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Reloading of /etc/resolv.conf
- From: Sebastian Kienzl <seb at riot dot org>
- To: libc-alpha at sourceware dot org
- Date: Mon, 15 Mar 2010 18:23:06 +0100
- Subject: Reloading of /etc/resolv.conf
Hello!
There's a patch in the wild against the resolver which makes it reload
/etc/resolv.conf on change, see
http://sources.redhat.com/ml/libc-alpha/2004-09/msg00130.html
However, this patch actually doesn't work properly for multi-threaded
programs, as only one thread will notice the change and refresh its
resolver state. I've attached a proper patch. It's for 2.5 but it should
work with current versions, too.
Even though the patch may not be interesting for upstream, I decided to
let you know about this problem, since the mentioned patch seems to be
used by at least Debian and Ubuntu.
Regards,
Seb.
diff -ur resolv.orig/res_libc.c resolv/res_libc.c
--- resolv.orig/res_libc.c 2005-11-01 01:06:40.000000000 +0100
+++ resolv/res_libc.c 2010-03-15 14:13:18.000000000 +0100
@@ -22,7 +22,7 @@
#include <arpa/nameser.h>
#include <resolv.h>
#include <bits/libc-lock.h>
-
+#include <sys/stat.h>
/* The following bit is copied from res_data.c (where it is #ifdef'ed
out) since res_init() should go into libc.so but the rest of that
@@ -89,12 +89,34 @@
return (__res_vinit(&_res, 1));
}
+static time_t resconf_mtime;
+__libc_lock_define_initialized (static, resconf_mtime_lock);
+
+/* Check if the modification time of resolv.conf has changed.
+ If so, have all threads re-initialize their resolver states */
+static void
+__res_check_resconf (void)
+{
+ struct stat statbuf;
+ if (stat (_PATH_RESCONF, &statbuf) == 0) {
+ __libc_lock_lock (resconf_mtime_lock);
+ if (statbuf.st_mtime != resconf_mtime) {
+ resconf_mtime = statbuf.st_mtime;
+ atomicinclock (lock);
+ atomicinc (__res_initstamp);
+ atomicincunlock (lock);
+ }
+ __libc_lock_unlock (resconf_mtime_lock);
+ }
+}
+
/* Initialize resp if RES_INIT is not yet set or if res_init in some other
thread requested re-initializing. */
int
__res_maybe_init (res_state resp, int preinit)
{
if (resp->options & RES_INIT) {
+ __res_check_resconf ();
if (__res_initstamp != resp->_u._ext.initstamp) {
if (resp->nscount > 0) {
__res_iclose (resp, true);