When your users can't access your site
An unreachable site represents one of the most critical situations for an online business. Every minute of downtime translates into lost revenue, customer trust, and potentially search rankings. The problem is that causes can be multiple and located at different levels of the technical stack.
Unreachability can stem from DNS (the domain doesn't resolve), network (the server isn't reachable), the server itself (web service stopped, crash), SSL certificate (expired or misconfigured), or the application (fatal error, maintenance). Each cause requires a different resolution approach.
The critical first step is determining whether the problem is global (everyone is affected) or localized (only certain users or networks). This distinction immediately guides the diagnosis. Proactive monitoring like MoniTao allows you to detect unreachability before your users notice.
A site can become unreachable for many reasons. Here are the most frequent causes, classified by technical layer:
A methodical diagnosis allows you to quickly identify the failing layer:
Here are the essential commands to diagnose an unreachable site:
# 1. Check DNS resolution
dig example.com +short
nslookup example.com
# 2. Test network connectivity
ping -c 4 example.com
traceroute example.com
# 3. Test web port
nc -zv example.com 80
nc -zv example.com 443
# 4. Test HTTP/HTTPS
curl -I http://example.com
curl -I https://example.com
# 5. View SSL details
curl -vI https://example.com 2>&1 | grep -A 6 "Server certificate"
openssl s_client -connect example.com:443 -servername example.com
# 6. Check certificate expiration
echo | openssl s_client -connect example.com:443 2>/dev/null | openssl x509 -noout -datesThese commands walk through each layer: DNS → Network → Port → HTTP → SSL. Identify where it blocks and focus your efforts on that layer.
Proactive monitoring detects unreachability before your users:
Most common sudden causes: deployment that broke something, SSL certificate that just expired, resource limit reached (OOM), hosting provider outage, or server reboot without service auto-start.
Probably a DNS cache issue. Your WiFi has the old IP cached while 4G is trying to resolve to the new one. Wait for DNS propagation (TTL) or flush DNS cache on your devices.
Configure MoniTao with short intervals (1 minute) and immediate SMS alerts. You'll be notified within 60-90 seconds of the incident starting.
Depends on the configured TTL (Time To Live). With a short TTL (300s), propagation in minutes. With a long TTL (86400s), up to 24-48h. In practice, most is propagated in 1-4 hours.
If using Let's Encrypt: certbot renew. Otherwise, regenerate a certificate with your provider. Meanwhile, you can temporarily redirect to HTTP (not recommended) or put up a maintenance page.
Check your server logs (systemctl status, dmesg, /var/log/). The problem may be in your configuration, not the host. Test with curl from the server itself (localhost) to isolate the issue.
An unreachable site is a business emergency. The key is having a structured diagnosis methodology that walks through each technical layer: DNS, network, server, web service, application. With the right tools and approach, you can identify the cause in minutes.
MoniTao lets you detect unreachability before your users through continuous multi-point monitoring. With instant alerts and detailed history, you can react quickly and analyze root causes to prevent recurrence. Don't let an outage impact your business for hours.
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