Custom DNS resolver on Debian 12

Add name server:

nameserver xx.xx.xx.xx

Disable settings overwrite on system reboot:

#       domain-name, domain-name-servers, domain-search, host-name,
#       dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,

Alternatively:

[main]
dns=none