Cloudflare Error 520 and 521: Troubleshooting Network Nodes and Proxy Layers

<p style="line-height: 2;"><span style="font-size: 19px;"><strong>Key Takeaways</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">· Cloudflare 520 and 521 errors indicate that its edge nodes cannot communicate properly with the origin server.</span></p><p style="line-height: 2;"><span style="font-size: 16px;">· A 520 error usually means the origin returned an empty response or unknown error, while a 521 error means the origin refused the connection outright.</span></p><p style="line-height: 2;"><span style="font-size: 16px;">· Troubleshooting requires distinguishing whether the target website is down, Cloudflare is blocking the request, or the local proxy network node timed out.</span></p><p style="line-height: 2;"><span style="font-size: 16px;">· In data collection scenarios, using a high-purity residential proxy network and configuring appropriate session timeouts can effectively reduce the occurrence of these errors.</span></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>What are 520 and 521 Errors?</strong></span></h2><p style="line-height: 2;"><span style="font-size: 16px;">When users or scraping programs access a website deployed with Cloudflare CDN, if Cloudflare's edge nodes cannot obtain a normal response from the origin server, it returns a 5xx series error code to the client.</span></p><p style="line-height: 2;"><span style="font-size: 16px;"><strong>520 Error: </strong></span><span style="font-size: 16px;">Cloudflare successfully connected to the origin, but the origin returned an empty response, an unknown error, or simply disconnected.</span></p><p style="line-height: 2;"><span style="font-size: 16px;"><strong>521 Error: </strong></span><span style="font-size: 16px;">Cloudflare attempted to connect to the origin, but the origin server is offline or directly refused Cloudflare's network connection request.</span></p><p style="line-height: 2;"><span style="font-size: 16px;">Both errors point to the same issue: the communication link between Cloudflare and the origin is broken. However, when using proxies for data collection, the causes are often more complex.</span></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>Why Data Collection and Proxy Scenarios Encounter 520 and 521</strong></span></h2><p style="line-height: 2;"><span style="font-size: 16px;">In regular browsing, this is usually a problem for the website administrator to solve. However, when using a proxy pool for large-scale data collection, these two errors usually occur for three reasons:</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>First, a real fault on the target website</strong></span><span style="font-size: 16px;"><br>The target origin is indeed down or undergoing maintenance. At this time, regardless of the proxy IP used, it will return 520 or 521.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Second, Cloudflare's deceptive risk control blocking</strong></span><span style="font-size: 16px;"><br>This is the most common situation in scraping scenarios. When Cloudflare detects abnormal high-frequency access or anomalous fingerprint features from your proxy IP, to block automated programs, it sometimes deliberately returns a 520 or 521 page instead of directly returning a 403 or 429. This is actually a covert banning strategy.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Third, unstable proxy node network</strong></span><span style="font-size: 16px;"><br>If the proxy node used is too far from the target server, or the proxy server itself is overloaded, it causes a TCP handshake timeout. Cloudflare may mistakenly judge the origin as unresponsive, thereby triggering a 520 error.</span></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>Network Node and Proxy Layer Troubleshooting Checklist</strong></span></h2><p style="line-height: 2;"><span style="font-size: 16px;">When your scraping program frequently encounters 520 or 521 errors, please follow these no-code operational steps for troubleshooting:</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Step 1: Confirm the target website status</strong></span><span style="font-size: 16px;"><br>First, use a regular local network to access the target URL directly in a browser. If local access also errors out, it means the target website itself has a problem, and you can only wait for them to recover. If local access works but proxy access errors out, proceed to the next step.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Step 2: Check proxy IP purity</strong></span><span style="font-size: 16px;"><br>Extract a new IP from the proxy dashboard or switch proxy nodes (for example, switching from datacenter proxies to the high-purity </span><a href="https://www.b2proxy.com/pricing/residential-proxies" target="_blank"><span style="color: rgb(9, 109, 217); font-size: 16px;">residential proxies</span></a><span style="color: rgb(9, 109, 217); font-size: 16px;"> </span><span style="font-size: 16px;">provided by B2Proxy), and try accessing again. If changing to a residential IP returns a normal 200 status code, it means the previous IP has been blacklisted by Cloudflare's risk control system.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Step 3: Check proxy session and timeout settings</strong></span><span style="font-size: 16px;"><br>If you are using pay-per-traffic proxies or a high-frequency rotating proxy pool, try enabling "Sticky Sessions" to keep the same IP connected for a period of time. At the same time, check the "connection timeout" setting in the proxy client or automation tool, and it is recommended to extend it appropriately to avoid handshake interruptions due to network fluctuations.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Step 4: Verify DNS resolution nodes</strong></span><span style="font-size: 16px;"><br>Sometimes the proxy gateway does not correctly resolve the target domain's CDN node. You can use third-party website speed testing tools to compare response conditions from nodes in different regions to confirm whether there is a local network failure in the region where the proxy exit is located.</span></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>Common Error Troubleshooting Comparison Table</strong></span></h2><p style="line-height: 2;"><img src="https://www.b2proxy.com/static/ad/6116a8f175ee6a72826b63cf118063f0.png" alt="2.2.png" data-href="https://www.b2proxy.com/static/ad/6116a8f175ee6a72826b63cf118063f0.png" style=""></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>How to Avoid These Errors with High-Quality Proxy Networks</strong></span></h2><p style="line-height: 2;"><span style="font-size: 16px;">To thoroughly reduce the occurrence of 520 and 521 errors, the core lies in improving the stability of the proxy network and IP reputation.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Use highly reputable residential proxy networks</strong></span><span style="font-size: 16px;"><br>Datacenter IP ranges are often a key focus for Cloudflare and are highly prone to triggering 5xx-level blockages. The high-purity residential proxies provided by B2Proxy originate from real home broadband networks, possess extremely high reputation, and can smoothly pass Cloudflare's basic checks just like ordinary real users.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Reasonably configure protocols and geo-targeting</strong></span><span style="font-size: 16px;"><br>For network node selection, try to choose exit IPs with closer geographic distances. For example, when scraping US websites, prioritize using B2Proxy's US residential proxies. At the same time, proxy networks that support the SOCKS5 protocol are more stable when handling long connections and handshakes.</span></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>Establish automated health check mechanisms</strong></span><span style="font-size: 16px;"><br>Build simple status code monitoring into the scraping system. When a certain proxy node continuously returns 520 or 521 multiple times, automatically trigger IP rotation instead of blindly retrying, thereby saving time and bandwidth costs.</span></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>Frequently Asked Questions</strong></span></h2><p style="line-height: 2;"><span style="font-size: 16px;"><strong>Q: Are 520 and 521 errors because Cloudflare itself is broken?</strong></span><span style="font-size: 16px;"><br>A: No. Cloudflare is just a middleman; the error means it cannot reach the website's real server or has been refused by the origin server.</span></p><p style="line-height: 2;"><span style="font-size: 16px;"><strong>Q: If a proxy scraper encounters a 520 error, is the proxy broken?</strong></span><span style="font-size: 16px;"><br>A: Not necessarily. It could be that the target website is down, or your proxy IP has been deliberately disguised by Cloudflare's risk control system using a 520 error page for blocking.</span></p><p style="line-height: 2;"><span style="font-size: 16px;"><strong>Q: Why can a browser open a webpage, but the scraping program reports a 520?</strong></span><span style="font-size: 16px;"><br>A: The TLS fingerprints and request header features of browsers are different from those of scraping programs. If the proxy IP has poor reputation, Cloudflare will only block the scraping program's requests while letting real browsers through.</span></p><p style="line-height: 2;"><span style="font-size: 16px;"><strong>Q: How to quickly test if a proxy IP has been banned by Cloudflare?</strong></span><span style="font-size: 16px;"><br>A: Use that proxy IP to access several different large websites using Cloudflare nodes (such as well-known forums or blogs). If all websites report 520 or 521 errors, it means the IP has been blacklisted.</span></p><p style="line-height: 2;"><br></p><h2 style="line-height: 2;"><span style="font-size: 24px;"><strong>Conclusion</strong></span></h2><p style="line-height: 2;"><span style="font-size: 16px;">The core of solving 520 and 521 errors lies in accurately determining whether the problem is at the target origin, the network node, or the proxy layer. In data collection operations, choosing a residential proxy provider like </span><a href="https://www.b2proxy.com/pricing/isp-proxies" target="_blank"><span style="color: rgb(9, 109, 217); font-size: 16px;">B2Proxy</span></a><span style="font-size: 16px;">, which possesses a high-purity IP pool and flexible session management capabilities, can effectively avoid risk control blockages caused by poor IP reputation. Establishing a healthy node monitoring and rotation mechanism is the key to ensuring the long-term stability of the collection link.</span></p><p style="line-height: 2;"><span style="font-size: 16px;"> </span></p><p><br></p>