add_response_header

Add Response Header Plugin

This plugin modifies HTTP response headers as they pass through the proxy. It conditionally appends a value to the Message header when it equals foo, and unconditionally removes the Welcome header from all responses. Use this plugin when you need to manipulate response headers based on their current values, inject metadata into responses, or remove sensitive headers before responses reach clients. It operates during the response headers processing phase.

How It Works

1. The proxy receives an HTTP response from the upstream server and invokes the plugin's on_http_response_headers callback.
2. The plugin reads the Message response header and checks if its value equals "foo".
3. If the Message header equals "foo", the plugin appends "bar" to it (resulting in "foo, bar"). If the header doesn't exist or has a different value, no modification is made.
4. The plugin unconditionally removes the Welcome header from the response, regardless of its value.
5. The plugin returns Action::Continue, forwarding the modified response headers to the client.

Implementation Notes

• Appending headers: The Message header is modified using the "add" operation, which appropriately appends values automatically separated by commas.
• Removing headers: The Welcome header is removed unconditionally. In Rust, this is achieved by trying to set the header to None.
• Error handling considerations: Missing headers need to be handled carefully depending on the target language. The Go implementation uses defer recover() to respond with a 500 error if panics occur during header manipulation, while Rust handles them gracefully via unwrap_or_default().

Configuration

No configuration required.

Build

Build the plugin for any supported language from the plugins/ directory:

# Rust
bazelisk build //samples/add_response_header:plugin_rust.wasm

# C++
bazelisk build //samples/add_response_header:plugin_cpp.wasm

# Go
bazelisk build //samples/add_response_header:plugin_go.wasm

Test

Run the unit tests defined in tests.textpb:

# Using Docker (recommended)
docker run -it -v $(pwd):/mnt \
    us-docker.pkg.dev/service-extensions-samples/plugins/wasm-tester:main \
    --proto /mnt/samples/add_response_header/tests.textpb \
    --plugin /mnt/bazel-bin/samples/add_response_header/plugin_rust.wasm

# Using Bazel (all languages)
bazelisk test --test_output=all //samples/add_response_header:tests

Expected Behavior

Derived from tests.textpb:

Scenario	Description
NoDefaultResponseHeader	Applies no changes if the required header is missing.
LeavesResponseHeader	Makes no modifications if the header value does not match the expected string.
ExtendsResponseHeader	Appends a value to the header when the original value matches.
RemovesResponseHeader	Unconditionally removes the specified header.
RequestAndResponse	Confirms behavior correctly matches the response phase.
CaseInsensitiveLookup	Successfully detects and appends to the header regardless of its casing.
CaseInsensitiveRemoval	Successfully removes the specified header even when casing differs.

Available Languages

• Rust
• C++
• Go