Web crawling is the process of programmatically browsing and indexing websites. Search engines like Google use web crawlers to discover and catalog the billions of pages on the internet. Many businesses also leverage web crawling to extract publicly available data from websites for market research, price monitoring, lead generation and more.
However, as crawlers retrieve data from websites automatically and at scale, they can put a heavy load on web servers. Website owners increasingly try to detect and block suspicious crawlers to prevent content theft, spam, and denial-of-service attacks. Getting blocked is a constant challenge for anyone building a web crawler.
In this guide, we‘ll share proven techniques to crawl websites efficiently while avoiding detection and banning. Follow these best practices to keep your web scrapers running smoothly.
Why Websites Block Web Crawlers
Not all bots are bad bots. Googlebot and other search engine crawlers provide a valuable service by helping people find relevant information online. Many websites actually want to be indexed by search engines to attract more traffic and customers.
The problem is when websites detect large-scale, automated crawling that appears malicious. Here are some reasons a crawler might get blocked:
- Too many requests too quickly, overloading the server
- Not respecting robots.txt rules that define off-limits pages
- Accessing pages a normal user wouldn‘t, like post-login pages
- Suspicious request patterns without typical browser headers and behavior
As data becomes more valuable, an increasing number of sites are using anti-bot solutions to protect their content. To avoid disruptions to your crawling projects, you need to make your web scrapers appear as human-like as possible.
Header Spoofing
One of the first things anti-bot systems look at is the HTTP headers of incoming requests, especially the User-Agent string. By default, popular crawling libraries send a generic User-Agent like "python-requests/2.25.1".
In contrast, requests from real web browsers contain much more detailed User-Agent strings, like:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/93.0.4577.82 Safari/537.36To slip past basic bot checks, set your crawler‘s User-Agent to match what a browser would send, including specifics like the operating system and browser version. You can find lists of common User-Agent strings online.
In addition to User-Agent spoofing, it‘s a good idea to set other typical request headers like Accept, Accept-Language, and Referer. Replicate all the headers a real browser would send for the most human-like appearance.
Request Randomization
Predictable crawling patterns make it easier for anti-bot systems to identify scrapers. Even with a legitimate User-Agent, sending requests with the exact same headers each time can get you blocked.
Introduce some randomness by choosing from a pool of browser-like User-Agent strings, referrers, and other header values for each request. Rotating headers prevents leaving a consistent fingerprint.
Adding random delays between requests is another best practice. A few seconds of sleep makes your crawler seem more human compared to firing off requests in rapid succession. Randomizing the length of delay further obscures robotic activity.
IP Rotation
Advanced bot detection tools look beyond just headers to the IP address making the requests. Too many requests from a single IP in a short period of time triggers rate limiting and blocking.
The solution is to spread requests across many different IP addresses using a proxy server or pool of proxies. Proxies act as intermediaries, forwarding requests to the destination server with their IP address instead of yours.
There are dedicated rotating proxy services for web scraping that automatically switch the IP used after a certain number of requests or time interval. Choosing proxies in the same geographies as your target users makes your traffic look more natural.
Be careful with free public proxies – they tend to be unreliable and may already be blocked. Stick with reputable paid proxy providers for the best results.
Avoiding Honeypots
Some sites create honeypots or traps to catch crawlers. An example is including links that are invisible to human users but still appear in the raw HTML. When a scraper follows those links, the anti-bot system knows it‘s not a regular visitor.
To avoid honeypot detection, program your crawler to only interact with elements visible in a rendered web page. Resist the temptation to crawl every link discovered. If something seems suspcious, like a link unrelated to the site‘s main content, it‘s safer to skip it.
Headless Browsers
Simple crawlers work by making HTTP requests to get the HTML of pages and then parsing that HTML for links and data. While lightweight and fast, this approach can‘t execute JavaScript code that modern websites rely on to load content dynamically.
Headless browser tools like Puppeteer and Selenium solve this by automating full web browsers like Chrome and Firefox. With a headless browser, you can click buttons, fill out forms, and wait for JS-driven elements to appear before extracting data, more closely mimicking human behavior.
Headless browsers also help manage cookies and other stateful information across requests. The downside is they consume more system resources compared to simple HTTP scrapers.
Respecting Robots.txt
Ethical crawlers respect the rules set forth in a website‘s robots.txt file. This file specifies which pages and directories are disallowed for scrapers. While it‘s not a strict defense, blatantly violating robots.txt is a surefire way to get your crawler blocked and your IP address banned.
Configure your crawler to check for a robots.txt file and parse its rules before scraping a new domain. Many popular crawling frameworks have built-in support for robots.txt handling. It‘s not just polite, it‘s a best practice all professional scrapers should follow.
Monitoring for Blocking
Sometimes even with the best practices, determined websites can still detect and block crawlers. It‘s important to build monitoring into your crawler to alert you of potential bans so you can pause and investigate.
CAPTCHA challenges are a common anti-bot technique. If you start seeing CAPTCHA pages instead of the expected content, your crawler may have been flagged. Drastic drops in success rate or increases in connection errors are other signs of blocking.
Have an action plan for mitigating damage when your crawler gets blocked. Rotating to new proxy IPs, slowing down the crawl rate, and improving your bot-detection avoidance techniques can often get you unblocked.
Build vs Buy
While it‘s certainly possible to build a stealthy web crawler yourself with enough time and expertise, there are pre-built scraping solutions that can save you development effort. Products like Scrapy Cloud, Zyte, and SerpApi offer APIs with rotating proxies, CAPTCHA solving, and other anti-bot evasion features built-in.
The choice of building your own crawler versus using an off-the-shelf tool comes down to your specific needs and resources. Custom web scrapers give you full control and unlimited flexibility but require ongoing development and maintenance. Paid APIs are easier to get started with but can become expensive at scale and may not cover every website you need.
Conclusion
Web crawling is getting more difficult as websites deploy increasingly sophisticated anti-bot countermeasures. While there‘s no perfect solution, you can keep your crawlers running longer by following the techniques covered in this guide:
- Spoof headers to look like a browser
- Randomize headers and crawl patterns
- Use proxy servers for IP rotation
- Avoid links invisible to human users
- Render JavaScript like a real browser
- Respect robots.txt rules
- Monitor for blocking and adapt
The most important rule when writing any kind of web crawler is to be a good bot. Crawl at a reasonable rate, don‘t overload servers, and respect rules about what content should be accessed. By combining ethical practices with technical countermeasures, you can still leverage web crawling to power your business while playing fair with website owners.
Building a robust, scalable crawler is a big undertaking, but the rewards are immense. Programmatically accessing web data opens up a world of possibilities for research, automation, and data-driven decision making.
