netdev - Re: Trying to implement secondary loopback

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Date:	Wed, 13 Mar 2013 12:13:46 +0100
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Thomas Martitz <thomas.martitz@....fraunhofer.de>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	herbert@...dor.apana.org.au, ebiederm@...ssion.com
Subject: Re: Trying to implement secondary loopback

On Tue, Mar 12, 2013 at 1:05 PM, Thomas Martitz
<thomas.martitz@....fraunhofer.de> wrote:
> Hello folks,
>
> (I'm sending this mail a second time (with a few people in CC) because I
> received no response.)
>
> I'm Thomas Martitz, working on my Master Thesis: an Ethernet NIC. As part of
> my work I want to develop a custom loopback interface (for testing
> purposes).
>
> However I'm facing a problem: It doesn't appear to be possible for a second
> loopback device to exist. Even when copying (and module'ifying)
> drivers/net/loopback.c the resulting device doesn't behave like the standard
> lo interface.
>
> After examing the code I found this line (in loopback_net_init()):
>
> loopback.c:206         net->loopback_dev = dev;
>
> This suggests that each network namespace can only have one loopback
> interface. And indeed after modifying my custom interface to include this
> particular line it begins to work, but obviously at the same time the
> standard lo interface stops working.
>
> So my questions are:
> * Is this on purpose/expected?
> * Is there anyway around it so that I can have a custom loopback interface
> without touching lo's functionality.
> * Generally, what's the proper way (if any) implement a secondary loopback
> interface?

The only really question that matters is, why do you need a second
loopback interface?
Why can't you use any other pseudo interface?

-- 
Thanks,
//richard
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