Vodafone sends malformed Authentication option

[Freax13]

Starting with 10.5.0 dhcpcd adds a "Reconfigure Accept" option to REBIND packets by default (see a7935e2). This causes my ISP's (Vodafone) DHCPv6 server to respond with a malformed "Authentication" option with 33-byte "Authentication Information" field (as opposed to the normal 17-byte field defined in RFC 3315):

(Wireshark's dissector seemingly parses this extra data into additional fields, but that's because it only considers the length of the option and not the Protocol and Algorithm fields.)

dhcpcd detects this malformed data and rejects the reply:
Sep 12 18:12:48 router dhcpcd[3452928]: enp88s0: authentication failed from fe80::2: Invalid argument
Sep 12 18:12:48 router dhcpcd[3452928]: enp88s0: authentication failed from fe80::2: Invalid argument
Sep 12 18:12:49 router dhcpcd[3452928]: enp88s0: authentication failed from fe80::2: Invalid argument
Sep 12 18:12:49 router dhcpcd[3452928]: enp88s0: authentication failed from fe80::2: Invalid argument

To work around this, one can suppress the "Reconfigure Accept" option in packets sent by dhcpcd by adding this option to the config:

nooption dhcp6_reconfigure_accept

This fixed the issue for me.

I don't think this is an issue with dhcpcd and I don't think anything needs to be done to fix this. I just wanted to write this down in case anybody else encounters similar issues.