This bug was fixed in the package isc-dhcp - 4.3.5-3ubuntu4
---------------
isc-dhcp (4.3.5-3ubuntu4) bionic; urgency=medium
* dhclient-script.linux: handle empty case also when waiting for ipv6 link
local DAD. (LP: #1718568)
-- Dan Streetman <dan.street...@canonical.com> Thu, 12 Oct 2017
08:48:06 -0400
** Changed in: isc-dhcp (Ubuntu Bionic)
Status: In Progress => Fix Released
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https://bugs.launchpad.net/bugs/1718568
Title:
dhclient-script fails to wait for link-local address
Status in isc-dhcp package in Ubuntu:
Fix Released
Status in isc-dhcp source package in Trusty:
In Progress
Status in isc-dhcp source package in Xenial:
In Progress
Status in isc-dhcp source package in Zesty:
Won't Fix
Status in isc-dhcp source package in Artful:
In Progress
Status in isc-dhcp source package in Bionic:
Fix Released
Status in isc-dhcp package in Debian:
New
Bug description:
[impact]
bug 1633479 made a change to isc-dhcp to wait for an interface's link-local
ipv6 address to switch from 'tentative' to normal, because all link-local
addresses briefly go through a 'tentative' state while the kernel is performing
ipv6 link-local 'duplicate address detection' (DAD). While in the 'tentative'
state, dhclient can't take over the interface and send out
dhcpv6 requests; it must wait until DAD completes.
However, the change made in that bug does not account for the case
where the 'tentative' check is done before the interface has even set
up a link-local address; its case statement assumes if there is no
'tentative' or 'dadfailed' string in the output, the link-local
address is ready to use. When the address check finds no address at
all, this will return as successful, even though it shouldn't, and
dhclient will fail to get the dhcpv6 address.
[test case]
on a system that is configured for dhcpv6 on one or more of its
interfaces, repeatedly try to get the dhcpv6 address. For interfaces
that are slower to actually set up their initial tentative link-local
address, they will occasionally fail, since the current code is a race
between the kernel adding the tentative link-local address, and the
dhclient-script.linux code checking the interface for a tentative
address.
with the patch to correct this, even interfaces slow to add their
tentative link-local address should correctly wait for the address to
get added, and then transition from tentative to normal, and then
begin the dhcpv6 process.
[regression potential]
errors in this function can cause dhclient to fail to get a ipv6
address for an interface; regression would happen if this patch makes
it fail more than it already is failing, but would not cause other
failures or problems after getting an ipv6 address; this patch will
affect only startup-time.
[other info]
related bug 1633479
[original description]
Summary:
========
If a interface does not yet have a link-local address (as it may have
just been brought up), dhclient -6 <ifname> will fail. The built-in
"wait for link-local address" loop does not function properly, causing
DHCP failure.
Discussion:
===========
In trying to configure isc-dhcp-client 4.3.5-3ubuntu1 for IPv6 on
Ubuntu 17.04, I was finding that on boot I was getting failures with
the logged message "no link-local IPv6 address for <ifname>"
I found that it took several seconds for the link-local address to be
assigned when the interface came up (in this case, the ISP/modem-
facing interface), and worked around it with a script that looks at
/sbin/ifconfig $IFACE | /bin/fgrep -q 'scopeid 0x20'
and loops for a fixed number of times for that to be successful.
On looking at /sbin/dhclient-script it appears that it *tries* to do
the same thing in
# set the link up and wait for ipv6 link local dad to finish
ipv6_link_up_and_dad()
this code sets
out=$(ip -6 -o address show dev "$dev" scope link)
then checks it with a case statement inside of a loop for
case " $out " in
*\ dadfailed\ *)
error "$dev: ipv6 dad failed."
return 1;;
*\ tentative\ *) :;;
*) return 0;;
esac
If there is no link-local address, $out will be empty. The default
case is taken, and the loop exits immediately:
$ echo "'$out'" ; case " $out " in
> *\ dadfailed\ *)
> echo "dadfailed"
> ;;
> *\ tentative\ *)
> echo "tentative"
> ;;
> *)
> echo "default"
> esac
''
default
As a result, there is no "wait for link-local address" and when there
is no link-local address, dhclient fails later on.
Possible Fix:
=============
Adding "the missing case" for "no address" case that continues the
loop is one possible solution.
. case " $out " in
. *\ dadfailed\ *)
. error "$dev: ipv6 dad failed."
. return 1;;
. *\ tentative\ *) :;;
+ " ")
+ :
+ ;;
. *) return 0;;
. esac
At least in my situation, this prevents the failure of dhclient due to
the link-local address not being "ready" yet.
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