Re: Improved handling of changing DNS resolvers

[David Gibson <david@gibson.dropbear.id.au>]

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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).

> Right -- the main motivation behind this (other than simplicity) is that
> we can close /etc/resolv.conf before sandboxing.
> 
> However, we could keep a handle on it, just like we do for PID and pcap
> files, while still unmounting the filesystem.
> 
> And we could also use inotify to detect changes I guess -- we do the
> same to monitor namespaces in pasta mode (see pasta_netns_quit_init()).

All true, but I'm not sure those are actually the most pressing issues
we'll face with a host network change.

> > libslirp's current approach is to DNAT a single address exposed to the guest
> > to one of the resolvers configured on the host. The problem here is that if that
> > one resolver goes down, the guest can't resolve DNS names. We're
> > considering changing so that instead of a single address, we expose a set of
> > MAXNS addresses, and DNAT those 1:1 to the DNS resolvers registered with
> > the host. Because the DNAT table lives on the host side, we can refresh the
> > guest's resolvers whenever the host's resolvers change, but without the need to
> > expire a DHCP lease (even with short leases, the guest will still lose
> > connectivity
> > for a time).
> > 
> > Does this sound like an approach Passt would be open to adopting as well?
> 
> Yes, definitely, patches would be very welcome.

Hm, that's doesn't fit that easily into the passt model.  For the most
part we don't NAT at all, we only have a couple of special cases where
we do.  Because of that the problem with adding any extra NAT case is
address allocation.  Currently we use the host's gateway address,
which mostly works but is a bit troublesome.  I have some ideas I
think will work better, but those don't necessarily get us more
available addresses.

> Note that David (Cc'ed) is currently working on a generalised/flexible
> address mapping mechanism, some kind of (simple) NAT table as far as I
> understood it.

That's a bit overstating it.  I'm making our current single NAT case
(translating host side loopback to gateway address on the guest) more
configurable.  I have plans (or at least ideas) for a more generalized
NAT mechanism, but I'm really not implementing that yet.  What I'm
doing now is kind of a soft prerequisite for that rework though (as
well as useful in its own right).

> This might even address your DNS idea already, I'm not sure, I'd wait
> for him to comment.

Hadn't considered specifically that model, but it's a reasonbly
natural extension of it (address allocation is still a complication).
I'll certainly consider this case when I do more on this.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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