Debian Upgrade Buster To Bullseye

This article contains all problems that I encountered when upgrading Debian from 10/Buster to 11/Bullseye.

isc-dhcp-server failed to start

Unfortunately, all you see at first using systemctl status isc-dhcp-server is

Sep 30 20:51:28 host systemd[1]: isc-dhcp-server.service: Control process exited, code=exited, status=1/FAILURE
Sep 30 20:51:28 host systemd[1]: isc-dhcp-server.service: Failed with result 'exit-code'.
Sep 30 20:51:28 host systemd[1]: Failed to start LSB: DHCP server.

journalctl -xe contains relevant information:

Okt 01 09:22:26 host dhcpd[47376]: Server starting service.
Okt 01 09:22:28 host isc-dhcp-server[47361]: Starting ISC DHCPv4 server: dhcpd.
Okt 01 09:22:28 host isc-dhcp-server[47361]: Launching IPv6 server only.
Okt 01 09:22:28 host dhcpd[47388]: Wrote 0 NA, 0 TA, 0 PD leases to lease file.
Okt 01 09:22:28 host dhcpd[47388]: 
Okt 01 09:22:28 host dhcpd[47388]: No subnet6 declaration for vmnet (2001:0DB8::/32).
Okt 01 09:22:28 host dhcpd[47388]: ** Ignoring requests on vmnet.  If this is not what
Okt 01 09:22:28 host dhcpd[47388]:    you want, please write a subnet6 declaration
Okt 01 09:22:28 host dhcpd[47388]:    in your dhcpd.conf file for the network segment
Okt 01 09:22:28 host dhcpd[47388]:    to which interface vmnet is attached. **

The simple solution is to add the required subnet declaration in /etc/dhcp/dhcpd6.conf:

subnet6 2001:0DB8::/32 {
	range6 2001:0DB8::1 2001:0DB8::efff;
}