2 files changed, 24 insertions, 4 deletions

diff --git a/sbin/dhclient/dhclient.c b/sbin/dhclient/dhclient.c
index 070fcf5374fc3..ec4ad127a4a70 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 fb9d9f17048c0..2f28c57225747 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 }