How to detect the physical connected state of a network cable connector

Question

In a Linux environment  I need to detect the physical connected or disconnected state of an RJ45 connector to its socket   Preferably using BASH scripting only   The following solutions which have been proposed on other sites do NOT work for this purpose    Using  ifconfig  - since a network cable may be connected but the network not properly configured or not currently up  Ping a host - since the product will be within a LAN using an unknown network configuration and unknown hosts    Isn t there some state which can be used in the  proc file system  everything else is in there    How is the Linux world suppose to have their own version of the Windows bubble that pop up from the icon tray indicating that you ve just unplugged the network cable     Kent Fredric and lothar  both of your answers satisfy my need    thanks a lot   Which one I ll use    I still don t know   I guess I can t put you both down as the correct answer   And its probably fair for you that I do choose one   Flip a coin I guess   Again  thanks

User · Answer

cat  sys class net ethX is by far the easiest method   The interface has to be up though  else you will get an invalid argument error   So first   ifconfig ethX up   Then   cat  sys class net ethX

User · Answer

Use  ip monitor  to get REAL TIME link state changes

User · Answer

on arch linux   im not sure on other distros  you can view the operstate  which shows up if connected or down if not the operstate lives on    sys class net  interface name here  operstate  you can also put watch  watch -d -n -1  sys class net  interface name here  operstate

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tail -f  var log syslog   grep -E  link  up down     or for me faster gets   tail -f  var log syslog   grep  link   up  down      It will listen to the syslog file   Result  if disconnect and after 4 seconds connect again    Jan 31 13 21 09 user kernel   19343 897157  r8169 0000 06 00 0 enp6s0  link down Jan 31 13 21 13 user kernel   19347 143506  r8169 0000 06 00 0 enp6s0  link up

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I use this command to check a wire is connected   cd  sys class net  grep    eth0 operstate   If the result will be up or down  Sometimes it shows unknown  then you need to check   eth0 carrier   It shows 0 or 1

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Somehow if you want to check if the ethernet cable plugged in linux after the commend   ifconfig eth0 down   I find a solution  use the ethtool tool    ethtool -t eth0 The test result is PASS The test extra info  Register test   offline          0 Eeprom test     offline          0 Interrupt test  offline          0 Loopback test   offline          0 Link test    on offline          0   if cable is connected link test is 0 otherwise is 1

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You can use ethtool     sudo ethtool eth0 Settings for eth0      Supported ports    TP       Supported link modes    10baseT Half 10baseT Full                             100baseT Half 100baseT Full                             1000baseT Full     Supports auto-negotiation  Yes     Advertised link modes   10baseT Half 10baseT Full                             100baseT Half 100baseT Full                             1000baseT Full     Advertised auto-negotiation  Yes     Speed  1000Mb s     Duplex  Full     Port  Twisted Pair     PHYAD  0     Transceiver  internal     Auto-negotiation  on     Supports Wake-on  umbg     Wake-on  g     Current message level  0x00000007  7      Link detected  yes   To only get the Link status you can use grep     sudo ethtool eth0   grep Link     Link detected  yes

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On OpenWRT the only way to reliably do this  at least for me  is by running these commands    Get switch name swconfig list    assuming switch name is  quot switch0 quot  swconfig dev switch0 show   grep link     Possible output root OpenWrt    swconfig dev switch0 show   grep link          link  port 0 link up speed 1000baseT full-duplex txflow rxflow         link  port 1 link up speed 1000baseT full-duplex txflow rxflow eee100 eee1000 auto         link  port 2 link up speed 1000baseT full-duplex txflow rxflow eee100 eee1000 auto         link  port 3 link down         link  port 4 link up speed 1000baseT full-duplex eee100 eee1000 auto         link  port 5 link down         link  port 6 link up speed 1000baseT full-duplex txflow rxflow  This will show either  quot link down quot  or  quot link up quot  on every port of your switch

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I was using my OpenWRT enhanced device as a repeater  which adds virtual ethernet and wireless lan capabilities  and found that the  sys class net eth0 carrier and opstate values were unreliable  I played around with  sys class net eth0 1 and  sys class net eth0 2 as well with  at least to my finding  no reliable way to detect that something was physically plugged in and talking on any of the ethernet ports  I figured out a bit crude but seemingly reliable way to detect if anything had been plugged in since the last reboot poweron state at least  which worked exactly as I needed it to in my case    ifconfig eth0   grep -o  RX packets  0-9      grep -o   0-9      You ll get a 0 if nothing has been plugged in and something   0 if anything has been plugged in  even if it was plugged in and since removed  since the last power on or reboot cycle   Hope this helps somebody out at least

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There exists two daemons that detect these events   ifplugd and netplugd

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You want to look at the nodes in     sys class net    I experimented with mine    Wire Plugged in    eth0 carrier 1 eth0 operstate unknown   Wire Removed    eth0 carrier 0 eth0 operstate down   Wire Plugged in Again   eth0 carrier 1 eth0 operstate up   Side Trick  harvesting all properties at once the easy way    grep    eth0      This forms a nice list of  key value pairs

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Some precisions and tricks   I do all this as normal user  not root  Grab infos from dmesg  Using dmesg is one of the 1st things to do for inquiring current state of system   dmesg   sed   eth  Link is h   x p  d    could answer something like    936536 904154  e1000e  eth0 NIC Link is Down   or   936555 596870  e1000e  eth0 NIC Link is Up 100 Mbps Full Duplex  Flow Control  Rx Tx   depending on state  message could vary depending on hardware and drivers used   Nota  this could by written dmesg grep eth  Link is tail -n1 but I prefer using sed   dmesg   sed   eth  Link is h   x s    Link is    p  d  Up 100 Mbps Full Duplex  Flow Control  Rx Tx  dmesg   sed   eth  Link is h   x s    Link is    p  d  Down  Test around  sys pseudo filesystem  Reading or writting under  syscould break your system  especially if run as root  You ve been warned  -   This is a pooling method  not a real event tracking   cd  tmp grep -H    sys class net eth0   2 gt  dev null  gt ethstate while   read -t 1 do     grep -H    sys class net eth0   2 gt  dev null           diff -u ethstate -           tee  gt  patch -p0            grep      done   Could render something like  once you ve unplugged and plugged back  depending         -   2016-11-18 14 18 29 577094838  0100   sys class net eth0 carrier 0   sys class net eth0 carrier changes 9   sys class net eth0 duplex unknown   sys class net eth0 operstate down   sys class net eth0 speed -1     -   2016-11-18 14 18 48 771581903  0100   sys class net eth0 carrier 1   sys class net eth0 carrier changes 10   sys class net eth0 duplex full   sys class net eth0 operstate up   sys class net eth0 speed 100    Hit Enter to exit loop   Nota  This require patch to be installed  In fine  there must already be something about this     Depending on Linux Installation  you could add if-up and if-down scripts to be able to react to this kind of events   On Debian based  like Ubuntu   you could store your scripts into   etc network if-down d  etc network if-post-down d  etc network if-pre-up d  etc network if-up d   see man interfaces for more infos

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On the low level  these events can be caught using rtnetlink sockets  without any polling  Side note  if you use rtnetlink  you have to work together with udev  or your program may get confused when udev renames a new network interface   The problem with doing network configurations with shell scripts is that shell scripts are terrible for event handling  such as a network cable being plugged in and out   If you need something more powerful  take a look at my NCD programming language  a programming language designed for network configurations   For example  a simple NCD script that will print  cable in  and  cable out  to stdout  assuming the interface is already up    process foo         Wait for device to appear and be configured by udev      net backend waitdevice  eth0          Wait for cable to be plugged in      net backend waitlink  eth0          Print  cable in  when we reach this point  and  cable out        when we regress      println  cable in        or pop bubble  Network cable in         rprintln  cable out      or rpop bubble  Network cable out                                  just joking  there s no pop bubble   in NCD yet         internally  net backend waitlink   uses rtnetlink  and net backend waitdevice   uses udev   The idea of NCD is that you use it exclusively to configure the network  so normally  configuration commands would come in between  such as   process foo         Wait for device to appear and be configured by udev      net backend waitdevice  eth0          Set device up      net up  eth0          Wait for cable to be plugged in      net backend waitlink  eth0          Add IP address to device      net ipv4 addr  eth0    192 168 1 61    24        The important part to note is that execution is allowed to regress  in the second example  for instance  if the cable is pulled out  the IP address will automatically be removed

User · Answer

You can use ifconfig     ifconfig eth0 up   ifconfig eth0   If the entry shows RUNNING  the interface is physically connected  This will be shown regardless if the interface is configured   This is just another way to get the information in  sys class net eth0 operstate

User · Answer

Most modern Linux distributions use NetworkManager for this  You could use D-BUS to listen for the events   If you want a command-line tool to check the status  you can also use mii-tool  given that you have Ethernet in mind

[linux] How to detect the physical connected state of a network cable/connector?

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