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DNS Troubleshooting Cheat Sheet: 10 Checks Every Sysadmin Should Know

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DNS Troubleshooting Cheat Sheet: 10 Checks Every Sysadmin Should Know

Tags: dns, networking, devops, webdev

Every sysadmin has been there — a website goes down, email stops delivering, or SSL certificates expire at 2 AM. Having a systematic troubleshooting workflow saves hours of guesswork.

Here are 10 essential DNS and network checks I run on every issue, with the commands and free online tools to do them instantly.

1. DNS Propagation Check

When you update DNS records, changes don't happen instantly. DNS propagation can take up to 48 hours depending on TTL values.

What to check: Query multiple global DNS servers to see if your changes have propagated.

dig @8.8.8.8 example.com A
dig @1.1.1.1 example.com A
dig @9.9.9.9 example.com A
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Or check 30+ servers at once with DNS Propagation Checker.

2. WHOIS / RDAP Lookup

Domain expired? Ownership dispute? WHOIS tells you registration dates, registrar, and nameservers.

whois example.com
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RDAP is the modern replacement — structured JSON, no rate limiting. Try it with WHOIS Lookup.

3. SSL Certificate Verification

Expired SSL = browser warnings = lost visitors. Always check certificate expiry, chain, and protocol support.

openssl s_client -connect example.com:443 -servername example.com
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For a visual breakdown: SSL Certificate Checker.

4. SPF Record Validation

SPF tells receiving mail servers which IPs are allowed to send email for your domain. A misconfigured SPF record = emails landing in spam.

dig example.com TXT | grep "v=spf1"
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Check syntax and lookup count (max 10!) with SPF Checker.

5. DKIM Record Check

DKIM adds a cryptographic signature to outgoing emails. If your DKIM record is missing or wrong, deliverability tanks.

dig selector._domainkey.example.com TXT
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Validate your DKIM setup: DKIM Checker.

6. DMARC Policy Check

DMARC ties SPF and DKIM together and tells receivers what to do with failing emails (none, quarantine, reject).

dig _dmarc.example.com TXT
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Analyze your policy: DMARC Checker.

7. IP Blacklist Check

If your server IP is on a DNSBL blacklist, your emails will be rejected. Check regularly.

# Check against Spamhaus
dig +short 4.3.2.1.zen.spamhaus.org
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Check 50+ blacklists at once: IP Blacklist Checker.

8. Reverse DNS (PTR) Lookup

Many mail servers reject email from IPs without valid PTR records. Your PTR should match your mail server hostname.

dig -x 93.184.216.34
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Quick check: Reverse DNS Lookup.

9. MX Record Lookup

Email not working? First check if MX records are properly configured and pointing to the right mail servers.

dig example.com MX
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Check MX priority and reachability: MX Lookup.

10. Port Connectivity Test

Firewall blocking traffic? Check if specific ports are open and reachable from outside your network.

nc -zv example.com 443
nc -zv example.com 25
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Test any port remotely: Port Checker.

Quick Reference Table

Issue Check Record Type
Site not resolving DNS Propagation A / AAAA
Domain expired WHOIS Lookup —
SSL warning Certificate Check —
Email in spam SPF + DKIM + DMARC TXT
Email rejected IP Blacklist + PTR PTR
Email not arriving MX Lookup MX
Service unreachable Port Check —

Wrapping Up

These 10 checks cover 90% of DNS and network issues I encounter. Having them in a systematic workflow means less guesswork and faster resolution.

All the tools mentioned above are free at DNS Robot — no signup, no rate limits, works in 12 languages.

What's your go-to troubleshooting workflow? Drop it in the comments.

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SHAIK VAHID •

Perfect Helpful toolkit for Web Hosting Support teams.