default local DNS caching name server

Simo Sorce simo at
Sat Apr 12 20:09:47 UTC 2014

On Sat, 2014-04-12 at 13:04 -0400, Chuck Anderson wrote:
> On Sat, Apr 12, 2014 at 12:06:23PM -0400, Paul Wouters wrote:
> > On Sat, 12 Apr 2014, Chuck Anderson wrote:
> > 
> > >Okay, so here is where you and I differ then.  We need a solution to
> > >run everywhere, on every system, in every use case.
> > 
> > Sounds like wanting ponies? Obviously I fully agree with a solution that
> > works everywhere, all the time, for everyone, however the want it :)
> > 
> > > The local DNS
> > >daemon (note that I didn't say "cache" this time) should be a part of
> > >the Base OS like init/systemd is.  It should be small, unobtrusive,
> > >and do very little, namely the one thing we need: handle failover
> > >between multiple DNS servers.  I would use the term "DNS proxy" but
> > >that term is too overloaded with other connotations and preconceived
> > >ideas.
> > 
> > Handling failover requires keeping state of previous queries and
> > outstanding requests to determine which servers are bad or not. Mind
> > you, unbound allows you to set a max TTL on any record received using
> > cache-max-ttl=0, so you can very easilly implement this idea. I think
> > it is a bad idea, because your solution violates your own principle
> > above: it interferes with my use case of optimising DNS caches, reducing
> > unneccessary latency, and doing things like pre-fetching of low TTL
> > records.
> Of course there would be /some/ state kept.  It just wouldn't cache
> the data, it would only use the state of recent queries and response
> times to determine if that resolver was dead and start sending those
> queries to another resolver.  It would basically do exactly what
> glibc's stub resolver does now, but ONCE for the entire system rather
> than having each process do that independently.  I would want this
> daemon to be as lightweight as possible to minimize any interference
> with optimising DNS caches, latency, etc. and so that it could be used
> everywhere, just like systemd is used on all Fedora systems and some
> form of "init" is used on all Linux systems.
> Another way to think of this is to separate out the built-in logic in
> unbound/BIND/dnsmasq/etc. that determines when an authoritative server
> is dead and apply it to all queries that are made by glibc's stub
> resolver.  Or separate out the logic that glibc uses to determine when
> a nameserver in /etc/resolv.conf is dead and make that a system-wide
> daemon.

You can do this today, just write a nsswitch module to handle the host
database and connect to it via a pipe from all processes.

> > In DNS, the publisher of data tells you how long the data should be valid
> > for. If they want the record not to be cached at all, they can set the TTL
> > to 0. Why should we deploy a daemon that does not provide the very useful
> > feature of caching in general (especially when doing DNSSEC validation)
> > when people who wish to not get cached already have a means out, publish
> > records with TTL=0? If you want to be Akamai, you can!
> Because things get messy once you start caching on the end-user
> system.

Citation please ?
What kind of messy ? If you properly handle TTLs what gets messed up ?
*especially* if unbound is configured to automatically flush caches when
you change networks.

>  Sure, you can optionally have that messiness (and I'd argue
> that for Fedora Workstation that would be a sane default) but for
> Fedora Server I think it is too heavyweight of a solution to run
> everywhere, and you agreed that running this in VMs is probably not
> desired.
> If the lightweight dnslookupd process is configured to forward the
> request to a local unbound+dnssec-triggerd, then everything from that
> point will work in the same way it does today with local caching, TTL
> handling, DNSSEC, etc.  But that should be /optional/.  I'm arguing
> that dnslookupd should be on by default everywhere.

Can you substantiate what is lightweight for you ?
I have unbound running on my machine and it is basically unnoticeable.
The resident memory is 15MiB, with the data and all right around the
same size of other similar daemons like polkitd system-journald dhclient
dbus-daemon, all stuff you already run your servers and I have never
heard anybody call them *heavy* weight.

> > >dnslookupd keeps track of up/down DNS servers via some health check
> > >mechanism, and switches between them appropriately.
> > 
> > I tend to call heartbeats/keepalives "make deads". They often do the
> > opposite. Why invent a whole new health check protocol when you can
> > simple send DNS queries and use strategies to prefer the nearest/fastest
> > servers already. These kind of selection/preference protocols are part
> > of any decent DNS implementation. There is no need to re-invent the
> > wheel.
> It doesn't need to do active heartbeats--it could passively watch
> queries/responses that it is forwarding to the resolver and decide
> based on that if a server is dead and stop querying it until the next
> one fails, etc. just like glibc does today.

Are you volunteering for writing this daemon ?
Because if you are not then we are wasting time I would think.

> For the use cases you desire with full caching and DNSSEC, dnslookupd
> shouldn't get in the way.  All applications/glibc would query
>, which would immediately forward all those requests to the
> local unbound+dnssec-triggerd setup.  Dnslookupd would only take
> action if unbound died for some reason (and if there was an alternate
> DNS resolver to switch to).

I still fail to see what is the point of writing yet another DNS caching
(but non-caching) dameon when we already have unbound, and as Paul said,
if you really want to, you can tell it to override TTLs to 0 causing it
to effectively not cache data.

I see no point in writing something new in this area.


Simo Sorce * Red Hat, Inc * New York

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