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

1 files changed, 4 insertions, 1 deletions

diff --git a/include/resolv.h b/include/resolv.h
index e3d4fd1..78da374 100644
--- a/include/resolv.h
+++ b/include/resolv.h
@@ -176,7 +176,8 @@ struct __res_state {
 	int	res_h_errno;		/*%< last one set for this context */
 	int	_vcsock;		/*%< PRIVATE: for res_send VC i/o */
 	u_int	_flags;			/*%< PRIVATE: see below */
-	u_int	_pad;			/*%< make _u 64 bit aligned */
+	u_short	reload_period;		/*%< seconds between stat(resolv.conf)*/
+	u_short	_pad;			/*%< make _u 64 bit aligned */
 	union {
 		/* On an 32-bit arch this means 512b total. */
 		char	pad[72 - 4*sizeof (int) - 3*sizeof (void *)];
@@ -188,6 +189,8 @@ struct __res_state {
 		} _ext;
 	} _u;
 	u_char	*_rnd;			/*%< PRIVATE: random state */
+	struct timespec	conf_mtim;	/*%< mod time of loaded resolv.conf */
+	time_t		conf_stat;	/*%< time of last stat(resolv.conf) */
 };
 
 typedef struct __res_state *res_state;