Previously the bridge MTU was set to ETHERMTU and could not be changed. Since

we can only bridge interfaces with the same value it meant that all members had
to be set at ETHERMTU as well.

Allow the first member to be added to define the MTU for the bridge, the check
still applies to all additional members.

Print an informative message if the MTU is incorrect [1]

Requested by:	Niki Denev [1]
Approved by:	mlaier (mentor)
MFC after:	3 days

2 files changed, 9 insertions, 1 deletions

diff --git a/share/man/man4/if_bridge.4 b/share/man/man4/if_bridge.4
index 0a927e5..0a8c624 100644
--- a/share/man/man4/if_bridge.4
+++ b/share/man/man4/if_bridge.4
@@ -133,6 +133,9 @@ the filter for processing.
 Note that packets to and from the bridging host will be seen by the
 filter on the interface with the appropriate address configured as well
 as on the interface on which the packet arrives or departs.
+.Pp
+The MTU of the first member interface to be added is used as the bridge MTU,
+all additional members are required to have exactly the same value.
 .Sh EXAMPLES
 The following when placed in the file
 .Pa /etc/rc.conf
diff --git a/sys/net/if_bridge.c b/sys/net/if_bridge.c
index c7987c2..bd0958e 100644
--- a/sys/net/if_bridge.c
+++ b/sys/net/if_bridge.c
@@ -716,8 +716,13 @@ bridge_ioctl_add(struct bridge_softc *sc, void *arg)
 	if (ifs == NULL)
 		return (ENOENT);
 
-	if (sc->sc_ifp->if_mtu != ifs->if_mtu)
+	/* Allow the first member to define the MTU */
+	if (LIST_EMPTY(&sc->sc_iflist))
+		sc->sc_ifp->if_mtu = ifs->if_mtu;
+	else if (sc->sc_ifp->if_mtu != ifs->if_mtu) {
+		if_printf(sc->sc_ifp, "invalid MTU for %s\n", ifs->if_xname);
 		return (EINVAL);
+	}
 
 	if (ifs->if_bridge == sc)
 		return (EEXIST);