Ubuntu usage

Baseline firmware is usually enough on Ubuntu.

Reset the Ethernet interface

Run the following commands after reflashing the tether gateway or changing its Ethernet MAC. Replace eth0 with your tether interface.

sudo dhclient -6 -r eth0
sudo dhclient -6 eth0

See the status

Run the following commands to see the status. Replace eth0 with your tether interface. Replace eth0 with your tether interface.

ip -6 route show default
ip -6 route show dev eth0
ip -6 addr show dev eth0
ip -6 neigh show dev eth0

Example default route (proto ra = Router Advertisement):

default via fe80::f4ce:36ff:fe04:73b1 dev eth0 proto ra metric 1024 expires 1747sec

Live GW valid/GONE monitor

The idea with this monitoring is the same as in a Windows PowerShell loop:

• one line per second

• green for valid

• red for GONE

Replace eth0 with your tether interface. Stop with Ctrl+C.

IF=eth0
while true; do
  line=$(ip -6 route show default dev "$IF" 2>/dev/null | head -1)
  if [ -n "$line" ]; then
    via=$(echo "$line" | sed -n 's/.* via \([^ ]*\).*/\1/p')
    metric=$(echo "$line" | sed -n 's/.* metric \([0-9]*\).*/\1/p')
    expires=$(echo "$line" | sed -n 's/.* expires \([^ ]*\).*/\1/p')
    proto=$(echo "$line" | sed -n 's/.* proto \([^ ]*\).*/\1/p')
    printf '\033[32m%s  GW valid  expires=%s  via=%s  metric=%s  proto=%s\033[0m\n' \
      "$(date +%H:%M:%S)" "${expires:-n/a}" "$via" "${metric:-?}" "${proto:-?}"
  else
    printf '\033[31m%s  GW GONE\033[0m\n' "$(date +%H:%M:%S)"
  fi
  sleep 1
done

• expires=…sec - Kernel countdown until the RA route expires. Should jump back when a new RA arrives (~60 s on baseline).

• GW GONE - Rare on Ubuntu with baseline firmware. Investigate if the tether gateway stopped sending RAs or the link is down.

Extra: capture Router Advertisements

To capture Router Advertisements, use the following command:

sudo tcpdump -ni eth0 'icmp6 and ip6[40] == 134'