I Always Wanted a Tool Like This — So I Built One
I always wanted a tool that could check multiple domain names across multiple TLDs in one go — and remember the results. So I built one. Free, self-hosted, open source.
If you've ever started a new project, you know the drill.
You come up with a name you love. You head over to a registrar, type it in, and find out it's taken. You try a variation. Also taken. You add a TLD. Taken. You open three more browser tabs, start checking .io, .ai, .co one at a time, and twenty minutes later you've lost the thread of what you were actually trying to build.
The tools that exist either charge you for bulk lookups, require an account, send your ideas to someone else's server, or give you one domain at a time in a slow, clunky interface. None of them remember what you checked last week. None of them let you come back and re-verify the ones that were available.
I wanted something simple: paste a list of names, pick a few TLDs, see everything at once, and have the results saved so I could check back later. That tool didn't exist the way I wanted it, so I built it.
What It Does
Domain Checker is a self-hosted web app that lets you check domain availability in bulk — multiple names, multiple TLDs, all at once.
You paste your domain names into a text area (or upload a .txt file), select the TLDs you care about from a checkbox grid — .com, .io, .ai, .co, .dev, .app and more — and hit check. The app runs concurrent lookups using RDAP, the modern open standard that registrars themselves use, and returns color-coded results in seconds.
AVAILABLE shows in gold. TAKEN in brick red. ERROR in ochre if a lookup fails. Clean, instant, no ambiguity.
But the part I find most useful day-to-day is persistence. Every result is saved to a local SQLite database. Come back tomorrow, next week, whenever — your results are still there. And with one click you can re-check everything marked AVAILABLE to see if anything has changed. Because it does change, sometimes overnight.
Built to Be Self-Hosted
The whole app runs in Docker. No account required, no API key, no subscription. Your domain name ideas stay on your own server.
git clone https://github.com/knightsri/domain-checker
cd domain-checker
docker-compose up -dThat's the entire setup. Open your browser and it's running.
It's also a PWA — installable on desktop and mobile like a native app. Check domains from your phone, your laptop, anywhere. The UI is built with a Bauhaus-inspired theme (warm, geometric, functional — inspired by Paul Klee's color grid paintings) that makes it genuinely pleasant to use.
Under the Hood
The backend is FastAPI with async RDAP lookups via httpx — up to 5 concurrent requests so large batches don't drag. The frontend is React + Vite with Tailwind CSS. Storage is SQLite, mounted as a Docker volume so data survives container restarts.
A few features worth calling out specifically:
Smart TLD handling — paste myidea.com and it checks that exact domain. Paste myidea with .ai and .io selected and it expands automatically. Mixed lists just work.
Recheck available domains — one button re-runs checks only on domains marked AVAILABLE. TAKEN domains are skipped by default since they're unlikely to have changed.
IANA TLD validation — the app fetches the authoritative TLD list from IANA and warns you when a custom TLD might return unreliable results.
CSV export — download your results anytime for sharing or archiving.
Run It
The full source is on GitHub at github.com/knightsri/domain-checker under MIT license. Contributions welcome.
If you've ever wished for a tool like this — now it exists.
What domain names are you sitting on? Drop them in the comments.
