svn commit: r334443 - head/sbin/dhclient

[Conrad Meyer]

Author: cem
Date: Thu May 31 19:36:24 2018
New Revision: 334443
URL: https://svnweb.freebsd.org/changeset/base/334443

Log:
  dhclient(8): allow to supersede interface-mtu option
  
  In some cases broken DHCP servers might send invalid MTU value, so allow to
  use 'supersede' in dhclient.conf to override this. When superseded value is
  0, MTU value is not updated at all.
  
  PR:		206721
  Submitted by:	novel@
  Reported by:	<jimp AT pfsense.org>
  MFC after:	37 minutes (if you care about 11, please MFC to 11.2)
  Relnotes:	yes (potentially surprising behavior change w/ broken dhcpd mtu)
  Differential Revision:	https://reviews.freebsd.org/D15484

Modified:
  head/sbin/dhclient/dhclient.c
  head/sbin/dhclient/dhclient.conf.5

Modified: head/sbin/dhclient/dhclient.c
==============================================================================
--- head/sbin/dhclient/dhclient.c	Thu May 31 16:48:08 2018	(r334442)
+++ head/sbin/dhclient/dhclient.c	Thu May 31 19:36:24 2018	(r334443)
@@ -849,11 +849,23 @@ bind_lease(struct interface_info *ip)
 
 	opt = &ip->client->new->options[DHO_INTERFACE_MTU];
 	if (opt->len == sizeof(u_int16_t)) {
-		u_int16_t mtu = be16dec(opt->data);
-		if (mtu < MIN_MTU)
-			warning("mtu size %u < %d: ignored", (unsigned)mtu, MIN_MTU);
+		u_int16_t mtu = 0;
+		bool supersede = (ip->client->config->default_actions[DHO_INTERFACE_MTU] ==
+			ACTION_SUPERSEDE);
+
+		if (supersede)
+			mtu = getUShort(ip->client->config->defaults[DHO_INTERFACE_MTU].data);
 		else
+			mtu = be16dec(opt->data);
+
+		if (mtu < MIN_MTU) {
+			/* Treat 0 like a user intentionally doesn't want to change MTU and,
+			 * therefore, warning is not needed */
+			if (!supersede || mtu != 0)
+				warning("mtu size %u < %d: ignored", (unsigned)mtu, MIN_MTU);
+		} else {
 			interface_set_mtu_unpriv(privfd, mtu);
+		}
 	}
 
 	/* Write out the new lease. */

Modified: head/sbin/dhclient/dhclient.conf.5
==============================================================================
--- head/sbin/dhclient/dhclient.conf.5	Thu May 31 16:48:08 2018	(r334442)
+++ head/sbin/dhclient/dhclient.conf.5	Thu May 31 19:36:24 2018	(r334443)
@@ -38,7 +38,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 1, 1997
+.Dd May 31, 2018
 .Dt DHCLIENT.CONF 5
 .Os
 .Sh NAME
@@ -227,6 +227,14 @@ rather than any value supplied by the server, these va
 in the
 .Ic supersede
 statement.
+.Pp
+Some options values have special meaning:
+.Bl -tag -width indent
+.It Ar interface-mtu
+Any server-supplied interface MTU is ignored by the client if a
+.Ic supersede
+zero value is configured.
+.El
 .It Xo
 .Ic prepend No { Op Ar option declaration
 .Oo , Ar ... option declaration Oc }