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 an upgrade payload, one can set custom filters for the given upgrade type, up to and including only using the router filter to send the HTTP data upstream.


Buffering is generally not compatible with upgrades, so if the Buffer filter is configured in the default HTTP filter chain it should probably be excluded for upgrades by using upgrade filters and not including the buffer filter in that list.

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.

HCM Upgrade Enabled

Route Upgrade Enabled

Upgrade Enabled

T (Default)

T (Default)


T (Default)




T (Default)






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 or HTTP/3 hops

While HTTP/2 and HTTP/3 support for WebSockets is off by default, Envoy does support tunneling WebSockets over HTTP/2 and above 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 —-> HTTP/1 >—- 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 for HTTP/2 via Extended CONNECT (RFC 8441) support, turned on by setting allow_connect to true at the second layer Envoy.

For HTTP/3 there is parallel support configured by the alpha option allow_extended_connect as there is no formal RFC yet.

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.


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 and above, 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.


When doing non-wildcard domain matching for CONNECT requests, the CONNECT target is matched rather than the Host/Authority header. You may need to include the port (e.g. hostname:port) to successfully match.

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 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.


This mode of CONNECT support can create major security holes if not configured correctly, as the upstream will be forwarded unsanitized headers if they are in the body payload.

Please use with caution!


For an example of proxying connect, please see configs/proxy_connect.yaml

For an example of terminating connect, please see configs/terminate_http1_connect.yaml and configs/terminate_http2_connect.yaml


For CONNECT-over-TLS, Envoy can not currently be configured to do the CONNECT request in the clear and encrypt previously unencrypted payload in one hop.

To send CONNECT in plaintext and encrypt the payload, one must first forward the HTTP payload over an “upstream” TLS loopback connection to encrypt it, then have a TCP listener take the encrypted payload and send the CONNECT upstream.

Tunneling TCP over HTTP

Envoy also has support for tunneling raw TCP over HTTP CONNECT or HTTP POST requests. Find below some usage scenarios.

HTTP/2+ CONNECT 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 CONNECT >— [L1 Envoy] —> raw SMTP >— [Client]

HTTP/1.1 CONNECT can be used to have TCP client connecting to its own destination passing through an HTTP proxy server (e.g. corporate proxy not supporting HTTP/2):

[HTTP Server] —> raw HTTP >— [L2 Envoy] —> HTTP tunneled over HTTP/1.1 CONNECT >— [L1 Envoy] —> raw HTTP >— [HTTP Client]


When using HTTP/1 CONNECT you will end up having a TCP connection between L1 and L2 Envoy for each TCP client connection, it is preferable to use HTTP/2 or above when you have the choice.

HTTP POST can also be used to proxy multiplexed TCP when intermediate proxies that don’t support CONNECT.

An example set up proxying HTTP would look something like this:

[TCP Server] —> raw TCP >— [L2 Envoy] —> TCP tunneled over HTTP/2 or HTTP/1.1 POST >— [Intermediate Proxies] —> HTTP/2 or HTTP/1.1 POST >— [L1 Envoy] —> raw TCP >— [TCP Client]


Examples of such a set up can be found in the Envoy example config directory.

For HTTP/1.1 CONNECT, try either:

$ envoy -c configs/encapsulate_in_http1_connect.yaml --base-id 1
$ envoy -c configs/terminate_http1_connect.yaml --base-id 1

For HTTP/2 CONNECT, try either:

$ envoy -c configs/encapsulate_in_http2_connect.yaml --base-id 1
$ envoy -c configs/terminate_http2_connect.yaml --base-id 1

For HTTP/2 POST, try either:

$ envoy -c configs/encapsulate_in_http2_post.yaml --base-id 1
$ envoy -c configs/terminate_http2_post.yaml --base-id 1

In all cases you will be running a first Envoy listening for TCP traffic on port 10000 and encapsulating it in an HTTP CONNECT or HTTP POST request, and a second one listening on 10001, stripping the CONNECT headers (not needed for POST request), and forwarding the original TCP upstream, in this case to

Envoy waits for the HTTP tunnel to be established (i.e. a successful response to the CONNECT request is received), before starting to stream the downstream TCP data to the upstream.

If you want to decapsulate a CONNECT request and also do HTTP processing on the decapsulated payload, the easiest way to accomplish it is to use internal listeners.



CONNECT-UDP is in an alpha status and may not be stable enough for use in production. We recommend to use this feature with caution.

CONNECT-UDP (RFC 9298) allows HTTP clients to create UDP tunnels through an HTTP proxy server. Unlike CONNECT, which is limited to tunneling TCP, CONNECT-UDP can be used to proxy UDP-based protocols such as HTTP/3.

CONNECT-UDP support is disabled by default in Envoy. Similar to CONNECT, it can be enabled through the upgrade_configs by setting the value to the special keyword CONNECT-UDP. Like CONNECT, CONNECT-UDP requests are forwarded to the upstream by default. connect_config must be set to terminate the requests and forward the payload as UDP datagrams to the target.

Example Configuration

The following example configuration makes Envoy forward CONNECT-UDP requests to the upstream. Note that the upgrade_configs is set to CONNECT-UDP.

27      filters:
28      - name:
29        typed_config:
30          "@type":
31          codec_type: HTTP3
32          stat_prefix: ingress_http
33          route_config:
34            name: local_route
35            virtual_hosts:
36            - name: local_service
37              domains:
38              - "*"
39              routes:
40              - match:
41                  connect_matcher:
42                    {}
43                route:
44                  cluster: cluster_0
45          http_filters:
46          - name: envoy.filters.http.router
47            typed_config:
48              "@type":
49          http3_protocol_options:
50            allow_extended_connect: true
51          upgrade_configs:
52          - upgrade_type: CONNECT-UDP
53  clusters:
54  - name: cluster_0

The following example configuration makes Envoy terminate CONNECT-UDP requests and send UDP payloads to the target. As in this example, the connect_config must be set to terminate CONNECT-UDP requests.

26      filters:
27      - name:
28        typed_config:
29          "@type":
30          codec_type: HTTP3
31          stat_prefix: ingress_http
32          route_config:
33            name: local_route
34            virtual_hosts:
35            - name: local_service
36              domains:
37              - "*"
38              routes:
39              - match:
40                  connect_matcher:
41                    {}
42                route:
43                  cluster: service_google
44                  upgrade_configs:
45                  - upgrade_type: CONNECT-UDP
46                    connect_config:
47                      {}
48          http_filters:
49          - name: envoy.filters.http.router
50            typed_config:
51              "@type":
52          http3_protocol_options:
53            allow_extended_connect: true
54          upgrade_configs:
55          - upgrade_type: CONNECT-UDP
56  clusters:
57  - name: service_google

Tunneling UDP over HTTP


Raw UDP tunneling is in an alpha status and may not be stable enough for use in production. We recommend to use this feature with caution.

Apart from CONNECT-UDP termination, as described in the section above, Envoy also has support for tunneling raw UDP over HTTP CONNECT or HTTP POST requests, by utilizing the UDP Proxy listener filter. By default, UDP tunneling is disabled, and can be enabled by setting the configuration for tunneling_config.


Currently, Envoy only supports UDP tunneling over HTTP/2 streams.

By default, the tunneling_config will upgrade the connection to create HTTP/2 streams for each UDP session (a UDP session is identified by the datagrams 5-tuple), according to the Proxying UDP in HTTP RFC. Since this upgrade protocol requires an encapsulation mechanism to preserve the boundaries of the original datagram, it’s required to apply the HTTP Capsule session filter. The HTTP/2 streams will be multiplexed over the upstream connection.

As opposed to TCP tunneling, where downstream flow control can be applied by alternately disabling the read from the connection socket, for UDP datagrams, this mechanism is not supported. Therefore, when tunneling UDP and a new datagram is received from the downstream, it is either streamed upstream, if the upstream is ready or halted by the UDP Proxy. In case the upstream is not ready (for example, when waiting for HTTP response headers), the datagram can either be dropped or buffered until the upstream is ready. In such cases, by default, downstream datagrams will be dropped, unless buffer_options is set by the tunneling_config. The default buffer limits are modest to try and prevent a lot of unwanted buffered memory, but can and should be adjusted per the required use-case. When the upstream becomes ready, the UDP Proxy will first flush all the previously buffered datagrams.


If POST is set, the upstream stream does not comply with the connect-udp RFC, and instead it will be a POST request. The path used in the headers will be set from the post_path field, and the headers will not contain the target host and target port, as required by the connect-udp protocol. This option should be used carefully.

Example Configuration

The following example configuration makes Envoy tunnel raw UDP datagrams over an upgraded CONNECT-UDP requests to the upstream.

32        session_filters:
33        - name: envoy.filters.udp.session.http_capsule
34          typed_config:
35            '@type':
36        tunneling_config:
37          # note: proxy_host supports string substitution, for example setting "%FILTER_STATE("
38          # will take the target host value from the session's filter state.
39          proxy_host:
40          # note: target_host supports string substitution, for example setting "%FILTER_STATE("
41          # will take the target host value from the session's filter state.
42          target_host:
43          # note: The target port value can be overridden per-session by setting the required port value for
44          # the filter state key ``udp.connect.target_port``.
45          default_target_port: 443
46          retry_options:
47            max_connect_attempts: 2
48          buffer_options:
49            max_buffered_datagrams: 1024
50            max_buffered_bytes: 16384
51          headers_to_add:
52          - header:
53              key: original_dst_port