Truncated response over UDP -> immediately retry over TCP

This reduces latency, because when the client retries, or if the
query padding was large enough, we can reply from the cache or
even immediately.

dnscrypt-proxy/proxy.go

@@ -376,6 +376,10 @@ func (proxy *Proxy) processIncomingQuery(serverInfo *ServerInfo, clientProto str
 			serverInfo.noticeBegin(proxy)
 			if serverProto == "udp" {
 				response, err = proxy.exchangeWithUDPServer(serverInfo, sharedKey, encryptedQuery, clientNonce)
+				if err == nil && len(response) >= MinDNSPacketSize && response[2]&0x02 == 0x02 {
+					dlog.Debug("Truncated response over UDP, retrying over TCP")
+					response, err = proxy.exchangeWithTCPServer(serverInfo, sharedKey, encryptedQuery, clientNonce)
+				}
 			} else {
 				response, err = proxy.exchangeWithTCPServer(serverInfo, sharedKey, encryptedQuery, clientNonce)
 			}