[Dnsmasq-discuss] Dnsmasq switching to using secondary DNS exclusively

[Simon Kelley]

Michael Kraft wrote:
> I have a WRT54GLv1 with Tomato 1.25 on it.  Tomato uses Dnsmasq 2.47 for 
> DNS.  I realize the latest Dnsmasq is 2.49, but I don't see anything in 
> the change log related to what I'm seeing.   I have tomato set up to 
> cache DNS (150 entries) and handle the DNS queries itself.  This results 
> in a dnsmasq.conf of:
> 
> pid-file=/var/run/dnsmasq.pid
> interface=br0
> resolv-file=/etc/resolv.dnsmasq
> addn-hosts=/etc/hosts.dnsmasq
> expand-hosts
> min-port=4096
> dhcp-range=192.168.1.100,192.168.1.149,255.255.255.0,10080m
> dhcp-option=3,192.168.1.1
> dhcp-lease-max=255
> dhcp-authoritative
> dhcp-host=00:13:CE:2A:1E:82,192.168.1.6,10080m
> 
> The resolv.dnsmasq is:
> nameserver 68.87.64.146
> nameserver 68.87.75.194
> 
> resolv.conf is:
> nameserver 127.0.0.1
> 
> So my primary DNS is 68.87.64.146 and my secondary DNS is 68.87.75.194.
> 
> Every now and then the router starts using the secondary DNS 
> exclusively.  During this time the dnsmasq.conf and resolv.dnsmasq don't 
> change.  I can get the router to start using the primary DNS again by 
> killing and restarting Dnsmasq (or refreshing the DHCP release from my 
> ISP). 
> 
> It's my understanding that the secondary DNS should only be used if the 
> primary DNS doesn't answer the DNS query, but at the times I'm 
> experiencing the problem the primary DNS is responding to queries so 
> Dnsmasq should be using that.  There is nothing in the logs that seem to 
> indicate any kind of problem as far as I can tell.  The only log entries 
> I ever see relating to nameservers look like the following with the 
> secondary listed first and the primary listed second:
> 
> Aug 11 23:55:55 ? daemon.info dnsmasq[23367]: using nameserver 
> 68.87.75.194#53
> Aug 11 23:55:55 ? daemon.info dnsmasq[23367]: using nameserver 
> 68.87.64.146#53
> 
> 
> Since my ISP's secondary DNS is a lot farther away geographically from 
> me than the primary, this not only slows down DNS queries, but causes 
> performance issues with Limelight, Akamai and any other server that 
> determines the server farm, by the DNS location.
> 
> My question is why would Dnsmasq be switching to using the secondary DNS 
> exclusively and how (short of removing the secondary DNS server 
> manually) can I stop it from doing this?


Dnsmasq doesn't treat these servers as primary and secondary - it 
considers them equal. The reason that the server in use changes is that 
dnsmasq sometimes sends a query to both servers, to see which one is 
fastest. Whichever replies first gets used until the next time the 
"race" gets run.

You can change this behaviour with "strict-order", but be aware that 
will make performance much worse if the first server ever does fail.


Cheers,

Simon.