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Re: ipt_ROUTE and destination MAC address

To: netdev@xxxxxxxxxxx
Subject: Re: ipt_ROUTE and destination MAC address
From: "junk" <junk@xxxxxxxxxxx>
Date: Fri, 11 Feb 2005 15:17:12 +0100
In-reply-to: <>
References: <S262133AbVBJPBt/> <> <>
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On Fri, 11 Feb 2005 01:07:55 +0100 (CET), Henrik Nordstrom wrote
> On Thu, 10 Feb 2005, junk wrote:
> > i'm coding a virtual interface. That virtual interface has to receive 
> > packets
> > coming on eth0. For that purpose, i'm using ipt_ROUTE. That works great, i 
> > can
> > see my packets arriving on red0 (my virtual interface).
> >
> > The problem is that the destination MAC is the one of eth0, so, it seems the
> > kernel doesn't really deliver the packet to my driver. I can see it in 
> > tcpdump
> > but my driver receive function is never called.
> It is not due to the destination MAC, but to what ipt_ROUTE does.
> ipt_ROUTE reoutes the packet as if it came in on the other interface,
>  all done at the IP layer in the kernel, it does not resubmit the 
> packet to the driver level.
> The MAC is not modified as this is not relevant to the IP layer, and 
> there really isn't any reason why it should be modified either. The 
> MAC used in received skbufs is the MAC the sending station was 
> addressing the packet to, not the MAC of the receiving interface. 
> Usually they are the same, but not always.
> Can you give an more detailed example of what it is you are trying 
> to accomplish or why you need a custom virtual interface driver? It 
> is possible (or even likely) there is other better tools for the job.
> Also I am a little confused on your virtual interface driver. 
> Normally virtual interface drivers does not have a receive function, 
> only a transmit function called when packets are routed to be 
> transmitted on the interface.. How you make packets arrive at the 
> interface driver is up to you (emulated hardware or whatever).
> Regards 
> Henrik
> -
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The purpose is to run a redundant network.

My virtual interface has to duplicate packets it receive from software, and
send each copy of it on eth0 and eth1 respectively.

From the application, there is only one interface: red0.

red0 has to:
- duplicate packets having red0 as outgoing interface (real output iface are
- receive packets from eth0/eth1, discarding the copy (as eth0 and eth1
receive the same data), pass the packet to application
- alert userland if eth0 or eth1 comes down

The problem is that I want my driver to know about incoming packets, not only
the software.

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