On Mon, Sep 04, 2017 at 02:57:48AM +0000, sauteeed via RT wrote:
> Software Version: BIND-9.9.6-P1
> OS: CentOS 6.4
> Subject:error sending response: unset
> 
> 
> Bug Detail
> ===========
> Hi,
> 
> I got 2 servers running the same version of BIND and OS, but one of
> them periodically encounters the problem "error sending response:
> unset", and when this took place, server just failed to send DNS
> responses to the clients. I don't see any difference between these 2
> servers themselves. Maybe something outside could lead it to such a
> case?

9.9.6-P1 is a very old version of BIND (nearly 3 years old).  You should
be using the latest release of BIND 9.9.

For the log message you are seeing, please look at this knowledge base article:
https://kb.isc.org/article/AA-00717

You may find it useful to email bind-users list where you might raise
questions like this, instead of creating bug tickets.

		Mukund