plugin/file: guard against cname loops (#4387)

Automatically submitted.

plugin/file/lookup.go

@@ -3,6 +3,7 @@ package file
 import (
 	"context"
 
+	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin/file/rrutil"
 	"github.com/coredns/coredns/plugin/file/tree"
 	"github.com/coredns/coredns/request"
@@ -29,7 +30,6 @@ const (
 // Lookup looks up qname and qtype in the zone. When do is true DNSSEC records are included.
 // Three sets of records are returned, one for the answer, one for authority  and one for the additional section.
 func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string) ([]dns.RR, []dns.RR, []dns.RR, Result) {
-
 	qtype := state.QType()
 	do := state.Do()
 
@@ -62,6 +62,16 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 		elem, wildElem *tree.Elem
 	)
 
+	loop, _ := ctx.Value(dnsserver.LoopKey{}).(int)
+	if loop > 8 {
+		// We're back here for the 9th time; we have a loop and need to bail out.
+		// Note the answer we're returning will be incomplete (more cnames to be followed) or
+		// illegal (wildcard cname with multiple identical records). For now it's more important
+		// to protect ourselves then to give the client a valid answer. We return with an error
+		// to let the server handle what to do.
+		return nil, nil, nil, ServerFailure
+	}
+
 	// Lookup:
 	// * Per label from the right, look if it exists. We do this to find potential
 	//   delegation records.
@@ -105,6 +115,7 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 			// Only one DNAME is allowed per name. We just pick the first one to synthesize from.
 			dname := dnamerrs[0]
 			if cname := synthesizeCNAME(state.Name(), dname.(*dns.DNAME)); cname != nil {
+				ctx = context.WithValue(ctx, dnsserver.LoopKey{}, loop+1)
 				answer, ns, extra, rcode := z.externalLookup(ctx, state, elem, []dns.RR{cname})
 
 				if do {
@@ -156,6 +167,7 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 	if found && shot {
 
 		if rrs := elem.Type(dns.TypeCNAME); len(rrs) > 0 && qtype != dns.TypeCNAME {
+			ctx = context.WithValue(ctx, dnsserver.LoopKey{}, loop+1)
 			return z.externalLookup(ctx, state, elem, rrs)
 		}
 
@@ -192,6 +204,7 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 		auth := ap.ns(do)
 
 		if rrs := wildElem.TypeForWildcard(dns.TypeCNAME, qname); len(rrs) > 0 {
+			ctx = context.WithValue(ctx, dnsserver.LoopKey{}, loop+1)
 			return z.externalLookup(ctx, state, wildElem, rrs)
 		}
 

plugin/pkg/upstream/upstream.go

@@ -32,7 +32,6 @@ func (u *Upstream) Lookup(ctx context.Context, state request.Request, name strin
 	req.SetEdns0(uint16(size), do)
 
 	nw := nonwriter.New(state.W)
-
 	server.ServeDNS(ctx, nw, req)
 
 	return nw.Msg, nil