net: don't require recursion be available in DNS responses

Fixes #12778

Change-Id: I2ca53180d46180b951749abe453fd560d0f1d9d6
Reviewed-on: https://go-review.googlesource.com/16950Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient.go

@@ -40,7 +40,7 @@ func reverseaddr(addr string) (arpa string, err error) {
 func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs []dnsRR, err error) {
 	addrs = make([]dnsRR, 0, len(dns.answer))
 
-	if dns.rcode == dnsRcodeNameError && dns.recursion_available {
+	if dns.rcode == dnsRcodeNameError {
 		return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
 	}
 	if dns.rcode != dnsRcodeSuccess {

src/net/dnsclient_test.go

@@ -92,3 +92,26 @@ func TestIssue8434(t *testing.T) {
 		t.Fatalf("IsTemporary = false for err = %#v; want IsTemporary == true", err)
 	}
 }
+
+// Issue 12778: verify that NXDOMAIN without RA bit errors as
+// "no such host" and not "server misbehaving"
+func TestIssue12778(t *testing.T) {
+	msg := &dnsMsg{
+		dnsMsgHdr: dnsMsgHdr{
+			rcode:               dnsRcodeNameError,
+			recursion_available: false,
+		},
+	}
+
+	_, _, err := answer("golang.org", "foo:53", msg, uint16(dnsTypeSRV))
+	if err == nil {
+		t.Fatal("expected an error")
+	}
+	de, ok := err.(*DNSError)
+	if !ok {
+		t.Fatalf("err = %#v; wanted a *net.DNSError", err)
+	}
+	if de.Err != errNoSuchHost.Error() {
+		t.Fatalf("Err = %#v; wanted %q", de.Err, errNoSuchHost.Error())
+	}
+}

src/net/dnsclient_unix.go

@@ -182,7 +182,11 @@ func tryOneName(cfg *dnsConfig, name string, qtype uint16) (string, []dnsRR, err
 				continue
 			}
 			cname, rrs, err := answer(name, server, msg, qtype)
-			if err == nil || msg.rcode == dnsRcodeSuccess || msg.rcode == dnsRcodeNameError && msg.recursion_available {
+			// If answer errored for rcodes dnsRcodeSuccess or dnsRcodeNameError,
+			// it means the response in msg was not useful and trying another
+			// server probably won't help. Return now in those cases.
+			// TODO: indicate this in a more obvious way, such as a field on DNSError?
+			if err == nil || msg.rcode == dnsRcodeSuccess || msg.rcode == dnsRcodeNameError {
 				return cname, rrs, err
 			}
 			lastErr = err