Hey all, I recently stumbled across a bug in dhcrelay6 where a relayed DHCPv6 packet from a client was being ignored by my DHCPv6 server. A network trace indicated that the relayed packets had a IPv6 link-local address for the link-address field in the relayed packet. The analogous field in a DHCPv4 packet is used to inform the DHCPv4 server of the subnet the client is in.
I traced this down to a hard-coded setting of the link-address field to the client-interface's link-local address: https://github.com/openbsd/src/blob/5990a1963d9d3a8702389a957caa067859ba104b/usr.sbin/dhcrelay6/dhcrelay6.c#L581 Per RFC8415 19.1.1<https://datatracker.ietf.org/doc/html/rfc8415#section-19.1.1> dhcrelay6 should be placing the globally-scoped address assigned to the client interface in this field. If the relay agent received the message to be relayed from a client, the relay agent places a globally scoped unicast address (i.e., GUA or ULA) from a prefix assigned to the link on which the client should be assigned leases into the link-address field. Inverting the condition so it does NOT select link-local addresses, and eliminating the zero-ing of s6_addr[2] and s6_addr[3] fixes the bug and allows clients to get DHCPv6 assignments via dhcrelay6: https://github.com/openbsd/src/blob/5990a1963d9d3a8702389a957caa067859ba104b/usr.sbin/dhcrelay6/dispatch.c#L191-L200 After implementing the change and running the patched version locally, I have confirmed that it properly relays dhcpv6 packets. Thanks, Asa Yeamans
