bin/57957: dhcpcd fails to unconfigure addresses from released lease

[campbell+netbsd%mumble.net@localhost]

>Number:         57957
>Category:       bin
>Synopsis:       dhcpcd fails to unconfigure addresses from released lease
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Feb 23 22:00:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        netbsd-10
>Organization:
The NetBSDCDCPCHCPR Foundation
>Environment:
NetBSD nanocons.local 10.0_RC3 NetBSD 10.0_RC3 (GENERIC64) #15: Wed Jan 17 05:31:14 UTC 2024  root@manticore.local:/usr/obj/10/evbarm64/sys/arch/evbarm/compile/GENERIC64 evbarm
>Description:
My network had an IPv6 prefix, say 2001:db8:0:1111::/64, delegated from my upstream ISP by DHCPv6-PD.

Downstream to my LAN, my router serves this prefix and default route through SLAAC and DHCPv6.

On my LAN is a NetBSD device whose interface ure0 is configured with dhcpcd, say to 2001:db8:0:1111::1234.

The prefix and prefix length changed upstream, say to 2001:db8:0:2220::/60, so all the downstream DHCPv6 leases stopped working.

DHCP server didn't do FORCERENEW, so I tried `dhcpcd -n ure0' to force renewal, giving ure0 a new address 2001:db8:0:2220::1234.  But ure0 still had the old address 2001:db8:0:1111::1234 and kept using it as a source address, so IPv6 connectivity was effectively lost.

Tried `dhcpcd -k ure0', but that's broken (https://gnats.NetBSD.org/57956).  Tried `dhcpcd -U ure0' to dump why the old address 2001:db8:0:1111::1234 is still configured, and that failed too.

Ran `dhcpcd -k' to make it release leases on all interfaces (fortunately I still have console access to this machine), and it removed the _new_ (working) address 2001:db8:0:2220::1234 but left the _old_ (now broken) address 2001:db8:0:1111::1234 in place.
>How-To-Repeat:
1. change DHCP server to new configuration
2. dhcpcd -n
3. dhcpcd -k
4. observe old configuration is still there
>Fix:
Yes, please!