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FreeBSD-EN-16:10.dhclient                                       Errata Notice
                                                          The FreeBSD Project

Topic:          Better handle unknown options received from a DHCP server

Category:       core
Module:         dhclient
Announced:      2016-08-12
Credits:        Microsoft OSTC
Affects:        FreeBSD 10.3
Corrected:      2016-05-06 05:44:12 UTC (stable/10, 10.3-STABLE)
                2016-08-12 04:01:16 UTC (releng/10.3, 10.3-RELEASE-p7)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

The dhclient(8) utility is used to request an IP address from a DHCP server.
Some implemenations of DHCP servers can use "options" to pass extra
information to dhclient.

II.  Problem Description

In Azure, the DHCP server adds a private option (id 0xf5), which contains
binary form of an IPv4 address.  Once this option is converted to string
form, it could contain '$', for example:

IPv4 address: 100.72.36.54
binary form: 0x64 0x48 0x24 0x36
string form: "dH$6"

In this case, dhclient(8) exits upon "illegal" options as shown above, thus
the an Azure virtual machine will fail to obtain an IP address, and fail to
start.

III. Impact

The virtual machine in Azure may not set IP address properly and becomes
inaccessible.

IV.  Workaround

No workaround is available, however it is presumed this issue only affects
FreeBSD running in Azure.

V.   Solution

1) Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date.
Afterward, reboot the system or restart dhclient(8).

2) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Afterward, reboot the system or restart dhclient(8).

3) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/EN-16:10/dhclient.patch
# fetch https://security.FreeBSD.org/patches/EN-16:10/dhclient.patch.asc
# gpg --verify dhclient.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.

Restart the applicable daemons, or reboot the system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r299156
releng/10.3/                                                      r303984
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-16:10.dhclient.asc>
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