ipv6 dhcp server not handing out addresses

[robert at spotswood-computer.net]

I've trying to retire an old Debian server (v7 Wheezy). I've new one built
(really a VM) and installed (v9 - Stretch). One by one, I'm moving the
services over. Going well, until I hit the IPv6 dhcp server. The ipv4 dhcp
server went smooth.

The old server is running isc-dhcp-server 4.2.2, while the new server is
running isc-dhcp-server 4.3.5.

I copied the configuration file, but not the lease database from old
server. Then I stopped the old ipv6 (and ipv4) dhcp servers and started
the new ones. The ipv6 dhcp server starts, and is listening, but it is not
handing out addresses. I tested with two Windows machine: ipconfig
/release6 then ipconfig /renew6. Both machines had an ipv6 address from
the old dhcp server, so it's not a client problem, and can renew said
address.

Out of frustration, I copied the old database to the new server and
restarted. Still not working.

I finally fired up wireshark on the client, and the problem seems to be
there are no advertise reply to the solicit from the client, which does
show up in the dhcpd logs. So the server sees the request, but doesn't
answer it.

I checked the ip6tables and everything is accept, so it's not a firewall
issue. Any ideas?

== config file ==
default-lease-time 6048;
max-lease-time 6048;
log-facility local7;
ddns-updates on;
ddns-update-style interim;
update-static-leases on;
authoritative;
#log-facility debug;

subnet6 fd00:220:0:1::/64 {
	#Range for clients
	range6 fd00:220:0:1::601 fd00:220:0:1::800;
	#Additional options
	option dhcp6.name-servers fd00:220:0:1::40, fd00:220:0:1::50;
	option dhcp6.domain-search "redacted.name";
}