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What is TLS fingerprinting?

TLS fingerprinting is a technique used by servers to identify the client making a connection by analyzing the unique characteristics of the TLS handshake. While changing the User-Agent header might have been enough in the past, modern servers use much more sophisticated detection methods.
Even if two requests have identical HTTP headers, they can be distinguished by their TLS handshake parameters, making traditional HTTP-only spoofing ineffective.

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How TLS fingerprinting works

When a client initiates a TLS connection, it sends a ClientHello message containing:
  • Cipher suites - The encryption algorithms the client supports
  • TLS extensions - Additional capabilities and features
  • Extension order - The sequence in which extensions are sent
  • Elliptic curves - Supported curves for key exchange
  • Signature algorithms - Methods for certificate verification
  • Compression methods - Data compression preferences
Servers can analyze these parameters to create a unique “fingerprint” that identifies the client software.
Real browsers like Chrome, Firefox, and Safari each have distinct TLS fingerprints based on their underlying TLS libraries and version-specific implementations.

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Example: JA3 fingerprinting

One common TLS fingerprinting method is JA3, which creates a hash from: 
SSL Version, Accepted Ciphers, List of Extensions, Elliptic Curves, Elliptic Curve Formats
For example, a Chrome browser might produce: 
771,4865-4866-4867-49195-49199-49196-49200-52393-52392-49171-49172-156-157-47-53,0-23-65281-10-11-35-16-5-13-18-51-45-43-27-17513,29-23-24,0
This string is then MD5 hashed to produce the JA3 fingerprint.

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Why TLS fingerprinting matters

Servers use TLS fingerprinting to:
  1. Detect bots - Identify automated clients that don’t match real browser fingerprints
  2. Prevent scraping - Block requests from non-browser clients
  3. Enforce policies - Require specific client versions or security features
  4. Track users - Create persistent identifiers across sessions
Standard HTTP libraries like Go’s net/http, Python’s requests, or Node’s https module all have distinctive TLS fingerprints that are easily detected and blocked by anti-bot systems.

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How this library solves the problem

The TLS Client library uses uTLS to mimic real browser TLS fingerprints. Instead of using Go’s standard TLS implementation, it allows you to:
  • Specify browser profiles - Mimic Chrome, Firefox, Safari, and other browsers
  • Match exact versions - Use fingerprints from specific browser versions
  • Include all details - Replicate cipher suites, extensions, curves, and their ordering

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Example comparison

Standard Go client: 
import "net/http"

client := &http.Client{}
resp, err := client.Get("https://example.com")
// TLS fingerprint: Easily identified as Go's crypto/tls
TLS Client with browser profile: 
import (
    tls_client "github.com/bogdanfinn/tls-client"
    "github.com/bogdanfinn/tls-client/profiles"
)

client, _ := tls_client.NewHttpClient(tls_client.NewNoopLogger(),
    tls_client.WithClientProfile(profiles.Chrome_144),
)
resp, err := client.Get("https://example.com")
// TLS fingerprint: Indistinguishable from Chrome 144

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Testing your fingerprint

You can verify your TLS fingerprint using these services: 
resp, err := client.Get("https://tls.peet.ws/api/all")
if err != nil {
    log.Fatal(err)
}

body, _ := io.ReadAll(resp.Body)
fmt.Println(string(body))
// Shows your TLS fingerprint details and detected client
Use these testing services during development to ensure your client profile correctly mimics the intended browser.

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Beyond TLS fingerprinting

While TLS fingerprinting is powerful, modern anti-bot systems also analyze:
  • HTTP/2 fingerprints - SETTINGS frames, stream priorities, header ordering
  • HTTP/3 fingerprints - QUIC settings, frame types, connection parameters
  • Header consistency - Matching headers to known browser patterns
  • Behavioral patterns - Request timing, navigation flows, JavaScript execution
This library addresses TLS, HTTP/2, and HTTP/3 fingerprinting through its client profile system.

Client profiles

Learn about browser profile system

HTTP protocols

Understand HTTP/1.1, HTTP/2, HTTP/3 support

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