I have to secure interfaces on Linux servers by setting bonding interfaces. The cabling information is not accurate.

Is there a simple way to know if two interfaces are connected to the same LAN?

Some interfaces have no IP, if possible I would prefer to not set dummies IPs.

I finally did it using arping, it was already installed on the servers:

ifconfig eth2 up 
ifconfig ethO up 
tcpdump -i eth2 -c 3 arp net

and in another terminal :

arping -D -I eth0

tcpdump should displays lines like that:

16:15:43.032103 arp who-has (Broadcast) tell
16:15:44.032277 arp who-has (Broadcast) tell
16:15:45.032441 arp who-has (Broadcast) tell

*-D is optional but it gives a nice source address.

  • 3
    Try to send an arp broadcast message on one interface and see if you can see it on the other one – Ulrich Dangel Jul 4 '14 at 12:46
  • @UlrichDagel I didn't managed to specifically generate ethernet datagrams on one interface. Do you know a standard command to do that ? – Emmanuel Jul 4 '14 at 13:49
  • you can either write your own tool or use somthing like scapy, arpscan etc and check in parallel with tcpdump on the other deice – Ulrich Dangel Jul 5 '14 at 1:36

Some ideas, assuming interfaces are eth0 and eth1:

  • Sniff on both interfaces at the same time for non unicast traffic. You should see all packets twice

    ( tcpdump -nni eth0 -c 10 broadcast or multicast & tcpdump -nni eth1 -c 10 broadcast or multicast & ) | sort
  • Probe with an IP-less protocol.

    For example with this tool to generate DHCP requests: http://www.latinsud.com/pub/dhd/dhd.c

    ( sleep 1; ./dhd eth1 > /dev/null ) & tcpdump -nni eth0 udp and port 68

    You should see something like this:

    14:46:16.449738 IP > BOOTP/DHCP, Request from 00:50:56:99:76:cb, length 300
    14:46:16.650330 IP > BOOTP/DHCP, Request from 00:50:56:99:76:cb, length 300

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