[systemd-devel] Temporarily add DHCP DNS as fallback

[Jurek Olden]

On 08.10.19 21:10, Kenneth Porter wrote:
> --On Tuesday, October 08, 2019 11:12 AM +0200 Jurek Olden
> <jurek.olden at in.tum.de> wrote:
> 
>> For instance when using a WLAN to connect via AnyConnect, i need to be
>> able to resolve the AnyConnect server via the local DNS.
>> Company networks with local name resolution would be another example.
>>
>> Is there a way to make systemd (i use networkd and resolved) use the
>> DHCP provided DNS to resolve anything the static DNS can't (or when the
>> static one isn't reachable yet) and then go back to the static DNS?
> 
> This is a tricky problem and not really specific to systemd. It's a
> resolver-on-VPN-client problem. When I've used AnyConnect on Windows,
> the Cisco client intercepts my local DNS and replaces it with the remote
> DNS, so my local servers and workstations can no longer be resolved.
> That means I can't pull email from my LAN server while I'm connected to
> the VPN. I've used the open source VPN client on my Linux gateway and
> ignored the remote DNS but that has the downside that I can't resolve
> hosts on the remote LAN. How would the client know which DNS to ask for
> any name? It would need to ask both and then return a not-found only if
> both failed. I don't think any resolvers or DNS servers do this. But I'd
> be interested in any that had this kind of fallback feature.

I think i have mixed two things together here, first there is what you
call the resolver-on-VPN problem and that is not trivial.

But is it possible to configure systemd-resolved so that it tries the
DHCP DNS if the static DNS is not reachable at all (no NXDOMAIN, just
not routable/no response)?

That would already help me out tremendously.

Regards,
Jurek Olden