Optionally pad outgoing frames to the minimum of 60 bytes (excl. FCS)

before tagging them.  This can help to work around brain-damage in some
switches that fail to pad a frame after untagging it if its length drops
below the minimum.  This option is blessed by IEEE Std 802.1Q (2003 Ed.),
paragraph C.4.4.3.b.  It's controlled by sysctl net.link.vlan.soft_pad.

Idea by:	az
MFC after:	1 week

2 files changed, 57 insertions, 2 deletions

diff --git a/share/man/man4/vlan.4 b/share/man/man4/vlan.4
index 91dc78c..622620d 100644
--- a/share/man/man4/vlan.4
+++ b/share/man/man4/vlan.4
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 25, 2006
+.Dd August 11, 2006
 .Dt VLAN 4
 .Os
 .Sh NAME
@@ -79,6 +79,27 @@ The parent interface is likely to be an Ethernet card connected
 to a properly configured switch port.
 The VLAN tag should match one of those set up in the switched
 network.
+.Pp
+Initially
+.Nm
+assumes the same minimum length for tagged and untagged frames.
+This mode is selected by the
+.Xr sysctl(8)
+variable
+.Va net.link.vlan.soft_pad
+set to 0 (default).
+However, there are network devices that fail to adjust frame length,
+should it fall below the allowed minimum due to untagging.
+Such devices should be able to interoperate with
+.Nm
+after changing the value of
+.Va net.link.vlan.soft_pad
+to 1.
+In the latter mode,
+.Nm
+will pad short frames before tagging them
+so that their length stays not less than the minimum value
+after untagging by the non-compliant devices.
 .Sh HARDWARE
 The
 .Nm
@@ -205,7 +226,8 @@ can be corrected manually if used in conjunction with such parent interface.
 .Sh SEE ALSO
 .Xr kqueue 2 ,
 .Xr miibus 4 ,
-.Xr ifconfig 8
+.Xr ifconfig 8 ,
+.Xr sysctl 8
 .Sh BUGS
 No 802.1Q features except VLAN tagging are implemented.
 .Pp
diff --git a/sys/net/if_vlan.c b/sys/net/if_vlan.c
index 9883066..498d2a2 100644
--- a/sys/net/if_vlan.c
+++ b/sys/net/if_vlan.c
@@ -132,6 +132,10 @@ SYSCTL_DECL(_net_link);
 SYSCTL_NODE(_net_link, IFT_L2VLAN, vlan, CTLFLAG_RW, 0, "IEEE 802.1Q VLAN");
 SYSCTL_NODE(_net_link_vlan, PF_LINK, link, CTLFLAG_RW, 0, "for consistency");
 
+static int soft_pad = 0;
+SYSCTL_INT(_net_link_vlan, OID_AUTO, soft_pad, CTLFLAG_RW, &soft_pad, 0,
+	   "pad short frames before tagging");
+
 static MALLOC_DEFINE(M_VLAN, VLANNAME, "802.1Q Virtual LAN Interface");
 
 static eventhandler_tag ifdetach_tag;
@@ -809,6 +813,35 @@ vlan_start(struct ifnet *ifp)
 		}
 
 		/*
+		 * Pad the frame to the minimum size allowed if told to.
+		 * This option is in accord with IEEE Std 802.1Q, 2003 Ed.,
+		 * paragraph C.4.4.3.b.  It can help to work around buggy
+		 * bridges that violate paragraph C.4.4.3.a from the same
+		 * document, i.e., fail to pad short frames after untagging.
+		 * E.g., a tagged frame 66 bytes long (incl. FCS) is OK, but
+		 * untagging it will produce a 62-byte frame, which is a runt
+		 * and requires padding.  There are VLAN-enabled network
+		 * devices that just discard such runts instead or mishandle
+		 * them somehow.
+		 */
+		if (soft_pad) {
+			static char pad[8];	/* just zeros */
+			int n;
+
+			for (n = ETHERMIN + ETHER_HDR_LEN - m->m_pkthdr.len;
+			     n > 0; n -= sizeof(pad))
+				if (!m_append(m, min(n, sizeof(pad)), pad))
+					break;
+
+			if (n > 0) {
+				if_printf(ifp, "cannot pad short frame\n");
+				ifp->if_oerrors++;
+				m_freem(m);
+				continue;
+			}
+		}
+
+		/*
 		 * If underlying interface can do VLAN tag insertion itself,
 		 * just pass the packet along. However, we need some way to
 		 * tell the interface where the packet came from so that it