Today I was asked for some information concerning a server that could not reach its default gateway fe80::1 over eth1 (and the same for a working server).

Instead of searching for the same or similar commands every time, I will now document a oneliner :-)

ssh $server 'set -x;
	ip a show dev eth1;
	tcpdump -e -i any -nnnn -vv icmp6 &
	sleep 1;
	ping6 -c 2 fe80::1%eth1;
	sleep 1;
	kill %%'


  • I start tcpdump first in the background so the sequence of commands can continue, later I kill it
  • the sleeps are probably not necessary, but included just in case there is a delay in the packets
  • set -x prints out all commands before they are executed - good to copy/paste and mail it to someone since they will now know the parameters
  • ip a show dev eth1: gets important starting data about eth1 (ip addresses and mac address)
  • tcpdump flags:
    • -e: show mac addresses and packet direction (In, Out)
    • -i any: capture traffic on any interface (verify for asymmetric routing)
    • -nnnn: don’t resolve any ips (not really useful and potentially confusing for local stuff)
    • -vv: be more verbose
    • icmp6: I’m only intereseted in pings over ipv6
  • ping flags:
    • -c 2: I send two pings out, kind of a control; don’t leave the parameter out, because by default ping will ping forever
      • fe80::1: this is our (local) default gateway on any ipv6 network
      • eth1: I’m pinging a link-local address and want to diagnose traffic over eth1
        • if you didn’t know about the ‘ip%dev’ syntax, Google it…
  • There is more you may want to throw in, depending on the circumstances; these are useless for a link-local address:
    • ip ro get $ip: show the gateway and source ip that will be used to reach $ip
    • traceroute -n $ip: document the intermediate hops (gateways)