How to inject fullsize 802.1q-tagged frame through BPF?

[Eugene Perevyazko]

On Thu, May 15, 2008 at 12:21:23PM -0400, Sam Leffler wrote:
> Eugene Perevyazko wrote:
> > Is there a way to inject full-mtu vlan-tagged frame through BPF on parent 
> >device?
> > I'd like not to open several tenths of BPFs for each vlanN if it's 
> > possible.
> 
> bpf write code in the kernel calculates header size when packets are 
> injected.  This is likely based on the frames being stock 802.3 so would 
> require mods to the bpf code to handle the additional space for the tag.

I've made a simple patch that works for me on FreeBSD 6.2-RELEASE #1.
Not sure if this is the way to do it, but it "just works". May be someone
else will need such feature too.

Eugene Perevyazko


--- bpf.c.old	Mon May 19 10:47:55 2008
+++ bpf.c	Mon May 19 10:57:44 2008
@@ -64,6 +64,7 @@
 #include <sys/socket.h>
 
 #include <net/if.h>
+#include <net/ethernet.h>
 #include <net/bpf.h>
 #include <net/bpfdesc.h>
 
@@ -617,7 +618,9 @@
 		return (0);
 
 	bzero(&dst, sizeof(dst));
-	error = bpf_movein(uio, (int)d->bd_bif->bif_dlt, ifp->if_mtu,
+        /* Use VLAN_MTU capability, if supported */
+	error = bpf_movein(uio, (int)d->bd_bif->bif_dlt, 
+	    ifp->if_mtu + ( (ifp->if_capenable & IFCAP_VLAN_MTU) ? ETHER_VLAN_ENCAP_LEN : 0 ),
 	    &m, &dst, d->bd_wfilter);
 	if (error)
 		return (error);