Show HN: A local MitM proxy to control TLS fingerprints

impersonate-proxy A local MITM proxy that lets you control TLS fingerprints (JA3/JA4), HTTP/2 fingerprints, HTTP header order, User-Agent, and source IP headers — all from a single YAML config file. A Chrome extension is included for toggling the proxy and switching fingerprint profiles directly from the browser toolbar without restarting the proxy. Intended for authorized security testing of WAF bot-detection systems. Route curl, browsers, or Playwright through the proxy to observe how different fingerprint combinations are classified. How it works curl / browser / Playwright │ HTTP CONNECT (to proxy) ▼ ┌─────────────────────────────────────────┐ │ impersonate-proxy │ │ │ │ MITM TLS ◄──────────────► uTLS │ │ (our CA cert) (custom JA3/4) │ │ │ │ Header rewriter (UA, order, add/del) │ │ HTTP/2 framer (SETTINGS, WINDOW_UPDATE│ │ pseudo-header order) │ └─────────────────────────────────────────┘ │ Custom TLS ClientHello + HTTP/2 ▼ Target server / WAF Prerequisites macOS or Linux (amd64 / arm64) Go 1.22+ macOS brew install go Linux The distro-packaged Go is often outdated. Install the official binary directly: # Download and extract (replace 1.22.5 with the latest from https://go.dev/dl/) curl -OL https://go.dev/dl/go1.22.5.linux-amd64.tar.gz sudo rm -rf /usr/local/go sudo tar -C /usr/local -xzf go1.22.5.linux-amd64.tar.gz # Add to PATH (add this line to ~/.bashrc or ~/.zshrc to make it permanent) export PATH=$PATH:/usr/local/go/bin Verify: go version # go version go1.22.5 linux/amd64 ARM64 (Raspberry Pi, AWS Graviton, etc.): replace linux-amd64 with linux-arm64 in the download URL. ARM64 (Raspberry Pi, AWS Graviton, etc.): replace linux-amd64 with linux-arm64 in the download URL. Setup 1. Clone and build git clone https://github.com/ytkoka/impersonate-proxy.git cd impersonate-proxy make build 2. Generate the MITM CA certificate The CA is generated automatically on first run. Start the proxy once to create ca.crt and ca.key: make run # 2026/04/22 12:00:00 generated CA certificate → ca.crt # 2026/04/22 12:00:00 listening on 127.0.0.1:8080 preset=chrome Stop it with Ctrl-C. 3. Trust the CA certificate Clients need to trust your MITM CA so they don't reject the proxy-generated leaf certificates. macOS system keychain (affects all apps): make trust-ca # runs: sudo security add-trusted-cert ... Linux system trust (affects all apps; requires ca-certificates package): # Debian / Ubuntu sudo cp ca.crt /usr/local/share/ca-certificates/impersonate-proxy.crt sudo update-ca-certificates # RHEL / Fedora / Amazon Linux sudo cp ca.crt /etc/pki/ca-trust/source/anchors/impersonate-proxy.crt sudo update-ca-trust curl only (no system-wide change): curl --cacert ca.crt ... Playwright / Node.js: export NODE_EXTRA_CA_CERTS="$(pwd)/ca.crt" Firefox: Preferences → Privacy & Security → View Certificates → Authorities → Import ca.crt Configuration Edit config.yaml before starting the proxy. All fields have defaults — you only need to specify what you want to override. listen: "127.0.0.1:8080" mgmt_listen: "127.0.0.1:8081" # management API used by the Chrome extension (empty to disable) ca_cert: "ca.crt" ca_key: "ca.key" tls: # TLS fingerprint preset (controls JA3 / JA4) # Options: chrome | firefox | safari | edge | ios | random | golang preset: "chrome" http: # Override User-Agent (leave empty to pass through the client's UA) user_agent: "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36" # Spoof source IP: sets both X-Forwarded-For and True-Client-IP to this value, # replacing any values the client may have already set (leave empty to disable) # client_ip: "1.2.3.4" # Emit headers in this order; headers not listed are appended after header_order: - "Host" - "User-Agent" - "Accept" - "Accept-Language" - "Accept-Encoding" - "Connection" # Add or overwrite headers add_headers: Accept-Language: "ja,en-US;q=0.9,en;q=0.8" # Remove headers before forwarding remove_headers: [] http2: enabled: true # SETTINGS frame entries — id and order both affect the HTTP/2 fingerprint. # RFC 7540 §11.3 IDs: # 1=HEADER_TABLE_SIZE 2=ENABLE_PUSH 3=MAX_CONCURRENT_STREAMS # 4=INITIAL_WINDOW_SIZE 5=MAX_FRAME_SIZE 6=MAX_HEADER_LIST_SIZE settings: - { id: 1, val: 65536 } # Chrome defaults shown here - { id: 2, val: 0 } - { id: 4, val: 6291456 } - { id: 6, val: 262144 } # Connection-level WINDOW_UPDATE increment window_update: 15663105 # Order of pseudo-headers in the HEADERS frame pseudo_header_order: [method, authority, scheme, path] Management API When the proxy starts it also exposes a lightweight HTTP API on mgmt_listen (default 127.0.0.1:8081). The Chrome extension uses this to read and update settings at runtime without restarting the proxy. You can also call it directly with curl: # Read current settings curl http://127.0.0.1:8081/api/config # Switch to Firefox fingerprint and set a spoofed IP curl -s -X POST http://127.0.0.1:8081/api/config \ -H "Content-Type: application/json" \ -d '{"tls_preset":"firefox","client_ip":"203.0.113.1","user_agent":""}' # Switch to an arbitrary JA3/JA4 fingerprint at runtime — same fields as the # config.yaml custom_hello block, sent as JSON (see "Custom TLS fingerprint" below) curl -s -X POST http://127.0.0.1:8081/api/config \ -H "Content-Type: application/json" \ -d '{ "tls_preset": "custom", "custom_hello": { "cipher_suites": [2570, 4865, 4866, 4867, 49195, 49199, 49196, 49200, 52393, 52392, 49171, 49172, 156, 157, 47, 53], "curves": ["X25519", "P256", "P384"], "versions": ["1.3", "1.2"], "extensions": [2570, 0, 23, 65281, 10, 11, 35, 16, 5, 18, 13, 51, 45, 43, 27, 21] }, "client_ip": "", "user_agent": "" }' Changes take effect immediately for new connections. Set mgmt_listen: "" to disable the API entirely. Browser fingerprint reference Custom TLS fingerprint (preset: "custom") The built-in presets (chrome, firefox, safari, …) cover the most common cases. When you need to match a specific browser version or a fingerprint that differs from those presets, set preset: "custom" and provide a custom_hello block. How JA3 / JA4 map to config fields JA3 and JA4 are one-way hashes — you cannot reverse a hash back to a spec. Find the underlying parameters for the target browser with tls.peet.ws or Wireshark, then paste them into custom_hello. JA3 and JA4 are one-way hashes — you cannot reverse a hash back to a spec. Find the underlying parameters for the target browser with tls.peet.ws or Wireshark, then paste them into custom_hello. Chrome 131 example tls: preset: "custom" custom_hello: cipher_suites: # hex IDs; 0x0a0a = GREASE placeholder (randomised per connection) - 0x0a0a - 0x1301 # TLS_AES_128_GCM_SHA256 - 0x1302 # TLS_AES_256_GCM_SHA384 - 0x1303 # TLS_CHACHA20_POLY1305_SHA256 - 0xc02b # ECDHE-ECDSA-AES128-GCM-SHA256 - 0xc02f # ECDHE-RSA-AES128-GCM-SHA256 - 0xc02c # ECDHE-ECDSA-AES256-GCM-SHA384 - 0xc030 # ECDHE-RSA-AES256-GCM-SHA384 - 0xcca9 # ECDHE-ECDSA-CHACHA20-POLY1305 - 0xcca8 # ECDHE-RSA-CHACHA20-POLY1305 - 0xc013 # ECDHE-RSA-AES128-SHA - 0xc014 # ECDHE-RSA-AES256-SHA - 0x009c # RSA-AES128-GCM-SHA256 - 0x009d # RSA-AES256-GCM-SHA384 - 0x002f # RSA-AES128-SHA - 0x0035 # RSA-AES256-SHA curves: # X25519 | X25519Kyber768 | P256 | P384 | P521 - "X25519Kyber768" - "X25519" - "P256" versions: # TLS versions to advertise - "1.3" - "1.2" extensions: # extension type IDs in order (controls JA3 extensions component) - 0x0a0a # GREASE - 0 # server_name (SNI) - 23 # extended_master_secret - 65281 # renegotiation_info - 10 # supported_groups - 11 # ec_point_formats - 35 # session_ticket - 16 # ALPN - 5 # status_request - 18 # signed_certificate_timestamp - 13 # signature_algorithms - 51 # key_share - 45 # psk_key_exchange_modes - 43 # supported_versions - 27 # compress_certificate - 17513 # application_settings (ALPS) - 0x0a0a # GREASE - 21 # padding Supported extension type IDs Runtime updates: preset: "custom" is not limited to config.yaml — it can also be switched to at runtime via the management API (POST /api/config with a custom_hello object, see Management API) or from the Chrome extension's TLS Preset dropdown, without restarting the proxy. Runtime updates: preset: "custom" is not limited to config.yaml — it can also be switched to at runtime via the management API (POST /api/config with a custom_hello object, see Management API) or from the Chrome extension's TLS Preset dropdown, without restarting the proxy. Usage Start the proxy make run # Kills any previous instance on port 8080, rebuilds, and starts. To switch fingerprint profiles, edit config.yaml and re-run make run. curl # With CA trusted system-wide (after make trust-ca): curl --proxy http://127.0.0.1:8080 https://tls.peet.ws/api/all # Without system trust — pass CA explicitly: curl --proxy http://127.0.0.1:8080 --cacert ca.crt https://tls.peet.ws/api/all Chrome extension The chrome-extension/ directory contains a Manifest V3 extension that controls the proxy from the browser toolbar. Installation: Open chrome://extensions in Chrome Enable Developer mode (toggle in the top-right corner) Click Load unpacked and select the chrome-extension/ folder Controls: User-Agent scope: The extension changes the HTTP User-Agent header only. JavaScript's navigator.userAgent is controlled by Chrome itself and is not affected. To spoof both simultaneously, launch Chrome with --user-agent="..." alongside the proxy settings. User-Agent scope: The extension changes the HTTP User-Agent header only. JavaScript's navigator.userAgent is controlled by Chrome itself and is not affected. To spoof both simultaneously, launch Chrome with --user-agent="..." alongside the proxy settings. Playwright (Node.js) const { chromium } = require('playwright'); const browser = await chromium.launch(); const context = await browser.newContext({ proxy: { server: 'http://127.0.0.1:8080' }, }); // If CA is not in the system keychain, set before launching: // NODE_EXTRA_CA_CERTS=./ca.crt node script.js const page = await context.newPage(); await page.goto('https://tls.peet.ws/api/all'); Playwright (Python) from playwright.sync_api import sync_playwright with sync_playwright() as p: browser = p.chromium.launch() context = browser.new_context(proxy={"server": "http://127.0.0.1:8080"}) page = context.new_page() page.goto("https://tls.peet.ws/api/all") Set NODE_EXTRA_CA_CERTS (Node) or REQUESTS_CA_BUNDLE (Python) if the CA is not trusted system-wide. Verifying fingerprints tls.peet.ws returns the full fingerprint breakdown for any request it receives. Pipe the output through jq or Python for a readable view: curl -s --proxy http://127.0.0.1:8080 --cacert ca.crt \ https://tls.peet.ws/api/all | python3 -m json.tool Key fields to check: Project structure impersonate-proxy/ ├── main.go # Entry point ├── config/config.go # YAML config struct and defaults ├── fp/dialer.go # uTLS dialer — TLS fingerprint presets ├── h2fp/conn.go # HTTP/2 framer — SETTINGS / WINDOW_UPDATE / pseudo-header control ├── mitm/ca.go # MITM CA: generate, cache, and serve leaf certs ├── proxy/proxy.go # Proxy server: CONNECT handling, protocol branch, runtime config ├── rewrite/headers.go # HTTP header rewriting (UA, order, add/remove, IP spoof) ├── mgmt/server.go # Management HTTP API (/api/config GET + POST) ├── chrome-extension/ │ ├── manifest.json # Manifest V3 │ ├── popup.html # Toolbar popup UI │ ├── popup.css │ ├── popup.js # Proxy toggle + management API client │ └── icon.svg ├── config.yaml # Default configuration └── Makefile Makefile targets Cleanup Remove the binary and generated CA files: make clean If you added the CA to the macOS system keychain, remove it through Keychain Access (search for "impersonate-proxy CA") or: sudo security delete-certificate -c "impersonate-proxy CA" /Library/Keychains/System.keychain Limitations MITM only: The proxy decrypts and re-encrypts traffic. Clients must trust the generated CA. No HTTP/2 from client: The client→proxy leg uses HTTP/1.1 (via CONNECT). Only the proxy→server leg uses HTTP/2 with custom fingerprints. Chunked request bodies: Requests with Transfer-Encoding: chunked bodies are not currently supported. No QUIC / HTTP/3: Out of scope. User-Agent (HTTP header only): The proxy rewrites the User-Agent HTTP header, but JavaScript's navigator.userAgent is set by the browser independently and is unaffected. Use Chrome's --user-agent launch flag to override both simultaneously. Legal notice This tool is intended for authorized security testing only — for example, testing WAF and bot-detection configurations on systems you own or have explicit written permission to test. Using this tool against systems without authorization may violate applicable laws (such as the Computer Fraud and Abuse Act, Japan's Unauthorized Computer Access Law, or equivalent legislation in your jurisdiction) and the terms of service of the target. The authors accept no liability for misuse. Acknowledgements uTLS — TLS fingerprint customization tls.peet.ws — Fingerprint inspection API used in examples JA4+ — Fingerprinting standard reference