plugin/file: run additional processing for CNAME/DNAME answers (#8337)

externalLookup, which resolves the chase for ordinary CNAME, wildcard
CNAME, and DNAME answers, returned a nil additional section. So a chased
SRV/MX/SVCB/HTTPS answer with an in-bailiwick target was missing the
target's A/AAAA glue, unlike the direct path.

Run additionalProcessing at externalLookup's return points so all three
callers add the glue, and add ordinary-CNAME, wildcard-CNAME, and DNAME
regression cases.

Fixes #6628

Signed-off-by: Saleh <root@lr0.org>

plugin/file/lookup.go

@@ -350,7 +350,10 @@ func (z *Zone) authority(do bool, result Result) []dns.RR {
 	return z.ns(do)
 }
 
-// externalLookup adds signatures and tries to resolve CNAMEs that point to external names.
+// externalLookup adds signatures and tries to resolve CNAMEs that point to
+// external names. It also runs additional-section processing on the resolved
+// answer so in-bailiwick SRV/MX/SVCB/HTTPS targets get their A/AAAA glue, like
+// the direct path.
 func (z *Zone) externalLookup(ctx context.Context, state request.Request, tr *tree.Tree, elem *tree.Elem, rrs []dns.RR) ([]dns.RR, []dns.RR, []dns.RR, Result) {
 	qtype := state.QType()
 	do := state.Do()
@@ -369,7 +372,7 @@ func (z *Zone) externalLookup(ctx context.Context, state request.Request, tr *tr
 	if elem == nil || (qtype == dns.TypeNS || qtype == dns.TypeSOA && targetName == z.origin) {
 		lookupRRs, result := z.doLookup(ctx, state, targetName, qtype)
 		rrs = append(rrs, lookupRRs...)
-		return rrs, z.authority(do, result), nil, result
+		return rrs, z.authority(do, result), z.additionalProcessing(rrs, do), result
 	}
 
 	i := 0
@@ -392,12 +395,12 @@ Redo:
 		if elem == nil || (qtype == dns.TypeNS || qtype == dns.TypeSOA && targetName == z.origin) {
 			lookupRRs, result := z.doLookup(ctx, state, targetName, qtype)
 			rrs = append(rrs, lookupRRs...)
-			return rrs, z.authority(do, result), nil, result
+			return rrs, z.authority(do, result), z.additionalProcessing(rrs, do), result
 		}
 
 		i++
 		if i > 8 {
-			return rrs, z.ns(do), nil, Success
+			return rrs, z.ns(do), z.additionalProcessing(rrs, do), Success
 		}
 
 		goto Redo
@@ -414,7 +417,7 @@ Redo:
 		}
 	}
 
-	return rrs, z.ns(do), nil, Success
+	return rrs, z.ns(do), z.additionalProcessing(rrs, do), Success
 }
 
 // findDelegation returns the first zone cut between the zone apex and qname.

plugin/file/lookup_test.go

@@ -408,6 +408,48 @@ var additionalTestCases = []test.Case{
 			test.AAAA("other.example.org. 1800 IN AAAA 2001:db8::20"),
 		},
 	},
+	{
+		// An ordinary CNAME chased to an SRV still needs the target's glue (#6628).
+		Qname: "cname-srv.example.org.", Qtype: dns.TypeSRV,
+		Answer: []dns.RR{
+			test.CNAME("cname-srv.example.org. 1800 IN CNAME srv.example.org."),
+			test.SRV("srv.example.org. 1800 IN SRV 10 50 8080 host.example.org."),
+			test.SRV("srv.example.org. 1800 IN SRV 20 50 8080 host.example.org."),
+		},
+		Ns: additionalAuth,
+		Extra: []dns.RR{
+			test.A("host.example.org. 1800 IN A 192.0.2.10"),
+			test.AAAA("host.example.org. 1800 IN AAAA 2001:db8::10"),
+		},
+	},
+	{
+		// A wildcard CNAME chased to an SRV.
+		Qname: "foo.wild.example.org.", Qtype: dns.TypeSRV,
+		Answer: []dns.RR{
+			test.CNAME("foo.wild.example.org. 1800 IN CNAME srv.example.org."),
+			test.SRV("srv.example.org. 1800 IN SRV 10 50 8080 host.example.org."),
+			test.SRV("srv.example.org. 1800 IN SRV 20 50 8080 host.example.org."),
+		},
+		Ns: additionalAuth,
+		Extra: []dns.RR{
+			test.A("host.example.org. 1800 IN A 192.0.2.10"),
+			test.AAAA("host.example.org. 1800 IN AAAA 2001:db8::10"),
+		},
+	},
+	{
+		// A DNAME substitution landing on an SRV (#6628).
+		Qname: "srv.dname.example.org.", Qtype: dns.TypeSRV,
+		Answer: []dns.RR{
+			test.DNAME("dname.example.org. 1800 IN DNAME dsub.example.org."),
+			test.CNAME("srv.dname.example.org. 1800 IN CNAME srv.dsub.example.org."),
+			test.SRV("srv.dsub.example.org. 1800 IN SRV 10 50 8080 host.example.org."),
+		},
+		Ns: additionalAuth,
+		Extra: []dns.RR{
+			test.A("host.example.org. 1800 IN A 192.0.2.10"),
+			test.AAAA("host.example.org. 1800 IN AAAA 2001:db8::10"),
+		},
+	},
 }
 
 func TestAdditionalSectionDeduplication(t *testing.T) {
@@ -504,4 +546,14 @@ mixed		IN SRV	10 50 8080 host.example.org.
 ; Two MX records with distinct targets.
 two		IN MX	10 host.example.org.
 		IN MX	20 other.example.org.
+
+; A CNAME resolving to an SRV: the ordinary-CNAME chase must still add glue.
+cname-srv	IN CNAME	srv.example.org.
+
+; A wildcard CNAME resolving to an SRV.
+*.wild		IN CNAME	srv.example.org.
+
+; A DNAME whose subtree holds an SRV: the DNAME chase must add glue too.
+dname		IN DNAME	dsub.example.org.
+srv.dsub	IN SRV	10 50 8080 host.example.org.
 `