Fix review comments

upstream/dns_upstream.go

@@ -27,7 +27,7 @@ func NewDnsUpstream(endpoint string, proto string, tlsServerName string) (Upstre
 
 	var tlsConfig *tls.Config
 
-	if tlsServerName != "" {
+	if proto == "tcp-tls" {
 		tlsConfig = new(tls.Config)
 		tlsConfig.ServerName = tlsServerName
 	}
@@ -101,6 +101,9 @@ func (u *DnsUpstream) exchange(proto string, query *dns.Msg) (r *dns.Msg, err er
 		conn.Close() // Not giving it back
 	}
 
-	u.transport.Yield(conn)
+	if err == nil {
+		// Return it back to the connections cache if there were no errors
+		u.transport.Yield(conn)
+	}
 	return r, err
 }

upstream/helpers.go

@@ -72,8 +72,7 @@ func CreateResolver(bootstrap string) *net.Resolver {
 			PreferGo: true,
 			Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
 				var d net.Dialer
-				conn, err := d.DialContext(ctx, network, bootstrap)
-				return conn, err
+				return d.DialContext(ctx, network, bootstrap)
 			},
 		}
 	}

upstream/upstream.go

@@ -52,4 +52,6 @@ func (p *UpstreamPlugin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *
 }
 
 // Name implements interface for CoreDNS plugin
-func (p *UpstreamPlugin) Name() string { return "upstream" }
+func (p *UpstreamPlugin) Name() string {
+	return "upstream"
+}