plugin/file: fix crash (#1301)

When z.Tree.Prev returns zero we should break out of this loop, not
use elem as if nothing has happened.

Can be triggered by sending edns0 to unsigned zone.

plugin/file/lookup.go

@@ -235,7 +235,10 @@ func (z *Zone) Lookup(state request.Request, qname string) ([]dns.RR, []dns.RR,
 
 	ret := z.soa(do)
 	if do {
-		deny, _ := z.Tree.Prev(qname) // TODO(miek): *found* was not used here.
+		deny, found := z.Tree.Prev(qname)
+		if !found {
+			goto Out
+		}
 		nsec := z.typeFromElem(deny, dns.TypeNSEC, do)
 		ret = append(ret, nsec...)
 

test/file_serve_test.go

@@ -0,0 +1,102 @@
+package test
+
+import (
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestZoneEDNS0Lookup(t *testing.T) {
+	t.Parallel()
+	log.SetOutput(ioutil.Discard)
+
+	name, rm, err := TempFile(".", `$ORIGIN example.org.
+@	3600 IN	SOA sns.dns.icann.org. noc.dns.icann.org. (
+		2017042745 ; serial
+		7200       ; refresh (2 hours)
+		3600       ; retry (1 hour)
+		1209600    ; expire (2 weeks)
+		3600       ; minimum (1 hour)
+	)
+
+        3600 IN NS a.iana-servers.net.
+	3600 IN NS b.iana-servers.net.
+
+www     IN A 127.0.0.1
+www     IN AAAA ::1
+`)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	// Corefile with for example without proxy section.
+	corefile := `example.org:0 {
+       file ` + name + `
+}
+`
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeMX)
+	m.SetEdns0(4096, true)
+
+	r, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Could not exchange msg: %s", err)
+	}
+	if r.Rcode == dns.RcodeServerFailure {
+		t.Fatalf("Rcode should not be dns.RcodeServerFailure")
+	}
+}
+
+func TestZoneNoNS(t *testing.T) {
+	t.Parallel()
+	log.SetOutput(ioutil.Discard)
+
+	name, rm, err := TempFile(".", `$ORIGIN example.org.
+@	3600 IN	SOA sns.dns.icann.org. noc.dns.icann.org. (
+		2017042745 ; serial
+		7200       ; refresh (2 hours)
+		3600       ; retry (1 hour)
+		1209600    ; expire (2 weeks)
+		3600       ; minimum (1 hour)
+	)
+
+www     IN A 127.0.0.1
+www     IN AAAA ::1
+`)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	// Corefile with for example without proxy section.
+	corefile := `example.org: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("example.org.", dns.TypeMX)
+	m.SetEdns0(4096, true)
+
+	r, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Could not exchange msg: %s", err)
+	}
+	if r.Rcode == dns.RcodeServerFailure {
+		t.Fatalf("Rcode should not be dns.RcodeServerFailure")
+	}
+}