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, 15 insertions, 3 deletions

diff --git a/share/man/man5/resolver.5 b/share/man/man5/resolver.5
index 4116f23..a3c5e05 100644
--- a/share/man/man5/resolver.5
+++ b/share/man/man5/resolver.5
@@ -28,7 +28,7 @@
 .\"     @(#)resolver.5	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd December 25, 2013
+.Dd October 12, 2015
 .Dt RESOLVER 5
 .Os
 .Sh NAME
@@ -175,6 +175,19 @@ the resolver from obeying the standard
 and
 .Sy search
 rules with the given name.
+.It Sy reload-period: Ns Ar n
+The resolver checks the modification time of
+.Pa /etc/resolv.conf
+every
+.Ar n
+seconds.
+If
+.Pa /etc/resolv.conf
+has changed, it is automatically reloaded.
+The default for
+.Ar n
+is two seconds.
+Setting it to zero disables the file check.
 .El
 .Pp
 Options may also be specified as a space or tab separated list using the
@@ -191,8 +204,7 @@ If more than one instance of these keywords is present,
 the last instance will override.
 .Pp
 The keyword and value must appear on a single line, and the keyword
-(e.g.\&
-.Sy nameserver )
+.Pq for example, Sy nameserver
 must start the line.
 The value follows the keyword, separated by white space.
 .Sh FILES