Question 1

What is a user agent string and what information does it contain?

Accepted Answer

User agent (UA) string is HTTP header sent by browsers/apps identifying themselves to servers. Format: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 Chrome/120.0.0.0". Contains: browser name and version (Chrome 120), rendering engine (AppleWebKit, Gecko, Trident), operating system (Windows 10, macOS, iOS, Android), device type (desktop, mobile, tablet), sometimes: language, architecture (64-bit), brand (Samsung, Apple). Used for: analytics, feature detection, mobile optimization, bot detection, browser support warnings. Note: UA strings can be spoofed - not fully reliable for security. This tool parses UA strings into structured, readable information.

Question 2

Why do all browsers start with "Mozilla/5.0" in their user agent?

Accepted Answer

Historical quirk from browser wars. Netscape Navigator called itself "Mozilla" (Mosaic Killer). Early websites checked "if contains Mozilla" to serve advanced features. Internet Explorer wanted those features, added "Mozilla/4.0 (compatible; MSIE)" to UA string. Other browsers followed to avoid being blocked. Result: all modern browsers claim "Mozilla/5.0" for compatibility even though Netscape is long dead. Shows browser evolution: Mozilla/5.0 (compatibility) → actual browser (Chrome, Safari, Firefox) → rendering engine (WebKit, Gecko). Legacy from 1990s web, persists for backward compatibility. Ignore "Mozilla/5.0", focus on actual browser identifier. This tool extracts real browser name despite Mozilla prefix.

Question 3

How do I detect mobile devices using user agent strings?

Accepted Answer

Check for mobile indicators in UA: "Mobile", "Android", "iPhone", "iPad", "iPod", specific devices (Samsung, Huawei). Desktop: "Windows NT", "Macintosh", "Linux x86_64". Tablets: "iPad", "Android" without "Mobile". Example mobile: "Mozilla/5.0 (iPhone; CPU iPhone OS 16_0 like Mac OS X) ... Mobile/15E148". Better approach: use JavaScript: navigator.userAgentData (modern), window.matchMedia("(max-width: 768px)") (responsive), navigator.maxTouchPoints > 0 (touch device). Challenges: UA strings unreliable (spoofed, fragmented), new devices constantly added, tablets sometimes report as desktop. This tool detects device type from UA with fallback indicators for edge cases.

Question 4

What are the differences between browser rendering engines (WebKit, Gecko, Blink)?

Accepted Answer

Rendering engine converts HTML/CSS/JS to visual page. WebKit: used by Safari, older Chrome, many mobile browsers. Open source (Apple). Blink: Google's fork of WebKit (2013), used by Chrome, Edge, Opera, Brave. Most popular. Gecko: Firefox and Mozilla products. Independent codebase. Trident: legacy Internet Explorer (deprecated). EdgeHTML: legacy Edge (deprecated). Differences affect: CSS rendering (subtle layout differences), JavaScript performance, supported web features. For developers: test across engines, feature detection over browser detection, use caniuse.com for compatibility. UA strings show engine: "AppleWebKit/537.36" or "Gecko/20100101". Modern sites use feature detection, not engine sniffing. This tool identifies rendering engine from UA string.

Question 5

How do I identify bots and crawlers from user agent strings?

Accepted Answer

Bots identify themselves in UA strings: Googlebot: "Googlebot/2.1", Bingbot: "bingbot/2.0", other crawlers: "Slurp", "DuckDuckBot", "Baiduspider", "ia_archiver" (Internet Archive). Social bots: "facebookexternalhit", "Twitterbot", "LinkedInBot". Monitoring: "Pingdom", "UptimeRobot". Good bots identify honestly. Bad bots: spoof browser UA to avoid detection, high request rate, ignore robots.txt. Detection strategies: check UA for known bot patterns, verify with reverse DNS (Googlebot IPs), rate limiting, CAPTCHA for suspicious patterns, analyze behavior (no JS execution, no image loading). Whitelist known good bots, block/throttle suspicious ones. This tool highlights bot identifiers in UA strings and provides bot detection patterns.

Question 6

Should I use user agent parsing for browser feature detection?

Accepted Answer

No - use feature detection instead of UA sniffing. Problems with UA: unreliable (can be spoofed), fragmented (thousands of variations), outdated quickly (new browsers, versions), doesn't detect actual capabilities. Better approach: JavaScript feature detection: "if ('geolocation' in navigator)", Modernizr library, @supports in CSS, progressive enhancement. Example: instead of "if UA contains Chrome", use "if ('IntersectionObserver' in window)". UA useful for: analytics (understanding user demographics), debugging (reproducing issues), serving different assets (mobile vs desktop HTML), blocking known bad bots. Use UA for stats, not for functionality decisions. This tool parses UA for analysis, not as replacement for feature detection.

Question 7

How do I handle User-Agent Client Hints in modern browsers?

Accepted Answer

User-Agent Client Hints (UA-CH) is modern replacement for UA strings. Instead of full string, server requests specific hints via headers: Sec-CH-UA (browser), Sec-CH-UA-Platform (OS), Sec-CH-UA-Mobile (mobile flag). Benefits: privacy (less fingerprinting), opt-in (server requests what it needs), structured (not parsing strings). Request hints: Accept-CH: Sec-CH-UA-Platform-Version (server header). JavaScript: navigator.userAgentData.getHighEntropyValues(["platform"]). Adoption: Chrome/Edge support (2021+), Firefox/Safari gradual adoption, UA string deprecated but remains for legacy. Migration: support both UA-CH and traditional UA parsing during transition. This tool parses traditional UA strings with note about UA-CH migration.

Question 8

What are common user agent spoofing techniques and why do they happen?

Accepted Answer

Spoofing = sending fake UA string. Reasons: privacy (avoid tracking), bypass restrictions (access desktop site on mobile), scraping (avoid bot detection), automated testing, emulation. Techniques: browser developer tools (change UA), browser extensions, curl/wget --user-agent flag, headless browsers (Puppeteer, Selenium). Detection: behavior analysis (bot-like patterns), canvas fingerprinting, TLS fingerprinting, JavaScript challenges, honeypot fields. Privacy tools: Tor Browser (standardized UA), Brave (reduced fingerprinting), privacy extensions randomize UA. For websites: don't rely solely on UA for security, combine with other signals, respect privacy settings. This tool shows what UA string reveals, highlighting privacy implications.

User Agent Parser

Example User-Agents

Desktop Browsers

Mobile Browsers

Bots & Crawlers

Development Tools

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References & Citations

Key Security Terms

Browser Fingerprinting

Frequently Asked Questions