Support CNAMEs and the ANY query type

Add support for CNAMEs both inside and outside of the zones set by this plugin.

Add support for the ANY question type.

CNAME support is robust against excessive stack depth and loops.

Includes tests for all changes.

Author: fuhry

records.go

@@ -9,6 +9,8 @@ import (
 	"github.com/miekg/dns"
 )
 
+const maxCnameStackDepth = 10
+
 // Records is the plugin handler.
 type Records struct {
 	origins []string // for easy matching, these strings are the index in the map m.
@@ -34,15 +36,38 @@ func (re *Records) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 
 	nxdomain := true
 	var soa dns.RR
+	cnameStack := make(map[string]struct{}, 0)
+
+resolveLoop:
 	for _, r := range re.m[zone] {
+		if _, ok := cnameStack[qname]; ok {
+			log.Errorf("detected loop in CNAME chain, name [%s] already processed", qname)
+			goto servfail
+		}
+		if len(cnameStack) > maxCnameStackDepth {
+			log.Errorf("maximum CNAME stack depth of %d exceeded", maxCnameStackDepth)
+			goto servfail
+		}
+
 		if r.Header().Rrtype == dns.TypeSOA && soa == nil {
 			soa = r
 		}
 		if r.Header().Name == qname {
 			nxdomain = false
-			if r.Header().Rrtype == state.QType() {
+			if r.Header().Rrtype == state.QType() || r.Header().Rrtype == dns.TypeCNAME || state.QType() == dns.TypeANY {
 				m.Answer = append(m.Answer, r)
 			}
+			if r.Header().Rrtype == dns.TypeCNAME {
+				cnameStack[qname] = struct{}{}
+				qname = r.(*dns.CNAME).Target
+				if plugin.Zones(re.origins).Matches(qname) == "" {
+					// if the CNAME target isn't a record in this zone, break and return.
+					// The administrator can configure the `finalize` plugin (https://coredns.io/explugins/finalize/)
+					// to complete resolution of these names.
+					break resolveLoop
+				}
+				goto resolveLoop
+			}
 		}
 	}
 
@@ -64,6 +89,15 @@ func (re *Records) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 
 	w.WriteMsg(m)
 	return dns.RcodeSuccess, nil
+
+servfail:
+	m.Rcode = dns.RcodeServerFailure
+	m.Answer = nil
+	if soa != nil {
+		m.Ns = []dns.RR{soa}
+	}
+	w.WriteMsg(m)
+	return dns.RcodeServerFailure, nil
 }
 
 // Name implements the plugin.Handle interface.

records_test.go

@@ -7,7 +7,7 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 
-	"github.com/caddyserver/caddy"
+	"github.com/coredns/caddy"
 	"github.com/miekg/dns"
 )
 
@@ -76,6 +76,25 @@ func TestLookupNoSOA(t *testing.T) {
 records {
         example.org.   60  IN MX 10 mx.example.org.
         mx.example.org. 60 IN A  127.0.0.1
+		cname.example.org.	60	IN CNAME	mx.example.org.
+		dualstack.example.org.	60	IN	A	127.0.0.1
+		dualstack.example.org.	60	IN	AAAA	::1
+		cnameloop1.example.org.	60	IN	CNAME	cnameloop2.example.org.
+		cnameloop2.example.org.	60	IN	CNAME	cnameloop1.example.org.
+		cnameext.example.org.	60	IN	CNAME	mx.example.net.
+
+		cnamedepth.example.org.	60	IN	CNAME	cnamedepth1.example.org.
+		cnamedepth1.example.org.	60	IN	CNAME	cnamedepth2.example.org.
+		cnamedepth2.example.org.	60	IN	CNAME	cnamedepth3.example.org.
+		cnamedepth3.example.org.	60	IN	CNAME	cnamedepth4.example.org.
+		cnamedepth4.example.org.	60	IN	CNAME	cnamedepth5.example.org.
+		cnamedepth5.example.org.	60	IN	CNAME	cnamedepth6.example.org.
+		cnamedepth6.example.org.	60	IN	CNAME	cnamedepth7.example.org.
+		cnamedepth7.example.org.	60	IN	CNAME	cnamedepth8.example.org.
+		cnamedepth8.example.org.	60	IN	CNAME	cnamedepth9.example.org.
+		cnamedepth9.example.org.	60	IN	CNAME	cnamedepth10.example.org.
+		cnamedepth10.example.org.	60	IN	CNAME	cnamedepth11.example.org.
+		cnamedepth11.example.org.	60	IN	A	127.0.0.1
 }
 `
 
@@ -122,6 +141,34 @@ var testCasesNoSOA = []test.Case{
 	{
 		Qname: "mx.example.org.", Qtype: dns.TypeAAAA,
 	},
+	{
+		Qname: "dualstack.example.org.", Qtype: dns.TypeANY,
+		Answer: []dns.RR{
+			test.A("dualstack.example.org. 60	IN	A 127.0.0.1"),
+			test.AAAA("dualstack.example.org. 60	IN	AAAA ::1"),
+		},
+	},
+	{
+		Qname: "cname.example.org.", Qtype: dns.TypeA,
+		Answer: []dns.RR{
+			test.CNAME("cname.example.org.	60	IN CNAME	mx.example.org."),
+			test.A("mx.example.org.	60	IN A	127.0.0.1"),
+		},
+	},
+	{
+		Qname: "cnameext.example.org.", Qtype: dns.TypeA,
+		Answer: []dns.RR{
+			test.CNAME("cnameext.example.org.	60	IN CNAME	mx.example.net."),
+		},
+	},
+	{
+		Rcode: dns.RcodeServerFailure,
+		Qname: "cnameloop1.example.org.", Qtype: dns.TypeA,
+	},
+	{
+		Rcode: dns.RcodeServerFailure,
+		Qname: "cnamedepth.example.org.", Qtype: dns.TypeA,
+	},
 }
 
 func TestLookupMultipleOrigins(t *testing.T) {