[CentOS] 6.x - find interface with link up

[Marko Vojinovic]

On Thursday 15 December 2011 16:04:35 Les Mikesell wrote:
> On Thu, Dec 15, 2011 at 3:39 PM,  <m.roth at 5-cent.us> wrote:
> >>>> In earlier versions 'mii-tool' would iterate over interfaces and
> >>>> show
> >>>> which have link up.   In 6.x it wants an interface as a parameter.
> >>>> What is the appropriate way to find which of some number of of
> >>>> interfaces are connected?   Better yet, what is the least typing
> >>>> to
> >>>> get the mac addresses of those interfaces?
> > 
> > Dumb question: in /etc/sysconfig/network-scripts, do the ifcfg-* have
> > HWADDRs?
> 
> They do, but the case I want to cover is where an existing server is
> cloned, or the disk has been moved from a failed chassis to a spare.
> And in this case the existing files will be wrong, and the nics will
> be named more or less randomly.   Assume the hands-on operators don't
> know much about linux and you can't actively help until they get at
> least one of the right IP's on the right NIC.
> 
> With 5.x I'd use mii-tool to find the connected interface (connecting
> one wire at a time if necessary), then ifconfig on that interface to
> get the hwaddr, then edit that into the ifcfg-* file for the names I
> want the active NICs to have, and reboot.   That's awkward enough to
> ask someone to do who already prefers windows, and it looks like it
> just got harder by having to explicitly run mii-tool for each possible
> interface (and we always have 4 to 6 per box).   There has to be a
> better way.   I thought 6.1 was going to have a new NIC name
> convention but I haven't had time to look into it and have to make
> something work now.

Your situation is the "textbook usecase" example for biosdevname
(http://linux.dell.com/biosdevname). It is a way to consistently name the 
network devices according to their physical location in a computer. For 
example, a typical NIC would not have a name "eth1", but "p2p3" (which means 
"port 3 of the NIC in PCI slot 2") or "em2" (which is "2nd ethernet port on 
the motherboard"). So once the hands-on operator plugs a cable into a 
particular port, you immediately know the corresponding interface name that 
the system will use for that connection. And in addition, this is MAC-address 
independent, so moving the hard drive from one box to the other requires 
basically no reconfiguration (as long as the operator plugs the same cables 
into the same sockets).

The biosdevname was first introduced in Fedora 15 
(http://fedoraproject.org/wiki/Features/ConsistentNetworkDeviceNaming), ie. 
only after RHEL 6 was already rolled out. Apparently, it has to cooperate 
intimately with the kernel, udev, initscripts, dracut, anaconda, kickstart, 
etc., --- so it is not just a userland app which one could yum install in a 
trivial way.

Therefore, somehow I doubt that CentOS 6 will ever see biosdevname implemented 
(maybe in the CentOSplus kernel and a "use at your own risk" label?), since it 
involves too many system changes and breaks backward compatibility. But RHEL 7 
is almost certainly going to have it, since this is actually the proper (and 
permanent) solution to the problem you have.

HTH, :-)
Marko