[systemd-devel] Temporarily add DHCP DNS as fallback

[Kenneth Porter]

--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.