How to See MAC Addresses

There are times when you lose connectivity with another server that is directly connected to your local network. Taking a look at the ARP table of the server from which you are troubleshooting will help determine whether the remote server's NIC is responding to any type of traffic from your Linux box. Lack of communication at this level may mean:

• Either server might be disconnected from the network.

• There might be bad network cabling.

• A NIC might be disabled or the remote server might be shut down.

• The remote server might be running firewall software such as iptables or the Windows XP built-in firewall. Typically in this case, you can see the MAC address, the server is running the correct software, but the desired communication doesn't appear to be occurring to the client on the same network.

Here is a description of the commands you may use to determine ARP values:

• The ifconfig -a command shows you both the NIC's MAC address and the associated IP addresses of the server that you are currently logged in to:

  [root@bigboy tmp]# ifconfig -a
  
  wlan0 Link encap:Ethernet HWaddr 00:06:25:09:6A:B5
  inet addr:192.168.1.100 Bcast:192.168.1.255 Mask:255.255.255.0
  UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
  RX packets:47379 errors:0 dropped:0 overruns:0 frame:0
  TX packets:107900 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:100
  RX bytes:4676853 (4.4 Mb) TX bytes:43209032 (41.2 Mb)
  Interrupt:11 Memory:c887a000-c887b000
  
  wlan0:0 Link encap:Ethernet HWaddr 00:06:25:09:6A:B5
  inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
  UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
  Interrupt:11 Memory:c887a000-c887b000
  [root@bigboy tmp]#
  

  
  

Here you can see the wlan0 interface has two IP addresses 192.168.1.100 and 192.168.1.99 tied to the NIC hardware MAC address of 00:06:25:09:6A:B5:

• The arp -a command will show you the MAC addresses in your server's ARP table and all the other servers on the directly connected network. Here we see we have some form of connectivity with the router at address 192.168.1.1:

  [root@bigboy tmp]# arp -a
  bigboypix (192.168.1.1) at 00:09:E8:9C:FD:AB [ether] on wlan0
  ? (192.168.1.101) at 00:06:25:09:6A:D7 [ether] on wlan0
  [root@bigboy tmp]#
  

  
  

• Make sure the IP addresses listed in the ARP table match those of servers expected to be on your network. If they don't, your server may be plugged into the wrong switch or router port.

• You should also check the ARP table of the remote server to see whether it is populated with acceptable values.