Re: Improved handling of changing DNS resolvers

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 23 Jan 2023 17:20:13 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Sat, Jan 21, 2023 at 10:47:03AM +0100, Stefano Brivio wrote:
> > Hi Noah,
> > 
> > Sorry for the delay, I didn't check pending mailing list posts for a
> > couple of days. Comments below:
> > 
> > On Tue, 17 Jan 2023 11:50:50 -0800
> > Noah Gold <nkgold@google.com> wrote:
> >   
> > > Hi folks,
> > > 
> > > libslirp and Passt have different approaches to sharing DNS resolvers with
> > > the guest system, each with their own benefits & drawbacks. On the libslirp
> > > project, we're discussing [1] how to support DNS failover. Passt already has
> > >  support for this, but there is a drawback to its solution which prevents us
> > > from taking a similar approach: the resolvers are read exactly once, so if the
> > > host changes networks at runtime, the guest will not receive the updated
> > > resolvers and thus its connectivity will break.  
> 
> So, passt/pasta kinda-sorta binds itself to a particular host
> interface, so DNS won't be the only issue if the host changes
> network.  For one thing, at least by default the guest gets the same
> IP as the host, so if the host IP changes the guest will get out of
> sync.  We'll mostly cope with that ok, but there will be some edge
> cases which will break (most obviously if after the network change the
> guest wants to talk to something at the host's old address / its
> current address).

Noah, by the way, if your usage for DNS failover is related to a
virtual machine being migrated to another host with different
addressing, mind that you could simply tell qemu to connect to a new
instance of passt. That's something you can't do with libslirp.

Would that solve your problem, or your issue is specifically related to
DNS failover without any VM migration playing a role?

-- 
Stefano