some question about "/sys/class/net/<iface>/operstate"

some question about "/sys/class/net/<iface>/operstate"

[杜英杰]

I want to use inotify to monitor /sys/class/net//operstate  to detect status of a iface in real time. 
when I ifdown && ifup eth3, the content of operstate changed, but the file's Modify time didn't change. 
I don't know the reason, is there any file which can be monitored by inotify to get iface status in real time? 
Much appreciation for any advice! 


below are my terminal msg:
  
[root@yingjie-4-5-0-20201109195739-1 11:10:39 0000:00:0c.0]$stat /sys/class/net/eth3/operstate 
File: ‘/sys/class/net/eth3/operstate’
Size: 4096            Blocks: 0          IO Block: 4096   regular file
Device: 14h/20d Inode: 25838       Links: 1
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-11-10 20:42:18.592000969 +0800
Modify: 2020-11-10 20:42:18.592000969 +0800
Change: 2020-11-10 20:42:18.592000969 +0800
Birth: -
[root@yingjie-4-5-0-20201109195739-1 11:10:50 0000:00:0c.0]$cat /sys/class/net/eth3/operstate 
up
You have new mail in /var/spool/mail/root
[root@yingjie-4-5-0-20201109195739-1 11:11:06 0000:00:0c.0]$ifdown eth3
[root@yingjie-4-5-0-20201109195739-1 11:11:13 0000:00:0c.0]$cat /sys/class/net/eth3/operstate 
down
[root@yingjie-4-5-0-20201109195739-1 11:11:16 0000:00:0c.0]$stat /sys/class/net/eth3/operstate 
File: ‘/sys/class/net/eth3/operstate’
Size: 4096            Blocks: 0          IO Block: 4096   regular file
Device: 14h/20d Inode: 25838       Links: 1
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-11-10 20:42:18.592000969 +0800
Modify: 2020-11-10 20:42:18.592000969 +0800
Change: 2020-11-10 20:42:18.592000969 +0800
Birth: -

Re: some question about "/sys/class/net/<iface>/operstate"

[Cong Wang]

On Tue, Nov 10, 2020 at 8:32 PM 杜英杰 <leondyj@pku.edu.cn> wrote:
>
> I want to use inotify to monitor /sys/class/net//operstate  to detect status of a iface in real time.
> when I ifdown &amp;&amp; ifup eth3, the content of operstate changed, but the file's Modify time didn't change.
> I don't know the reason, is there any file which can be monitored by inotify to get iface status in real time?
> Much appreciation for any advice!

You need to listen to netdev netlink messages for changes like
this. These messages are generated in real-time.

Thanks.

Re: some question about "/sys/class/net/<iface>/operstate"

[Stephen Hemminger]

On Wed, 11 Nov 2020 11:02:14 -0800
Cong Wang <xiyou.wangcong@gmail.com> wrote:

> On Tue, Nov 10, 2020 at 8:32 PM 杜英杰 <leondyj@pku.edu.cn> wrote:
> >
> > I want to use inotify to monitor /sys/class/net//operstate  to detect status of a iface in real time.
> > when I ifdown &amp;&amp; ifup eth3, the content of operstate changed, but the file's Modify time didn't change.
> > I don't know the reason, is there any file which can be monitored by inotify to get iface status in real time?
> > Much appreciation for any advice!  
> 
> You need to listen to netdev netlink messages for changes like
> this. These messages are generated in real-time.

The /sys and /proc are pseudo-filesystems. The file modify time and inotify do not work
as expected on these files. Cong is right you need to use netlink for this.

Re: some question about "/sys/class/net/<iface>/operstate"

[Erik Kline]

On Wed, Nov 11, 2020 at 11:55 AM Stephen Hemminger
<stephen@networkplumber.org> wrote:
>
> On Wed, 11 Nov 2020 11:02:14 -0800
> Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> > On Tue, Nov 10, 2020 at 8:32 PM 杜英杰 <leondyj@pku.edu.cn> wrote:
> > >
> > > I want to use inotify to monitor /sys/class/net//operstate  to detect status of a iface in real time.
> > > when I ifdown &amp;&amp; ifup eth3, the content of operstate changed, but the file's Modify time didn't change.
> > > I don't know the reason, is there any file which can be monitored by inotify to get iface status in real time?
> > > Much appreciation for any advice!
> >
> > You need to listen to netdev netlink messages for changes like
> > this. These messages are generated in real-time.
>
> The /sys and /proc are pseudo-filesystems. The file modify time and inotify do not work
> as expected on these files. Cong is right you need to use netlink for this.

Related question: could/should modify time and/or inotify be made to
work?  I genuinely don't know if that would even be possible (separate
from "desirable").

Re: some question about "/sys/class/net/<iface>/operstate"

[Stephen Hemminger]

On Wed, 11 Nov 2020 12:34:51 -0800
Erik Kline <ek@loon.com> wrote:

> On Wed, Nov 11, 2020 at 11:55 AM Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> >
> > On Wed, 11 Nov 2020 11:02:14 -0800
> > Cong Wang <xiyou.wangcong@gmail.com> wrote:
> >  
> > > On Tue, Nov 10, 2020 at 8:32 PM 杜英杰 <leondyj@pku.edu.cn> wrote:  
> > > >
> > > > I want to use inotify to monitor /sys/class/net//operstate  to detect status of a iface in real time.
> > > > when I ifdown &amp;&amp; ifup eth3, the content of operstate changed, but the file's Modify time didn't change.
> > > > I don't know the reason, is there any file which can be monitored by inotify to get iface status in real time?
> > > > Much appreciation for any advice!  
> > >
> > > You need to listen to netdev netlink messages for changes like
> > > this. These messages are generated in real-time.  
> >
> > The /sys and /proc are pseudo-filesystems. The file modify time and inotify do not work
> > as expected on these files. Cong is right you need to use netlink for this.  
> 
> Related question: could/should modify time and/or inotify be made to
> work?  I genuinely don't know if that would even be possible (separate
> from "desirable").

The problem is lots of data changes in /proc and /sys all the time, like every counter
value. So having any kind of notification would be a major performance hit.