When scraping many pages from a website, using the same user-agent consistently leads to the detection of a scraper. A way to bypass that detection is by faking your user agent and changing it with…
Web scraping is a task that has to be performed responsibly so that it does not have a detrimental effect on the sites being scraped. Web Crawlers can retrieve data much quicker, in greater depth than humans, so bad scraping practices can have some impact on the performance of the site.
If a crawler performs multiple requests per second and downloads large files, an under-powered server would have a hard time keeping up with requests from multiple crawlers. Since web crawlers, scrapers or spiders (words used interchangeably) don’t really drive human website traffic and seemingly affect the performance of the site, some site administrators do not like spiders and try to block their access. Most websites may not have anti-scraping mechanisms since it would affect the user experience, but some sites do block scraping because they do not believe in open data access.
In this article, we will talk about how websites detect and block spiders and techniques to overcome those barriers.
Basic Rule: “Be Nice”
An overarching rule to keep in mind for any kind of web scraping is
BE GOOD AND FOLLOW A WEBSITE’S CRAWLING POLICIES
Here are some of the best practices you can follow to overcome the detection.
1. Respect Robots.txt
Web spiders should ideally follow the robot.txt file for a website while scraping. It has specific rules for good behavior such as how frequently you can scrape, which pages allow scraping, and which ones you can’t. Some websites allow Google to scrape their websites, by not allowing any other websites to scrape. This goes against the open nature of the Internet and may not seem fair but the owners of the website are within their rights to resort to such behavior.
You can find the robot.txt file on websites. It is usually the root directory of a website – http://example.com/robots.txt.
If it contains lines like the ones shown below, it means the site doesn’t like and does not want to be scraped.
However, since most sites want to be on Google, arguably the largest scraper of websites globally, they do allow access to bots and spiders.
What if you need some data, that is forbidden by Robots.txt. You could still go and scrape it. Most anti-scraping tools kick in when you are scraping pages that are not allowed by Robots.txt.
What do these tools look for – is this client a bot or a human. And how do they find that? By looking for a few indicators that humans do and bots don’t. Humans are random, bots are not. Humans are not predictable, bots are.
Here are a few easy giveaways that you are bot/scraper/crawler –
- scraping too fast and too many pages, faster than a human ever can
- following the same pattern while crawling. For example – go through all pages of search results, and go to each result only after grabbing links to them. No human ever does that.
- too many requests from the same IP address in a very short time
- not identifying as a popular browser. You can do this by specifying a ‘User-Agent’.
- using a user agent string of a very old browser
The points below should get you past most of the basic to intermediate anti-scraping mechanisms used by websites.
2. Make the crawling slower, do not slam the server, treat websites nicely
Web scraping bots fetch data very fast, but it is easy for a site to detect your scraper as humans cannot browse that fast. The faster you crawl, the worse it is for everyone. If a website gets too many requests than it can handle it might become unresponsive.
Make your spider look real, by mimicking human actions. Put some random programmatic sleep calls in between requests, add some delays after crawling a small number of pages and choose the lowest number of concurrent requests possible. Ideally put a delay of 10-20 seconds between clicks and not put much load on the website, treating the website nice.
Use auto throttling mechanisms which will automatically throttle the crawling speed based on the load on both the spider and the website that you are crawling. Adjust the spider to an optimum crawling speed after a few trials runs. Do this periodically because the environment does change over time.
3. Do not follow the same crawling pattern
Humans generally will not perform repetitive tasks as they browse through a site with random actions. Web scraping bots tend to have the same crawling pattern because they are programmed that way unless specified. Sites that have intelligent anti-crawling mechanisms can easily detect spiders by finding patterns in their actions
Incorporate some random clicks on the page, mouse movements and random actions that will make a spider look like a human.
4. Disguise your requests by rotating IPs or Proxy Services
When scraping, your IP address can be seen. A site will know what you are doing and if you are collecting data. They could take data such as – user patterns or experience if they are first time users.
Multiple requests coming from the same IP will lead you to get blocked, which is why we need to use multiple addresses. When we send requests from a proxy machine, the target website will not know where the original IP is from, making the detection harder.
Create a pool of IPs that you can use and use random ones for each request. Along with this, you have to spread a handful of requests across multiple IPs.
There are several methods can be used to change your outgoing IP.
Services such as VPNs, shared proxies and TOR can help. In addition, various commercial providers also provide services for automatic IP rotation. A lot of companies now provide residential IPs to make scraping even easier – but most are expensive.
5. Rotate User Agents
A user agent is a tool that tells the server which web browser is being used. If the user agent is not set, websites won’t let you view content. You can get your User-Agent by typing ‘what is my user agent’ in Google’s search bar.
You can also check your user-string here:
Every request made from a web browser contains a user-agent header and using the same user-agent consistently leads to the detection of a bot. The only way to make your User-Agent appear more real and bypass detection is to fake the user agent.
How to Spoof a User-Agent:
Create a list of user agents and pick a random one for each request to prevent getting blocked. Set your user agent to a common web browser instead of the default user agent.
Set your user-agent to a common web browser instead of using the default user-agent (such as wget/version or urllib/version). You could even pretend to be the Google Bot: Googlebot/2.1 if you want to have some fun! (http://www.google.com/bot.html)
A user-agent string listing to get you started can be found here:
6. Check if Website is Changing Layouts
Some websites make it tricky for scrapers, serving slightly different layouts.
For example, in a website pages 1-20 will display a layout, and rest of the pages may display something else. To prevent this, check if you are getting data scraped using XPaths or CSS selectors. If not, check how the layout is different and add a condition in your code to scrape those pages differently.
7. Use a headless browser
The problem with this is that when doing any kind of web scraping, the content is rendered by the JS code and not the raw HTML response the server delivers.
Blacklisting can be prevented is by using a headless browser. A headless browser is one that works like any other browser; only they aren’t visual on a desktop. This means there is no graphical interface. Instead of interacting with element, you can automate everything with a command-line interface. This can help you to stay undetected while web scraping
Keep in mind that headless browsers like Selenium and Puppeteer use a lot of resources (RAM, CPU, Bandwidth etc) in comparison to script-based approaches. You can check out these web scraping tutorials on using headless browsers:
Web scraping tutorial using a headless browser:
8. Beware of Honey Pot Traps
Honeypots are systems set up to lure hackers and detect any hacking attempts that try to gain information. It is usually an application that imitates the behavior of a real system. Some websites install honeypots, which are links invisible to normal users but can be seen by web scrapers.
When following links always take care that the link has proper visibility with no nofollow tag. Some honeypot links to detect spiders will have the CSS style display:none or will be color disguised to blend in with the page’s background color.
This detection is obviously not easy and requires a significant amount of programming work to accomplish properly, as a result this technique is not widely used on either side – the server side or the bot or scraper side.
9. Avoid scraping data behind a login
Login is basically permission to get access to web pages. Some websites like Indeed and Facebook do not allow permission.
If a page is protected by login, the scraper would have to send some information or cookies along with each request to view the page. This makes it easy for the target website to see requests coming from the same address. They could take away your credentials or block your account.
Its generally preferred to avoid scraping websites that have a login as you will get blocked easily, but one thing you can do is imitate human browsers whenever authentication is required you get the target data you need.
How can websites detect web scraping?
Websites can use different mechanisms to detect a scraper/spider from a normal user. Some of these methods are enumerated below:
- Unusual traffic/high download rate especially from a single client/or IP address within a short time span.
- Repetitive tasks performed on the website – based on an assumption that a human user won’t perform the same repetitive tasks all the time.
- Detection through honeypots – these honeypots are usually links which aren’t visible to a normal user but only to a spider. When a scraper/spider tries to access the link, the alarms are tripped.
How to address this detection?
Spend some time upfront and investigate the anti-scraping mechanisms used by a site and build the spider accordingly, it will provide a better outcome in the long run and increase the longevity and robustness of your work.
How to easily know if a site doesn’t support data scraping?
Check the robots.txt file, which is usually in the root directory of a website e.g http://example.com/robots.txt.
If it contains lines like the ones shown below, it means the site doesn’t like and want scraping.
However, since most sites want to be on Google (arguably the largest scraper of websites globally) they do allow access to bots and spiders.
This line is for preventing well-behaved bots or the bots which respect robots.txt.
What happens when you get banned ?
There are two simple ways to ban a webspider/webscraper – either by banning all accesses from a particular IP or by banning all accesses that use a specific id to access the server (most browsers and web spiders identify themselves whenever they request a page by user agents. Chrome browser for example uses
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.149 Safari/537.36
The banning can be temporary or permanent.
Temporary blocks can last minutes or hours.
Permanent bans go against the open nature of the Internet but some sites resort to this “scorch the internet” measure.
How do you find out if a website has blocked you ?
If any of the following signs appear on the site that you are crawling, it is usually a sign of being blocked or banned.
- CAPTCHA pages
- Unusual content delivery delays
- Frequent response with HTTP 404, 301 or 50x errors
Frequent appearance of these HTTP status codes is also indication of blocking
- 301 Moved Temporarily
- 401 Unauthorized
- 403 Forbidden
- 404 Not Found
- 408 Request Timeout
- 429 Too Many Requests
- 503 Service Unavailable
A comprehensive list of HTTP return codes (successes and failures) can be found here. It will be worth your time to read through these codes and be familiar with them.
All these ideas above provide a starting point for you to build your own solutions or refine your existing solution. If you have any ideas or suggestions, please join the discussion in the comments section.
Thank you for reading.
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