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Lately, after setting up an instance of an isc kea dhcp6 server , I noticed that after a reboot it would be inactive, although it had been started properly. Turned out that it simply couldn't bind the link-local address.

Oct 10 16:27:04 wyvern kea-dhcp6[1180]: WARN  DHCPSRV_OPEN_SOCKET_FAIL failed to open socket: Failed to open multicast socket on interface enp2s0, reason: Failed to open link-local socket on interface enp2s0: Failed to bind socket 13 to fe80::6a1d:efff:fe2d:6399/port=547: Cannot assign requested address
Oct 10 16:27:04 wyvern kea-dhcp6[1180]: INFO  DHCP6_OPEN_SOCKETS_FAILED maximum number of open service sockets attempts: 0, has been exhausted without success
Oct 10 16:27:04 wyvern kea-dhcp6[1180]: WARN  DHCPSRV_NO_SOCKETS_OPEN no interface configured to listen to DHCP traffic

I tried a good many things, especially really making sure that all the After=network-online.target etc. were present. In the end, I just settled for the simplest but up today flawlessly working version:

[Service]
ExecStartPre=/bin/sleep 5