Guides Hub

DNS

DNS_PROBE_FINISHED_NXDOMAIN Fix Checklist

DNS_PROBE_FINISHED_NXDOMAIN? Fix it in 4 checks: DNS records, nameserver delegation, resolver cache, domain status. Free instant check, no sign-up.

DNS

SPF Permerror: Too Many DNS Lookups

SPF permerror? Get under the 10 DNS lookup limit in 3 steps: count lookups, flatten includes, drop dead mechanisms. Free instant check, no sign-up.

HTTP

ERR_TOO_MANY_REDIRECTS: Find and Break the Redirect Loop

ERR_TOO_MANY_REDIRECTS? Locate the loop in 4 checks: origin rules, CDN settings, cookies, HTTPS enforcement. Free instant redirect trace, no sign-up.

SSL

SSL Certificate Expired: Renewal and Recovery Checklist

SSL certificate expired or expiring? Recover in 4 steps: renew, deploy to every endpoint, verify the chain, automate. Free instant check, no sign-up.

RDAP / WHOIS

RDAP vs WHOIS: Key Differences

RDAP vs WHOIS: structured JSON vs legacy text, what each returns, and when WHOIS is still required. Free instant RDAP domain lookup, no sign-up.

General

Site Unreachable? DNS, SSL, and HTTP Diagnostic Checklist

Site unreachable? Isolate the failing layer in a fixed order: DNS first, then SSL, then HTTP. One free instant check runs all three, no sign-up.

Domain does not resolve

Use DNS, propagation, and nameserver guides to separate record errors from cache delays.

Mail delivery or auth fails

Trace SPF, DMARC, blacklist, and DNS issues before changing mail infrastructure.

HTTPS or certificate warning

Check expiry, SAN mismatch, wildcard coverage, and certificate chain issues.

Redirect loop or access failure

Start with redirect behavior, response codes, and broad DNS/HTTP diagnosis guides.

4 guides

DNS Lookup

4 guides

Deliverability

4 guides

SSL Check

4 guides

HTTP Diagnostics

4 guides

RDAP / WHOIS

4 guides

Propagation

4 guides

Port Scan

2026-08-27

HTTP Error 500.19 Internal Server Error (IIS Config)

HTTP Error 500.19 is IIS failing to read a config file, not your app crashing. The HRESULT hex points to bad XML, a lock, or permissions. Free instant check.

2026-08-27

HTTP Error 502.5 ANCM Process Failure (ASP.NET Core)

HTTP Error 502.5 means IIS ran but the ASP.NET Core Module couldn't start your app. Check the runtime, stdout log, and processPath. Free instant check.

2026-08-26

444 Connection Closed Without Response (nginx)

444 Connection Closed Without Response means nginx dropped your request with no reply. Check 3 things: reachability, TLS/SNI, WAF rules. Free instant check.

2026-08-26

550 5.7.350 Remote Server Returned Message Detected as Spam

550 5.7.350 means a recipient server flagged your outbound mail as spam. Fix it in 3 checks: SPF, DKIM, and DMARC on your sending domain. Free instant check.

2026-08-25

SEC_ERROR_REVOKED_CERTIFICATE in Firefox: Fix

SEC_ERROR_REVOKED_CERTIFICATE: Firefox found the cert on a revocation list. Server serving a revoked cert, or a local proxy? Free instant check, no sign-up.

2026-08-25

554 5.4.14 Hop Count Exceeded (Mail Loop): Fix

554 5.4.14 Hop count exceeded means your mail is looping. Trace the loop via Received headers and MX routing, then fix it. Free instant check, no sign-up.