netdev - Trying to implement secondary loopback

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Message-ID: <513F1A17.1000809@hhi.fraunhofer.de>
Date:	Tue, 12 Mar 2013 13:05:43 +0100
From:	Thomas Martitz <thomas.martitz@....fraunhofer.de>
To:	netdev@...r.kernel.org
CC:	davem@...emloft.net, edumazet@...gle.com,
	herbert@...dor.apana.org.au, ebiederm@...ssion.com
Subject: Trying to implement secondary loopback

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?

Many thanks in advance.

Best regards.

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