Refining cURL User Agents: Avoid Blocks & Boost Proxies

Getting Started with cURL and User Agents

Meet cURL, short for 'Client URL'. It's a handy command-line tool designed for transferring data using a variety of network protocols like HTTP, HTTPS, FTP, and more. Developers and system administrators often rely on cURL for quick interactions with web services, file transfers, and other network communications.

Because cURL operates using standard internet protocols, every HTTP or HTTPS request it sends carries extra information. A crucial piece of this information is the HTTP header, which contains details like the 'User-Agent'.

HTTP headers primarily serve to send metadata about your system, helping the server tailor its response correctly. Specifically, the User-Agent string provides clues about your operating system, browser, and sometimes other configuration details.

However, User Agents aren't just for technical adjustments; they can also be used for tracking user activity. Consequently, altering the User-Agent is a common practice in automated tasks such as web scraping to prevent detection and blocking. This guide will walk you through using cURL, setting custom User Agents, and strategies for reducing the chances of getting blocked.

Understanding cURL User Agents

The Basics of Using cURL

Good news: cURL usually comes pre-installed on most modern operating systems, including Windows 10+, macOS, and many Linux distributions. You can typically access it directly from your command-line interface (like Terminal or Command Prompt). If needed, you can find more details on getting cURL set up in our guide on how cURL handles data transfers.

For this tutorial, we'll focus on making HTTP(S) requests with cURL.

By default, cURL performs a GET request. So, to fetch the content of a webpage, you just need to type curl followed by the URL:

curl

Executing this command will likely print the raw HTML source of the page to your console, which might look a bit chaotic at first glance.

It's worth noting that cURL adheres strictly to the provided URL. If the server responds with a redirect (telling the client to look at a different URL), cURL won't follow it automatically. You'll see a message indicating the resource has moved. To make cURL follow redirects, use the -L option:

curl -L

Attempting to fetch `https://www.example.com` without `-L` might result in just getting the redirect notice, not the actual page content.

Finally, you can include custom headers in your request. This is done using the -H flag, followed by the header string:

curl \
  -L \
  -H "X-Custom-Header: SomeValue"

Remember that cURL's command-line options are flexible; their order generally doesn't matter. These two commands achieve the same result:

curl -L -H "X-Custom-Header: SomeValue" https://api.example.com
curl -H "X-Custom-Header: SomeValue" -L

Decoding the User Agent

A User-Agent string is essentially a text label sent with your request, giving the server hints about the client software making the request. The server uses this information to potentially adjust the content it sends back. Here’s an example of a common User-Agent string:

While User Agents were initially intended to help servers deliver compatible content, they're now also widely used for website analytics and user tracking. A unique User-Agent can help identify a specific user session (even across different IP addresses), and servers might use it to block certain types of clients or suspected bots.

Therefore, when using scripts for tasks like data gathering, regularly changing or 'rotating' User Agents often becomes essential to avoid having your access restricted by web servers.

Modifying the User Agent in cURL Requests

You can easily specify a custom User-Agent string for your cURL request using the -A (or --user-agent) option:

curl \
  -A "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/108.0.0.0 Safari/537.36"

Note that cURL doesn't validate the string you provide; you can technically set it to anything. However, using a nonsensical or obviously fake User-Agent might cause some websites to immediately block your request.

A typical User-Agent string follows a conventional structure, though the specifics vary. Let's dissect the example above:

1. Mozilla/5.0: This token is largely historical, dating back to the Netscape era. It's kept for compatibility, as some very old web servers might expect it.

2. (X11; Linux x86_64): This part usually contains platform details. Here, it indicates the X Window System (common on Unix-like systems), the Linux operating system, and a 64-bit processor architecture.

3. AppleWebKit/537.36: This specifies the browser's layout engine – in this case, WebKit (used by Safari, Chrome, and others) version 537.36.

4. (KHTML, like Gecko): Another compatibility token. KHTML was the engine for the Konqueror browser, and 'like Gecko' suggests compatibility with the engine used by Firefox.

5. Chrome/108.0.0.0: This identifies the browser itself (Google Chrome) and its version number.

6. Safari/537.36: Since Chrome uses WebKit, which originated with Safari, this token is often included, sometimes indicating the WebKit version again or a Safari version for compatibility checks.

While you can technically craft any User-Agent string, creating a completely custom one isn't generally recommended. Firstly, older servers might misinterpret an unconventional string format.

More importantly, an overly unique User-Agent makes your client much easier to single out and track. Techniques like browser fingerprinting rely partially on unique identifiers like the User-Agent. Often, the best strategy is to use common, generic User-Agent strings that blend in with regular web traffic.

Configuring a Default User Agent for cURL

If you find yourself needing the same User-Agent for multiple cURL commands, you can set a default one to avoid typing the -A option every time. This involves creating or editing a configuration file named _curlrc (on Windows) or .curlrc (on Unix-like systems including macOS and Linux).

• Windows: Look for or create the file at C:\Users\<YourUsername>\_curlrc.

• Unix-like systems (Linux, macOS): Look for or create the file at ~/.curlrc (in your home directory).

If the file doesn't exist, simply create it using a plain text editor. Inside the file, add the following line, replacing the placeholder with your desired User-Agent string:

user-agent="Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/109.0.0.0 Safari/537.36"

Save the file. Now, you can verify that cURL is using your default User-Agent by running a request with the verbose flag (-v). Using a tool like Evomi's IP checker is a good way to see the headers being sent:

curl -v

The output will be quite detailed. Scroll towards the beginning of the output (look for lines starting with '>'), and you should see the User-Agent: header reflecting the value you set in your configuration file.