HTTP upgrades

Envoy Upgrade support is intended mainly for WebSocket and CONNECT support, but may be used for arbitrary upgrades as well. Upgrades pass both the HTTP headers and the upgrade payload through an HTTP filter chain. One may configure the upgrade_configs with or without custom filter chains. If only the upgrade_type is specified, both the upgrade headers, any request and response body, and HTTP data payload will pass through the default HTTP filter chain. To avoid the use of HTTP-only filters for upgrade payload, one can set up custom filters for the given upgrade type, up to and including only using the router filter to send the HTTP data upstream.

Upgrades can be enabled or disabled on a per-route basis. Any per-route enabling/disabling automatically overrides HttpConnectionManager configuration as laid out below, but custom filter chains can only be configured on a per-HttpConnectionManager basis.

Note that the statistics for upgrades are all bundled together so WebSocket and other upgrades statistics are tracked by stats such as downstream_cx_upgrades_total and downstream_cx_upgrades_active

Websocket over HTTP/2 hops

While HTTP/2 support for WebSockets is off by default, Envoy does support tunneling WebSockets over HTTP/2 streams for deployments that prefer a uniform HTTP/2 mesh throughout; this enables, for example, a deployment of the form:

[Client] —- HTTP/1.1 —- [Front Envoy] —- HTTP/2 —- [Sidecar Envoy —- H1 —- App]

In this case, if a client is for example using WebSocket, we want the Websocket to arrive at the upstream server functionally intact, which means it needs to traverse the HTTP/2 hop.

This is accomplished via extended CONNECT support, turned on by setting allow_connect true at the second layer Envoy. The WebSocket request will be transformed into an HTTP/2 CONNECT stream, with :protocol header indicating the original upgrade, traverse the HTTP/2 hop, and be downgraded back into an HTTP/1 WebSocket Upgrade. This same Upgrade-CONNECT-Upgrade transformation will be performed on any HTTP/2 hop, with the documented flaw that the HTTP/1.1 method is always assumed to be GET. Non-WebSocket upgrades are allowed to use any valid HTTP method (i.e. POST) and the current upgrade/downgrade mechanism will drop the original method and transform the Upgrade request to a GET method on the final Envoy-Upstream hop.

Note that the HTTP/2 upgrade path has very strict HTTP/1.1 compliance, so will not proxy WebSocket upgrade requests or responses with bodies.

CONNECT support

Envoy CONNECT support is off by default (Envoy will send an internally generated 403 in response to CONNECT requests). CONNECT support can be enabled via the upgrade options described above, setting the upgrade value to the special keyword “CONNECT”.

While for HTTP/2, CONNECT request may have a path, in general and for HTTP/1.1 CONNECT requests do not have a path, and can only be matched using a connect_matcher

Envoy can handle CONNECT in one of two ways, either proxying the CONNECT headers through as if they were any other request, and letting the upstream terminate the CONNECT request, or by terminating the CONNECT request, and forwarding the payload as raw TCP data. When CONNECT upgrade configuration is set up, the default behavior is to proxy the CONNECT request, treating it like any other request using the upgrade path. If termination is desired, this can be accomplished by setting connect_config If it that message is present for CONNECT requests, the router filter will strip the request headers, and forward the HTTP payload upstream. On receipt of initial TCP data from upstream, the router will synthesize 200 response headers, and then forward the TCP data as the HTTP response body.

Warning

This mode of CONNECT support can create major security holes if configured correctly, as the upstream will be forwarded unsanitized headers if they are in the body payload. Please use with caution

Tunneling TCP over HTTP/2

Envoy also has support for transforming raw TCP into HTTP/2 CONNECT requests. This can be used to proxy multiplexed TCP over pre-warmed secure connections and amortize the cost of any TLS handshake. An example set up proxying SMTP would look something like this

[SMTP Upstream] — raw SMTP — [L2 Envoy] — SMTP tunneled over HTTP/2 — [L1 Envoy] — raw SMTP — [Client]

Examples of such a set up can be found in the Envoy example config directory If you run bazel-bin/source/exe/envoy-static –config-path configs/encapsulate_in_connect.v3.yaml –base-id 1 and bazel-bin/source/exe/envoy-static –config-path configs/terminate_connect.v3.yaml you will be running two Envoys, the first listening for TCP traffic on port 10000 and encapsulating it in an HTTP/2 CONNECT request, and the second listening for HTTP/2 on 10001, stripping the CONNECT headers, and forwarding the original TCP upstream, in this case to google.com.