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.

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
| Layer | What you can control |
|---|---|
| TLS | Cipher suites, extensions, their order (JA3 / JA4) via uTLS presets or a fully custom custom_hellospec |
| HTTP/1.1 | Header order, User-Agent, add/remove any header, IP spoofing ( X-Forwarded-For/True-Client-IP) |
| HTTP/2 | SETTINGS values & order, WINDOW_UPDATE, pseudo-header order (HTTP/2 fingerprint) |

macOSor- Linux(amd64 / arm64)
Go 1.22+

brew install goThe distro-packaged Go is often outdated.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.):replacelinux-amd64withlinux-arm64in the download URL.

git clone https://github.com/ytkoka/impersonate-proxy.git cd impersonate-proxy make build
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 withCtrl-C`.

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

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]
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:

| Endpoint | Method | Description |
|---|---|---|
| /api/config | GET | Return active settings as JSON, including the current custom_hello |
| /api/config | POST | Update TLS preset (including a fully custom custom_hello), client IP, and User-Agent |

```

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. Setmgmt_listen: ""` to disable the API entirely.

| Browser | TLS preset | HTTP/2 SETTINGS | WINDOW_UPDATE |
|---|---|---|---|
| Chrome | chrome | 1:65536,2:0,4:6291456,6:262144 | 15663105 |
| Firefox | firefox | 1:65536,4:131072,5:16384 | 12517377 |
| Safari | safari | 1:4096,3:100,4:2097152,6:16384 | 10485760 |

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

| Fingerprint component | Config field | Notes |
|---|---|---|
| TLS version range | versions | Min/max are derived automatically |
| Cipher suite list + order | cipher_suites | Use 0x0a0aas a GREASE placeholder; uTLS randomises it per connection |
| Extension type IDs + order | extensions | Order directly controls the JA3 extensions component; values matching the GREASE pattern ( 0xXAXA) are randomised per connection |
| Supported groups (curves) | curves | Also controls which key shares are sent |

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

| ID | Name | Notes |
|---|---|---|
| 0xXAXA(any GREASE pattern) | GREASE | Randomised per connection |
| 0 | server_name (SNI) | |
| 5 | status_request | OCSP stapling |
| 10 | supported_groups | Uses the curveslist |
| 11 | ec_point_formats | Fixed: uncompressed (0) |
| 13 | signature_algorithms | Chrome-like defaults |
| 16 | ALPN | Advertises h2,http/1.1 |
| 18 | signed_certificate_timestamp | |
| 21 | padding | BoringSSL-style padding |
| 23 | extended_master_secret | |
| 27 | compress_certificate | |
| 28 | record_size_limit | Fixed: 0x4001 |
| 35 | session_ticket | |
| 43 | supported_versions | Uses the versionslist |
| 45 | psk_key_exchange_modes | PSK with DHE |
| 50 | signature_algorithms_cert | Chrome-like defaults |
| 51 | key_share | Key shares for X25519 and P256 (from curves) |
| 17513 | application_settings (ALPS) | Advertises h2 |
| 65281 | renegotiation_info | |
| other | GenericExtension | Sent with empty payload |

Runtime updates:preset: "custom"is not limited toconfig.yaml— it can also be switched to at runtime via the management API (POST /api/configwith acustom_helloobject, see Management API) or from the Chrome extension's TLS Preset dropdown, without restarting the proxy.

```
make run

Kills any previous instance on port 8080, rebuilds, and starts.

`` To switch fingerprint profiles, editconfig.yamland re-runmake run`.

```

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
`` Thechrome-extension/` directory contains a Manifest V3 extension that controls the proxy from the browser toolbar.

Installation:

Open chrome://extensionsin Chrome
Enable Developer mode(toggle in the top-right corner)
Click Load unpackedand select thechrome-extension/folder

Controls:

| Control | What it does |
|---|---|
| Proxy toggle | Enables / disables Chrome's proxy setting (routes traffic through :8080) |
| TLS Preset | Switches the uTLS fingerprint preset (chrome / firefox / safari / edge / ios / random / golang / custom) |
| Cipher Suites / Curves / TLS Versions / Extensions | Shown when Custom (JA3/JA4)is selected — the same fields ascustom_helloinconfig.yaml, letting you dial in an arbitrary JA3/JA4 fingerprint without editing YAML or restarting the proxy |
| Client IP | Sets X-Forwarded-ForandTrue-Client-IPon every request |
| User-Agent | Overrides the HTTP User-Agentheader |
| Apply button | POSTs the new settings to the management API; takes effect immediately |
| API field | Address of the management API (default http://127.0.0.1:8081) |

User-Agent scope:The extension changes the HTTPUser-Agentheaderonly. JavaScript'snavigator.userAgentis controlled by Chrome itself and is not affected. To spoof both simultaneously, launch Chrome with--user-agent="..."alongside the proxy settings.

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');
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.

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:

| Field | Description |
|---|---|
| tls.ja3_hash | JA3 fingerprint hash |
| tls.ja4 | JA4 fingerprint string |
| http2.akamai_fingerprint | HTTP/2 fingerprint string (SETTINGS + WINDOW_UPDATE + pseudo-header order) — field name is defined by the tls.peet.ws API |
| http1.headers | Header names in the order received by the server |
| user_agent | User-Agent as seen by the server |
| ip | Source IP as seen by the server — verify X-Forwarded-For/True-Client-IPspoofing here |

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
| Target | Description |
|---|---|
| make build | Compile the binary |
| make run | Build, kill any existing instance, and start |
| make trust-ca | Add ca.crtto the macOS system keychain (requires sudo) |
| make clean | Remove the binary, ca.crt, andca.key |

Remove the binary and generated CA files:

make cleanIf 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- 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: chunkedbodies are not currently supported.
- No QUIC / HTTP/3: Out of scope.
- User-Agent (HTTP header only): The proxy rewrites the- User-AgentHTTP header, but JavaScript's- navigator.userAgentis set by the browser independently and is unaffected. Use Chrome's- --user-agentlaunch flag to override both simultaneously.

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.

uTLS — TLS fingerprint customization
tls.peet.ws — Fingerprint inspection API used in examples
JA4+ — Fingerprinting standard reference

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