Re: [gentoo-user] Eth0 interface not found - udev

[Stroller]

On 7 April 2013, at 16:35, Pandu Poluan wrote:
> On Apr 7, 2013 3:56 PM, "Stroller" <stroller@××××××××××××××××××.uk> wrote:
>
>> AIUI the motive for these changes are so that you can unpack an enterprise-type server, the ones with two NICs on the motherboard, and always know which NIC is which. You can then unpack a pallet load of them, and deploy them without any need for determining which is which or for any manual intervention. This is actually pretty important and useful, but I'm not sure this has all been done the best way.
>
> AFAICT, on-board NICs have sequential MAC Adresses, with the one labeled "Port 1" has the smallest MAC Address. So far, *all* Linux distros I've used on a server will reliably name "Port X" as "eth$((X-1))". So it's never a puzzle as to which port bears which "ethX" moniker.

I would expect this to be the case, too, but I'm told it's not always so - you cannot be certain of it.

I think that the kernel allocates interfaces to NICs in the order in which they're found - eth0 to the first one, eth1 to the second, and so on. A pair of on-board NICs may be allocated interface IDs in the same order as their MACs, but they may not be - especially if, for some reason, one responds abnormally slowly to probing from the kernel. 

A really good long discussion of this is available at [1], see also [2]:

   Without biosdevname, you get all ethX names - they're just in completely
   non-deterministic order.  Often times after the first non-deterministic
   order is set in 70-persistent-net.names, and with no other configuration
   changes to your system, on reboot you'll get those same names for those
   devices again, but only because no renames are actually taking place -
   the kernel accidentally names them in the same way each time.
   
   You cannot swizzle them within the ethX namespace and have it work -
   it's racy and failure-prone.  You must change out of ethX in order to
   get consistency at all.

> The new naming scheme, however, is much less intuitive. Where originally I just immediately use eth0, now I have to enumerate the monikers first, because even between servers of the same model (let's say, HP's DL360 G7), the PCI attachment point might differ.

I agree. However, attempts to solve this in kernel (I think *several* of them), which would have allowed the eth0, ethX namespaces to be retained, were rejected. See [3].

I believe that HP shared involvement in this - I think they collaborated with Dell on how the BIOS would declare the NICs in a way that would be available to the kernel.

Stroller.



[1] http://marc.info/?l=linux-netdev&m=128163454631618&w=3
[2] http://lists.us.dell.com/pipermail/linux-poweredge/2010-November/043586.html
[3] http://marc.info/?l=linux-netdev&m=128518030400371&w=3