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Proxy glossary

What is a TLS fingerprint?

A TLS fingerprint is a short signature of the options a client offers in its TLS ClientHello, which sites use to tell HTTP clients and browsers apart.

TLS fingerprint explained

Also called JA3 or JA4.

Every HTTPS connection opens with a ClientHello listing the TLS versions, cipher suites, extensions and curves the client supports, in a particular order. Browsers, curl, Python and Go each build that message differently. JA3 hashes five of those fields into a 32-character string; JA4 is a newer format that sorts some fields so the result is steadier.

A site or its CDN can compare the fingerprint with the User-Agent header. A request that claims to be Chrome but has the ClientHello of a Python library is easy to spot, before any page is served and without looking at the IP.

A proxy does not change any of this. Through a CONNECT tunnel your client performs the TLS handshake directly with the site, so the ClientHello leaves your machine and arrives unchanged. Only the source IP differs. Changing exit IP therefore fixes IP reputation problems and does nothing for fingerprint ones.

Fingerprints identify a kind of client, not a person. Thousands of ordinary users share the same Chrome fingerprint. What stands out is a mismatch between the fingerprint and the rest of the request.

For example

Your script gets a 403 from a shop through a residential proxy, but the same URL loads in your browser through the same proxy. The IP is not the difference. The site is reading the ClientHello, and the browser and the script send different ones.

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