User-Agent rotation involves dynamically switching browser identifiers during web scraping to mimic diverse user requests, while avoiding detection requires combining this with randomized request timing and proxy rotation to create natural-looking traffic patterns that bypass anti-bot systems.
In this guide, we’ll explore how to use User-Agent rotation in cURL to improve the effectiveness of automated web scraping. You’ll learn:
- The importance of the User-Agent header and its role in HTTP requests.
- How to customize and rotate User-Agent strings in cURL.
- Practical methods to bypass detection when performing web scraping tasks.
Let’s dive in and make your automated requests more robust and secure!
What Is a User Agent and Why Does It Matter in Web Scraping?
A User-Agent is a string included in the HTTP header of web requests that identifies the software making the request. This string can convey details such as the browser, operating system, and device type. Web servers use this information to tailor content to the client or to detect non-human activity, such as scraping bots.
Here’s an example of a real User-Agent string from a Chrome browser:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36
For web scraping, using a generic or default User-Agent (e.g., from cURL) is a red flag for anti-bot systems, potentially leading to blocked requests. Rotating User-Agent headers is a key technique to mimic human-like activity, enhancing the effectiveness of automated scraping while avoiding detection.
What Is the Default cURL User Agent , and Why Is It a Problem for Web Scraping?
Just like most HTTP clients, cURL sets the User-Agent header when making an HTTP request. The default cURL user agent string is:
curl/X.Y.Z
Where X.Y.Z is the version of cURL installed on your machine.
To verify that, make a cURL request to the /user-agent endpoint of the httpbin.io project. This API returns the User-Agent header string set by the caller.
Make a GET request to /user-agent with cURL using the following command:
curl "https://httpbin.io/user-agent"
Note: On Windows, replace curl with curl.exe to avoid aliasing issues in PowerShell.
The endpoint should return something like this:
{ "user-agent": "curl/8.4.0" }
As you can see, the user agent set by cURL is curl/8.4.0. This clearly identifies the request as coming from cURL, which can be problematic as anti-bot solutions could easily block such requests.
How to Set cURL User Agent Header
There are two approaches to setting a user agent in cURL. Let’s explore them both!
Set a Custom User Agent Directly
cURL has an option to specify the User-Agent string directly with the -A or --user-agent option:
curl -A "<user-agent_string>" "<url>"
Consider the following example:
curl -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36" "https://httpbin.io/user-agent"
The output will be:
{ "user-agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36" }
To unset the User-Agent header entirely, pass an empty string to -A:
curl -A "" "https://httpbin.io/headers"
The result will be:
{ "headers": { "Accept": [ "*/*" ], "Host": [ "httpbin.io" ] } }
To set the User-Agent header to a blank string, pass a single space to -A:
curl -A " " "https://httpbin.io/headers"
Set a Custom User Agent Header
Alternatively, you can set the User-Agent header like any other HTTP header using the -H or --header option:
curl -H "User-Agent: <user-agent_string>" "<url>"
For example:
curl -H "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36" "https://httpbin.io/user-agent"
The result will be:
{ "user-agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36" }
How to Rotate User-Agent Headers in cURL for Web Scraping
Using a fixed User-Agent header in cURL can trigger anti-bot systems when making automated requests at scale. To reduce detection, rotating User-Agent headers simulates requests from different browsers and devices. Here’s how to implement it:
Steps to Rotate User Agents:
- Collect user agents: Create a list of real-world User-Agent strings from various devices and browsers.
- Set up rotation logic: Randomly select a User-Agent string for each request.
- Integrate into cURL requests: Apply the selected User-Agent string dynamically.
Bash Implementation
Store a list of user agents in an array:
user_agents=( "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36" "Mozilla/5.0 (Macintosh; Intel Mac OS X 14.5; rv:126.0) Gecko/20100101 Firefox/126.0" # ... "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:126.0) Gecko/20100101 Firefox/126.0" )
Implement a function to randomly select a user agent:
get_random_user_agent() {
local count=${#user_agents[@]}
local index=$((RANDOM % count))
echo "${user_agents[$index]}" }
Use the function to set the user agent in cURL:
user_agent=$(get_random_user_agent) curl -A "$user_agent" "https://httpbin.io/user-agent"
PowerShell Implementation
Store a list of user agents in an array:
$user_agents = @(
"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36" "Mozilla/5.0 (Macintosh; Intel Mac OS X 14.5; rv:126.0) Gecko/20100101 Firefox/126.0" # ... "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:126.0) Gecko/20100101 Firefox/126.0" )
Create a function to randomly pick a user agent:
function Get-RandomUserAgent { $count = $user_agents.Count $index = Get-Random -Maximum $count return $user_agents[$index] }
Use the function to set the user agent in cURL:
$user_agent = Get-RandomUserAgent curl.exe -A "$user_agent" "https://httpbin.io/user-agent"
Conclusion
In this guide, you learned why setting the User-Agent header in an HTTP client is important and how to do it in cURL. By rotating user agents, you can reduce the risk of detection and blocking when making automated requests. For more advanced solutions, consider integrating a proxy with cURL to further enhance your web scraping capabilities.
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