protocol

User Agent

An HTTP header string identifying the browser, version, and operating system to the web server, checked by anti-bot systems against other request properties.

The User-Agent is an HTTP request header string that identifies the client software making the request — browser name, version, rendering engine, and operating system. A typical Chrome browser sends a User-Agent like `Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36`. Python's `requests` library sends `python-requests/2.31.0` by default.

Anti-bot systems use the User-Agent as one signal in a multi-factor analysis. Setting a realistic Chrome User-Agent is necessary but not sufficient — the User-Agent is cross-referenced against TLS fingerprint (does the cipher suite match this browser version?), HTTP/2 settings, and browser fingerprint properties (does `navigator.userAgent` match the HTTP header?). Inconsistencies across these layers are high-confidence bot signals.

AlterLab automatically sets and maintains consistent User-Agent strings across all request layers — HTTP header, `navigator.userAgent` JavaScript property, and `sec-ch-ua` client hint headers — matching the combination to the Chromium version in use. Developers do not need to manage User-Agent strings manually.

Related Terms

TLS Fingerprint
A signature derived from TLS handshake parameters a client sends when opening an HTTPS connection, used by anti-bot systems to classify traffic.
Browser Fingerprinting
A technique anti-bot systems use to identify bots by collecting dozens of browser attributes including Canvas, WebGL, and timing signatures.
Anti-Bot Protection
Systems deployed by websites to detect and block automated traffic using TLS fingerprinting, CAPTCHA challenges, and behavioural analysis.
HTTP/2 Fingerprint
A network-level identifier from HTTP/2 handshake parameters including SETTINGS frame values and header ordering, used alongside TLS fingerprinting.
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