Re: [gentoo-user] Without udev, who/what names ethernet devices?

[Samuli Suominen]

On 07/06/13 18:05, Chris Stankevitz wrote:
> On Fri, Jun 7, 2013 at 1:06 AM, Alan McKinnon <alan.mckinnon@×××××.com> wrote:
>>> Regarding this string "eth0":
>>>
>>> 1. What does this string represent?  Is it a file on a filesystem?
>>> (no!)  Is it okay for me to call it an "ethernet *device*"
>>
>> It's just a name.
>
> I'm interested in a bit more resolution here.  I believe we
> established that the name "eth0" is given by the kernel.  Presumably
> these names are made available through system calls.  What is the name
> of the system call that provides these monikers?  I'm curious by what
> moniker systemcalls refer to these devices.  For example, is the
> systemcall that retrieves these monikers called "GetEthernetNames"?

http://www.kernel.org/doc/htmldocs/device-drivers/API-device-rename.html

and top of this file will tell how the names calculate:

http://cgit.freedesktop.org/systemd/systemd/tree/src/udev/udev-builtin-net_id.c#n20

>
>> What you CANNOT do with udev is eg switch the names eth0 and eth1 around
>> after the kernel has named them. That was tried for years, it doesn't
>> work. So now udev never interferes with kernel namespace, it create it's
>> own namespace
>
> Okay.  From your description I conclude that there are two classes of
> names for ethernet devices.  "Kernel ethernet names" and "udev
> ethernet names".  When a userland utility (wuch as ifconfig) takes an
> "ethernet device name" as input are they expecting a "kernel ethernet
> name" or the "udev ethernet name"?  That question can be answered with
> a simple "kernel" or "udev" but I'm interested in a little more

everything will use the same, which is whatever you last used to rename 
the device as udev is userspace just as much `ip` command from iproute2 
package is
boot -> kernel assigns eth0 -> the name is now eth0 to userspace -> at 
this point you rename it to something else, using udev, ip, ipconfig, or 
whatever and use it everywhere, or don't rename at all and keep using 
the kernel assigned name

case1:
boot using net.ifnames=0 -> kernel assigns eth0 -> udev sees 
net.ifnames=0 and doesn't rename to anything -> user uses eth0 in 
whatever he is doing

case2:
boot with the default which is net.ifnames=1 -> kernel assigns to eth0 
-> udev sees net-ifnames=1 and reads it's rules and renames accordingly 
to for example enp1s0

> detail.  What do these userland utilities do with the name?  Are these
> utilities calling systemcalls such as
> GivePropertiesOnEthernetNameInKernelNamespace("eth0")?  Are they
> establishing a connection to a udev server and querying based on udev
> namespace names?
>
> The goal of these questions is not for me to determine if udev is good
> or bad, debate global warming, etc.  I just want to technically
> understand these names, what their differences are, where they come
> from, how they are referred in various function calls, etc.

hope above links help explaining in better...

>
> Chris
>