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From: "Christian S.J. Peron" <csjp@FreeBSD.org>
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Subject: Re: How to inject fullsize 802.1q-tagged frame through BPF?
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Eugene,

Thanks for the report. I will look into this. Although this patch technically
works, I think the right solution to this problem is to introduce a new DLT_
for VLAN encapsulated frames. I will look into this problem in a bit more
detail.

On Mon, May 19, 2008 at 04:07:22PM +0300, Eugene Perevyazko wrote:
> 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);
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