The checker fetches exactly one requested URL and a few public files (robots.txt, sitemap.xml, llms.txt, security.txt and others). It does not crawl websites. Its user agent is: RelaconCheck/1.0 (+https://relacon.at/website-check/bot)
The requested URL (following up to 5 redirects), its http:// and www/non-www variants, /robots.txt, /sitemap.xml, /llms.txt, /llms-full.txt, /.well-known/security.txt, /humans.txt, /favicon.ico, the web app manifest linked in the head, the MTA-STS policy at https://mta-sts.<domain>/.well-known/mta-sts.txt, a deliberately non-existent URL for the 404 test, up to 5 images by HEAD, the og:image and DNS records (SPF, DMARC, DKIM, MX, CAA, _mta-sts).
To detect bot challenges, the checker also fetches the requested page once with the Googlebot token and once with the GPTBot token in the user agent. This request names itself as a RELACON probe so that nobody mistakes it for the real crawler: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html) RelaconCheck-probe (+https://relacon.at/website-check/bot)
Site owners can ask for their domain to be excluded. Send an email to rl@relacon.at with the subject "Opt-out Website-Check" and the domain.
The checker has SSRF protection (http/https only, ports 80/443, no internal addresses) and rate limits. No content of other sites is published, only measurements and short excerpts such as the title or meta description.