Subject: bug#20816: Excessive negative cache time for DNS lookups

From: [email-masked] (Ludovic Courtès)
Date: Mon, 15 Jun 2015 22:02:50 +0200

Mark H Weaver <[email-masked]> skribis:

> On my GuixSD system, DNS lookup failures (due to lack of network) are
> cached for at least 4 minutes, and possibly much longer (I've not had
> the patience to wait until it expires).
>
> Even after restarting nscd, the negative cache entry persists for about
> another 20 seconds.

To reproduce it, I ran ‘guix system vm’ on this:

--8<---------------cut here---------------start------------->8---
(use-modules (gnu) (guix))
(use-package-modules base)

(operating-system
  (host-name "komputilo")
  (timezone "Europe/Berlin")
  (locale "en_US.UTF-8")
  (bootloader (grub-configuration (device "/dev/sdX")))
  (file-systems (cons (file-system
                        (device "root")
                        (title 'label)
                        (mount-point "/")
                        (type "ext4"))
                      %base-file-systems))
  (packages (cons* (canonical-package glibc) %base-packages))
  (services (map (lambda (mservice)
                   (mlet %store-monad ((service mservice))
                     (if (memq 'nscd (service-provision service))
                         (nscd-service (nscd-configuration
                                        (debug-level 10)))
                         mservice)))
                 %base-services)))
--8<---------------cut here---------------end--------------->8---

Then I had one console running “tail -f /var/log/nscd.log”, and another
one where I would run “ping www.gnu.org.”

Initially looking up www.gnu.org fails (because the network is down),
and nscd.log indeed shows:

--8<---------------cut here---------------start------------->8---
Mon Jun 15 21:04:20 2015 - 276: handle_request: request received (Version = 2) from PID 296
Mon Jun 15 21:04:20 2015 - 276: 	GETAI (www.gnu.org)
Mon Jun 15 21:04:20 2015 - 276: Haven't found "www.gnu.org" in hosts cache!
Mon Jun 15 21:04:20 2015 - 276: add new entry "www.gnu.org" of type GETAI for hosts to cache (first)
--8<---------------cut here---------------end--------------->8---

For subsequent requests, it shows fewer details, meaning that the cached
failure is used:

--8<---------------cut here---------------start------------->8---
Mon Jun 15 21:04:25 2015 - 276: handle_request: request received (Version = 2) from PID 297
Mon Jun 15 21:04:25 2015 - 276: 	GETFDHST
Mon Jun 15 21:04:25 2015 - 276: provide access to FD 6, for hosts
--8<---------------cut here---------------end--------------->8---

Twenty seconds after the original failure, we see:

--8<---------------cut here---------------start------------->8---
Mon Jun 15 21:04:40 2015 - 276: pruning hosts cache; time 1434395080
Mon Jun 15 21:04:40 2015 - 276: considering GETAI entry "www.gnu.org", timeout 1434395080
--8<---------------cut here---------------end--------------->8---

So cache expiry seems to be working as expected.

Now, if we enable networking with “dhclient -v eth0” and see what
happens, we get again a series of lookup failures, even after the
www.gnu.org entry has been removed from cache; apparently new failures
get cached.  Eventually, after a bunch of cache prunes (so a few times
20 seconds), lookup succeeds.

The situation is different if you use ‘getent hosts’ instead of ‘ping’–
note that the latter uses getaddrinfo whereas the former uses
gethostbyname{,v6}, which is cached differently.  Here we get a
successful lookup within 20 seconds after networking has been put up.

So my guess is that the GETAI cache is racy or something.

I’ve run nscd with Helgrind, which reports a few things (see attached
file), but I suspect most of it are false positives.

Ludo’.