netdev - Re: [iproute2] finding the name of a TAP created with %d

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Message-ID: <20210406171532.dlcvtm5wzmuemhwk@eve.qyliss.net>
Date:   Tue, 6 Apr 2021 17:15:32 +0000
From:   Alyssa Ross <hi@...ssa.is>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org
Subject: Re: [iproute2] finding the name of a TAP created with %d

On Tue, Apr 06, 2021 at 09:22:31AM -0700, Stephen Hemminger wrote:
> On Tue, 6 Apr 2021 13:42:40 +0000
> Alyssa Ross <hi@...ssa.is> wrote:
>
> > If I do
> >
> > 	ip tuntap add name tap%d mode tap
> >
> > then a TAP device with a name like "tap0", "tap1", etc. will be created.
> > But there's no way for me to find out which name was chosen for the
> > device created by that command.
>
> Use a follow on ip link show or look in sysfs.
>
> > Perhaps ip should print the name of tuntap devices after they're
> > created?
>
> You can already do that with followon command, or use batch to put two commands together.

I don't think that's reliable, is it?  What happens if I create one
device, but by the time I do ip link show another one has been created?
There's no way to know for sure that the last device was created by the
command I ran.

As I understand it, the only way to avoid a race like that for sure
would be to look at the ifr_name returned by the kernel from TUNSETIFF.

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