Hi Mathieu,

I haven't explained well enough.

You are right, NM doesn't handle /etc/resolv.conf differently in
Quantal.  What has changed is the IP address that nm-dnsmasq listens on.
This change *will* have an impact on some machines. A machine that has
no file at /etc/resolv.conf, or an empty file there, falls back to the
default "nameserver 127.0.0.1".  (As resolv.conf(5), says "If no
nameserver  entries  are  present,  the default  is  to use the name
server on the local machine.") Such a machine has working name service
with Precise NM, but loses name service as soon as Quantal NM is
installed and restarted.

Please note that I am not proposing that we do anything to
/etc/resolv.conf. My proposal is that the administrator be *warned* that
/etc/resolv.conf is missing.

I also agree with the line of argument that administrators who customize
their systems are responsible for adapting these customizations as
necessary. Nevertheless I think we should do something in this case for
two reasons. First, there were quite a few images derived from 12.04
which lacked /etc/resolv.conf. This was in part because at least two
well known image-building tools failed to install the symlink on the
generated image. So there are probably quite a few machines out there
that lack /etc/resolv.conf, and in many cases the users won't have
noticed any problem because name service worked on them for the reason
described earlier. Second, because of broken images on one hand and
inexperience with resolvconf on the other, some admins have undoubtedly
deleted /etc/resolv.conf in order to "fix" name service; they will be
not be happy if name service breaks again on the next upgrade.  I have
summarized these issues at
https://bugs.launchpad.net/ubuntu/+source/resolvconf/+bug/1000244/comments/66

In order to confine warnings only to cases where we know that name
service will break, we would issue the warning on upgrade to the new
network-manager package only if we see that /etc/resolv.conf is missing
and nm-dnsmasq is in use ("dns=dnsmasq" in /etc/NM/NM.conf).  If
/etc/resolv.conf is an empty file (as opposed to absent) then we should
regard that as a deliberate configuration and issue no warning, since I
haven't seen any reports of empty /etc/resolv.conf's arising from faulty
images or tools.

Less technically speaking, in the past few months I have corresponded
with a number of Ubuntu users about resolvconf. What I repeatedly hear
are complaints along the lines of: "you have changed the way UNIX has
worked for decades" and "resolvconf isn't user-friendly" and "Ubuntu has
fogotten the principle of K.I.S.S.".  Obviously I don't share these
feelings, but I do want to avoid making any more Ubuntu users feel this
way if we can reasonably help it. And here is a case where I think we
can easily avoid making some users angry with us.

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https://bugs.launchpad.net/bugs/1060200

Title:
  Detect in the postinst that resolvconf is installed but
  /etc/resolv.conf is not a symlink to ../run/resolvconf/resolv.conf

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