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Re: [Xen-devel] xen 2.0, networking, bridging, and nfsroot

To: James Harper <JamesH@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] xen 2.0, networking, bridging, and nfsroot
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>
Date: Wed, 11 Aug 2004 13:57:22 +0100
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>, brianw@xxxxxxxxxxxx, Adam Heath <doogie@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxxx, Ian.Pratt@xxxxxxxxxxxx
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> Isn't the misfeature you describe a function of the brctl utilities? Couldn't 
> they be modified to add an option to 'brctl addif', eg 'inherit' so to make 
> br0 inherit eth0's address you might use this syntax:

The current situation as implemented by the bridge code is
inconsistent (at least in my view).

If I create a bridge and add eth0 to it, I can still send packets
via eth0 with the original IP address -- I just can't receive
them until I move the IP address to the bridge.

> brctl could then internally do it this way:
> 1. create bridge br0
> 2. give br0 eth0's ip address
> 3. up br0
> 4. add eth0 to br0
> 5. remove address from eth0
> but maybe the problem is that 3+4+5 need to be done in an atomic operation 
> and there's no way to do that from userspace.

I'm not sure how atomic it would need to be to keep nfsroot happy
-- certainly all the pages of the brctl binary and libraries had
better already be paged in.

Also, you need to worry about transferring all the routes that
were pointing through eth0...
> another solution which would maybe solve the nfsroot problem is to have the 
> kernel configure a bridge at boot time via kernel parameters, eg 
> bridge=br0,eth0 and then do everything on br0 from the start rather than 
> starting with eth0 and moving to br0 at some later stage.

That could possibly work, though I believe such boot time options
are becoming deprecated in favour of initrd's in 2.6.

> I think it would be incorrect to just always automatically add the ip address 
> of an interface being added though.

You haven't totally convinced me ;-)  The fact that you can still
TX makes me think it might just be a bug.

Has anyone got the time/inclination to take this up with
<bridge@xxxxxxxx> or search the archives ?


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