From 0237abdc788b71df5c36e54c83c9062825c43959 Mon Sep 17 00:00:00 2001
From: marius <marius@FreeBSD.org>
Date: Thu, 7 Jun 2018 15:51:23 +0000
Subject: [PATCH] MFC: r334443 (by cem@) MF stable/11: r334787

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 When superseded value is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>
Approved by:	re (gjb)
Relnotes:	yes (potentially surprising behavior change w/ broken dhcpd mtu)
Differential Revision:	https://reviews.freebsd.org/D15484
---
 sbin/dhclient/dhclient.c      | 18 +++++++++++++++---
 sbin/dhclient/dhclient.conf.5 | 10 +++++++++-
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/sbin/dhclient/dhclient.c b/sbin/dhclient/dhclient.c
index 070fcf5374f..ec4ad127a4a 100644
--- a/sbin/dhclient/dhclient.c
+++ b/sbin/dhclient/dhclient.c
@@ -828,11 +828,23 @@ 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. */
diff --git a/sbin/dhclient/dhclient.conf.5 b/sbin/dhclient/dhclient.conf.5
index fb9d9f17048..2f28c572257 100644
--- a/sbin/dhclient/dhclient.conf.5
+++ b/sbin/dhclient/dhclient.conf.5
@@ -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 values can be defined
 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 }
-- 
2.45.0