On Fri, 18 Aug 2017, Dan wrote:

> Dear All,
>
> I wonder if anyone can help with the following strange network problem
> I'm having on my SL7.3 machine, please:
>
> Every day, just after midday, I suddenly lose access to DNS name
> resolution, and therefore become unable to interact with any remote
> hosts, even though my network interface is still up.  Manually taking
> the network interface down then up again restores name resolution
> (although bringing the interface back up is surprisingly slow), but
> only for a few minutes before it fails again.  These cycles go on
> until about 1pm, after which all is fine once more.

This could be a red herring, so beware of wild goose chases.

You appear to be on timezone +0100 (from other header info I'm
going to assume BST). I've seen software break for just that one hour
when there has been a confusion over BST/GMT.
(IIRC the reason was that the date/time was rounded to the nearest day
by adding 12hrs and truncating.)

> If I try to ping the nameserver (by IP address) during one of the
> periods of name resolution failure, I'm met with ICMP "destination
> unreachable" packets originating from my own IP address on the
> relevant interface.  (Whereas pinging the nameserver at any other time
> produces successful responses.)

Does ping -n have the same problem ?