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

Automatically submitted.

test/file_loop_test.go

@@ -0,0 +1,49 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+const loopDB = `example.com. 500 IN SOA ns1.outside.com. root.example.com. 3 604800 86400 2419200 604800
+example.com. 500 IN NS ns1.outside.com.
+a.example.com. 500 IN CNAME b.example.com.
+*.foo.example.com. 500 IN CNAME bar.foo.example.com.`
+
+func TestFileLoop(t *testing.T) {
+	name, rm, err := test.TempFile(".", loopDB)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	// Corefile with for example without proxy section.
+	corefile := `example.com:0 {
+		file ` + name + `
+	}`
+
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("something.foo.example.com.", dns.TypeA)
+
+	r, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Could not exchange msg: %s", err)
+	}
+
+	// This should not loop, don't really care about the correctness of the answer.
+	// Currently we return servfail in the file lookup.go file.
+	// For now: document current behavior in this test.
+	if r.Rcode != dns.RcodeServerFailure {
+		t.Errorf("Rcode should be dns.RcodeServerFailure: %d", r.Rcode)
+
+	}
+}