dhclient-script will not work with read-only /etc

[Scott Moser]

dhclient-script attempts to support read-only root , but it wont correctly work.
For a read-only root to work, and support dhclient-script writing to /etc/resolv.conf, then /etc/resolv.conf would have to be a symlink to a writeable location (ie, /run).

To that end, dhclient-script attempts to check and see if / is going to be mounted 'rw' and only wait for that case if it is.

The problem is that it basically has the following logic:
  if root_will_be_mounted_rw; then
      wait-for-writable /etc/
  fi
  new_resolv_conf=/etc/resolv.conf.dhclient-new
  write new file to to $new_resolv_conf
  change attributes of $new_resolv_conf to match resolv.conf
  mv $new_resolv_conf /etc/resolv.conf

That doesn't work if /etc is read-only and /etc/resolv.conf is a symlink for the following reasons:
 * $new_resolv_conf is being written to /etc/ (and wont be writable)
 * the mv will actually break the symlink and try to create a new file in /etc

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: isc-dhcp-client 4.1.1-P1-17ubuntu9
ProcVersionSignature: Ubuntu 3.0.0-11.18-generic 3.0.4
Uname: Linux 3.0.0-11-generic x86_64
ApportVersion: 1.23-0ubuntu1
Architecture: amd64
Date: Fri Sep 23 11:50:56 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100318)
ProcEnviron:
 SHELL=/bin/bash
 PATH=(custom, user)
 LANG=en_US.UTF-8
SourcePackage: isc-dhcp
UpgradeStatus: Upgraded to oneiric on 2010-11-15 (311 days ago)

[Scott Moser (smoser)]

I'm attaching this as a patch rather than a merge proposal as there is a upload in the queue right now for oneiric, and I would not suggest this until the opening of 'p'.

[Scott Moser (smoser)]

The following are comments on the changes in the patch

fix updating of /etc/resolv.conf for read-only

This patch makes the following changes:
 * instead of writing to a temp file in /etc/, write directly to
   /etc/resolv.conf
 * replace the multiple '>>' with a single '>' and braces. This means
   the file will be opened for write once rather than append many times.
 * opening for write (truncate) keeps the ownership of the file if it exists,
   and correctly follows a symlink.
   This makes the chmod and chown --reference un-necessary
 * use 'grep -i' rather than 'sed' to get the old nameserver entries from
   existing resolv.conf. This is how the linux.udeb does it, and I find it more clean.
 * changes 'wait_for_rw' to take a file argument rather than hard coding /etc

Note, there is an issue with this patch if /etc/resolv.conf is a symlink and
is the symlink (*not the target*) is not owned by root. To demonstrate:
  $ rm -f /tmp/foo /tmp/bar
  $ ln -s /tmp/foo /tmp/bar
  $ sudo sh -c 'echo hi > /tmp/foo'
  $ sudo sh -c 'echo himom > /tmp/bar'
  sh: /tmp/bar: Permission denied

The only solution I see for this is to attempt to resolve the target of the
symlink (readlink -f) and writing directly there instead of through the
symlink.

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "proposed patch" of this bug report has been identified as being a patch in the form of a debdiff. The ubuntu-sponsors team has been subscribed to the bug report so that they can review and hopefully sponsor the debdiff. In the event that this is in fact not a patch you can resolve this situation by removing the tag 'patch' from the bug report and editing the attachment so that it is not flagged as a patch. Additionally, if you are member of the ubuntu-sponsors please also unsubscribe the team from this bug report.

[This is an automated message performed by a Launchpad user owned by Brian Murray. Please contact him regarding any issues with the action taken in this bug report.]

[Launchpad Janitor (janitor)]

This bug was fixed in the package isc-dhcp - 4.1.1-P1-17ubuntu11

---------------
isc-dhcp (4.1.1-P1-17ubuntu11) precise; urgency=low

  * debian/dhclient-script.linux: fix for read-only /etc (LP: #857524)
 -- Scott Moser <email address hidden> Mon, 24 Oct 2011 22:28:35 -0400

[laszlo gombos (gombi)]

This change makes dhclient-script dependent on the existence of /etc/fstab. If /etc/fstab does not exist, dhclient fails to grab an IP.