User-Agent Rotation Guide: 4 Techniques to Avoid Getting Blocked

When scraping data from websites that use sophisticated web application firewalls for content protection, user-agent rotation is a critical part of staying undetected. Without it, repeated requests from the same user-agent (UA) will flag your activity and can trigger rate-limits or even blocks. In this post, I will walk you through how to implement dynamic User-Agent rotation using Python and the fake_useragent library and apply them in Selenium-based scraping workflows including other alternative techniques.

Why Rotate User-Agents?

Websites inspect the User-Agent HTTP header to determine the client making the request to make sure whether it’s a real browser, bot, or script. Rotating UAs makes your scraper appear like different real browsers (Chrome, Safari, Edge) across different OS platforms. It’s like changing your disguise every few seconds. There are multiple approaches to implement user-agent rotation depending on your tech stack and scraping setup. Below, I have demonstrated one practical method using the fake-useragent Python library and other alternative methods of generating user-agent strings.

Parse your user-agent and learn more about them here.

Step-by-Step: How I Implemented User-Agent Rotation in my Academic Project

One of the easiest ways to rotate user-agents in Python is by using the fake-useragent library. This helps simulate traffic from different browsers and operating systems and makes your scraping activity appear more like normal user behavior. Below is a practical example that installs the library, generates a random user-agent (with a fallback for reliability), and injects it into a Selenium session. Just make sure the generated user-agent matches your actual browser version as misalignment can be detected by modern anti-bot systems.

1. Install the Library

pip install fake-useragent

2. Generate Random User-Agent with a Fallback

from fake_useragent import UserAgent

fallback_ua = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 Chrome/116.0.0.0 Safari/537.36'

try:
    ua = UserAgent(
        browsers=['chrome', 'safari'], 
        os='win',
        min_percentage=2.0, 
        fallback=fallback_ua
    )
    user_agent = ua.random
except Exception as e:
    print(f" Failed to generate user-agent. Using fallback: {e}")
    user_agent = fallback_ua

Important: The user-agent string must align with the actual browser version being automated. For instance, if your Selenium driver launches Chrome version 116, ensure your user-agent string reflects Chrome/116. Mismatched versions can be detected by advanced anti-bot tools through JavaScript APIs like navigator.userAgentData, navigator.plugins, and navigator.webdriver.

3. Set User-Agent in Selenium

After generating the user-agent, we need to inject it into the options of the Selenium Web Driver.

options.add_argument(f"user-agent={user_agent}")

Alternative Ways to Rotate User-Agents

1. Maintain Your Own User-Agent Pool

Create a local .txt, .json, or .csv file with real user-agent strings (e.g., only Chrome 116 if your browser is Chrome 116). Example:

import random

with open('useragents_chrome_116.txt') as f:
    user_agents = f.read().splitlines()

user_agent = random.choice(user_agents)

2. Use an API Service

You can use third party API services to obtain fresh user-agents.

3. Hardcode Smart Defaults

For quick and small projects, rotate through a small hardcoded list of trusted UA strings that match your browser version:

user_agents = [
    "Mozilla/5.0 ... Chrome/116 ...",
    "Mozilla/5.0 ... Chrome/116 ...",
    # Add more here
]
user_agent = random.choice(user_agents)

Conclusion

Rotating user-agent strings is not just a best practice but it is a necessity when scraping modern websites protected by advanced bot mitigation systems. By dynamically changing your UA headers to mimic real browsers and aligning them with the actual browser version you are automating, you reduce the risk of detection and increase the longevity of your scraping sessions. These techniques are shared for educational purposes to help developers, data scientists, and researchers understand how websites work and how to test responsibly. Always respect each site’s terms of use, robots.txt, and legal guidelines when conducting automated data access.

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