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520 Status Code – What Is It and How to Avoid It?

Encountering the obscure HTTP status code 520 can be frustrating for developers and scrapers alike…

Common Technical Causes

While generic, 520 errors typically indicate an issue on the origin infrastructure prevented processing the request successfully. Some common technical triggers include:

  • Application crashes – Runtime errors like infinite loops, unhandled exceptions, or resource exhaustion can cause the app server to crash entirely. This leads to a service outage where no new requests can be handled.

  • Load balancer failures – Sites sitting behind load balancers depend on them to route traffic. If the load balancer goes down, requests cannot reach the backend servers.

  • Reverse proxy cache problems – Sites often use reverse proxy caching tiers to improve performance. Cache errors or overfilled caches can result in invalid responses.

  • Back-end communication failures – Microservices architectures have many internal touch points. Any disruption in services communicating can propagate invalid responses.

For example, a major outage of Amazon S3 in 2017 caused widespread 520 errors across thousands of sites relying on it. Even a short 15 minute S3 disruption was enough to generate 5.5 billion failed requests according to Catchpoint.

Cloud infrastructure dependencies like this highlight the fragility of modern web architectures. Just a small hiccup in one link of the chain can ripple outward.

Why Scrapers Trigger 520 Errors

Scrapers and bots receive 520 errors frequently because sites actively try to detect and block them. Automated scraping traffic looks suspicious for many reasons:

  • Repeated access patterns – Scrapers often hit sites in very systematic ways, unlike humans browsing organically. This stands out.

  • No browser fingerprints – Scrapers lack fingerprints from real browsers like fonts, plugins, and hardware specs. Missing fingerprints are red flags.

  • Stateless requests – Scrapers fail to handle cookies properly or maintain sessions. Hopping between pages statelessly is abnormal behavior.

  • Data center server IPs – Cheap scrapers often run on cloud servers with IP ranges known for abuse. Sites blacklist these data center ranges.

  • Overly high speed – Scraper requests come in extremely fast without plausible human delays. The inhuman speeds indicate automation.

  • No mouse movements – Scrapers don‘t produce mouse movements, clicks, and scrolling like a real user because they render via API requests rather than full browser emulation.

Let‘s explore some common tactics sites use to detect scrapers…

Evasion Techniques to Appear More Human

The best way to avoid 520 blocks is making scrapers act human. Some techniques to seem more user-like:

Rotate Random User Agents

Use a large set of common real-world user agents and randomly assign them across requests. Don‘t repeat the same static user agent. Sites log the user agents accessing them and look for outliers.

Having a diverse pool of mobile and desktop agents from different browsers, OSes and devices prevents your scraper from standing out as an anomaly.

Handle Cookies and Sessions

Properly handle cookies by storing and resending them. Reuse browser sessions across multiple page views. Stateless cookie-less access tends to attract suspicion, since normal users maintain session state.

Implement Plausible Delays

Add randomized delays between requests to mimic human browsing patterns. Delays of 1-7 seconds between actions, and 15-45 seconds between site visits work well.

Never scrape at full automated speed with no delays at all. That‘s instant bot detection!

Slow Down Scraping Speed

Limit the requests per minute to act more human. Scrapers can process pages insanely fast, but real users browse leisurely. Take your time!

Distribute Scraping Over Time

Rather than pounding a site continuously in one sitting, spread scraping out over multiple days or weeks. This keeps site volume low and more stealthy.

Leverage Proxy Rotation

Rotate through many residential proxy IPs to avoid patterns tied to individual scraping servers. Proxy services make this easy…

Specialized Web Scraping Proxies

An effective technique is using proxy services tailored to web scraping. Scraping proxies manage thousands of frequently rotated IPs to distribute requests across.

Popular scraping proxy providers include BrightData, SmartProxy, and Soax. Here‘s how they work…

These companies maintain large pools of residential IPs worldwide, usually 50,000-100,000+. The IPs come from real home and mobile internet connections via ISP partnerships.

Scrapers connect through the proxy service API which randomly assigns an IP to each request. The proxies handle routing traffic, managing cookies/sessions, implementing delays, retrying errors, and basically mimicking organic browsing.

This removes a lot of the scraping logic burden from developers. The end result is requests coming from thousands of real residential IPs that are hard to distinguish from normal user traffic.

According to BrightData CEO Or Lenchner, "Our proxies look so human that we constantly get pirated logins for streaming sites from real viewers who think they‘re residential IPs."

Scraping proxies start around $100/month for 5GB-40GB of traffic which suits most small scrapers. The largest plans offer 100-500+ GB/month for heavy usage.

Compared to the hassle of maintaining your own proxies, scraping services provide turnkey access to clean IPs. Their APIs also simplify proxy management compared to self-rotating.

Headless Browsers

For heavily protected sites, scrapers may still get blocked even using proxies. In these cases, running headless browsers like Puppeteer, Playwright, or Selenium/WebDriver may succeed where proxies fail.

Headless Chrome and Firefox perform a full browser render via automation. This provides the most comprehensive mimic of real human users.

Of course, running at scale requires significant server resources for CPU, RAM, and network. Large scrapers may spend $10k+/month on cloud servers to distribute browser instances.

There‘s also the complexity of orchestrating and load balancing headless browsers. So proxies tend to be more cost-efficient for most scrapers.

But when proxies fail against aggressive bot mitigation, browser automation is the foolproof scraping solution…

Long-Term Strategies to Maintain Access

The scraping game is constantly evolving as sites deploy newer bot defenses. Scrapers need to continuously adapt their tactics.

Beyond the evasion techniques discussed above, some long-term strategies can help sustain scraper access over the years:

  • Regularly acquire fresh residential IPs – Gradually cycle through new proxies and ISPs as old IPs get flagged. This churn is the cost of doing business.

  • Contribute to open-source scraping tools – Give back to projects like Scrapy, Puppeteer, and friends. This builds goodwill with communities that may share tips.

  • Participate in industry groups – Join scraping forums and attend web security conferences. Learn the latest trends and techniques.

  • Don‘t overscrape – Restrain scraping volume to avoid greedy abuse. Fly under the radar. Think marathon not sprint.

With dedication and vigilance, your well-behaved scraper can enjoy long prosperous access without pesky blocks.

Now let‘s move on to debugging techniques when you do run into errors…

Step-By-Step Guide To Debugging 520 Blocks

If your scraper starts throwing 520 codes, here are systematic steps to help diagnose what‘s failing:

Review server-side logs – The backend application logs may provide more details about any crashes or failures. Though often you won‘t have access.

Inspect scraper verbose logs – Log all request headers, post data, cookies, user agents, and other details on your scraper for comparison.

Check for explicit block pages – The response content may directly say the access is blocked, rate limited or otherwise restricted. This makes issues obvious.

Look for suspicious response headers – Headers like X-Google-GFE-Request-Policy indicate Google Cloud intercepted the request as abusive. Watch for unusual security-related headers.

Toggle different user agents – Try switching user agents to see if blocks are tied to specific ones. Rotate between desktop, mobile, and less common browsers.

Test with different proxies – If you control proxies directly, test rotating across different IPs and providers to isolate blocks.

Validate all scraper headers – Double check the scraper is sending normal headers like user agent, accept, encoding, origin, etc.

Check cookies and sessions – Make sure your cookie handling logic works properly and maintains state across requests. Test this.

Request individual pages – Narrow down which URLs trigger blocking versus those allowed. This can help identify patterns.

Slow down request rate – Temporarily slow down requests per minute and add larger delays to verify if it‘s a rate limit.

Retry requests – Retry blocked requests in case it was a transient error. But don‘t retry endlessly.

Contact proxy support – If using a paid proxy service, their support team may be able to investigate blocked IPs or other issues.

With diligence, you can usually narrow down the causes of 520 errors. But prevention is easier than curing – so ensure your scraper is well-programmed up front!


While annoyingly vague, 520 errors ultimately mean the server failed to generate a valid response for the request. In web scraping, these are often active bot blocks.

Sites block scrapers through various means, so configuring your scraper to appear human is critical. Use proxy rotation, realistic delays, cookie handling, fingerprinting evasion and more. Scraping-focused proxies make this easier.

When you do hit blocks, comprehensive debugging and incremental testing can usually isolate the root causes – whether technical faults or anti-scraper defenses. Tracing headers and response codes reveals useful clues.

With proper design, even the most sophisticated sites can usually be scraped successfully. Just stay vigilant, keep adapting your techniques, and respect reasonable site terms to maintain long-term access.

Scraping without tripping alarms takes dedication. But the arms race with always-advancing bot mitigation means the learning never stops!

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