resolver: automatically reload /etc/resolv.conf

On each resolver query, use stat(2) to see if the modification time
of /etc/resolv.conf has changed.  If so, reload the file and reinitialize
the resolver library.  However, only call stat(2) if at least two seconds
have passed since the last call to stat(2), since calling it on every
query could kill performance.

This new behavior is enabled by default.  Add a "reload-period" option
to disable it or change the period of the test.

Document this behavior and option in resolv.conf(5).

Polish the man page just enough to appease igor.

https://lists.freebsd.org/pipermail/freebsd-arch/2015-October/017342.html

Reviewed by:	kp, wblock
Discussed with:	jilles, imp, alfred
MFC after:	1 month
Relnotes:	yes
Sponsored by:	Dell Inc.
Differential Revision:	https://reviews.freebsd.org/D3867

2 files changed, 51 insertions, 2 deletions

diff --git a/lib/libc/resolv/res_init.c b/lib/libc/resolv/res_init.c
index 4dbd6d1..087e6a8 100644
--- a/lib/libc/resolv/res_init.c
+++ b/lib/libc/resolv/res_init.c
@@ -78,6 +78,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/socket.h>
+#include <sys/stat.h>
 #include <sys/time.h>
 
 #include <netinet/in.h>
@@ -227,6 +228,7 @@ __res_vinit(res_state statp, int preinit) {
 	statp->pfcode = 0;
 	statp->_vcsock = -1;
 	statp->_flags = 0;
+	statp->reload_period = 2;
 	statp->qhook = NULL;
 	statp->rhook = NULL;
 	statp->_u._ext.nscount = 0;
@@ -321,6 +323,22 @@ __res_vinit(res_state statp, int preinit) {
 
 	nserv = 0;
 	if ((fp = fopen(_PATH_RESCONF, "re")) != NULL) {
+	    struct stat sb;
+	    struct timespec now;
+
+	    if (_fstat(fileno(fp), &sb) == 0) {
+		statp->conf_mtim = sb.st_mtim;
+		if (clock_gettime(CLOCK_MONOTONIC_FAST, &now) == 0) {
+		    statp->conf_stat = now.tv_sec;
+		} else {
+		    statp->conf_stat = 0;
+		}
+	    } else {
+		statp->conf_mtim.tv_sec = 0;
+		statp->conf_mtim.tv_nsec = 0;
+		statp->conf_stat = 0;
+	    }
+
 	    /* read the config file */
 	    while (fgets(buf, sizeof(buf), fp) != NULL) {
 		/* skip comments */
@@ -666,6 +684,10 @@ res_setoptions(res_state statp, const char *options, const char *source)
 		} else if (!strncmp(cp, "no-check-names",
 				    sizeof("no-check-names") - 1)) {
 			statp->options |= RES_NOCHECKNAME;
+		} else if (!strncmp(cp, "reload-period:",
+				    sizeof("reload-period:") - 1)) {
+			statp->reload_period = (u_short)
+				atoi(cp + sizeof("reload-period:") - 1);
 		}
 #ifdef RES_USE_EDNS0
 		else if (!strncmp(cp, "edns0", sizeof("edns0") - 1)) {
diff --git a/lib/libc/resolv/res_state.c b/lib/libc/resolv/res_state.c
index 59c9430..87643422 100644
--- a/lib/libc/resolv/res_state.c
+++ b/lib/libc/resolv/res_state.c
@@ -26,6 +26,8 @@
  */
 
 #include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/time.h>
 #include <netinet/in.h>
 #include <arpa/nameser.h>
 #include <resolv.h>
@@ -59,13 +61,38 @@ res_keycreate(void)
 	res_thr_keycreated = thr_keycreate(&res_key, free_res) == 0;
 }
 
+static res_state
+res_check_reload(res_state statp)
+{
+	struct timespec now;
+	struct stat sb;
+
+	if ((statp->options & RES_INIT) == 0 || statp->reload_period == 0) {
+		return (statp);
+	}
+
+	if (clock_gettime(CLOCK_MONOTONIC_FAST, &now) != 0 ||
+	    (now.tv_sec - statp->conf_stat) < statp->reload_period) {
+		return (statp);
+	}
+
+	statp->conf_stat = now.tv_sec;
+	if (stat(_PATH_RESCONF, &sb) == 0 &&
+	    (sb.st_mtim.tv_sec  != statp->conf_mtim.tv_sec ||
+	     sb.st_mtim.tv_nsec != statp->conf_mtim.tv_nsec)) {
+		statp->options &= ~RES_INIT;
+	}
+
+	return (statp);
+}
+
 res_state
 __res_state(void)
 {
 	res_state statp;
 
 	if (thr_main() != 0)
-		return (&_res);
+		return res_check_reload(&_res);
 
 	if (thr_once(&res_init_once, res_keycreate) != 0 ||
 	    !res_thr_keycreated)
@@ -73,7 +100,7 @@ __res_state(void)
 
 	statp = thr_getspecific(res_key);
 	if (statp != NULL)
-		return (statp);
+		return res_check_reload(statp);
 	statp = calloc(1, sizeof(*statp));
 	if (statp == NULL)
 		return (&_res);