[Bug 1123425] Re: nscd: Either fix automatic hosts cache invalidation or add resolvconf update script to invalidate the hosts cache

[Ubuntu Foundations Team Bug Bot]

*** This bug is a duplicate of bug 1123424 ***
    https://bugs.launchpad.net/bugs/1123424

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https://bugs.launchpad.net/bugs/1123425

Title:
  nscd: Either fix automatic hosts cache invalidation or add resolvconf
  update script to invalidate the hosts cache

Status in “eglibc” package in Ubuntu:
  New
Status in “eglibc” package in Debian:
  Unknown

Bug description:
  When nscd is running and the hosts cache is enabled and resolv.conf
  changes, the hosts cache needs to be invalidated, but this does not
  currently happen.

  I discovered this while running nscd with the hosts cache enabled. I
  connected to a VPN whose internal nameservers resolve certain domain
  names differently from external nameservers: for a given name,
  external nameservers resolve the name to the IP address of the
  company's reverse proxy whereas the internal nameservers resolve the
  name to an internal IP address. After I connected to the VPN my
  resolv.conf file was correctly updated by resolvconf such that the VPN
  nameserver was listed first, but nscd continued to supply the old
  external IP address out of its cache. Analogous problem on
  disconnecting from the VPN. (When resolvconf is installed it updates
  the resolver configuration file resolv.conf. It actually writes to
  /run/resolvconf/resolv.conf to which /etc/resolv.conf is a symbolic
  link.)

  I would have expected that nscd would invalidate its hosts cache
  automatically when resolv.conf changed. I initially thought that this
  was the point of the patch discussed here:

       http://www.eglibc.org/archives/patches/msg00977.html

  which I believe has since been integrated into Ubuntu nscd. But
  experimentation proves that nscd does *not* invalid its hosts cache
  when resolv.conf changes... at least, not under the circumstances
  described above.

  Now I understand that when resolv.conf changes, nscd does re-read the
  resolv.conf file and so does use the correct nameservers for new
  queries, but does not clear its cache.

  If nscd is supposed to invalidate its hosts cache when resolv.conf
  changes then please fix the bug which causes this to fail to happen.

  If it was not the intent to include that functionality in nscd, then
  please add a resolvconf update script that invalidates the hosts cache
  when resolv.conf is changed by resolvconf.

  This can be implemented in two parts. First, add a new "invalidate-
  hosts" method to the initscript which invalidates the hosts cache,
  making use of nscd's "--invalidate" option.  Then include a file named
  /etc/resolvconf/update-libc.d/000nscd in the nscd package with the
  following content.

      #!/bin/sh
      [ -x /etc/init.d/nscd ] && /etc/init.d/nscd invalidate-hosts

  The code in the initscript could look something like the following.

  --- nscd_ORIG	2012-10-04 04:13:52.000000000 +0200
  +++ nscd	2013-02-12 21:00:33.191132785 +0100
  @@ -70,6 +70,11 @@
   	return 0
   }
   
  +invalidate_hosts()
  +{
  +	"$DAEMON" --invalidate hosts
  +}
  +
   case "$1" in
   start)
   	log_daemon_msg "Starting $DESC" "$NAME"
  @@ -118,8 +123,16 @@
   		1) log_success_msg "running." ; exit 0 ;;
   	esac
   	;;
  +invalidate-hosts)
  +	log_daemon_msg "Invalidating hosts cache of $DESC"
  +	status || invalidate_hosts 
  +	case "$?" in
  +		0) log_end_msg 0 ; exit 0 ;;
  +		*) log_failure_msg " (failed)" ; exit 1 ;;
  +	esac
  +	;;
   *)
  -	echo "Usage: /etc/init.d/$NAME {start|stop|force-reload|restart|status}" >&2
  +	echo "Usage: /etc/init.d/$NAME {start|stop|force-reload|restart|status|invalidate-hosts}" >&2
   	exit 1
   	;;
   esac

  Should you implement this, please Suggest resolvconf (>= 1.70) and Conflict with resolvconf (<< 1.70), since those older versions of resolvconf restarted nscd if resolv.conf changed and nscd had its hosts cache enabled.
  -- 
  Thomas Hood

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