[dns_sd] Send an EDNS0 query by default (#3586)

Based on https://groups.google.com/d/topic/prometheus-users/02kezHbuea4/discussion

Does not attempt to handle a situation where the server does not understand
EDNS0, however that is an unlikely case, and the behaviour of such ancient
systems is hard to predict in advance, so if it does come up, it will need
to be handled on a case-by-case basis.

discovery/dns/dns.go

@@ -295,7 +295,7 @@ func lookupFromAnyServer(name string, qtype uint16, conf *dns.ClientConfig, logg
 
 	for _, server := range conf.Servers {
 		servAddr := net.JoinHostPort(server, conf.Port)
-		msg, err := askServerForName(name, qtype, client, servAddr, false)
+		msg, err := askServerForName(name, qtype, client, servAddr, true)
 		if err != nil {
 			level.Warn(logger).Log("msg", "DNS resolution failed", "server", server, "name", name, "err", err)
 			continue
@@ -311,8 +311,8 @@ func lookupFromAnyServer(name string, qtype uint16, conf *dns.ClientConfig, logg
 }
 
 // askServerForName makes a request to a specific DNS server for a specific
-// name (and qtype).  Retries in the event of response truncation, but
-// otherwise just sends back whatever the server gave, whether that be a
+// name (and qtype).  Retries with TCP in the event of response truncation,
+// but otherwise just sends back whatever the server gave, whether that be a
 // valid-looking response, or an error.
 func askServerForName(name string, queryType uint16, client *dns.Client, servAddr string, edns bool) (*dns.Msg, error) {
 	msg := &dns.Msg{}
@@ -327,10 +327,9 @@ func askServerForName(name string, queryType uint16, client *dns.Client, servAdd
 		if client.Net == "tcp" {
 			return nil, fmt.Errorf("got truncated message on TCP (64kiB limit exceeded?)")
 		}
-		if edns { // Truncated even though EDNS is used
-			client.Net = "tcp"
-		}
-		return askServerForName(name, queryType, client, servAddr, !edns)
+
+		client.Net = "tcp"
+		return askServerForName(name, queryType, client, servAddr, false)
 	}
 	if err != nil {
 		return nil, err