Envoy WASM Filters in Rust

~20 mins
tl;dr: Example filter for conditionally adding HTTP headers on-the-fly.


I have had a renewed interest in WASM ever since I read the Mozilla WASI announcement and some of its supporting literature. A few things clicked for me after that, most prominently the potential for use beyond the browser. The compile once, run anywhere aspects echo the Linux container revolution of the last decade, but with true sandboxing, faster starts and without the baggage of a Linux userspace.

The poster child of that revolution was of course Docker. It arguably ushered in a new paradigm for packaging and deploying software to cloud and enterprise computing landscapes that was both efficient and cost-effective. Sure, the backing company is an omnishambles, but its tech concepts live on in various forms.

Docker achieved all of this despite not being the first to utilise the Linux kernel’s cgroups(7) and namespaces(7) features in a cohesive manner, nor Linux even being the first to offer such virtualised isolation at all. Other OSes have famously had similar offerings since the early noughties, notably FreeBSD’s Jails and Solaris' Zones (which hold a special place in my heart from my time interning at Sun).

So why, then, did Docker beat the others? The answer I often see touted elsewhere and that I personally believe is developer experience. It democratised those Linux container primitives through an abstraction, provided a simplistic product engineer focused CLI workflow, and defined an immutable image format solving for the “works on my machine” problem. Those product engineers could now build, ship and run their workloads on any server with a Docker daemon.

WASI implementers will face this same hurdle to become successful on the server, but can benefit from being more integrated to tooling and platforms. This can already be seen with the burgeoning support for compile targets in the toolchains of languages like Rust, C++, Go and AssemblyScript (a TypeScript subset), as well as edge compute platforms like Fastly’s Lucet and Cloudflare’s Workers, Ethereum through eWasm and even good ol' Kubernetes through Krustlet.

Incidentally, Krustlet now supports multiple providers (Wasmtime, waSCC) but I view this (in keeping with the container metaphor) as being less analogous to the orchestration system wars of Kubernetes/Mesos/Rancher/Swarm/Nomad and more to container runtime e.g. Docker/runc/CRI-O/Rocket. Wasmtime aims for strict adherence to WASI, and waSCC is an interesting approach based on the Actor model, though I’m not clear on its relationship to the WASI spec.


Envoy has supported WASM extensions for a while now (even predating WASI, but there are plans afoot to re-align). It does this by implementing the proxy-wasm spec, an open standard ABI for interoperability between WASM VMs and a host proxy. The idea being that in the future if a great extension exists for say HAProxy, then it would also be usable in Envoy, Nginx and any other proxy also implementing this spec. An ambitious venture for sure.

Anyway, Envoy is leading the charge on this by a long shot at the moment and this is useful for me because it is fast becoming the universal data plane for services meshes, including the one we operate at my current gig as our platform SOA backbone: Istio.

Being early adopters of Istio we now have a few hundred services in our platform’s mesh. We built the platform atop Kubernetes and have found the pod a useful axiom for offering cross-cutting functionality to our users through the sidecar pattern. For example: injecting secrets, pub/sub over CloudEvents or rotating DB credentials.

Taking care of common technological concerns like these allow our users to have increased focus on building differentiating product features.

Now, to automatically bolt-on these functional building blocks we invariably utilise Kubernetes controllers or mutating webhooks to inject additional containers alongside the “primary” container during admission. To make that concrete to the more Kubernetes-initiated reader, 6/6 Running is not an uncommon sight in a kubectl get pods output from our platform.

Despite the overwhelming majority of the sidecars we write (or reuse from the community) being in Go, and even after careful attention being paid to ensuring low and stable resource requirements, they do still add up. This is especially true when you have large clusters full of workloads utilising them (a curse of success I suppose).

However, since we are using Istio there is another avenue that avoids additional containers (for bolting on network related functionality at least) and that is the Envoy filter.

Envoy Filters

In an Istio-enabled pod there is a necessary Envoy sidecar container (so if you’re following, that’s 2/2 Running on our platform by default!). It acts as the gatekeeper to the pod’s network thanks to a Kubernetes CNI plugin manipulating the pod iptables. So all pod packets ingress and egress route through that Envoy, and Envoy filters can be used manipulate them as they traverse the proxy’s internal network and application protocol processing stack.

It is worth mentioning that Envoy has a strong catalogue of native filters available and Istio in turn has facilities for enabling them through the EnvoyFilter CRD. So that should always be your first port of call. But what do you do if you have a use case that is not covered in a native filter?

You have two options:

  1. Write a new filter into Envoy’s source and compile a custom version.

    This is tried and true, but requires you to maintain a custom supply chain for Envoy and keep it rebased. You are also limited to C++ in terms of languages (unless you get creative) given that is what Envoy is written in.

  2. Write a new filter and dynamically load it at runtime using a WASM VM.

    This is why I’m writing this fieldnote today. Doing this is becoming increasingly accessible to your average Joe like me. Through the proxy-wasm spec implementation, Envoy is allowing embedded but securely isolated binary extension without the need to hack up the source. You can do this right now with SDKs in Rust C++, AssemblyScript and even early Go support.

Example: Writing a HTTP Header Augmenting Filter

Earlier I was talking about functional building blocks offered to our platform users. One such example is an injected sidecar that refreshes tokens from an IdP and supplies them to the primary container, either via a UNIX file descriptor or by POST’ing to a local HTTP endpoint. The primary container can subsequently use the token in RPCs to upstream integrations without concerning itself with token lifecycle.

Sidecar use case

NOTE: There’s absolutely no need to do this for inter-mesh RPCs of course. Istio (SPIFFE) workload identity is state-of-the-art. The use case here is to federate workload identity beyond the mesh itself across multiple network hops e.g. to legacy on-premises services.

Now I should start by saying this ain’t broke and I’m not paid to fix things that ain’t broke at work. We wrote it quickly and it does its thing.

On the other hand, it is objectively sub-optimal. For one, it is yet another sidecar container. Worse than that, though, it puts the onus of responsibility back on the product engineers to write code that either polls or uses inotify(7) on that token file, or otherwise uses a dedicated HTTP handler to receive the tokens.

I had been revisiting Rust recently and so last weekend I wanted to see if I could improve on this use case with an Envoy filter, using the nascent Rust proxy-wasm SDK.

As it transpired, it is entirely possible to get rid of that additional sidecar container and instead utilise the one sidecar container that cannot be gotten rid of (Envoy) to procure, refresh and conditionally add the IdP token to outbound HTTP headers.

So that’s no additional platform sidecar overhead, no additional coding for product engineers:

Sidecar-less use case

To prove the concept I implemented a generic HTTP filter that can augment requests with additional headers automatically discovered from a 3rd party endpoint at regular intervals. The full example is on GitHub but I’ll talk through some key parts below.

NOTE: the header-providing 3rd party service can be any configured Envoy cluster. So in an Istio context, this could be another sidecar available over loopback in the same pod, or some external centralised service perhaps in the greater mesh authorising based on SPIFFE identity, or even outside of the mesh authorising on Kubernetes service account token or cloud IAM for example, all the while benefiting from circuit breakers, retries, load balancing and other usual Istio-Envoy goodness.

Getting booted

I found the documentation sparse but the traits easy enough to decipher. The key thing to know is there’s seemingly 3 “Contexts” available:

  1. Root
  2. HTTP
  3. Stream

Root is a singleton that should be initialised as the WASM VM boots. It is the right place to setup shared data and timers. HTTP and Stream are called during HTTP and TCP filter chains respectively, though I suspect the latter is more nuanced than that. I only made use of the Root and HTTP contexts in my example.

To register context implementations there’s the special _start() function called by the Envoy host when initialising.

pub fn _start() {
    proxy_wasm::set_root_context(|context_id| -> Box<dyn RootContext> {
        CONFIGS.with(|configs| {
                .insert(context_id, FilterConfig::default());

        Box::new(RootHandler { context_id })
    proxy_wasm::set_http_context(|_context_id, _root_context_id| -> Box<dyn HttpContext> {
        Box::new(HttpHandler {})

This is also seemed to be the most fitting place for me to set the log level.

Configuring the filter

For my configuration I created a Serde type to deserialise from JSON because JSON works best with how the host Envoy wants to do configuration.

#[derive(Deserialize, Debug)]
struct FilterConfig {
    /// The Envoy cluster name housing a HTTP service that will provide headers
    /// to add to requests.
    header_providing_service_cluster: String,

    /// The path to call on the HTTP service providing headers.
    header_providing_service_path: String,

    /// The authority to set when calling the HTTP service providing headers.
    header_providing_service_authority: String,

    /// The length of time to keep headers cached.
    #[serde(with = "serde_humanize_rs")]
    header_cache_expiry: Duration,

To get some configuration values into the filter there’s an on_configure() hook method called on the RootContext as the WASM VM boots, and this can be married with the get_configuration() method for actually getting the configuration bytes.

Populating the token

Another useful method on the RootContext is on_tick() which is a ticker controlled by set_tick_period(). I use it to dispatch calls to the header providing endpoint (e.g. the IdP) on an interval.

fn on_tick(&mut self) {
        // Log the action that is about to be taken.
        match self.get_shared_data(CACHE_KEY) {
            (None, _) => debug!("initialising cached headers"),
            (Some(_), _) => debug!("refreshing cached headers"),

        CONFIGS.with(|configs| {
            configs.borrow().get(&self.context_id).map(|config| {
                // Dispatch an async HTTP call to the configured cluster.
                        (":method", "GET"),
                        (":path", &config.header_providing_service_path),
                        (":authority", &config.header_providing_service_authority),
                .map_err(|e| {

However this is not your typical RPC. It is async from the caller’s perspective and you have to play ball with Envoy’s internal processing stack. The other side can be grabbed when the on_http_call_response() hook method triggers.

fn on_http_call_response(
        &mut self,
        _token_id: u32,
        _num_headers: usize,
        body_size: usize,
        _num_trailers: usize,
    ) {
        // Gather the response body of previously dispatched async HTTP call.
        let body = match self.get_http_call_response_body(0, body_size) {
            Some(body) => body,
            None => {

        // Store the body in the shared cache.
        match self.set_shared_data(CACHE_KEY, Some(&body), None) {

Using shared data

The snippets above make passing reference to “shared data”. There are facilities in the proxy-wasm ABI for storing and retrieving data in a safe manner. In this example I refresh the headers to be added to outbound requests on a recurring tick, and cache them in shared data in the RootContext, out-of-band from the HTTP filter chains.

The final piece of the puzzle is to retrieve the currently cached, to-be-inserted headers during the hot path of an outbound request, and insert them into the payload.

This is done on the HttpContext, when the on_http_request_headers() hook method triggers on the outbound request.

impl HttpContext for HttpHandler {
    fn on_http_request_headers(&mut self, _num_headers: usize) -> Action {
        match self.get_shared_data(CACHE_KEY) {
            (Some(cache), _) => {
                    "using existing header cache: {}",

                match self.parse_headers(&cache) {
                    Ok(headers) => {
                        for (name, value) in headers {
                            self.set_http_request_header(&name, value.as_str())


Deploying it

In the hack directory I have a Docker compose stack complete with source, destination and header providing containers, and an Envoy container configured with the currently compiled filter. It mimics the Kubernetes/Istio pod network setup and I found it useful for locally developing the filter.

Testing the real deal was a little trickier. Manually distributing the Envoy filter binary to a test Kubernetes cluster such that it could be utilised by an Istio EnvoyFilter resource necessitated jumping through a few hoops, but only because I like making things difficult it seems. For the record there are promising tools like Solo.io’s wasme suite and AssemblyHub solving the filter distribution problem. Additionally, with OCI registries like AWS’s ECR starting to support OCI artifact types, there is nothing stopping use of them as a WASM module registry in addition to your typical OCI images.

Anyway, my approach was simply to use a ConfigMap to hold the WASM binary. The binaryData field is esoteric but has actually existed since Kubernetes 1.10. Doing it this way went a bit sideways when I realised the ConfigMap resource has a size limit of 1mb, presumably hamstrung by etcd value limits, and my un-optimised Rust compiler was producing WASM binaries in excess of that. What followed was a few rounds of optimisation:

  1. 2.1mb –> 1.7mb after reducing macro usage in the code.
  2. 1.7mb –> 372kb when compiled with lto=true and opt-level=s.
  3. 372kb –> 131kb when compiled through wasm-pack.

Nice. That was more than enough for Kubernetes to accept my WASM binary as a ConfigMap. I made use of Kustomize’s files feature to do the serialisation on the fly.

So now that the binary was in a binary ConfigMap in my cluster, I needed to get it loaded into the target pod’s Envoy. This is not as simple as editing a Deployment spec’s mounts because that Envoy is itself injected by Istio.

Fortunately there’s a handy mount annotation that can be put on the Deployment spec.

      sidecar.istio.io/userVolumeMount: >
                '{ "filter":{"mountPath":"/etc/filter.wasm","subPath":"filter.wasm"} }'

This reflects in the injected Envoy container stanza. Then all you need is to mount the ConfigMap to the pod.

  - name: filter
      name: filter

With that done, the Envoy container has the custom filter available to its userspace at /etc/filter.wasm. The final piece is to tell the Envoy process to use it, and this is done by selecting it with an EnvoyFilter CRD loaded with our custom configuration.

apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
  name: sourceworkload
    - applyTo: HTTP_FILTER
        context: SIDECAR_OUTBOUND
              name: envoy.http_connection_manager
                name: envoy.router
        operation: INSERT_BEFORE
              configuration: |
                  "header_providing_service_cluster": "inbound|8081|mgmt-8081|mgmtCluster",
                  "header_providing_service_authority": "localhost"
              name: header_augmenting_filter
              rootId: header_augmenting_filter
                    filename: /etc/filter.wasm
                runtime: envoy.wasm.runtime.v8
                allow_precompiled: true
          name: envoy.filters.http.wasm
      app: sourceworkload

In this example I am sliding the filter into the HTTP outbound chain and using the existing mgmt cluster to call the header providing container over loopback. As mentioned at the start of this section, there are many ways to skin that cat.