60  IN  A   192.168.0.1
+ns2.sub.example.org.   60  IN  A   192.168.0.2
+
+;; Query time: 11 msec
+;; SERVER: 127.0.0.1#53(127.0.0.1)
+;; WHEN: Tue Feb 27 11:20:08 JST 2018
+;; MSG SIZE  rcvd: 174
+```

amazondns.go

@@ -68,6 +68,9 @@ L:
 		m.Answer = resp.Answer
 		m.Rcode = resp.Rcode
 
+		// It's useful resolving CNAME for DNS of ELB, RDS and so on
+		resolveCNAME(name, m)
+
 		// Overwrite authority and additional section
 		if len(m.Answer) > 0 {
 			m.Ns = zone.ns
@@ -96,6 +99,7 @@ L:
 
 	state.SizeAndDo(m)
 	m, _ = state.Scrub(m)
+
 	w.WriteMsg(m)
 	return dns.RcodeSuccess, nil
 }
@@ -124,7 +128,58 @@ func handleNotFound(zone *Zone, name string, m *dns.Msg) {
 	m.Rcode = dns.RcodeNameError
 }
 
-func (ad *AmazonDNS) fillResponse(state request.Request, m *dns.Msg) {
+func resolveCNAME(reqName string, res *dns.Msg) {
+	ignore := map[string]struct{}{}
+
+	for {
+		var name string
+		var cname string
+		var ttl uint32
+		cnameIndex := -1
+
+		// Find CNAME record
+		for i, rr := range res.Answer {
+			if rr.Header().Rrtype == dns.TypeCNAME {
+				name = rr.Header().Name
+
+				if name != reqName {
+					continue
+				}
+
+				if _, ok := ignore[name]; ok {
+					continue
+				}
+
+				cname = rr.(*dns.CNAME).Target
+				ttl = rr.Header().Ttl
+
+				cnameIndex = i
+				break
+			}
+		}
+
+		if cnameIndex == -1 {
+			break
+		}
+
+		var replaced bool
+
+		// Replace records belong to CNAME record
+		for _, rr := range res.Answer {
+			if rr.Header().Name == cname {
+				rr.Header().Name = name
+				rr.Header().Ttl = ttl
+				replaced = true
+			}
+		}
+
+		// Remove CNAME record
+		if replaced {
+			res.Answer = append(res.Answer[:cnameIndex], res.Answer[cnameIndex+1:]...)
+		} else {
+			ignore[name] = struct{}{}
+		}
+	}
 }
 
 // Name implements the Handler interface.

build.sh

@@ -1,18 +1,24 @@
 #!/bin/sh
 
-VERSION=0.1
-TAG=master
+VERSION=0.2
+TAG=b1ce9ed6e5b6178fbfa73d3764d25a6e1f20fc82
+CADDY_TAG=37b291f82c2083a378b698577640389686b0baf4
 
 if [ "$1" = "" ]; then
     docker run --rm \
         -v $(pwd):/go/src/github.com/wadahiro/coredns-amazondns \
         -v $(pwd)/.tmp:/go \
         -w /go/src/github.com/wadahiro/coredns-amazondns \
-        golang:1.9 ./build.sh $TAG
+        golang:1.9 ./build.sh $TAG $CADDY_TAG
 else 
     echo "Building CoreDNS:$1 with amazondns..."
 
     go get github.com/coredns/coredns
+
+    cd /go/src/github.com/mholt/caddy
+
+    git checkout $2
+
     cd /go/src/github.com/coredns/coredns
 
     git reset --hard HEAD