HTTP Header Checker

Run the HTTP header checker after deploying a new origin, switching CDN orange-cloud modes, or enabling HSTS. Cache-Control and CDN-specific headers explain why users see stale HTML despite a fresh deploy — compare ETag and max-age before purging blindly. If headers show a different Server banner than expected, confirm DNS A/AAAA with DNS Checker — you may be hitting an old load balancer IP.

Security reviews use HSTS, CSP, X-Frame-Options, Referrer-Policy, and Permissions-Policy to gauge baseline hardening. Missing HSTS on HTTPS-only sites leaves first-visit downgrade windows. CSP misconfiguration breaks scripts in production — header checker captures the live policy string for diffing against staging. SSL Certificate Checker validates the certificate; headers validate what the app sends after TLS terminates.

Redirect debugging benefits from seeing the full chain: marketing short links, apex→www rules, and http→https upgrades each add a hop. Support threads often need the exact 301 Location target — copy from the checker instead of guessing from browser address bars that may hide intermediate steps.

API and SPA teams inspect CORS headers (Access-Control-Allow-Origin) when browser consoles show blocked fetches. Headers prove what the edge returns to unauthenticated GET requests — distinct from POST preflight behavior you may still need to test separately.

Compliance and cookie-banner projects catalog Set-Cookie attributes: Secure, HttpOnly, SameSite. Header checker documents issuance on the landing URL — useful before privacy audits. DN01 does not scan entire sites for every cookie on every path; start with the URL under investigation.

Load balancers sometimes strip Server headers while adding Via — document both when hardening fingerprinting policy. Pen testers use header variance to map CDN vs origin; your hardening ticket should list intentional differences per hostname.

Mobile apps calling APIs may never show browser DevTools — paste failing API URLs into header checker to capture 401/403 response headers (WWW-Authenticate, Retry-After) for backend triage without reproducing on device.

Different headers at the same URL from office VPN vs mobile network often mean geo-routed CDNs or split A records — compare DNS Checker answers and re-fetch from the header tool's path. We show one fetch snapshot, not fifty global edge nodes.

301 vs 302 vs 307 matter for SEO and method preservation. The checker exposes status codes per hop — do not assume all redirects are equivalent. Meta refresh redirects in HTML bodies are outside HTTP header scope; view page source if Location is missing but browser still moves.

Stale Cache-Control with long max-age at the CDN while origin sends no-cache means edge configuration overrides origin — fix the CDN rule, purge, then re-check headers. Empty Cache-Control on static assets may cause excessive revalidation — intentional on HTML, wasteful on immutable assets.

Security headers present on www but missing on apex (or vice versa) usually mean incomplete vhost config. Check both hostnames separately. HSTS on only one label leaves sibling labels vulnerable — align TLS virtual hosts and re-run SSL Certificate Checker for SAN coverage.

Brotli and gzip Content-Encoding affect body delivery but not header semantics — do not confuse missing Content-Length on chunked responses with errors; focus on cache and security headers for policy audits.

HTTP headers exist above TCP/TLS. A valid certificate from SSL Certificate Checker does not imply correct caching or CSP. Conversely, perfect security headers on the wrong IP mean DNS still points elsewhere for some resolvers — triangulate with DNS Checker and DIG on A/AAAA.

CDN proxies terminate TLS and may strip or inject headers. Server and Via headers reveal how many proxies participated. Origin-only diagnostics require grey-cloud or direct-origin fetches outside the CDN — DN01 fetches the public URL you enter, typically through whatever path DNS defines today.

HSTS preload lists and certificate transparency are outside this tool's scope. Header checker shows Strict-Transport-Security on live responses — enough for many audits, not a substitute for preload registry submission or CT log monitoring.

Automate recurring header regression checks via the documented API after token registration. Browser UI remains best for ad-hoc investigations with copy buttons and localized labels — we do not sell a full SIEM or WAF analytics dashboard.

HTTP/3 and QUIC change how connections establish but many security headers remain HTTP semantic layer — checker documents headers on the fetched response path your URL negotiates today, not every experimental edge protocol variant.

Stale 304 Not Modified responses depend on If-None-Match client behavior — header checker on bare URL may show 200 while browsers with cached ETag see 304; document test conditions when comparing support screenshots.

DN01 does not execute JavaScript — SPAs that set security headers only after client render may mislead if you only inspect static hosting URLs.

curl -I is the operator standard but blocked on many corporate laptops and intimidating for non-CLI teammates. DN01 formats headers for copy-paste with redirect chains visible in one panel — same information class, gentler UX, eight locales.

Browser DevTools Network tabs excel at interactive debugging with POST bodies and WebSockets. Header checker targets quick GET/HEAD-style snapshots of response metadata for tickets — not full HAR exports or waterfall timing analysis.

Dedicated security scanners crawl sites, score headers, and track CVEs. DN01 provides honest single-URL fetches without a paid grade badge or mass crawl — pair with your scanner for depth, use DN01 for fast «what does this URL return right now?»

We do not follow arbitrary JavaScript redirects, authenticate to apps, or store historical header databases per customer. Rate limits protect the service. Propagation-style worldwide edge maps are out of scope — DNS Checker and DIG answer addressing; we answer HTTP metadata for the URL you submit.

Enterprise WAF teams maintain allowlists for header inspection bots — DN01 fetches resemble normal monitoring traffic but always coordinate with your security policy before automating high-volume checks against production. We report what we receive; we do not bypass bot management or solve CAPTCHA gates on your behalf.

Third-party tag managers inject scripts that alter perceived performance — header checker documents server declarations; Lighthouse and RUM tools measure client experience. Use both when marketing asks why «cache headers look fine but users see stale HTML» — edge worker middleware may rewrite bodies after origin responds.

Reverse proxy auth gateways return 302 to login with Set-Cookie — capture those headers when debugging SSO integrations; DN01 does not perform authenticated sessions but documents pre-auth redirects that confuse API clients expecting JSON.