openvpn dns issue

[Colin Law]

I wonder whether anyone can help with this rather odd problem.

I am using Ubuntu 16.10 with Unity Desktop.  I have vpn access to two
servers running openvpn, one a Pi running raspbian Jessie and the
other another Ubuntu Desktop running 14.04. I can connect to either of
the remote systems via the vpn and everything seems to be working
fine.  The problem arises when I disconnect  from the vpn and later
reconnect, I then find that I have no DNS available. Internet access
via IP is ok, it is just DNS that is not working. For example, dig
www.google.com just times out.

In the vpn servers I have specified
push "redirect-gateway def1 bypass-dhcp"
which I believe should send dns requests through the server, and using
tcpdump I can see that this is the case, dns requests are sent via
tun0.  However when I disconnect and reconnect I no longer see the
requests on tun0, but on eth0.  In order to recover I have to reboot
the PC (the client, I do not need to touch the servers) and then the
first time I connect all is well again.

It is not even necessary to reconnect to the same server to see the
problem, if I connect to server A, then disconnect and connect to
server B then again I see the dns problem

I can't see anything obvious in the logs and I have not managed to get
any clues via google.  Can anyone suggest what I might do to analyse
this?

Colin