F23 System Wide Change: Default Local DNS Resolver

[Simo Sorce]

On Mon, 2015-06-01 at 21:44 +0200, Reindl Harald wrote:
> Am 01.06.2015 um 21:38 schrieb Andrew Lutomirski:
> > On Mon, Jun 1, 2015 at 12:29 PM, Chris Adams <linux at cmadams.net> wrote:
> >> Once upon a time, Andrew Lutomirski <luto at mit.edu> said:
> >>> I'm with Jason here.  Glibc's resolver is amazingly buggy, and things
> >>> break randomly and unreproducibly when this happens.  A good setup
> >>> would have a local resolver and glibc would be configured to cache
> >>> nothing whatsoever.  Then, if you need to perform maintenance on the
> >>> local DNS cache, you can do it by flushing your local resolver rather
> >>> than trying to figure out how you're going to persuade every running
> >>> program to tell glibc to flush its cache.
> >>
> >> glibc doesn't have a cache, except each process caching the settings in
> >> /etc/resolv.conf.  That's part of the problem, because there's no way to
> >> cache "first server in resolv.conf is not responding", so each lookup
> >> has to figure that out for itself (many timeouts).
> >
> > Glibc caches *something* that enabled the bug I hit.  I don't know
> > exactly what it's trying to cache, but it's certainly stateful
> 
> it don't cache dns respones - try it out in your local network
> *client applications* may cache respones
> 
> try it out in your local network
> 
> * enter a non existing subdomain in firefox
> * add the hostname to your LAN nameserver
> * try again: firefox refuses
> * restart just firefox
> * it resolves without any delay
> 
> a) that proves no systemwide cachae
> b) it proves with introduce a local systemdwide cache
>     you introduce a problem not existing before
> 

If you have nscd running glibc caches, so it is a matter of
configuration.

The *only* reason why Firefox caches Names is because we do not have a
local dns caching resolver, so Firefox had to implement its own.

If you had a local caching resolver Firefox could be changed to stop
caching on its own instead. Which would be a plus, I often have way too
many tabs open to consider restarting firefox unless the website with
issues is really important. If i had a local resolver it would be easy
to just flush that one and have FF back in business immediately.

As you see it is a matter of perspective.

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York