Find every subdomain
of a domain.

Every hostname under a domain that ever appeared in a public TLS certificate — with certificate counts, first/last seen and live DNS status.

2 hosts · 236 ms

HostCertsFirst seenLast seenDNS
fastmail.help 4 2026-04-20 2026-08-18 ok
www.fastmail.help 5 2026-04-25 2026-08-23 ok

Programmatic access, with DNS A records: API GET /v1/hosts/fastmail.help (API key for paging).

Shown here: hostnames with a certificate that is valid now or expired within the last 90 days. The full history since 2013 — hostnames whose last certificate expired earlier — is a Pro plan method: GET /v1/hosts-history/fastmail.help.