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Old 10-28-2008, 03:21 AM
Clint Dilks
 
Default 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.


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Old 10-28-2008, 12:54 PM
"Filipe Brandenburger"
 
Default The relationship between udev and MAKEDEV

Hi,

On Tue, Oct 28, 2008 at 00:21, Clint Dilks <clintd@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
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Old 11-01-2008, 07:20 PM
Nifty Cluster Mitch
 
Default The relationship between udev and MAKEDEV

On Tue, Oct 28, 2008 at 09:54:07AM -0400, Filipe Brandenburger wrote:
> On Tue, Oct 28, 2008 at 00:21, Clint Dilks <clintd@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.

In the future may include a clean install. Thus the change
needs to be entered in your off line notebook so you can recall
the magic you are building in your box.

Check to see if the RPM builder marked it as a config file.

rpm -qc MAKEDEV
rpm -qV MAKEDEV

If it is a config file it will be paired with or as an *rpmnew or *rpmsave
file.
http://www.redhat.com/docs/books/max-rpm/max-rpm-html/s1-rpm-install-additional-options.html

If it is not a config file you will need to be able to regenerate it
from your notes.

Adding a local file still requires a notebook entry and also eliminates
the ability to do a simple search for *rpmnew or *rpmsave files. Sysadmins
need to watch for and as needed clean up these files....

$ sudo updatedb; locate -i rpm | egrep rpmnew|rpmsave

.....

The key to udev is that it is "udev - userspace device management". Today in
Linux most devices are under udev. Some devices are critical to system operation
and need to be in place before the user space 'udev' tools can
picks up the ball. The bonds depend on the distro..

http://git.kernel.org/?p=linux/hotplug/udev.git;a=blob;hb=HEAD;f=README
http://en.wikipedia.org/wiki/Udev

Another key is that the list of possible devices in a system is large,
very large. If you look at all the possible devices the list can
burst the limits imposed by major and minor device numbers. A number
of solutions surfaced to address and manage this overflow -- udev is
one of the solutions that seems to work.


--
T o m M i t c h e l l
Found me a new hat, now what?

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