mimic-rs

High-performance User-Agent to Sec-CH-UA Client Hints converter for Rust.

Overview

mimic-rs parses User-Agent strings from Chromium-based browsers and generates the corresponding Sec-CH-UA (Client Hints) headers. This is useful for:

• HTTP clients that need to send proper Client Hints headers
• Browser fingerprinting research
• Web scraping with accurate browser emulation
• Testing servers that rely on Client Hints

Features

• Zero-copy parsing where possible
• Support for Chrome, Edge, Brave, Opera, Vivaldi, Samsung Internet, Yandex, and generic Chromium
• Accurate greased brand generation matching Chromium's algorithm
• Platform detection (Windows, macOS, Linux, Android, Chrome OS)
• Mobile browser detection
• Optional serde support for serialization
• No unsafe code

Installation

Add to your Cargo.toml:

[dependencies]
mimic-rs = "0.1"

With serde support:

[dependencies]
mimic-rs = { version = "0.1", features = ["serde"] }

Usage

Basic Usage

use mimic_rs::ClientHints;

fn main() {
    let user_agent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 \
                      (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36";

    let hints = ClientHints::from_ua(user_agent).unwrap();

    println!("sec-ch-ua: {}", hints.sec_ch_ua());
    println!("sec-ch-ua-mobile: {}", hints.sec_ch_ua_mobile());
    println!("sec-ch-ua-platform: {}", hints.sec_ch_ua_platform());
}

Output:

sec-ch-ua: "Not A(Brand";v="8", "Chromium";v="120", "Google Chrome";v="120"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"

Manual Construction

use mimic_rs::{ClientHints, Brand, Platform};

let hints = ClientHints::new(
    Brand::Chrome,
    120,
    "120.0.6099.129",
    Platform::Windows,
    false,
);

println!("sec-ch-ua: {}", hints.sec_ch_ua());
println!("sec-ch-ua-full-version-list: {}", hints.sec_ch_ua_full_version_list());

Getting All Headers at Once

use mimic_rs::ClientHints;

let ua = "Mozilla/5.0 (Linux; Android 13) Chrome/120.0.0.0 Mobile Safari/537.36";
let hints = ClientHints::from_ua(ua).unwrap();

let (sec_ch_ua, sec_ch_ua_mobile, sec_ch_ua_platform) = hints.all_headers();

Supported Browsers

Supported Platforms

• Windows
• macOS
• Linux
• Android
• Chrome OS

API Reference

ClientHints

Main struct containing parsed browser information.

Methods

• from_ua(user_agent: &str) -> Result<ClientHints, ParseError> - Parse a User-Agent string
• new(brand, major_version, full_version, platform, is_mobile) - Create manually
• brand() -> Brand - Get the detected browser brand
• major_version() -> u32 - Get the major version number
• full_version() -> &str - Get the full version string
• platform() -> Platform - Get the detected platform
• is_mobile() -> bool - Check if mobile browser
• sec_ch_ua() -> String - Generate sec-ch-ua header value
• sec_ch_ua_full_version_list() -> String - Generate sec-ch-ua-full-version-list header
• sec_ch_ua_mobile() -> &'static str - Generate sec-ch-ua-mobile header (?0 or ?1)
• sec_ch_ua_platform() -> String - Generate sec-ch-ua-platform header
• all_headers() -> (String, &'static str, String) - Get all three main headers

Brand

Enum representing browser brands:

• Chrome, Edge, Brave, Opera, Vivaldi, Samsung, Yandex, Chromium

Platform

Enum representing operating systems:

• Windows, MacOS, Linux, Android, ChromeOS, Unknown

ParseError

Error types:

• EmptyUserAgent - Empty input string
• NotChromiumBased - Not a Chromium browser
• InvalidVersion - Could not parse version
• ChromeTokenNotFound - No Chrome/Chromium token found

Performance

The library is optimized for speed:

• No regex dependencies
• Minimal allocations
• Preallocated string buffers
• #[inline] on hot paths

Run benchmarks:

cargo bench

How Greased Brands Work

Chromium browsers include a "greased" fake brand in the sec-ch-ua header to prevent servers from relying on specific patterns. The greased brand changes based on the major version:

• Characters cycle through: , (, :, -, .
• Versions cycle through: 8, 99, 24
• Order of brands is permuted based on version

This library accurately replicates Chromium's greasing algorithm.

License

MIT License - see LICENSE for details.

Acknowledgments

Inspired by mimic (Go).