What's probably happening is this.

1) query arrives from client,
2) UDP socket opened, and query send from socket to more than one
upstream server.
3) Reply arrives from fastest upstream server, reply returned to client.
4) upstream socket closed.
5) reply arrives from slower upstream server -> destination unreachable.

dnsmasq does the "send the query to all available servers" trick every
once in a while to find the fastest one.

If this is what you're seeing, it's a feature, not a bug.

Cheers,

Simon.




On 27/06/17 06:56, Roger James wrote:
> I am seeing some rather perplexing behaviour regarding the reception of
> upstream IPV6 UDP DNS query responses on a Debian (raspian) system
> running on a Raspberry Pi. It appears that dnsmasq is not holding open
> an IPV6 UDP port to handle a response to an upstream query. What my
> network monitor shows is the upstream request going out and a reply
> coming back in. However the Pi rejects the incoming UDP packet with a
> ICMPV6 Destination unreachable (port unreachable).
> 
> I may be missing something obvious here, but it looks like dnsmasq is
> not holding open a listening port for the response.
> 
> Has anyone got any ideas on what is going, or can point me at some ways
> of tracking this down.
> 
> The configuration of dnsmasq is out of the box Debian.
> 
> Thanks,
> 
> Roger
> 
> 
> 
> _______________________________________________
> Dnsmasq-discuss mailing list
> Dnsmasq-discuss@lists.thekelleys.org.uk
> http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
> 

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