My application check /sys/class/net/eth0/carrier for ethernet status;
it works great on RPI 3B but broken on RPI 3B+.

If I plug out the RJ45 cable from both RPI 3B/3B+ :
the RPI 3B will change the carrier status from 1 to 0
but RPI 3B+ won't change anything.

Is there any attribute that my application can check for RPI 3B+ ethernet status?


Is there any attribute that my application can check for RPI 3B+ ethernet status?

Check the output of ip link show eth0. It should include NO-CARRIER if it is disconnected.

I can't promise this is the case (I don't have a 3B+ to check it on) if the kernel does not have access to the correct information, which the sysfs output implies, but it is worth a try.

  • Thanks for reply. but I always got following information from 『ip link show eth0』(no matter I plug out the cable or not): 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000 link/ether 00:1f:d8:00:bc:1b brd ff:ff:ff:ff:ff:ff – Wally Oct 15 '18 at 6:53

OK, Found two workarounds instead one true answer.
If ethernet cable is connected before RPI 3B+ booted, /sys/class/net/eht0/carrier will be 1 forever. (RPI 3B won't have this issue)

But we have the following two workarounds:
1. connect ehternet cable after RPI 3B+ boot into system
2. simple use "sudo ifconfig eth0 down" first, then "sudo ifconfig eth0 up" again. the RPI 3B+ /sys/class/net/eht0/carrier will works.

I believe this might be a Linux kernel (I'm using 4.14.56-v7+) bug since we can simply down & up trick to workaround this.

  • In my case, even though down/up (as suggested on item 2) sorts out the NO-CARRIER state, it does not affect anything physically in reality. The switch does not light the led as it should be expected and (more or less obviously!) I cannot ping other devices. – Richard Gomes Nov 8 '20 at 20:41

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