Five free tools for the jobs that come up constantly when you run servers: resolving DNS, reading a raw email, checking who owns a domain, working out what a cron expression does, and whether a port is actually reachable.
No signup, no rate-limit walls, no adverts. Where a tool can run in your browser it does — email headers in particular are parsed entirely on your machine and never sent anywhere.
Why these live here
DNS lookups are easy to get wrong in a hurry. A record can look correct in a control panel and still not resolve because of caching, a missing trailing dot, or a zone that has not been published yet. Seeing the whole record set at once tells you whether the problem is the record you changed or one you forgot.
Mail is where this gets expensive. When a client says their email is bouncing or landing in spam, the headers hold the answer, and almost nobody can read them. That is what the header analyser is for.
The port checker answers the other half of that question: whether the service is even reachable. It separates a dead daemon from a firewall, which is usually the first fork in a support call.
WHOIS answers the boring but costly questions: when does this expire, who is the registrar, has ownership actually transferred. It is worth running before you agree to host a client’s domain.
The cron calculator exists because cron syntax is one of those things nobody quite remembers, and a mistyped schedule can mean a backup that silently never runs.
If one of these is missing something you need, tell me and I will add it.