Website Down? How to Diagnose: Common Causes and a Complete Troubleshooting Guide

When a website suddenly goes down, many people's first reaction is that the server must be down, so they restart the server, restart Nginx, or even modify CDN settings. However, in real-world troubleshooting, a server outage is just one possibility. DNS resolution errors, CDN node failures, ISP routing issues, SSL certificate problems, local network cache, or even misconfigured firewall rules can all make a website appear "down."

Especially when you encounter situations like "it's down for me but fine for others," "works on Telecom but not on Mobile," or "inaccessible domestically but fine overseas," looking only at browser errors rarely gives you a clear answer. So, when a website goes down, the most important thing is not to immediately change configurations, but to first determine the scope of the problem, then systematically check the network, DNS, HTTP, CDN, and origin server layers. Below, we'll walk through the diagnostic steps in the order commonly used in operations.

1. Website Down? First, Determine Who Can't Access It

When you find a website is inaccessible, the first thing to do is not to log into the server, but to determine the scope of the problem, because "website down" can mean very different things.

If you don't first clarify the scope, you can easily head in the wrong direction. For example, if the site is only inaccessible on Guangdong Mobile, but you keep restarting the server, that won't solve anything. A more sensible approach is to first test the website from different regions and ISPs.

2. Use a Multi-Node Speed Test to Confirm If the Site Is Really Down

The result from your own computer only represents your current network path. For instance, if your office uses Telecom, even if the site loads fine, it doesn't prove that users on Unicom, Mobile, or in other regions aren't having issues.

When troubleshooting an inaccessible website, the simplest and most effective method is to use the Chahu website speed test tool for multi-node detection. By accessing the site from nodes in different regions and ISPs simultaneously, you can quickly see the overall connectivity. You don't need to dive into complex data initially; just focus on these key results:

• Which nodes can access the site normally

• Which regions are timing out

• Whether the issue is concentrated on one ISP

• Whether response times are significantly abnormal

• Whether there's a clear difference between domestic and overseas nodes

For example, if Telecom and Unicom nodes are mostly fine, but many Mobile nodes fail, it's less likely the server is completely down. You should investigate Mobile's routing, DNS routing, or CDN nodes.

If almost all regions time out, prioritize checking the origin server, CDN, web service, and server network.

Another common scenario: Chahu's multi-node test is all green, but only your computer can't access the site. In that case, check your local DNS, browser cache, proxy settings, or current network first, rather than touching the production server.

3. Use Ping to Check for Network Connectivity Issues

After determining the scope, the next step is to check Ping. Ping is best for quickly assessing basic network connectivity and for spotting significant latency or packet loss differences between nodes. Running Ping only from your computer still only shows the path from your network to the target server, so it's better to combine it with Chahu's online Ping for multi-region testing.

In the Ping results, focus on three aspects.

1. Are Most Nodes Getting a Response?

If multiple nodes across the country time out simultaneously, you need to check:

• Is the server IP online?

• Are CDN nodes having issues?

• Is the firewall blocking ICMP?

• Is there a network outage?

• Is the server experiencing a large-scale network failure?

But note: Ping failure doesn't necessarily mean the website is down.

Some servers, cloud providers, or CDN nodes actively block ICMP, so you can't rely solely on Ping results to judge website status.

2. Is There Significant Packet Loss?

If most regions are fine but a few show persistent packet loss, you need to investigate the network path for those regions.

For example: Shanghai Telecom is fine, Beijing Unicom is fine, but Guangdong Mobile shows heavy packet loss.

In this case, the server itself isn't completely unreachable; it's more likely a problem with a specific network segment or ISP routing.

3. Has Latency Suddenly Spiked?

If the site used to respond in tens of milliseconds, but suddenly many nodes show 150ms, 200ms, or higher, that's worth investigating.

Common causes include:

• Routing detours

• Cross-ISP congestion

• CDN routing to a distant node

• Server egress network issues

• Backbone line fluctuations

So Ping is better for determining "is there a network problem" rather than directly judging whether the website itself is functioning.

4. If Ping Is Fine, Check DNS Resolution

Many website access issues aren't because the server is down, but because the domain isn't resolving to the correct IP. When users visit a website, they don't connect directly to the server; they first use DNS to resolve the domain to an IP, then access the corresponding server or CDN node.

If there's a DNS issue, even if the server is perfectly fine, users may still not be able to access the site.

At this point, you can use an online DNS lookup tool to check the domain's resolution.

Key things to look at:

• A records

• AAAA records

• CNAME records

• NS records

• IPs returned from different regions

• Whether results are consistent across ISPs

1. Domain Doesn't Resolve to Any IP

If you can't find valid A, AAAA, or CNAME records, check:

• Whether DNS records were deleted

• Whether the DNS provider is having issues

• Whether the domain status is normal

• Whether NS settings were changed

• Whether DNSSEC configuration is problematic

2. Domain Still Resolves to an Old IP

This often happens after migrating servers or switching CDNs.

For example, the site has moved from an old server to a new one, but some local DNS servers still cache the old record. Until the TTL expires, some users will still be directed to the old IP.

This leads to:

Some people can access the site, while others can't.

3. Significant Differences in Resolution Across Regions

If the same domain resolves to clearly abnormal IPs in different regions, you need to check:

• Is CDN smart routing working correctly?

• Are there DNS cache anomalies?

• Are local DNS servers having issues?

• Is there DNS pollution or hijacking?

If the site behaves like "IP works, domain doesn't," DNS is a top priority to check.

5. DNS Is Fine, but the Site Still Won't Load: What Next?

If DNS resolution looks normal and Ping results are fine, but the webpage still won't load, shift your focus from "is the network reachable" to "is the web service responding." Check HTTP, HTTPS, ports, and the web server.

Check Ports 80 and 443

The most common ports for websites are:

• HTTP: 80

• HTTPS: 443

Just because the server responds to Ping doesn't mean the web service is running.

For example, these issues can all cause the site to be inaccessible:

• Nginx is stopped

• Apache service is down

• Port 443 is not open

• Cloud security group rules were changed

• System firewall accidentally blocked the port

• CDN can't reach the origin

• Origin port is misconfigured

So if the network is fine but browser requests time out, verify that the web server is actually listening on the expected port.

6. Use HTTP Status Codes to Narrow Down the Problem

The site might not be completely down. Sometimes the browser still receives a response, but it's an error like 403, 502, 503, or 504. Different status codes point to different areas:

If you see 502 errors, you probably don't need to focus on DNS; instead, check Nginx, PHP, application services, or CDN origin fetch. If you see many 504s, focus on origin response time, server load, database, and CDN origin fetch path. HTTP status codes won't tell you the root cause directly, but they help you narrow down the scope quickly.

7. If Only Some Regions Are Affected, Check the Route

If the site isn't down globally, DNS is fine, but certain regions or ISPs consistently fail, the network path is worth investigating.

For example:

Shanghai Telecom: OK
Beijing Unicom: OK
Guangzhou Mobile: Timeout

And the CDN IPs resolved in these regions are basically the same.

In this case, the problem might not be DNS, but the network path between users and the server or CDN node.

You can use Chahu's traceroute tool to observe the path packets take.

Focus on:

• At which hop does latency spike?

• Is there continuous packet loss?

• Is there a clear routing detour?

• Is the issue concentrated on one ISP?

• Is the CDN routing path reasonable?

For example, if Guangzhou users normally access a Hong Kong node with a short path, but the actual route detours through other regions or even overseas before returning, the experience will be poor even if the server itself is fine.

So, if the site is only inaccessible in certain regions, traceroute is often more valuable than repeatedly restarting the server.

8. Check CDN and WAF

Many websites now sit behind a CDN or DDoS-protected CDN, so users are actually accessing CDN edge nodes, not the origin. In such cases, the site being down doesn't necessarily mean the origin is at fault.

If multi-node tests show issues in some regions, check the CDN:

• Is the CNAME configured correctly?

• Are CDN nodes experiencing issues?

• Has the origin address been changed?

• Is the origin port correct?

• Does the origin firewall allow CDN origin-fetch IPs?

• Has the HTTPS certificate been synced?

• Are cache or redirect rules misconfigured?

If WAF is enabled, also check whether security policies are blocking legitimate users.

Common issues include:

• IP blacklist false positives

• Geographic access restrictions

• CC protection rules too strict

• Bot policy false positives

• User-Agent filter rules misconfigured

• Request rate limiting misconfigured

Sometimes only some users can't access the site, not because of network issues, but because security policies are blocking them.

9. For HTTPS Sites, Also Check SSL Certificates

Most official websites now use HTTPS, so certificate issues are common.

Typical symptoms include:

• Browser warning about insecure connection

• NET::ERR_CERT_DATE_INVALID

• Certificate domain mismatch

• TLS handshake failure

• HTTPS pages completely fail to load

Check:

• Is the SSL certificate expired?

• Is the certificate bound to the correct domain?

• Is the intermediate certificate complete?

• Has the CDN certificate been updated?

• Is the origin certificate valid?

• Are TLS protocol and cipher suite configurations compatible?

Especially after changing servers, CDNs, or reissuing certificates, it's common for the origin to be updated but the CDN to still use the old certificate. If HTTP works but HTTPS doesn't, prioritize checking this layer.

10. Different Symptoms, Different Priorities

In practice, you don't need to check everything every time. Prioritize based on the symptom.

The biggest advantage of this approach is that you don't always start from the server.

By first determining which layer the problem is likely in, you'll be much more efficient.

11. For Intermittent Issues, Set Up Continuous Monitoring

There's a type of failure even more troublesome than a complete outage: intermittent issues. For example, the site times out for a few minutes in the early morning but is fine during the day, or occasionally returns 502 under high load but recovers on its own. These problems are hard to catch by manually testing. By the time you notice user reports and check, the site may have already recovered.

In such cases, use Chahu website monitoring to continuously observe the site.

Besides standard HTTP(S) checks, you can monitor Ping, TCP, DNS, SSL, and more as needed.

For example:

• HTTP suddenly becomes inaccessible

• DNS resolution anomalies

• Ping latency spikes

• TCP port unreachable

• SSL certificate issues

• Response time consistently increasing

Historical monitoring records show when the failure occurred, how long it lasted, and when it recovered.

For sites that frequently have "users say it's down, but when I test it's fine," historical data is often more valuable than ad-hoc tests.

12. Overall Troubleshooting Sequence

If you're not sure where to start, follow this sequence:

Website down
    ↓
Chahu multi-node website speed test
    ↓
Determine if it's nationwide or regional
    ↓
Ping to check network connectivity
    ↓
DNS lookup to confirm resolved IP
    ↓
Traceroute to check network path
    ↓
Check HTTP status codes / ports 80 / 443
    ↓
Check CDN / WAF / SSL
    ↓
Check origin web service and server load

The core of this method isn't about using many tools, but about first determining which layer the problem belongs to.

The most troublesome part of a website being down isn't usually the complexity of the failure itself, but heading in the wrong direction from the start: restarting the server when it's an ISP routing issue, modifying Nginx when the domain still resolves to an old IP, or repeatedly checking CDN nodes when the SSL certificate has expired. These actions waste time and can even mess up configurations that were working fine.

In practice, start by using Chahu's multi-node speed test to confirm the scope, then narrow down layer by layer in the order of Ping, DNS, traceroute, HTTP/HTTPS, CDN, and origin server. Once you've identified which layer the problem is in, most "website suddenly down" issues become much easier to pinpoint.

Related Q&A

1. What's the difference between "This site can't be reached" and "Connection reset" in the browser?

They're quite different. "This site can't be reached" usually means the TCP connection failed to establish—for example, the server isn't listening on port 443, a firewall is blocking the request, or the IP is simply unreachable. The problem is at the connection level. "Connection reset" means the TCP handshake succeeded, but the server actively closed the connection. This is common with firewall blocks, WAF rule triggers, or the server internally rejecting the request. In practice, when I see "connection reset," I check security policies and WAF logs first, not port connectivity.

2. Can a server with 100% CPU usage cause the website to be inaccessible? What are the symptoms?

Yes, and the symptoms are distinctive. When the CPU is maxed out, the server's TCP stack still works, so Ping succeeds and TCP handshakes complete. But the web service (like Nginx or PHP-FPM) has no CPU resources to process requests. The browser will hang on "Establishing secure connection" or "Waiting for response," then time out with a 504 or a connection failure. If monitoring shows response times spiking from tens of milliseconds to seconds and then timing out, while CPU usage is near 100%, it's almost certainly a CPU bottleneck, not a network issue.

3. After deploying a CDN, some regions can't access the site, but direct IP access to the origin works. Where's the problem?

In this case, the problem is almost certainly at the CDN layer; the origin is fine. First, check the CDN console for node status and any announcements about node failures in that region. If there are none, have affected users flush their local DNS, because CDN routing is DNS-based, and they might be caching an old node IP. Then check the CDN's hotlink protection, IP blacklist/whitelist, and geographic access restrictions. Sometimes policies like "allow only mainland China" block overseas users.

4. How do you distinguish between a DDoS attack and a legitimate traffic spike causing the site to go down?

Attacks and traffic spikes look similar: both cause server load to spike, slower responses, and even timeouts. But there are clues. During an attack, access logs show a flood of abnormal requests from many different IPs, with messy or missing User-Agents, and URLs are highly concentrated (e.g., hitting one endpoint). With a legitimate spike, page distribution in logs is relatively even, and user behavior follows patterns. Also, if bandwidth is saturated but CPU is low, it's likely a volumetric attack. If CPU and database connections are maxed out but bandwidth is normal, it's more likely an application-layer attack or a genuine business surge.

5. When troubleshooting a website issue, should you look at server logs or monitoring data first?

Order matters. My habit is to check monitoring data first, then server logs. Monitoring data (like Chahu's multi-node probes) helps you quickly determine the scope and impact—whether it's nationwide or just one region. That's the first decision point. Once you have a direction, log into the server and check Nginx access and error logs for specific clues. If you start by digging through server logs, you can easily get lost in the volume, and if the problem is at the CDN or DNS layer, the server logs won't show anything abnormal—pure waste of time.