[FS#391] dnsmasq stops working properly if the fastest upstream DNS server returns a server failure

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - IronicSven (IronicSven) 

Attached to Project - LEDE Project
Summary - dnsmasq stops working properly if the fastest upstream DNS server returns a server failure
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - ** - Device problem occurs on
**Reproduced on TP-Link 1043nd v1 and TP-Link Archer C7 v2.

** - Software versions of LEDE release, packages, etc.
**Reboot (SNAPSHOT, r2961-5b089e4)
Dnsmasq version 2.76

** - Steps to reproduce
**Fastest upstream DNS server returns a server failure.

My provider is having some difficulties with his DNS servers this week. I noticed that if the fastest DNS server returns a server failure dnsmasq stops working properly because it ignores the replys of the slower DNS servers.

In google chrome ERR_NAME_RESOLUTION_FAILED appears and nslookup returns ** server can't find google.com: SERVFAIL

I don't use strict-order and it doesn't matter if the faulty upstream DNS server is the first or the last entry in the config as long as it returns the fastest reply.
I had to delete the upstream DNS server which returns the server failure from my config to get dnsmasq working again.

I was able to create a tcpdump and syslog while the DNS server 83.169.185.162 returned a server failure today.
- You can see in syslog.txt that the reply messages are missing until I delete 83.169.185.162 from the config.
- The tcpdump wan.pcap shows that 83.169.185.162 returns the fastest reply with a server failure and that the other DNS servers work properly but dnsmasq seems to ignore their replys.

One or more files have been attached.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=391