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dhclient(8): allow to supersede interface-mtu option

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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 When superseded value is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
This commit is contained in:
Conrad Meyer 2018-05-31 19:36:24 +00:00
parent bbf8c8faf0
commit f93497fe64
2 changed files with 24 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
sbin/dhclient/dhclient.c
View File

@ -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. */

10
sbin/dhclient/dhclient.conf.5
View File

@ -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 }