https://github.com/luckyframework/lucky/blob/baa251c24c1d582a1565b3712c565251a67c24b4/src/lucky/subdomain.cr#L5-L11 This option needs to be documented under this page https://luckyframework.org/guides/http-and-routing/routing-and-params#subdomains so you know to change to `tld_length = 2` for TLDs like `.co.uk`, or to `0` when working locally and using `localhost` as your domain.