Troubleshooting DHCPv6 Client XID Mismatches

An IPv6 WAN configured to obtain is address via DHCPv6 can suddenly find itself without an IPv6 address if the transaction ID for the IPv6 DHCP client does not match.

When this happens, a log message similar to the following may appear in the DHCP and/or System logs:

dhcp6c[xxxxx]: client6_recvadvert: XID mismatch

When this happens, the most common cause is having multiple running copies of the DHCPv6 client (dhcp6c) on the same interface. Both clients send out a request with different transaction IDs and then get confused by the responses.

When this happens, the quickest way to ensure the clients are reset is to kill the duplicates and start the client again.

From the shell or from Diagnostics > Command Prompt, first check for duplicate clients:

# ps uxawww | grep dhcp6c
root xxxxx 0.0 0.0 5780 1488 ?? INs Sat09PM 0:00.90 /usr/local/sbin/dhcp6c -d -c /var/etc/dhcp6c_wan.conf -p /var/run/ re1
root xxxxy 0.0 0.0 5780 1524 ?? Is Tue07AM 0:00.30 /usr/local/sbin/dhcp6c -d -c /var/etc/dhcp6c_wan.conf -p /var/run/ re1

Once it has been confirmed that a duplicate client is running, kill them:

# killall -9 dhcp6c

Then navigate to Interfaces > WAN, click Save, then click Apply Changes.

The WAN should now have its IPv6 address once again.