When it comes to web scraping, two issues are often discussed: one is how to avoid being allowed to collect public data by the target server, and the other is how to improve the quality of retrieved data. At the present stage, effective techniques can prevent the target website from allowing the collection of public data, such as the user's common proxy and practical IP address rotation. However, there is another technique that can play a similar role, but is often overlooked, and that is the use and optimization of HTTP headers. This approach also reduces the likelihood that web crawlers will be allowed to collect publicly available data from various data sources and ensures that high quality data is retrieved. Here's a look at five common headers:
1. HTTP Header User-Agent
HTTP Header User-Agent is a header containing application type, operating system, software, and version information. Its main role is to enable the server to identify the User's terminal device and provide the appropriate HTML layout for that device accordingly.
The user agent string is generated by a browser or application that can tell the web server details about the client device used by the user. For example, User-Agent can indicate that the request is coming from a particular browser (such as Chrome, Firefox, Safari, etc.), as well as the operating system used (such as Windows, macOS, iOS, Android, etc.) and device type (phone, tablet, PC, etc.). Based on this information, the server can determine which terminal device the request comes from, and return the corresponding page layout accordingly to ensure that a good user experience can be obtained on different devices.
Because User-Agent contains such a wealth of device information, many web servers often validate it as a first safeguard to identify suspicious requests. Some undesirable web crawlers or automated scripts may try to hide their identity. In order to avoid detection by the server, experienced crawlers will modify the content of the User-Agent Header, so that the server will mistake it as multiple natural users making requests, thus reducing the possibility of being identified as a crawler.
In order to avoid being banned or restricted by the target site, it is important to set the User-Agent Header properly. A common practice is to set the user-agent to a string similar to that of a particular browser or operating system, making the server think the request is coming from a normal User's device. In addition, crawlers can also use random or older versions of User-Agent strings, making it more difficult for the server to identify them.
2. HTTP Header Accept-Language
The Accept-Language Header passes information to the network server about which languages the client has and which specific language is preferred when the network server sends back a response. When the web server does not recognize the preferred language, a specific Header is usually used.
3. HTTP Header Accept-Encoding
Accept-Encoding Header informs the network server which compression algorithm to use when processing the request. In other words, when sent from a web server to a client, the server acknowledges information that can be compressed if it can handle it. When optimized with this Header, it saves traffic, which is better for both the client and the web server from a traffic load perspective.
4. Accept the HTTP Header
Accept Header belongs to the content negotiation category, and its purpose is to inform the network server what type of data format it can return to the client. If the Accept Header is configured properly, it makes the communication between the client and server more like real user behavior, reducing the likelihood that web crawlers will be allowed to collect publicly available data.
5. HTTP Header Referer
Before sending the request to the web server, the Referer Header provides the address of the web page the user was on before the request. The Referer Header doesn't really matter much when a website is trying to block the scraping process. A random real user is likely to be online several hours apart.
By understanding and optimizing these five common HTTP headers, web crawlers can better simulate real user behavior, reduce the risk of being banned by the target site, and ensure high-quality data is obtained. For data acquisition and capture work, reasonable use of these HTTP header information is an important means to improve the capture efficiency and data quality.