52 lines
2 KiB
Markdown
52 lines
2 KiB
Markdown
# Proxy Header Propagation Example
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This example demonstrates how to propagate HTTP headers from an incoming MCP server request to an outgoing MCP client request.
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This is particularly useful when building a gateway or proxy server that needs to forward authentication tokens, trace IDs, or other context-sensitive headers to downstream services.
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## Architecture
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This example runs three components in a single process:
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1. **Backend Server** (Port 8082):
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- Exposes a tool `echo_headers` that returns the headers it received.
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2. **Proxy Server** (Port 8081):
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- Exposes a tool `forward_headers`.
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- Acts as a client to the Backend Server.
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- Uses a custom `http.RoundTripper` (`HeaderForwardingTransport`) to inject headers from the context into outgoing requests.
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3. **Client**:
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- Connects to the Proxy Server.
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- Calls `forward_headers`.
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## How it works
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1. The Client calls `forward_headers` on the Proxy Server.
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2. The Proxy Server receives the request. The request context contains the HTTP headers in `req.Extra.Header`.
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3. The Proxy's tool handler extracts these headers and places them into a new `context.Context` using a specific key (`headerContextKey`).
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4. The Proxy uses an `mcp.Client` configured with a custom `HTTPClient` that uses `HeaderForwardingTransport`.
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5. `HeaderForwardingTransport.RoundTrip` inspects the context of outgoing requests. If it finds headers under `headerContextKey`, it adds them to the HTTP request.
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6. The Backend Server receives the request with the propagated headers.
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## Running the Example
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Run the example with:
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```bash
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go run main.go
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```
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You should see output indicating:
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1. Backend and Proxy servers starting.
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2. Gateway receiving headers.
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3. Client receiving the result, which contains the echoed headers.
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Example output:
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```
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2025/08/29 10:00:00 Starting Backend Server on :8082
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2025/08/29 10:00:00 Starting Gateway Server on :8081
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2025/08/29 10:00:00 Gateway received headers: map[...]
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2025/08/29 10:00:00 Client received result: ...
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```
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