[Dnsmasq-discuss] Intermittent DNSMASQ resolution failures

[Simon Kelley]

On 15/02/13 06:55, Stuart Wilson wrote:
> Noticed after sending the original email, the version I am using is
> 2.57. Could an upgrade fix this issue?


Unlikely, there are no bugs that correspond with the symptoms fixed in 
any releases for a long time. The most common reason for this is that 
one of the upstream servers is dodgy. Setting log-queries and then 
looking at the logs can often reveal which one.

Simon.

>
> /*Regards,
> Stuart
>
> */
> On 02/15/2013 01:32 AM, Stuart Wilson wrote:
>> Hi,
>>
>> I have noticed lately on several machines using the local cable ISP in
>> my neck of the woods, that using the DHCP supplied DNS servers from my
>> ISP is significantly slower than using a global DNS service like
>> OpenDNS. With that in mind I configured the Linux box I use at home to
>> use OpenDNS. It was working great, until I decided to fix it by adding
>> DNSMASQ as a local caching server to lighten the load. It works fine
>> most of the time, but sometimes I get intermittent failures to resolve
>> names. At first I just noticed delays getting to some websites, and
>> occasionally it would fail entirely. At times though it became
>> unacceptable and failed a lot. So, I started testing name resolution
>> in a shell using the "host" command, and found that it did indeed
>> sometimes give me a ";; connection timed out; no servers could be
>> reached" error. When I specifically ask the host command to query the
>> OpenDNS server directly, bypassing DNSMASQ, it never fails and is
>> always very fast.
>>
>> I got really curious about this and captured some packets with
>> Wireshark. First of a host query going through DNSMASQ that failed,
>> and then one going directly to the DNS server. I did indeed get no
>> reply back on the query that failed. The only difference I could find
>> between the packets being sent to OpenDNS by DNSMASQ, and those going
>> direclty from the OS to OpenDNS, is that the queries that failed from
>> DNSMASQ had the DF (don't fragment) bit set. Now it is quite possible
>> I'm missing something here, but it occurs to me that my using DNS
>> servers half way across the internet, rather than right down the
>> street at the local ISP, could be causing packets with the DF bit set
>> to get dropped. Is there any way to tell DNSMASQ to not set the DF
>> bit? Can anyone think of another reason why this is failing for me?
>> --
>>
>> /*Regards,
>> Stuart
>>
>>
>> */
>>
>>
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>
>
>
>
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