Subject: DNS server retries

DNS server retries

From: David Stuart <>
Date: Mon, 10 Sep 2012 09:59:50 -0400


We're using c-ares in some of our products and recently came across a
problem (which was the subject of one of my previous e-mails). But
basically we believe it boils down to the order in which DNS servers are
tried (or not) in the event of an NXDOMAIN response.

The (albeit fabricated) scenario goes a bit like this:

1) setup a hostname which only resolves while using a VPN (i.e.
non-publicly visible address)

2) Go into the windows adapter settings and change the binding order
such that the VPN is no longer on the top.

3) Try to resolve a machine name which is inside of the VPN network.

Of course we all agree that the "default" DNS server on the first
adapter should be tried first. But what happens after that is the
subject of the e-mail.

We're not totally sure how it's working in c-ares today, but it seems as
though even in the event of a NXDOMAIN response, the second adapter is
not tried (or at least not as soon as it should be)?

So the proposal on the table is that I would be willing to try to make
the needed modifications, assuming everyone is OK with taking the
product in that direction. Ideally I could get those changes "mainlined"
into the c-ares source. Let me know your thoughts.


David Stuart, CounterPath
Email: dstuart (at) counterpath (dot) com
Phone: (613) 254-8886 x2234  Web:
Address: 310 - 350 Terry Fox Drive, Kanata Ontario, K2K 2P5
Received on 2012-09-10