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I have an Ubuntu server in a local network that has a number of other devices on the network, both LAN and wireless. I would like to run a Bash script to find out the MAC address (only one per machine) of the interface that is in use on all the machines in the local network. This means depending on what someone's laptop was using (LAN or wireless or other) that it would return only address of the active interface.

I'm not sure which command to use (maybe arp?). ifconfig works only for my local machine, but I would like to do something like this regular expression from Stack Overflow question Best way to extract MAC address from ifconfig's output?.

How can I do this?

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lollercoaster
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2 Answers2

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Most unixes provide an "UP" flag on the flags line of the ifconfig(8) output for interfaces that are enabled, you could match on that. 

eth0      Link encap:Ethernet  HWaddr 00:01:29:a3:f6:1d  
          inet addr:192.168.0.254  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::201:29ff:fea3:f61d/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:35163720 errors:0 dropped:0 overruns:0 frame:0
          TX packets:36652372 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:32425323589 (32.4 GB)  TX bytes:4255769808 (4.2 GB)
          Interrupt:17

(note the UP on the 4th line)

However you can have more than one interface enabled, in which case you might have to look at routing to work out which interface would be used for the path to the default route (ifconfig -rn will help in this)

$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
.
.
.
0.0.0.0         192.168.0.1     0.0.0.0         UG        0 0          0 eth0

here the 0.0.0.0 entry is the default route, 192.168.0.1 will be the host that will be used as the default route and and eth0 is the interface the traffic will be passed over.

Drav Sloan
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This is easily doable with the UNIX arp command, but you'll only see MAC addresses for hosts that are in your ARP table. The best way to populate the table with every host on your subnet is to use nmap with the -sP arg.

In short:

netstat -rn (local subnet/mask)

then

/sbin/arp -n
caw
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