Subject: /sbin/dhclient: dhclient boothang bug
Package: isc-dhcp-client
Version: 4.1.1-P1-15+squeeze8
File: /sbin/dhclient
Severity: normal
Tags: patch
*** Please type your report below this line ***
Fixing the dhclient boot hang bug when it appeared after installing
readahead-fedora.
*************************************************************************************
The dhclient program is started indirectly by ifup, called in
/etc/init.d/networking
This bug delays boots while searching for internet connection on occasional
random boots, and possibly is related to the operation of startpar.
It is caused by the timing of dhclient in the boot process, whether it starts
in runlevel S or runlevel 2.
The networking init script that starts ifup which starts dhclient runs in
runlevel S. There is a delay before dhclient starts as ifup runs many other
scripts first.
The readahead-fedora package reduces this delay. A fast computer could also
affect it, or possibly the installation of new software that was started during
runlevel S.
If dhclient runs in S then the messages written to stderr will be displayed on
the console and the boot will be delayed until it has removed itself to the
background,
then runlevel 2 begins. In runlevel 2 the messages from runlevel S scripts
still running are automatically placed in the background by init so no delay
occurs.
Output from the console is displayed on screen during boot, providing this
feature is configured.
Output from stderr can be redirected when ifup is started.
2>/dev/null to redirect stderr, or possibly >/dev/null 2>&1 to redirect stdout
and stderr (stdout is not a problem here though.)
The process should be put in the background too, appending an &. A practical
test is to try variations with:
# ifdown wlan0
# ifup wlan0
So a simple solution is to change the line that calls ifup at boot, in this
section of /etc/init.d/networking:
case "$1" in
start)
process_options
log_action_begin_msg "Configuring network interfaces"
if ifup -a; then
log_action_end_msg $?
else
log_action_end_msg $?
fi
;;
to this: (careful here, "if ifup -a >/dev/null 2>&1 & ; then" failed with a
syntax error.)
case "$1" in
start)
process_options
log_action_begin_msg "Configuring network interfaces"
if ifup -a >/dev/null 2>&1 &
then
log_action_end_msg $?
else
log_action_end_msg $?
fi
;;
However, this also seems to ensure that dhclient runs later, in runlevel 2.
Typically on my system the timing would vary before. Placing it in the
background seems to give it a lower priority than the other processes. Writing
a new initscript giving an artificial delay in runlevel S using 'sleep' proved
this did work though.
It is also possible to recompile ifup so dhclient is started with the -nw
option by modifying the inet.defn file then compiling and installing the new
binary.
Just rename the old /sbin/ifup to /sbin/originalifup.bak and copy the new ifup
to /sbin and it works as before, using the Debian source package.
Ifupdown: Version: 0.6.10
Change this section in inet.defn so it includes -nw for the dhclient command as
shown below:
up
[[ifconfig %iface% hw %hwaddress%]]
dhclient3 -pf /var/run/dhclient.%iface%.pid -lf
/var/lib/dhcp3/dhclient.%iface%.leases %iface% \
if (execable("/sbin/dhclient3"))
dhclient -v -nw -pf /var/run/dhclient.%iface%.pid -lf
/var/lib/dhcp/dhclient.%iface%.leases %iface% \
elsif (execable("/sbin/dhclient"))
pump -i %iface% [[-h %hostname%]] [[-l %leasehours%]] \
elsif (execable("/sbin/pump") && mylinuxver() >= mylinux(2,1,100))
udhcpc -n -p /var/run/udhcpc.%iface%.pid -i %iface% [[-H %hostname%]] \
[[-c %client%]] \
elsif (execable("/sbin/udhcpc") && mylinuxver() >= mylinux(2,2,0))
dhcpcd [[-h %hostname%]] [[-i %vendor%]] [[-I %client%]] \
[[-l %leasetime%]] %iface% \
elsif (execable("/sbin/dhcpcd"))
There are various dependencies required to compile this package though. I am
not aware of any advantages to doing this, except dhclient prints a few lines
without delaying the boot, making it convenient for tracing the problem in
syslog and bootlog.
The disadvantage is it also stops messages in the terminal which would show if
a connection had been achieved when calling ifupdown manually.
As rsyslog starts near the beginning of runlevel 2, whether all the output from
dhclient appears in syslog or not shows if it appears in the console and thus
in the boot log. The first lines of output from dhclient:
Internet Systems Consortium DHCP Client 4.1.1-P1
Copyright 2004-2010 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
The first large group of messages written in syslog with the same time shown
are cached by the kernel and written when rsyslog starts, after this real time
logging begins.
I looked at the source code for dhclient, init and ifupdown and all seemed to
be operating normally, it was a matter of fine tuning the relationship between
them. As ifup runs dhclient in a local environment the environment variables
cannot be examined to disable writing to stderr at boot. It is possible
information on whether the system is booting could be obtained from elsewhere,
although I do not currently know how to do this myself.
I looked at the man page for startpar but not the source code. It is possible
some option for startpar could remove this problem too.
The reason dhclient prints out info then departs later is to provide the user
with feedback when it is called from the terminal. It becomes a daemon either
because it has made a connection or because it times out in which case it
continues trying to make a connection occasionally as a background process.
When it has become a daemon it does not print messages to stderr.
It is possible to determine how to call dhclient itself manually from looking
at the sequence in ifupdown, but it involves updating files and running other
commands as well which are done automatically by ifupdown. The actual
connection to the internet server is made by dhclient over a wired or wireless
network and it runs without regard to the existence of a network connection.
These changes may need to be adapted if the internet connection was managed by
a different package to ifupdown, such as Network Manager.
The init scripts often change in new versions of Debian too. But it could still
be useful to know what is causing the problem in order to find a solution.
-- System Information:
Debian Release: 6.0.8
APT prefers oldstable
APT policy: (500, 'oldstable')
Architecture: i386 (i686)
Kernel: Linux 3.2.0-0.bpo.4-686-pae (SMP w/1 CPU core)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Versions of packages isc-dhcp-client depends on:
ii debianutils 3.4 Miscellaneous utilities specific t
ii iproute 20100519-3 networking and traffic control too
ii isc-dhcp-common 4.1.1-P1-15+squeeze8 common files used by all the isc-d
ii libc6 2.11.3-4 Embedded GNU C Library: Shared lib
isc-dhcp-client recommends no packages.
Versions of packages isc-dhcp-client suggests:
pn avahi-autoipd <none> (no description available)
pn resolvconf <none> (no description available)
-- no debconf information
