[CFR] reflect resolv.conf update to running application

[Hajimu UMEMOTO]

Hi,

>>>>> On Sun, 21 Aug 2005 14:04:54 +1000
>>>>> Peter Jeremy <PeterJeremy at optushome.com.au> said:

PeterJeremy> Overall, I think that having applications see changes to /etc/resolv.conf
PeterJeremy> is a good idea.

Thanks.

PeterJeremy> On Sun, 2005-Aug-21 00:37:56 +0100, Robert Watson wrote:
>(1) Has anyone characterized the significant of the cost of doing a stat()
>    for every DNS lookup for a significant workload?  Does it matter?

PeterJeremy> I wrote a short program to run stat("/etc/resolv.conf") in a loop.
PeterJeremy> On a P-233 running 4.9, I got about 45000 iterations/sec.
PeterJeremy> On a P-120 running 5.4, I got about 10500 iterations/sec.
PeterJeremy> I don't think this matters - especially compared to the overheads
PeterJeremy> involved in sending and receiving the DNS UDP packets.  If you are
PeterJeremy> stating the same name frequently, it will be in the name cache so
PeterJeremy> the name lookups are fairly cheap.

Yes, I think a cost for stat() is considerably low than actuall DNS
lookup.

>(2) By reading the configuration file more frequently and more quickly
>    after a change, we increase the chances of a race condition in which
>    the resolve reads a partially written resolv.conf file during an
>    update.  Does this happen in practice?

I had care to don't update during the critical path.  For example, I
didn't change to use _res_init() in res_send().
I have been using this patch for myself about 3 months without any
problem.

PeterJeremy> This could be more of an issue, though I have no idea whether it
PeterJeremy> really is.  The easy work-around is to avoid updates.  Instead create
PeterJeremy> /etc/resolv.conf.tmp and rename it.

Yup, I forgot to mention.  Our dhclient updates resolv.conf frequently
even when it is not changed.  I think it is redundant, and I applied
following patch:

Index: sbin/dhclient/dhclient-script
diff -u sbin/dhclient/dhclient-script.orig sbin/dhclient/dhclient-script
--- sbin/dhclient/dhclient-script.orig	Fri Jun 10 12:41:18 2005
+++ sbin/dhclient/dhclient-script	Wed Jun 29 06:13:14 2005
@@ -152,6 +152,11 @@
 			cat /etc/resolv.conf.tail >>/etc/resolv.conf.std
 		fi
 
+		if diff /etc/resolv.conf.std /etc/resolv.conf >/dev/null 2>&1; then
+			rm -f /etc/resolv.conf.std
+			return 0
+		fi
+
 		# In case (e.g. during OpenBSD installs) /etc/resolv.conf
 		# is a symbolic link, take care to preserve the link and write
 		# the new data in the correct location.

I wish to commit this, too.

Sincerely,

--
Hajimu UMEMOTO @ Internet Mutual Aid Society Yokohama, Japan
ume at mahoroba.org  ume@{,jp.}FreeBSD.org
http://www.imasy.org/~ume/