How to set up a udev rule for ETH "link down"/"link up"?

Question

I like to switch on a green LED(connected through GPIOs), when eth0 is connected. When disconnected I like to switch the green LED of and a red one on.

Thought that udev is maybe the right place for it. I created the simple demo rule:

KERNEL=="eth0", SUBSYSTEM=="net", ACTION=="add", RUN+="/sbin/set_BSPleds eth0 on"

This rule should call a script when the eth0 is added. It was never executed. After I was looking to the udev monitor by entering "udevadm monitor -k -u" at the shell. There were no events coming when I unplug/plug the lan cable.

root@sama5d3xek:/etc/udev/rules.d# udevadm monitor -k -uh0
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

root@sama5d3xek:/etc/udev/rules.d#

Seems that there are no uevents for eth0. The ethernet driver is provided my ATMEL. I am building a custom Linux by the help of Yocto.

My question is, how to get the "link down"/"link up" events to udev? If it does not works with udev, what alternative way is the best?

This question could better be asked at http://unix.stackexchange.com/ — F. Hauri - Give Up GitHub, Nov 18 '16 at 13:00

score 3 · Answer 1 · edited May 23 '17 at 10:30

3

As others have already mentioned, it seems one can't use udev for this.

Ubuntu: wait for network link up and execute a bash command suggests

inotifywait -e modify /sys/class/net/eth0/carrier; echo 'Change detected'

(Currently, / is on nfs for my box, so I can't really say if it will work.)

In other posts, there are some concerns about using the inotify API on /sys: inotify_add_watch fails on /sys/class/net/eth0/operstate .

I think the Right Way(TM) do do this would be to use the netlink(7) API, preferably through a daemon such as netplugd.

Hope that helps :)

edited May 23 '17 at 10:30

Community

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1

answered Nov 25 '16 at 14:20

user2247306

183
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Even if you're `/` is on *nfs*, `/sys` is still mounted from your kernel. So this must work (or not work) same as on other `/` rootfs based... (read remarks about *inotify* on `/sys` partition. – F. Hauri - Give Up GitHub Nov 29 '16 at 20:56

F. Hauri - Give Up GitHub · Accepted Answer · 2016-11-21T22:13:29.533

2

Ethernet devices are devices, but connections are not.

You could trace connection through /dev/xconsole, dmesg or /var/log/kern.log.

Sample, using `rsyslog`:

You could (as root):

echo ':msg, contains, "Link is" |/run/netlink' >/etc/rsyslog.d/netlinkstate.conf
mkfifo /run/netlink
chgrp user /run/netlink
chmod 640 /run/netlink
service rsyslog force-reload

Then, logged as user, simply:

read line < /run/netlink

will wait for input from fifo, then hold until something happen.

state=${line#*Link is } eventtime=${line%% $HOSTNAME*}
echo $eventtime $state
2016-11-21T17:40:50.956181+01:00 Down

or

echo $eventtime $state
2016-11-21T17:40:50.956181+01:00 Up 100 Mbps Full Duplex, Flow Control: Rx/Tx

echo $eventtime ${state%% *}
2016-11-21T17:40:50.956181+01:00 Up

Under bash, you could use timeout for emptying fifo and read only last entry:

while read -t .01 entry </run/netlink;do line="$entry";done
state=${line#*Link is }
eventtime=${line%% $HOSTNAME*}
shortstate=${state%% *}

Nota: I've used /run to store fifo. This could be not the better place as this won't exist on next reboot.

edited Nov 21 '16 at 22:13

answered Nov 18 '16 at 13:06

F. Hauri - Give Up GitHub

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looks like I have to create a script which is started through systemV. This is permanently reading on dmesg and greping eth*? – Stefan Jaritz Nov 18 '16 at 13:16
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Or through `if-up` or `if-down` script. This depend on your Linux distro. – F. Hauri - Give Up GitHub Nov 18 '16 at 13:58
There is no common "distro". It is a embeeded Linux. – Stefan Jaritz Nov 18 '16 at 14:10
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I've posted a more detailled answer there: http://stackoverflow.com/a/40678535/1765658 – F. Hauri - Give Up GitHub Nov 19 '16 at 10:49
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Your edit was rejected because there is a lot of *bad practices* in it... Tracking an event would better be done by interacting with syslog. I suggest you post your proposed script as a new question. – F. Hauri - Give Up GitHub Nov 21 '16 at 16:14
As you're speaking about *no common distro*, I've posted a little sample using `rsyslog`. – F. Hauri - Give Up GitHub Nov 21 '16 at 16:47
my sample worked without rsyslog. The code works fine on my build machine. rRsyslog not installed on the embeeded linux system. Maybe someone knows good way to handle the events without extra tools/software – Stefan Jaritz Nov 22 '16 at 08:52
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You could follow `/var/log/kern.log` or `/proc/kmsg`, but don't pool event! – F. Hauri - Give Up GitHub Nov 22 '16 at 13:14

How to set up a udev rule for ETH "link down"/"link up"?

2 Answers2

Sample, using rsyslog:

Sample, using `rsyslog`: