Guides Hub

Short guides to help you interpret lookup results and resolve issues. Each guide follows the Problem → TOP 3 Causes → Tool Links → Resolution Checklist structure.

Most Common Issues

Start with the highest-frequency troubleshooting guides. These topics are the best candidates for landing traffic and fast diagnosis.

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.

Diagnose

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.

Diagnose

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.

Diagnose

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.

Diagnose

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.

Diagnose

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.

Diagnose

Browse by Symptom

If you know the symptom but not the root cause, start here and narrow the issue down with the matching tools.

Browse by Tool

Move from a guide into the exact utility you need. Each tool card groups the guides most likely to help you diagnose that area.

Categories

DNS (26)

Fix DNS resolution, propagation, and email authentication (SPF, DKIM, DMARC). Step-by-step guides with a free check on each.

SSL/TLS (12)

Resolve SSL certificate errors, chain and hostname mismatches, expiry, and HTTPS handshake failures. Free certificate check on each guide.

HTTP (29)

Diagnose HTTP status errors (4xx/5xx), redirect loops, connection failures, and security-header issues. Free reachability check on each guide.

Domain & Network (7)

Understand domain registration (RDAP/WHOIS), expiry and status flags, IP changes, and network ownership (ASN). Free lookups on each guide.

Recently Updated

Recently added or refreshed guides that reflect the current tool coverage.

2026-06-30

"Your IP Has Been Temporarily Blocked" Fix

"Your IP has been temporarily blocked"? Tell rate limiting, WAF, fail2ban, and flagged VPN/datacenter IPs in 4 checks. Free instant IP lookup, no sign-up.

Diagnose

2026-06-30

Filtered vs Closed Port in Nmap: What's the Difference

Nmap port "filtered" vs "closed"? What each state means — RST, dropped packets, or ICMP unreachable — checked in 4 steps. Free instant port check, no sign-up.

Diagnose

2026-06-29

No 'Access-Control-Allow-Origin' Header (CORS)

No 'Access-Control-Allow-Origin' header? Fix it in 3 checks: failed preflight, exact origin echoed back, and credentials mode. Free instant check, no sign-up.

Diagnose

2026-06-29

ERR_CONNECTION_CLOSED in Chrome: Fix Checklist

ERR_CONNECTION_CLOSED means a TCP FIN closed the connection mid-request. Check TLS, keep-alive, antivirus, and reachability. Free instant check, no sign-up.

Diagnose

2026-06-28

SMTP Port 25 Blocked: Why It Happens and the Fix

SMTP port 25 blocked usually means your ISP or cloud host bars outbound mail. Tell the cases apart and move to 587/465 in 4 checks. Free instant check, no sign-up.

Diagnose

2026-06-28

TXT Record Over 255 Characters: Split It Right

TXT record over 255 characters fails because each string caps at 255 bytes. Split it into quoted chunks the right way in 4 checks. Free instant check, no sign-up.

Diagnose

DNS (26)

Guides for diagnosing DNS record issues — propagation delays, A/AAAA misconfigurations, MX/SPF/DMARC email security.

523 Origin Is Unreachable (Cloudflare) Fix550 5.7.1: Fix SPF and Policy Rejection BouncesA/AAAA Record Troubleshooting ChecklistCDN Cache vs DNS PropagationCNAME vs A Record — When to Use WhichDKIM Selector Not Found: dkim=none and Missing KeyDKIM Verification Failed: dkim=fail and Body Hash MismatchDMARC none vs quarantine vs reject: Which Policy to UseDNS Propagation Delay: Why Changes Take Time to AppearDNS REFUSED: Fix the Policy Block ResponseDNS SERVFAIL: Fix Server Failure ResponsesDNS TTL Best Practices — Optimal Values for Every ScenarioDNS_PROBE_FINISHED_NXDOMAIN Fix ChecklistDNSSEC Validation Failed: Fix the SERVFAILEmail Going to Spam? SPF, DKIM, and DMARC SetupERR_NAME_NOT_RESOLVED FixMail Server Blacklist Removal ChecklistMTA-STS and TLS-RPT Setup: Enforce SMTP TLSMX/SPF/DMARC Basic ChecksNameserver Mismatch DiagnosisNo MX Record: Email Not DeliveringReverse DNS / PTR Setup GuideSMTP Port 25 Blocked: Why It Happens and the FixSPF Permerror: Too Many DNS LookupsSubdomain Takeover: Find Dangling DNS RecordsTXT Record Over 255 Characters: Split It Right

SSL (12)

Guides for SSL/TLS certificate problems — expiry, chain mismatches, and renewal procedures.

525 SSL Handshake Failed (Cloudflare) Fix526 Invalid SSL Certificate (Cloudflare) FixCAA Records: Restrict Which CAs Can Issue Your CertificatesCloudflare SSL Modes: Flexible vs Full vs Full (Strict)ERR_SSL_PROTOCOL_ERROR Fix GuideERR_SSL_VERSION_OR_CIPHER_MISMATCH FixNET::ERR_CERT_AUTHORITY_INVALID FixNET::ERR_CERT_COMMON_NAME_INVALID: Fix Certificate MismatchNET::ERR_CERT_DATE_INVALID FixSSL Certificate Expired: Renewal and Recovery ChecklistSSL Chain Missing / Domain Mismatch FixWildcard SSL Certificates — Setup and Pitfalls

HTTP (26)

Guides for HTTP issues — redirect loops, security header audits, and response code troubleshooting.

301 vs 302 Redirect Guide400 Bad Request Checklist403 Forbidden Checklist404 Not Found Root Cause Guide405 Method Not Allowed Checklist413 Payload Too Large Checklist429 Too Many Requests Checklist500 Internal Server Error Checklist502 Bad Gateway Checklist503 Service Unavailable Checklist504 Gateway Timeout ChecklistERR_CONNECTION_CLOSED in Chrome: Fix ChecklistERR_CONNECTION_REFUSED Fix GuideERR_CONNECTION_RESET Fix GuideERR_CONNECTION_TIMED_OUT Fix GuideERR_CONTENT_LENGTH_MISMATCH FixERR_EMPTY_RESPONSE FixERR_TOO_MANY_REDIRECTS: Find and Break the Redirect LoopHSTS Preload ChecklistHTTP Security Headers GuideMixed Content Blocked: Fix Insecure Resources on HTTPS PagesNo 'Access-Control-Allow-Origin' Header (CORS)Port 443 Closed ChecklistRisky Open Ports Security ChecklistTTFB / Compression / Cache-Control ChecklistX-Robots-Tag noindex Fix Guide

IP / Network (3)

Guides for IP and network diagnostics — public IP changes, proxy detection, and routing issues.

"Your IP Has Been Temporarily Blocked" FixIP Flagged as VPN, Proxy, or Hosting: What It MeansWhy Does My IP Address Change?

RDAP / WHOIS (2)

Guides for domain registration — RDAP vs WHOIS, registration status flags, and expiry checks.

clientHold / pendingDelete: What to Do at Each StageRDAP vs WHOIS: Key Differences

General (2)

Cross-domain diagnostic guides covering multiple tools and comprehensive troubleshooting workflows.

Domain Migration ChecklistSite Unreachable? DNS, SSL, and HTTP Diagnostic Checklist

How to Use Guides

1. Look Up

Look up IP, DNS, SSL, HTTP in Utilities.

2. Check Results

Review the status badge and checklist on the results page.

3. Apply Guide

Follow the resolution checklist in the related guide.

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