[Bug 1641328] Re: Ordering of mdns4_minimal and resolve in /etc/nsswitch.conf causes mDNS lookups to fail -- breaks network printing

Mon May 8 11:05:39 UTC 2017

Hi Mark!

Thank you for pointing me to this bug. I spent some time looking into
this.

It seems that there is currently no established mechanism for ordering
NSS modules into nsswitch.conf automatically. Instead every package is
doing its own parsing mangling of the file on install. libnss-resolve is
appending "[!UNAVAIL=return]" which causes the mdns4_minimal entry to
get shadowed, depending on installation order of the packages involved
(libnss-mdns and libnss-resolve).

A workaround was implemented in Debian in
https://anonscm.debian.org/cgit/collab-maint/nss-
mdns.git/commit/?id=44550bcc6b49116f927dea52395867ff47d7d3b9 (tests in
https://anonscm.debian.org/cgit/collab-maint/nss-
mdns.git/commit/?id=0e9b82b91cd9bf9464cf63df2c3f1cbbec3553e0) which
appears in Zesty (17.04) which will explain why Zesty appears to work
correctly. Though this is still a hack, I think it should work in both
ordering cases, so in the short term this should be fixed from a user's
perspective.

In the long term, Debian have a bug open (https://bugs.debian.org/cgi-
bin/bugreport.cgi?bug=846944) to fix the hack. It may be that mDNS
resolution moves to libnss-resolve, or that some kind of configuration
wrapper is arranged to improve on the ad-hoc mangling of nsswitch.conf.

So, for Ubuntu:

This is fixed in Zesty onwards.

The workaround for Yakkety (as you explained) is to edit
/etc/nsswitch.conf and bring "mdns4_minimal [NOTFOUND=return]" in front
of "resolve [!UNAVAIL=return]".

We could propose an update for Yakkety (16.10). But we have to do it
carefully - if there is a bug in the update, we could break DNS
resolution for all users. However, Debian have already written the
essential "swap it round" code so that will already have had some real
world testing.  But Yakkety has only two months left before it EOLs, and
the workaround for affected users is fairly trivial. So is it worth
trying? Opinions welcome.

For bug triage, there are effectively two bugs. 1) The fact that it
doesn't work for users in Yakkety; and 2) the bigger picture fix for the
hack or move of mDNS resolution to libnss-resolve. I see this bug as
having been reported for the first issue, and the Debian bug as tracking
the second. So I'll mark the systemd task as Invalid, as the fix for
this bug was in libnss-mdns (and we can track any SRU for libnss-mdns
here). I'm not linking the Debian bug, as that's the separate second
issue. If we want to track that in Launchpad, we should file a separate
bug for it, so as not to mix up the separate statuses of the two issues.

** Bug watch added: Debian Bug tracker #846944
   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=846944

** Changed in: systemd (Ubuntu)
       Status: Confirmed => Invalid

** Also affects: nss-mdns (Ubuntu Yakkety)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Yakkety)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu Yakkety)
       Status: New => Invalid

** Changed in: nss-mdns (Ubuntu)
       Status: Confirmed => Fix Released

** Changed in: nss-mdns (Ubuntu Yakkety)
       Status: New => Triaged

** Changed in: nss-mdns (Ubuntu Yakkety)
   Importance: Undecided => Medium

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https://bugs.launchpad.net/bugs/1641328

Title:
  Ordering of mdns4_minimal and resolve in /etc/nsswitch.conf causes
  mDNS lookups to fail -- breaks network printing

Status in nss-mdns package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Invalid
Status in nss-mdns source package in Yakkety:
  Triaged
Status in systemd source package in Yakkety:
  Invalid

Bug description:
  (See also libnss-resolve:amd64   231-9ubuntu1 amd64        nss module
  to resolve names via systemd-resolved)

  # fresh install of yakkety 

  mtearle at liberation:/etc$ lsb_release -a
  No LSB modules are available.
  Distributor ID:	Ubuntu
  Description:	Ubuntu 16.10
  Release:	16.10
  Codename:	yakkety

  # package details

  mtearle at liberation:~$ apt-cache policy libnss-resolve
  libnss-resolve:
    Installed: 231-9ubuntu1
    Candidate: 231-9ubuntu1
    Version table:
   *** 231-9ubuntu1 500
          500 http://mirror.rackspace.com/ubuntu yakkety-updates/main amd64 Packages
          100 /var/lib/dpkg/status
       231-9git1 500
          500 http://mirror.rackspace.com/ubuntu yakkety/main amd64 Packages
  mtearle at liberation:~$ apt-cache policy systemd
  systemd:
    Installed: 231-9ubuntu1
    Candidate: 231-9ubuntu1
    Version table:
   *** 231-9ubuntu1 500
          500 http://mirror.rackspace.com/ubuntu yakkety-updates/main amd64 Packages
          100 /var/lib/dpkg/status
       231-9git1 500
          500 http://mirror.rackspace.com/ubuntu yakkety/main amd64 Packages

  # attempt to ping VM elsewhere on network with mDNS hostname

  mtearle at liberation:/etc$ ping bazzavan.local
  ping: bazzavan.local: Name or service not known

  # can find both ipv4 and ipv6 addresses for the host

  mtearle at liberation:/etc$ avahi-resolve-host-name bazzavan.local
  bazzavan.local	fe80::a00:27ff:fea5:3f51

  mtearle at liberation:/etc$ avahi-resolve-host-name -4 bazzavan.local
  bazzavan.local	172.16.44.48

  # can ping it

  mtearle at liberation:/etc$ ping -c 1 172.16.44.48
  PING 172.16.44.48 (172.16.44.48) 56(84) bytes of data.
  64 bytes from 172.16.44.48: icmp_seq=1 ttl=64 time=0.265 ms

  --- 172.16.44.48 ping statistics ---
  1 packets transmitted, 1 received, 0% packet loss, time 0ms
  rtt min/avg/max/mdev = 0.265/0.265/0.265/0.000 ms

  # original ordering

  mtearle at liberation:/etc$ grep hosts /etc/nsswitch.conf 
  hosts:          files resolve [!UNAVAIL=return] mdns4_minimal [NOTFOUND=return] dns

  # go away and edit /etc/nsswitch.conf
  # change ordering of resolve and mdns4_minimal

  mtearle at liberation:/etc$ grep hosts /etc/nsswitch.conf 
  hosts:          files mdns4_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] dns

  # check mdns lookups now work, and it now pings

  mtearle at liberation:/etc$ ping -c 1 bazzavan.local
  PING bazzavan.local (172.16.44.48) 56(84) bytes of data.
  64 bytes from 172.16.44.48 (172.16.44.48): icmp_seq=1 ttl=64 time=0.161 ms

  --- bazzavan.local ping statistics ---
  1 packets transmitted, 1 received, 0% packet loss, time 0ms
  rtt min/avg/max/mdev = 0.161/0.161/0.161/0.000 ms

  # check libnss-resolve is still doing its thing

  mtearle at liberation:/etc$ ping -c 1 localhost.localdomain
  PING localhost.localdomain(localhost (::1%1)) 56 data bytes
  64 bytes from localhost (::1): icmp_seq=1 ttl=64 time=0.016 ms

  --- localhost.localdomain ping statistics ---
  1 packets transmitted, 1 received, 0% packet loss, time 0ms
  rtt min/avg/max/mdev = 0.016/0.016/0.016/0.000 ms

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