Port/telnet reachability, port scans, and live certificate inspection run from our servers, so they need an account (and are rate-limited). Sign in to use them. The in-browser tools on the Tools page need no sign-in.
Go to dashboard to sign inReachability and certificate checks run server-side from ackdelay.
Public targets only — private/internal ranges are blocked. Only probe hosts you own or are authorized to test.
Test whether a single TCP port is reachable (the "telnet host port" check) and how fast it connects.
Check a host against ~24 common service ports (FTP, SSH, HTTP/S, SMTP, RDP, SQL…). Authorized targets only.
Fetch and inspect the live TLS certificate served by a host — validity, expiry, SANs, issuer chain.
Paste a PEM certificate to decode its subject, issuer, validity, SANs, serial, and fingerprint.
Confirm a certificate matches its private key, or compare two certificates by fingerprint.