Wasm C++ filter

This sandbox demonstrates a basic Envoy Wasm filter written in C++ which injects content into the body of an HTTP response, and adds and updates some headers.

It also takes you through the steps required to build your own C++ Wasm filter, and run it with Envoy.

Step 1: Start all of our containers

First lets start the containers - an Envoy proxy which uses a Wasm Filter, and a backend which echos back our request.

Change to the examples/wasm-cc folder in the Envoy repo, and start the composition:

$ pwd
$ docker-compose build --pull
$ docker-compose up -d
$ docker-compose ps

    Name                     Command                State             Ports
wasm_proxy_1         /docker-entrypoint.sh /usr ... Up      10000/tcp,>8000/tcp
wasm_web_service_1   node ./index.js                Up

Step 2: Check web response

The Wasm filter should inject “Hello, world” at the end of the response body when you make a request to the proxy.

$ curl -s http://localhost:8000 | grep "Hello, world"
}Hello, world

The filter also sets the content-type header to text/plain, and adds a custom x-wasm-custom header.

$ curl -v http://localhost:8000 | grep "content-type: "
content-type: text/plain; charset=utf-8

$ curl -v http://localhost:8000 | grep "x-wasm-custom: "
x-wasm-custom: FOO

Step 3: Compile the updated filter

There are two source code files provided for the Wasm filter.

envoy_filter_http_wasm_example.cc provides the source code for the included prebuilt binary.

envoy_filter_http_wasm_updated_example.cc makes a few changes to the original.

The following diff shows the changes that have been made:

--- /source/generated/rst/start/sandboxes/_include/wasm-cc/envoy_filter_http_wasm_example.cc
+++ /source/generated/rst/start/sandboxes/_include/wasm-cc/envoy_filter_http_wasm_updated_example.cc
@@ -65,8 +65,8 @@
   for (auto& p : pairs) {
     LOG_INFO(std::string(p.first) + std::string(" -> ") + std::string(p.second));
-  addResponseHeader("X-Wasm-custom", "FOO");
-  replaceResponseHeader("content-type", "text/plain; charset=utf-8");
+  addResponseHeader("X-Wasm-custom", "BAR");
+  replaceResponseHeader("content-type", "text/html; charset=utf-8");
   return FilterHeadersStatus::Continue;
@@ -80,7 +80,7 @@
 FilterDataStatus ExampleContext::onResponseBody(size_t /* body_buffer_length */,
                                                 bool /* end_of_stream */) {
-  setBuffer(WasmBufferType::HttpResponseBody, 0, 12, "Hello, world");
+  setBuffer(WasmBufferType::HttpResponseBody, 0, 17, "Hello, Wasm world");
   return FilterDataStatus::Continue;


These instructions for compiling an updated Wasm binary use the envoyproxy/envoy-build-ubuntu image. You will need 4-5GB of disk space to accommodate this image.

Stop the proxy server and compile the Wasm binary with the updated code:

$ docker-compose stop proxy
$ docker-compose -f docker-compose-wasm.yaml up --remove-orphans wasm_compile_update

The compiled binary should now be in the lib folder.

$ ls -l lib
total 120
-r-xr-xr-x 1 root root 59641 Oct 20 00:00 envoy_filter_http_wasm_example.wasm
-r-xr-xr-x 1 root root 59653 Oct 20 10:16 envoy_filter_http_wasm_updated_example.wasm

Step 4: Edit the Dockerfile and restart the proxy

Edit the Dockerfile-proxy recipe provided in the example to use the updated binary you created in step 3.

Find the COPY line that adds the Wasm binary to the image:

FROM envoyproxy/envoy-dev:latest
COPY ./envoy.yaml /etc/envoy.yaml
COPY ./lib/envoy_filter_http_wasm_example.wasm /lib/envoy_filter_http_wasm_example.wasm
RUN chmod go+r /etc/envoy.yaml /lib/envoy_filter_http_wasm_example.wasm
CMD ["/usr/local/bin/envoy", "-c", "/etc/envoy.yaml", "--service-cluster", "proxy"]

Replace this line with the following:

COPY ./lib/envoy_filter_http_wasm_updated_example.wasm /lib/envoy_filter_http_wasm_example.wasm

Now, rebuild and start the proxy container.

$ docker-compose up --build -d proxy

Step 5: Check the proxy has been updated

The Wasm filter should instead inject “Hello, Wasm world” at the end of the response body.

$ curl -s http://localhost:8000 | grep "Hello, Wasm world"
}Hello, Wasm world

The content-type and x-wasm-custom headers should also have changed

$ curl -v http://localhost:8000 | grep "content-type: "
content-type: text/html; charset=utf-8

$ curl -v http://localhost:8000 | grep "x-wasm-custom: "
x-wasm-custom: BAR

See also

Envoy Wasm filter

Further information about the Envoy Wasm filter.

Envoy Wasm API(V3)

The Envoy Wasm API - version 3.

Proxy Wasm C++ SDK

WebAssembly for proxies (C++ SDK)