How to Detect Website Blocking? A Guide to Troubleshooting Domain, DNS, and Network Access Issues

When a website won't open, the most frustrating part is often not the "inaccessibility" itself, but the difficulty in pinpointing where the problem lies. Sometimes your own computer accesses it fine, but clients keep timing out; sometimes Wi-Fi fails, but switching to mobile data instantly restores access; and there's an even more headache-inducing scenario: the domain resolves correctly, the server isn't down, but the browser still says "can't connect," or returns a 403, Access Denied, or even redirects to a completely unfamiliar page.

These issues are often collectively referred to as "website blocking," but from a practical network troubleshooting perspective, "blocking" is not a single, fixed fault. DNS resolution errors, browser security mechanisms, CDN/WAF rules, server firewalls, and ISP routes can all produce similar symptoms. When performing a blocking check for your website, the real question isn't "is it blocked?" but rather: at which stage did the request fail?

1. What Are the Common Signs of Website Blocking?

Website access issues manifest in many ways, but a few are particularly typical.

The most common is some users can access the site, while others cannot at all. For example, Shanghai Telecom works fine, but Beijing Unicom times out; or your broadband fails, but mobile data works. This usually means the website itself isn't completely down; you should start investigating from ISPs, DNS, CDN nodes, or access policies.

Another scenario is that the domain resolves correctly, but the browser keeps timing out. For instance, DNS returns the correct IP, Ping shows no obvious issues, but HTTPS never establishes a connection. In this case, the problem might lie in TCP 443, TLS, CDN, firewalls, or the network path.

If you directly see:

403 Forbidden
Access Denied
Request Blocked

Then the situation is different. Receiving a 403 means the request actually reached the website, CDN, or WAF, but was rejected by security rules. Continuing to check DNS is pointless; you should focus on IP blacklists, WAF rules, bot protection, geo-restrictions, or server permission configurations.

Another obvious anomaly is: you type your domain, but it redirects to a completely unrelated website.

This requires checking both DNS and HTTP redirects. The issue could be DNS returning a wrong address, or the server, CDN, or website code configured with 301/302 redirects, or even malicious scripts injected into the site.

If the browser directly warns "Deceptive site," "Your connection is not private," or similar security alerts, the focus should shift from network routes to SSL certificates, website security status, malicious code, and third-party scripts.

So, the same phrase "website won't open" can stem from entirely different problems.

2. First Determine Which Layer Has the Problem

To troubleshoot website blocking, it's best to understand the entire access process.

When a user visits a website, the request typically goes through the following stages:

Enter domain
   ↓
DNS resolution
   ↓
Network routing
   ↓
TCP connection
   ↓
TLS handshake
   ↓
CDN / WAF
   ↓
Origin server
   ↓
Return HTTP page

If DNS fails at the first step, subsequent connections naturally cannot succeed; if DNS is fine but TCP 443 cannot establish a connection, the issue lies in the network, ports, or firewall; if TCP and TLS are both fine but you get a 403, the problem is likely at the CDN, WAF, or server access control layer.

This is why relying solely on a single "website blocking check" result is often inaccurate.

An effective troubleshooting approach combines DNS, multi-node access, Ping, TCP, HTTP/HTTPS, and traceroute rather than focusing on just one metric.

3. How to Detect Website Blocking: A Complete Troubleshooting Process

When troubleshooting such access issues, the worst thing is to jump to conclusions blindly. Following a fixed logical sequence helps avoid missing hidden faults and wasting time on wrong directions.

Step 1: Use Multi-Node Testing to Determine "Who Can't Access"

Don't use your own network as the sole standard. Your local computer only reflects your current city, ISP, and a single physical route. If your access is smooth, it doesn't mean users elsewhere can load the site properly.

The effective approach is to use a multi-node speed test and diagnostic tool like Chahu to pull up resolution and connectivity status across regions at once:

• Cover first-tier and core nodes like Beijing, Shanghai, Guangzhou;

• Cross-cover the three major backbone ISPs: Telecom, Unicom, and Mobile;

• Include overseas test points in Hong Kong, Japan, Singapore, the US, etc.

If the results show "Shanghai Unicom and overseas nodes all normal, but Beijing Telecom and Guangzhou Mobile consistently time out," you can immediately see this is not a server outage but a pattern with clear geographic or ISP characteristics. When using multi-node detection, the key is not the "normal/abnormal" checkmark but identifying the scope of impact at a glance.

Step 2: Compare DNS Resolution Results Across Regions

If anomalies are concentrated in specific regions or ISPs, the next step is to capture DNS records (A, AAAA, CNAME, NS).

Assuming your site doesn't use CDN or smart DNS, theoretically the IP returned nationwide should be identical:

example.com → 203.0.113.10

But if tests show that nodes in Guangzhou or Beijing resolve to a completely unfamiliar address:

example.com → 198.51.100.25

And this IP is neither your origin nor in your infrastructure list, you should suspect DNS resolution errors or hijacking.

However, there's a common misconception: "Different IPs across regions mean DNS is hijacked." That's not true. If your site uses a CDN, the system is designed to route users to the nearest edge node based on their location and network conditions, so different IPs are expected. The key to determining DNS issues isn't whether IPs are uniform, but whether the IP falls within your service provider's legitimate IP pool.

Additionally, if different networks return wildly different addresses, you can compare public DNS servers (e.g., 8.8.8.8, 1.1.1.1, 223.5.5.5). If authoritative DNS and public DNS resolve correctly, but a specific broadband's resolution is off, it's likely Local DNS cache pollution or ISP-level interference. If you recently changed domain resolution, also check TTL refresh time; old and new IPs alternating due to cache not expiring isn't truly "blocking."

Step 3: Test TCP Ports and HTTPS Channel

After DNS resolution is confirmed, verify network connectivity.

Ping can help you see latency and packet loss, but it's just a reference. Many site owners panic when Ping shows 100% packet loss, but it's meaningless: if the origin or CDN node has ICMP ping disabled, the page can still load instantly with HTTP 200 OK.

What really matters is whether TCP ports 80 and 443 can establish connections:

• Resolution is normal, but TCP 443 times out: the domain resolves correctly, but the HTTPS entry point is unreachable. Focus on firewall policies, cloud security group rules, CDN edge node reachability, and network link status.

• If TCP 443 is fine and TLS handshake succeeds, then look at the HTTP response.

Step 4: Interpret HTTP Status Codes

Status codes directly reveal the layer where the problem lies:

• 200 OK: Congratulations, the network and server responded. If the page still errors, check front-end scripts or resource loading.

• 301 / 302 Redirect: The request triggered a redirect. Always check where it redirects to: if it goes to an unfamiliar malicious URL, inspect Nginx rewrite rules, CDN 301 configurations, CMS plugins, or even server source code for hidden links.

• 403 Forbidden: The request reached the server or CDN but was blocked by rules. Look at WAF firewall logs, IP blacklists, geo-restrictions, bot protection, and Nginx access permissions.

• 429 Too Many Requests: Classic rate limiting; likely triggered Rate Limit protection rules or overly strict CC protection thresholds.

• 502 / 503 / 504: These usually indicate origin server crashes, backend application timeouts, or proxy servers failing to fetch origin data—completely different from network-level "blocking."

Step 5: Use Traceroute / MTR to Trace Packet Paths

If DNS, ports, and application configurations all check out, but connections from certain regions stall midway, it's time to use Chahu for traceroute.

A typical packet path is:

Local broadband → ISP access network → backbone network → cross-network/cross-border nodes → CDN edge node → final origin

Traceroute visually shows where your packets die. However, if you see consecutive * * * in the logs, don't immediately assume the network is cut—many backbone routers disable ICMP responses to prevent attacks. Only when nodes from multiple affected regions collectively lose packets around the same segment, and the final destination IP remains unreachable, can you confirm a line or backbone network fault.

4. How to Determine the Specific Cause from Test Results

Combining the above results usually gives a rough idea of which layer is problematic.

This approach is more practical than just looking at a "blocking check: abnormal" result because it tells you where to look next.

5. How to Distinguish DNS Hijacking, 403, and Browser Security Blocking

These three situations are often confused, but they are entirely different.

DNS Anomalies: Users May Connect to the Wrong Server

If a domain should resolve to your server or CDN but returns an unrelated address in certain networks, or accessing it leads to a strange page, you need to check DNS.

A reliable way is to compare:

• Authoritative DNS

• Different public DNS servers

• Nodes in different regions

• Different ISPs

If only one network's resolution is clearly abnormal while others are fine, the problem is likely within that network's DNS environment.

403: The Website Received the Request but Actively Rejected It

If the test results are:

DNS: Normal
TCP: Normal
TLS: Normal
HTTP: 403

Then stop worrying about "whether the network can reach."

The request has already reached the CDN, WAF, or server.

Next, check:

• WAF logs

• Firewall Rules

• IP blacklists

• Bot protection

• Geo Blocking

• Rate Limit

• Nginx access rules

Especially if you recently adjusted security policies and normal users suddenly see many 403s, the rules might be too strict.

Browser Security Warnings: Focus on Website Security and Certificates

If the browser warns about a dangerous site, certificate errors, or suspected malicious pages, the troubleshooting direction is completely different.

You should check:

• SSL certificate expiration

• Certificate domain match

• Certificate chain completeness

• Malicious JS injected into the site

• Abnormal iframes

• Injected redirects

• CMS or plugin compromise

This type of issue shouldn't revolve around Ping and traceroute.

6. What to Do After Confirming Website Blocking

Different problems require different solutions.

If it's a DNS anomaly, check A, AAAA, CNAME, NS, TTL, and DNSSEC, and confirm CDN domain configurations.

If it's CDN or WAF false blocking, review security logs to see which rules were hit. Often you don't need to disable the entire WAF; just adjust a specific IP, Bot, Rate Limit, or geo policy.

If it's an SSL/TLS anomaly, focus on certificate validity, domain match, intermediate certificate chain, SNI, and CDN certificate deployment status.

If it's a server firewall, check Security Groups, iptables, nftables, fail2ban, and Nginx/Apache allow/deny rules.

If it's confirmed as regional route or ISP network issues, continue analyzing with Ping, TCP, Traceroute, MTR, and multi-region tests. If your site uses CDN, check whether affected users are being routed to the same edge node.

During troubleshooting, the most important thing is not to change many configurations at once to restore access. Change one variable at a time and retest; otherwise, you won't know the real cause after the site recovers.

7. What Is the Correct Troubleshooting Sequence for Website Access Issues?

If you encounter similar issues in the future, follow this chain:

Website won't open
   ↓
Confirm origin server is normal
   ↓
Multi-region, multi-ISP testing
   ↓
Check DNS resolution
   ↓
Test Ping and TCP 80/443
   ↓
Check TLS and HTTP status codes
   ↓
Traceroute / MTR
   ↓
Check CDN, WAF, and server firewall

"Website blocking" is not a single fault type; it's a surface symptom. The key to efficient troubleshooting is determining at which step the request failed. If DNS resolution is wrong, check DNS; if DNS is fine but 443 can't connect, check routes, ports, and firewalls; if HTTP returns 403, look at CDN and WAF; if only a specific region or ISP is affected, combine multi-node tests and routing analysis.

Compared to relying solely on a "is the website blocked" check, a more reliable method is to evaluate DNS, multi-node connectivity, TCP/TLS, HTTP status codes, and network routes together. Once you find the problematic layer, the rest becomes much simpler.

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FAQ

Q1: My website is flagged by Google Chrome as "deceptive site" or "dangerous." How do I handle it?

A: This usually happens because the page contains injected malicious scripts, phishing links, or downloaded resources trigger security mechanisms. First, clean malicious files from the server and fix vulnerabilities. Then log in to Google Search Console (GSC), go to "Security & Manual Actions" - "Security Issues" to see the specific violating URLs. After cleaning, submit a review request directly in GSC; the warning usually lifts within 1 to 3 business days.

Q2: How do I determine if my domain is blocked by the Great Firewall?

A: The most obvious sign is: overseas nodes (Hong Kong, Japan, US, etc.) load instantly over HTTP/HTTPS with normal IP resolution; but all domestic ISP nodes experience TCP 443 timeouts or resets when accessing the domain, or the DNS-resolved IP is replaced with an unrelated invalid public IP. If only the server IP is blocked, it usually affects just that IP; if the domain's DNS is polluted, any DNS server in China will resolve to a fake IP.

Q3: I just switched to a new CDN or DDoS-protected IP, and some users see TLS/SSL certificate errors. What's the cause?

A: Two common reasons: first, the new certificate configuration on CDN edge nodes hasn't fully propagated, so some nodes still respond with old or default certificates; second, SNI (Server Name Indication) mismatch between the origin and CDN, or the origin has strict HTTPS validation enabled, causing CDN origin fetch failures. Wait 15–30 minutes and use SSL checking tools to test certificate chains from different nodes.

Q4: After enabling WAF, legitimate visitors frequently see CAPTCHAs or 403 errors. How can I optimize?

A: This is often because the WAF's CC protection or bot detection thresholds are too strict. For example, setting "requests per minute per IP" too low, or mistakenly blocking office/campus networks where hundreds of users share one public IP. Solution: check WAF blocking logs, change rules with high false-positive rates from "block" to "observe/challenge," and whitelist search engine crawlers.

Q5: My server is running fine, but the CDN frequently reports 502 Bad Gateway or 504 Gateway Timeout. Why?

A: 502/504 errors indicate CDN-to-origin fetch failures. The most common hidden cause is that the origin firewall (e.g., BT Panel firewall, cloud security groups, iptables, or fail2ban) has mistakenly blacklisted the CDN's origin-fetch IPs as attackers. In this case, add all official IP ranges published by the CDN provider to the origin firewall's whitelist.