Free diagnostic tools
Each tool works on its own, explains how it measures and where it stops, and shows a result before asking for anything. When a one-off check is not enough, every tool leads to continuous monitoring.
Website availability checker
Check whether a website responds, what status code it returns, how long the request took and where redirects lead. One observation from one location, with the method explained.
Runs from our probe, one location
TLS certificate checker
Inspect the certificate a server presents: validity, days until expiry, issuer, subject alternative names, chain length and the negotiated protocol.
Runs from our probe, one location
DNS lookup
Query a record type through a public resolver and see the normalized answer set.
Runs from our probe, one location
MX lookup
List the mail servers that receive email for a domain, with priorities, through a public resolver.
Runs from our probe, one location
HTTP response header inspector
See the headers a server returns for a URL: caching, content type, server, cookies policy and more, after following redirects.
Runs from our probe, one location
Redirect chain checker
Follow a URL hop by hop and see every redirect, its status code, cross-origin changes and protocol downgrades.
Runs from our probe, one location
Security header inspector
Review Strict-Transport-Security, Content-Security-Policy, X-Content-Type-Options, framing protection, Referrer-Policy and Permissions-Policy on a URL.
Runs from our probe, one location
Uptime and downtime calculator
Convert an availability target into allowed downtime per day, week, month, quarter and year, or compute observed availability from measured durations.
Runs in your browser
Cron expression explainer
Translate a five-field cron expression into words and list the next runs in any time zone, including daylight-saving edge cases.
Runs in your browser
robots.txt tester
Fetch a site's robots.txt, parse its groups and tell you whether a path is allowed or disallowed for a user agent, with the matching rule explained.
Runs from our probe, one location
Sitemap validator
Fetch a sitemap or sitemap index, count URLs, check lastmod formats, cross-host entries and size limits, and sample a few URLs.
Runs from our probe, one location
Domain expiry lookup
Look up registration expiry, registrar, status codes and name servers from the registry's RDAP service where supported.
Runs from our probe, one location
Response time checker
Measure how long one HTTP request takes and see where the time goes: DNS resolution, TCP connect, TLS handshake, time to first byte and download.
Runs from our probe, one location
HTTP status code checker
Request a URL and get the final HTTP status code explained, with the redirect chain that led there and what the code means for monitoring.
Runs from our probe, one location
HTTPS configuration checker
Review a site's HTTPS setup in one run: which TLS protocol versions it accepts, whether the certificate is trusted, HSTS, HTTP/2 via ALPN and whether plain HTTP redirects to HTTPS.
Runs from our probe, one location
Page metadata inspector
Fetch a page and review its head: title and description lengths, canonical, robots directives, hreflang set, Open Graph and Twitter tags, viewport, language and whether JSON-LD blocks parse.
Runs from our probe, one location
Host redirect checker
Request http and https, with and without www, and see where each variant lands, how many redirects it takes and whether they all agree on one canonical URL.
Runs from our probe, one location
SPF record checker
Look up a domain's SPF record, parse every mechanism, count the DNS lookups it needs against the limit of 10 and flag risky choices such as +all or ptr.
Runs from our probe, one location
DMARC record checker
Discover the DMARC policy that applies to a domain with the RFC 9989 DNS Tree Walk, parse the tags, grade the policy from none to reject and verify that external report destinations authorize your domain.
Runs from our probe, one location
Email DNS health check
One report over the DNS side of a domain's email: MX records and whether they resolve, SPF syntax and lookup count, DMARC policy, plus MTA-STS and TLS reporting declarations.
Runs from our probe, one location
DNS resolver comparison
Query a record at one of the zone's authoritative servers and at three public resolvers, then compare the four answers. A consistency check from one location, not a worldwide propagation map.
Runs from our probe, one location
Reverse DNS lookup
Find the PTR name for an IPv4 or IPv6 address and check that the name resolves back to the same address, the forward-confirmed check mail receivers apply.
Runs from our probe, one location
IPv6 readiness checker
See whether a hostname, its zone's name servers and its mail exchangers publish AAAA records, and whether those addresses are global unicast.
Runs from our probe, one location
TXT record lookup
List the TXT records of a name as the resolver returns them, with split strings joined, so you can confirm SPF, DKIM selectors, site verification tokens and other declarations.
Runs from our probe, one location
NS record lookup
Return the delegated name servers for a domain from a public resolver, the first thing to check when DNS behaves unexpectedly after a provider change.
Runs from our probe, one location
CAA record checker
Look up CAA records and see which certificate authorities are allowed to issue certificates for a domain, including wildcard issuance and incident reporting addresses.
Runs from our probe, one location
CNAME lookup
Resolve the CNAME of a hostname to see which target it aliases, the first step when a custom domain for a hosted service or status page does not work.
Runs from our probe, one location
JSON formatter and validator
Validate JSON in your browser, pretty-print or minify it and get the exact line and column of the first syntax error.
Runs in your browser
URL parser and encoder
Split a URL into scheme, host, port, path, query parameters and fragment, with every part decoded, and percent-encode or decode arbitrary text.
Runs in your browser
Base64 encoder and decoder
Encode UTF-8 text to Base64 or decode Base64 and base64url back to text, entirely in your browser, with byte counts and binary detection.
Runs in your browser
Unix timestamp converter
Convert Unix seconds or milliseconds to ISO 8601, HTTP date and your local time, or an ISO date back to epoch, with relative time and ISO week.
Runs in your browser
JWT decoder
Decode a JSON Web Token's header and payload in your browser, see standard claims with readable dates and whether the exp claim has passed. The signature is not verified.
Runs in your browser
Error budget calculator
Turn an SLO target and window into an error budget in minutes, then enter the downtime so far to see consumption, burn rate and the projected exhaustion day.
Runs in your browser
HTTP status code reference
A searchable reference of HTTP status codes with their meaning and what each one usually indicates when a monitor sees it.
Runs in your browser
CIDR and subnet calculator
Compute network address, netmask, wildcard, broadcast, usable host range and address counts for IPv4 prefixes, and network ranges for IPv6 prefixes.
Runs in your browser
How the tools work
Network tools run on isolated workers through the same safe outbound client our monitors use: public hostnames only, standard ports, validated redirects, strict time and size limits. Results are cached for a minute, kept private, excluded from sitemaps and never turned into indexable pages per domain. Calculators run entirely in your browser.