[Dnsmasq-discuss] Intermittent DNSMASQ resolution failures

[Stuart Wilson]

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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