How to find out if the ethernet cable is plugged into the physical interface without bringing up NIC

​

環境

• Red Hat Enterprise Linux

• 6.x

問題

• How to check if the ​

  ​ethernet​

  ​ cable is plugged into the physical interface without bringing up an interface?

決議

• For checking the status of the ​

  ​ethernet​

  ​ cable use ​

  ​ip link​

  ​ command.

1) If the ​

​ethernet​

​ cable is connected, and it is up also then it will show in the ​

​ip link​

​ command. Check the output of eth0.

​​Raw​​

# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 5000
    link/ether 00:16:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:16:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
      

In this case, eth0 <BROADCAST,MULTICAST,UP,LOWER_UP> is connected , and the status of the ​

​ethernet​

​ cable is also Up "state UP qlen 5000".

2) If ​

​ethernet​

​ cable is connected, but it is down then check below output:

# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN qlen 5000
    link/ether 00:16:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:16:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
      

In this case, eth0 <NO-CARRIER,BROADCAST,MULTICAST,UP> is connected, but the status of the ​

​ethernet​

​ cable is down.

3) If ​

​ethernet​

​ cable is not connected,then check below output:

In this nothing will come.

# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:16:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff