CentOS - Oct 2008 - The relationship between udev and MAKEDEV

Hi People,

Recently an issue where I was trying to changed the group and permission 
associated with the device /dev/net/tun.  As I knew devices were 
controlled by udev I went looking at its configuration and found this 
rule 50-udev.rules:KERNEL=="tun",                   
NAME="net/%k".
Some research on the web suggested that modifying the default rules was 
a bad idea and that you should in fact override them with your own 
custom rules.  So I tried setting up a custom rule, this failed.  I then 
tried modifying the default rule this also failed.  A helpful response 
from here pointed me to MAKEDEV and I was able to change 
/etc/makedev.d/01linux-2.6.x to do what I wanted.  But this has raised a 
couple of questions for me.

1.  If udev doesn't directly control the creation of this device why is 
there a udev rule for it?
2. Is modifying /etc/makedev.d/01linux-2.6.x likely to cause me issues 
in future ?

My investigation to date has led to me to believe that devices listed in 
/etc/udev/makedev.d/ are the devices that you always want to be present 
on a system rather than relying on something about your systems Hardware 
configuration to lead to a device being created.  Is this correct ?

Thank you for any thoughts / information you can share relating to this.

Hi,

On Tue, Oct 28, 2008 at 00:21, Clint Dilks <clintd at scms.waikato.ac.nz>
wrote:
> 2. Is modifying /etc/makedev.d/01linux-2.6.x likely to cause me issues in
> future ?
Possibly, since this file is owned by the MAKEDEV rpm. If there is an
upgrade in MAKEDEV, it may overwrite this file. Or if it doesn't, but
there was an addition of a new device there, you would not get the new
device. I would say it would be safer to create a separate file
instead of modifying those.
>From man MAKEDEV:
CONFIGURATION CONFLICTS: In the event that the set of configuration
files contains multiple rules for a given device name, MAKEDEV will
use all of them.  The end result is typically that the last rule given
 (either  by  virtue  of being  listed  below all other matching rules
in the same file, or by being listed in a file which is read after all
others which contain alternate rules) will apply.  MAKEDEV reads the
set of configuration files in sorted order, so this misfeature can be
exploited dependably.

So I believe if you restore the 01linux-2.6.x file to its original
state and create a 99local file with your customized rule, it should
work. Try to do that in a test environment and let us know how that
goes.

I cannot answer your other questions about why it's created with
MAKEDEV and not by udev itself, unfortunately I don't know enough of
udev or of /dev/net/tun to be able to tell why it's done like this.

HTH,
Filipe