[Bug 856984] Re: dhclient-script attempts to write /etc/resolv.conf before it is writable

Fri Sep 23 17:44:57 UTC 2011

This bug was fixed in the package isc-dhcp - 4.1.1-P1-17ubuntu10

---------------
isc-dhcp (4.1.1-P1-17ubuntu10) oneiric; urgency=low

  * make sure writing of /etc/resolv.conf actually waits until the file is
    writable (LP: #856984)
 -- Scott Moser <smoser at ubuntu.com>   Fri, 23 Sep 2011 11:26:29 -0400

** Changed in: isc-dhcp (Ubuntu)
       Status: New => Fix Released

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Title:
  dhclient-script attempts to write /etc/resolv.conf before it is
  writable

Status in “isc-dhcp” package in Ubuntu:
  Fix Released

Bug description:
  dhclient-script has a function in it 'wait_for_rw'. that basically tries to wait and sleep until /etc is writeable.
  The check of 'is writeable' is:
     [ -w /etc ] 

  For some period during early boot, that check is returning true, while
  subsequent actual write to /etc/resolv.conf is failing.

  I suspect what is happening is that '[ -w /etc ]' begins returning
  true as soon as 'mount -o remount,rw' issues the kernel call to
  remount read-write, but there is a period of time after that when the
  kernel is still refusing writes.

  I've seen this, and put some debug code into /sbin/dhclient-script,
  and sent /etc/init/network-* output to the console to see what was
  going on.

  I'm seeing this when booting the oneiric beta-2 cloud-image
  (20110921.1) under kvm with an attached virtual cdrom (with ovf data
  on it).

  I'll attach the whole console.log, but here is the interesting part:
  08  can't create /var/lib/dhcp3/dhclient.eth0.leases: No such file or directory
  09  Listening on LPF/eth0/52:54:00:12:34:56
  10  Sending on   LPF/eth0/52:54:00:12:34:56
  11  Sending on   Socket/fallback
  12  DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3
  13  DHCPOFFER of 10.0.2.15 from 10.0.2.2
  14  DHCPREQUEST of 10.0.2.15 on eth0 to 255.255.255.255 port 67
  15  DHCPACK of 10.0.2.15 from 10.0.2.2
  16  ls: cannot access /etc/resolv.conf: No such file or directory
  17  w=true W=false
  18  ls: cannot access /etc/resolv.conf: No such file or directory
  19  w=true W=false
  20  ls: cannot access /etc/resolv.conf: No such file or directory
  21  w=true W=true
  22  -rw-r--r-- 1 root root 0 Sep 23 01:46 /etc/resolv.conf
  23  can't create /var/lib/dhcp3/dhclient.eth0.leases: No such file or directory
  24  bound to 10.0.2.15 -- renewal in 39483 seconds.
  25  The disk drive for / is not ready yet or not present.
  26  Continue to wait, or Press S to skip mounting or M for manual recovery
  27  The disk drive for /tmp is not ready yet or not present.
  28  Continue to wait, or Press S to skip mounting or M for manual recovery
  29  cloud-init start-local running: Fri, 23 Sep 2011 01:46:13 +0000. up 6.83 seconds

  Lines 16 to 22 are output of my modified dhclient-script, from the following loop.
  |     while :; do
  |         w=false; W=false
  |         [ -w /etc ] && w=true
  |         { : >> /etc/resolv.conf ; } 2>/dev/null &&
  |            W=true
  |         echo "w=$w W=$W"
  |         ls -l /etc/resolv.conf
  |         $w && $W && break
  |         sleep .1
  |     done

  So, the '[ -w /etc ]' check was returning true, while the attempt to write to /etc/resolv.conf was returning false.  Its not as if this happened once, and / became rw in between.  It was happened 3 times.
  That demonstrates that the original /sbin/dhclient-script's check is failing, and its exiting the 'wait_for_rw' too early.

  The "is not ready yet" messages on lines 25 to 28 i believe are simply a result of the disk being slow, and the kernel taking a while to mount rw.
  On line 29, cloud-init 'start-local' comes up, which is an upstart job with 'start on mounted MOUNTPOINT=/'.

  So, a solution for this, which actually better answers the question of
  "is /etc/resolv.conf writable?" is the following:

  --- dhclient-script.dist        2011-09-22 22:16:52.188538963 -0400
  +++ dhclient-script     2011-09-22 22:18:07.158908650 -0400
  @@ -29,7 +29,7 @@ wait_for_rw() {

       # Wait for /etc/resolv.conf to become writable
       if [ "$rootmode" = "rw" ]; then
  -        while [ ! -w /etc ]; do
  +        while ! { : >> /etc/resolv.conf; } 2>/dev/null; do
               sleep 0.1
           done
       fi

  Further changes to dhclient-script are necessary since it actually
  tries to write to a temp file in /etc/resolv.conf.dhclient-new.  That
  should probably be to /run or another expected tmpfs as /etc/ might
  not be writable at all, and /etc/resolv.conf a link to a writeable
  location outside of /etc.

  ProblemType: Bug
  DistroRelease: Ubuntu 11.10
  Package: isc-dhcp-client 4.1.1-P1-17ubuntu9 [modified: sbin/dhclient-script]
  ProcVersionSignature: Ubuntu 3.0.0-11.18-virtual 3.0.4
  Uname: Linux 3.0.0-11-virtual x86_64
  ApportVersion: 1.23-0ubuntu1
  Architecture: amd64
  Date: Fri Sep 23 01:49:58 2011
  DhclientLeases:

  ProcEnviron:
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: isc-dhcp
  UpgradeStatus: No upgrade log present (probably fresh install)

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