Date: Fri, 16 Dec 2005 13:46:59 -0700 (MST) From: "John E. Hein" <jhein@timing.com> To: FreeBSD-gnats-submit@FreeBSD.org Cc: brooks@FreeBSD.org Subject: bin/90518: dhclient-script problems with symlink'd resolv.conf when /etc is read-only Message-ID: <200512162046.jBGKkxn7001129@Elmer.timing.com> Resent-Message-ID: <200512162050.jBGKo34l086570@freefall.freebsd.org>
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>Number: 90518
>Category: bin
>Synopsis: dhclient-script problems with symlink'd resolv.conf when /etc is read-only
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Fri Dec 16 20:50:03 GMT 2005
>Closed-Date:
>Last-Modified:
>Originator: John E. Hein
>Release: FreeBSD 4.8-RELEASE i386
>Organization:
Timing Solutions
>Environment:
System: noticed on 4.x; still appears to be a problem on -current
>Description:
Some situations - embedded products, for instance - call
for a read-only root (& /etc) file system. In such cases,
we sym-link /etc/resolv.conf to a writable file system
(e.g., /some/where/etc/resolv.conf).
In 4.x, the 'rm /etc/resolv.conf.std' in dhclient-script fails
and lines are appended to the actual resolv.conf file. Over
time, it can get quite large (in the case where new_domain_name
is empty and thus the rm /etc/resolv.conf is attempted)). In
this case the easy fix is 'cp /dev/null /etc/resolv.conf' rather
than using 'rm'.
In -current, the problem is similar but the effect can be
different. The way dhclient-script works there, a temporary
/etc/resolv.conf.std file is written to, then its contents is
cat'd to the real resolv.conf.
If /etc/resolv.conf.std does not exist on a read-only /etc,
resolv.conf will never populate.
If /etc/resolv.conf.std is a sym link (because the someone using
a read-only /etc has looked into dhclient-script and seen what
files it writes to), then you will hit the 'growing' resolv.conf
issue described above.
NB: I have not actually tried it on -current yet. I have just
looked at the add_new_resolv_conf() function. If I did not dig
deep enough, I apologize. I'll try to verify it on -current
when I get a chance.
>How-To-Repeat:
mount -ur /
Then run dhclient for your interface (using a dhcp server that
supplies DNS server info).
Observe the effects (or lack thereof) on resolv.conf
>Fix:
Here's a patch for -current. The choice of using TMPDIR
may be wrong. There could be a possible security issue,
but I'm not clear about all the ways dhclient-script can
be invoked. Writing to a less accessible directory might
be prudent. And using an env var here may allow a malicious
user could point TMPDIR somewhere undesirable.
Somewhere under /var/run might be a better place.
OpenBSD's version has the same problem.
Index: dhclient-script
===================================================================
RCS file: /base/FreeBSD-CVS/src/sbin/dhclient/dhclient-script,v
retrieving revision 1.10
diff -u -p -r1.10 dhclient-script
--- dhclient-script 8 Sep 2005 22:49:17 -0000 1.10
+++ dhclient-script 16 Dec 2005 20:31:52 -0000
@@ -134,21 +134,22 @@ add_new_resolv_conf() {
# thus broke the script. This code creates the resolv.conf if either
# are provided.
- rm -f /etc/resolv.conf.std
+ local tmpres=${TMPDIR:-/tmp}/resolv.conf.std
+ rm -f $tmpres
if [ -n "$new_domain_name" ]; then
- echo "search $new_domain_name" >>/etc/resolv.conf.std
+ echo "search $new_domain_name" >>$tmpres
fi
if [ -n "$new_domain_name_servers" ]; then
for nameserver in $new_domain_name_servers; do
- echo "nameserver $nameserver" >>/etc/resolv.conf.std
+ echo "nameserver $nameserver" >>$tmpres
done
fi
- if [ -f /etc/resolv.conf.std ]; then
+ if [ -f $tmpres ]; then
if [ -f /etc/resolv.conf.tail ]; then
- cat /etc/resolv.conf.tail >>/etc/resolv.conf.std
+ cat /etc/resolv.conf.tail >>$tmpres
fi
# When resolv.conf is not changed actually, we don't
@@ -156,8 +157,8 @@ add_new_resolv_conf() {
# If /usr is not mounted yet, we cannot use cmp, then
# the following test fails. In such case, we simply
# ignore an error and do update resolv.conf.
- if cmp -s /etc/resolv.conf.std /etc/resolv.conf; then
- rm -f /etc/resolv.conf.std
+ if cmp -s $tmpres /etc/resolv.conf; then
+ rm -f $tmpres
return 0
fi 2>/dev/null
@@ -168,8 +169,8 @@ add_new_resolv_conf() {
if [ -f /etc/resolv.conf ]; then
cat /etc/resolv.conf > /etc/resolv.conf.save
fi
- cat /etc/resolv.conf.std > /etc/resolv.conf
- rm -f /etc/resolv.conf.std
+ cat $tmpres > /etc/resolv.conf
+ rm -f $tmpres
# Try to ensure correct ownership and permissions.
chown -RL root:wheel /etc/resolv.conf
>Release-Note:
>Audit-Trail:
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