How to Set and Change User Agent when using curl

Jan 9, 2024 · 12 min read

As an avid web scraper, I rely on cURL daily to quickly test APIs and scrape data from websites. It's an indispensable tool for automated data collection. However, I learned early on that using the default static cURL user agent can earn you a one-way ticket to getting blocked if you aren't careful.

When a client like cURL sends a request to a server, it includes a user agent (UA) string to identify itself. This UA string gives away details like the application name and version. Many sites now actively block common user agents used by bots and scrapers to stop unwanted traffic.

So being able to change cURL's user agent is crucial for mimic legit web traffic and avoiding blocks. Trust me, you'll eventually want to unlock this skill if you plan to use cURL heavily.

In this beginner's guide, I'll walk through the process of changing the cURL user agent step-by-step. I'll also share some pro tips I've learned for picking smart UAs and programmatically rotating them to outsmart target sites.

Let's get started!

What Exactly is a User Agent?

First things first - what is a user agent?

A user agent string gives information about the application, device, and system making a request to a web server. Here’s an example UA from Chrome browser on a Windows laptop:

Mozilla/5.0 (Windows NT 10.0; Win64; x64)
AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36

This reveals useful bits like Chrome version, Windows OS version, and more. These details help servers identify clients to optimize and deliver the right content.

Now you may be wondering...

Why Should I Change cURL's Default User Agent?

When you send requests through cURL without setting a custom UA, here's what gets sent by default:


Short, sweet, and very bot-like!

This static UA instantly flags your requests as coming from an automation tool instead of a real browser. Many websites actively block traffic from any UAs with "curl" or uncommon client names.

So if you want to scrape or interact with sites at scale with cURL without getting blocked, changing the default UA is crucial. Mimicking a normal browser UA helps you hide in plain site amongst real visitors.

Option 1: Set a Custom Static User Agent

Let's start simple by setting a custom browser UA to mask cURL's identity.

The main options you can pass to change the user agent string are:

  • A or --user-agent: Sets UA directly
  • H: Sets any request header, including User-Agent
  • I prefer using the -A option since it's most explicit. Here's an example command:

    curl -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/109.0" <>

    This masks my request as coming from a Windows Firefox browser.

    You'll need to grab a legit UA string from an actual browser to populate the value passed to -A or -H. I like to collect a few different options from browsers on different devices and OS to mix it up.

    For example, I have a bank of strings mimicking:

  • Chrome on Windows
  • Safari on iPhone
  • Firefox on Linux
  • Having realistic and diverse user agents is key to avoiding patterns that could flag you as suspicious.

    Pro tip: Always double check any custom UA against an online parser to validate it looks legit before sending requests!

    Option 2: Randomly Rotate Multiple User Agents

    Setting a static UA is a good start, but spamming requests from the same exact UA can still get you noticed. The next level tactic is randomly rotating user agents programmatically. This makes your traffic blend in even better across different browsers.

    Here's an example bash script showing how I implement UA rotation with cURL:

      "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36"
      "Mozilla/5.0 (iPhone; CPU iPhone OS 12_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.1 Mobile/15E148 Safari/604.1"
      "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0"
    for i in {1..10}; do
       random_index=$((RANDOM % ${#user_agents[@]}))
       curl -A "$user_agent" $website

    I start by populating an array of legit browser UA strings I gathered earlier (Chrome, Safari, Firefox).

    Then inside a loop, I use bash's $RANDOM syntax to pick a random index value to select a different UA on each request. Lastly, I pass that UA string to cURL using my old friend -A to set it on the fly.

    After running this, I'd have 10 requests with rotating UAs instead of one static string getting blocked. Pretty slick!

    The key things to note with UA rotations in general:

  • Use a big list of strings from different platforms and browsers
  • Fetch new strings every so often as browsers update
  • Randomize which UA gets used on each request
  • This makes it extremely difficult for sites to detect any patterns and block you compared to cycling through a small list of UAs sequentially.

    Common Pitfalls and Troubleshooting

    I've picked up some hard-won knowledge around a few pitfalls that can trip you up when trying to change cURL UAs. Here are handy tips to dodge issues:

    Problem: Site still blocks you quickly, not fooled by UA changes

    Solution: Double check your strings match current browser formats exactly. I validate all mine through online parsers regularly to catch any red flags. Also use a wide pool and randomize which UAs get used.

    Problem: Requests fail with odd SSL errors after UA change

    Solution: Some sites may only allow browser connections and blacklist anything that says "curl". Try changing the client name in UA too. For example I've used "Python-urllib" before successfully.

    Problem: Custom UAs work at first but then get blocked after a while

    Solution: Rotate through a pool of strings often to avoid patterns. Also use proxies and throttle requests to stay sneaky. UA changes are just one evasion tactic.

    I always advise having solid monitoring around your scrapers to catch issues quickly when they crop up. Log which custom UAs worked and any suspicious blocking to optimize over time.

    Key Takeaways to Remember

    Changing cURL's user agent to mimic real browsers is crucial for avoiding blocks from sites trying to stop bots and scrapers. To recap some key pointers:

  • Set custom static user agents with A or H flags
  • Store a diverse pool user agent strings mimicking various browsers/devices
  • Refresh strings periodically as new browser versions release
  • Randomly rotate user agents programmatically to avoid patterns
  • Validate strings with online checkers and tweak if formatting looks suspicious
  • Combine UA changes with other evasion tactics like proxies for best results
  • And that's a wrap! Being able to tweak cURL user agents unlocks new possibilities for interfacing with restrictive sites at scale.

    I hope these practical examples give you a solid launch pad to start experimenting without getting immediately blocked. Changing the outward face you show target sites is pivotal for gathering intel under the radar.

    If you found this helpful, be sure to check out my other cURL tutorials on advanced tricks around scraping, automation, and more!

    Happy data hunting :)

    Frequently Asked Questions

    Is cURL a User-Agent?

    Yes, cURL sends its own default static user agent string in requests identifying itself and its version number. This often gets sites blocking it as bot traffic.

    How do I cURL a specific User-Agent?

    Use the -A flag followed by the full user agent string you want cURL to masquerade as. For example:

    curl -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/109.0"

    What is the user cURL command?

    There is no specific "user" command. To set a custom user agent with cURL, use existing flags like -A, --user-agent, -H, etc.

    Who is my User-Agent?

    When you browse websites, your browser automatically sends a default user agent identifying details like browser name and version, operating system, etc. You can see your browser's user agent by searching "what is my user agent".

    Why is cURL used?

    cURL is commonly used to transfer data to/from servers and APIs via the command line. Key reasons to use cURL include testing APIs, automation, downloading files, following redirects, and more.

    What is the default cURL user agent?

    If you don't set a custom user agent, cURL sends a default string identifying itself like:


    The format includes the name "curl" and the installed cURL version number.

    Can I create my own user agent?

    Yes, you can create fully custom user agent strings for cURL using any values you want. However, for scraping purposes, it's best to mimic existing browser user agents to appear more legitimate.

    What is user style agent?

    A user style agent (also user stylesheet) is an extension/add-on used by some browsers to customize page styling. It is not related to the general user agent concept indicating a browser/client identity.

    How do I change user agent?

    To change the cURL user agent, use flags like -A or -H followed by the desired browser user agent string. For browsers like Chrome/Firefox, you can install extensions to customize the default browser user agent.

    What is API curl?

    There is no such thing as "API curl". cURL is a command line tool used to transfer data using various protocols like HTTP, HTTPS, FTP, and more. It enables you to interact with API endpoints directly from your terminal.

    How do I send a curl request?

    To send a basic GET request with cURL, use syntax like:


    To send POST requests, use -X POST and -d to add a request body. There are many other options to customize cURL requests as needed.

    Is it safe to use curl?

    Yes, cURL is generally safe to use as long as you are interacting with trusted sites/servers and properly validating any inputs. As with any powerful tool, you must be careful to use cURL appropriately.

    How is user agent used?

    User agents allow servers to identify key details about the client sending requests such as browser, OS, device type, etc. Sites may use this info to optimize content delivery or block unwanted traffic.

    What is user agent name?

    The user agent name indicates the software/browser that generated the user agent string. For example "Chrome" or "Firefox".

    What is a user agent example?

    Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36

    This shows a Chrome browser user agent on Windows 10.

    What is the full form of cURL?

    cURL stands for "Client URL". It's named curl because it allows transferring data specified using URL syntax.

    Why is cURL called cURL?

    According to the creator Daniel Stenberg, the name originated from the phrase "see URL". This eventually became curled to cURL which resembles the C programming syntax.

    What language is cURL?

    cURL is written in the C programming language and relies on the libcurl C library to do its job.

    Why use curl for API?

    cURL allows manually testing APIs via the command line using simple syntax. It handles formatting requests, encoding data, configuring headers like authentication, and more that you would otherwise need to code.

    How do I save a curl file?

    To save the output content from a cURL request to a file, use syntax like:

    curl -o saved_file.html

    Does curl use HTTP by default?

    Yes, if you don't specify a protocol in the URL, cURL assumes HTTP by default when making requests.

    How do I test a user agent?

    To validate what details a user agent string contains, copy and paste it into a tool like It will parse the browser, OS, and other metadata.

    Is user agent private?

    No, the user agent string is automatically exposed in the headers of all HTTP requests from your browser. Websites and other clients you interact with are able to see its value.

    What are the two types of user agents?

    The two main types are browsers/clients that generate user agent strings, and servers/websites that receive and parse them.

    Can I fake user agent?

    Yes, it is possible to "spoof" user agents by changing them to values that don't match your actual browser or client. However, spoofing user agents to impersonate others may violate terms of service.

    Why is user agent important?

    User agents allow delivering tailored content optimized for each visitor's specific device capabilities, browser, and OS. Withoutawareness of user agent details, serving one-size-fits all content is difficult.

    What is user agent extension?

    A user agent switcher browser extension allows customizing and changing your browser's default user agent string to specified values. They are commonly used by web developers for testing.

    What is user agent spoofing?

    User agent spoofing refers to altering the browser/client user agent from its default value to pose as a different device or browser. This may allow accessing content or features limited to certain user agents.

    What is user agent switcher?

    A user agent switcher is an extension that gives users GUI controls to easily change their browser's default user agent string to values they specify, often to test sites. Popular switchers include User-Agent Switcher for Chrome and User-Agent Switcher for Firefox.

    How do I disable user agent styles?

    In Firefox, you can disable website-specific CSS rules targetting certain user agents by opening about:config and setting ua.generic.override to true. Chrome/Edge have similar settings to reset styles.

    Is curl a REST API?

    No, cURL is a command line tool while REST (Representational State Transfer) is an architectural style for designing APIs. However, cURL enables manually sending REST API requests without writing code.

    What is curl vs Postman?

    cURL operates directly from the terminal while Postman offers a graphical user interface. Both allow crafting API requests, but Postman includes more features like collections, environments, documentation, team collaboration tools, etc.

    What is cURL in testing?

    cURL is very useful in testing scenarios since it allows manually sending requests with different HTTP methods, headers, auth, etc to validate API functionality quickly from the command line.

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