Web crawler is a program or script that automatically crawls information on the World Wide Web according to certain rules. Crawlers can quickly complete the grasping task, saving time and cost. However, most crawlers may have encountered the situation that the IP was suddenly blocked by the website when they climbed. This is because the frequent crawling of crawlers will cause a load on the server, so most websites will set up access worm measures. Here are three common responses to access:
1. Limit the request Headers
The restriction of Headers request is a basic means of access worm, but with the continuous evolution of anti-crawler technology, the detection and restriction of Headers request has become more complicated. While emulating a browser's request Headers can simply fool some basic anti-crawler mechanisms, it may take more effort for a crawler to bypass these restrictions in the face of more stringent anti-crawler strategies.
A common anti-crawling strategy is to check the User-Agent. A User-Agent is an HTTP request header that contains information about the browser or crawler that sent the request. By modifying the User-Agent, the crawler can masquerade as a different browser, thus avoiding detection by the website. However, now some websites will detect the User-Agent, and if the requested User-Agent is found to be inconsistent with common browsers or has abnormal characteristics, it may be marked as a crawler and take corresponding restrictions.
Referer is also a common request header that is checked. Referer is used to indicate which page the current request is coming from. By checking the Referer, the website can determine whether the source of the request is legitimate. To bypass this detection, the crawler may need to set the correct Referer at the time of the request to make it appear to be a legitimate source of the jump.
2. Limit the request IP address
By monitoring the IP address of the source of the access request, the website determines whether it is crawling or malicious access. If a website detects frequent visits or a large number of requests coming from the same IP address, it is likely to blacklist the IP address, which in turn will cause the page to fail to open or return a 403 Deny access error.
To get around IP restrictions, crawler developers often use proxy IP. Proxy IP is the IP address provided by the intermediate server, which can hide the real IP address and make the crawler request appear to come from a different IP address, thus avoiding being blocked by the site. There are two types of proxy IP addresses: shared proxy IP addresses and exclusive proxy IP addresses.
Shared proxy IP: This type of proxy IP is shared by multiple users, so multiple crawlers or users may access it using the same IP address. This can lead to proxy IP being used so frequently that it is recognized as a proxy IP by some websites and subsequently blocked.
Exclusive proxy IP: This type of proxy IP is exclusive to one user, ensuring that the IP address is only used for crawler access. Exclusive proxy IP is relatively more stable because only you are using it, reducing the risk of being blocked. However, exclusive proxy IP costs more and requires a fee to purchase.
3. Limit request cookies
Some sites may require users to log in to access specific pages or data. When the crawler simulates login, the corresponding login Cookie is generated. However, the website also monitors the user's Cookie information, and if a large number of requests are found to be from the same Cookie, it is likely to be judged as crawlers and restricted. Therefore, crawlers need to dynamically manage cookies, ensure that the Cookie information is different for each request, and simulate multiple users for access.
In addition to the above three common anti-crawler measures, there are some more complex and covert anti-crawler techniques, such as captcha, dynamic rendering of pages, IP access frequency restrictions, etc. These techniques require more advanced anti-reptile strategies to cope with.