isc-dhcp-serverが起動しない

TL;DR

sudo systemctl status isc-dhcp-server.service

でエラーが出るときは

journalctl -xeu isc-dhcp-server.service

で確認してinterface関連のエラーだったら以下を設定する

/etc/default/isc-dhcp-server

INTERFACESv4="eth0"
INTERFACESv6=""

https://qiita.com/JhonnyBravo/items/210ec3c02e362da03dcd
https://tomsalmon.eu/2020/02/isc-dhcp-server-disable-dhcpv6-debian-9/

注意

isc-dhcp-serverはEOLだそうです
https://www.isc.org/blogs/isc-dhcp-eol/
代替はありますが乗り換えは少し工夫が必要かもしれません
https://en.wikipedia.org/wiki/Comparison_of_DHCP_server_software
https://www.reddit.com/r/openbsd/comments/somwlv/differences_of_dhcpd8_isc_vs_openbsd/
https://en.wikipedia.org/wiki/Kea_(software)
https://qiita.com/maclineto/items/840195c1392c2c65d306
https://github.com/rsaxvc/dnsmasq-mac-based-ipv4/blob/master/dnsmasq.conf.example