HTTP Intercepting Proxy (httpfromtcp)

A lightweight HTTP/1.1 intercepting proxy built on raw TCP using the httpfromtcp library. It accepts incoming HTTP requests, forwards them to an upstream server, logs request/response data, applies simple modification rules, and can replay captured requests.

What it does

• Acts as a forward HTTP proxy (client -> proxy -> upstream).
• Logs method, path, status, latency, and headers.
• Optionally logs request/response bodies (size-limited).
• Applies request/response rules from config (inject headers, rewrite paths, drop requests, override response headers/status).
• Stores requests to a JSONL file and replays them later.

Quick start

1. Configure your proxy in config.yaml.
2. Run the proxy:

go run cmd/proxy/main.go --config config.yaml

3. Send traffic through it:

curl -x http://localhost:8080 http://example.com/api/users

Replay captured requests

If store_requests_path is set, the proxy writes requests as JSONL. You can replay them later:

go run cmd/replay/main.go --config config.yaml

Or specify a file directly:

go run cmd/replay/main.go --file requests.jsonl --upstream example.com:80

Configuration

config.yaml example:

listen_addr: ":8080"
upstream_host: ""
log_bodies: true
max_body_bytes: 1024
json_logs: false
store_requests_path: "requests.jsonl"
upstream_timeout_sec: 10
rules:
  - match: "/api/users"
    inject_header: "X-Debug: true"
  - match: "/health"
    drop: true

Config options

• listen_addr: Address the proxy listens on (default :8080).
• upstream_host: Optional fixed upstream host (host:port). If empty, uses the incoming Host header or absolute URL.
• log_bodies: If true, logs request/response bodies.
• max_body_bytes: Max bytes to log from each body.
• json_logs: If true, logs JSON lines; otherwise pretty text.
• store_requests_path: JSONL file path for captured requests.
• upstream_timeout_sec: Timeout for reading upstream responses.
• rules: List of match rules applied to requests and responses.

Rule fields

• match: Path prefix to match (for example /api/users).
• inject_header: Adds a request header (format Key: Value).
• rewrite_path: Rewrites the request path to a fixed value.
• drop: Drops the request and returns 403.
• response_header: Adds a response header (format Key: Value).
• response_status: Overrides the response status code.

Example curl commands

Send a GET request via proxy:

curl -x http://localhost:8080 http://localhost:8090/api/students

Send a POST with JSON body:

curl -x http://localhost:8080 \
  -H "Content-Type: application/json" \
  -d '{"name":"Ada"}' \
  http://localhost:8090/api/students

Limitations

• HTTP only (no TLS interception). HTTPS requires MITM certificates, which are not implemented.
• Request parsing is limited by the underlying httpfromtcp buffer size.
• Keep-alive is disabled; the proxy sets Connection: close upstream.
• Chunked responses are de-chunked when possible; failures fall back to raw body.

Project structure

cmd/
  proxy/    main entrypoint for the proxy
  replay/   replays captured requests
internal/
  config/   config parsing
  forwarder/ upstream TCP client
  logger/   structured logging
  modifier/ request/response rules
  proxy/    proxy handler
  replay/   replay logic
  store/    request storage

Build

go build ./...