---
layout: docs
page_title: Connect - Envoy Integration
description: Consul Connect has first-class support for configuring Envoy proxy.
---

# Envoy Integration

Consul Connect has first class support for using
[Envoy](https://www.envoyproxy.io) as a proxy. Consul configures Envoy by
optionally exposing a gRPC service on the local agent that serves [Envoy's xDS
configuration
API](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-docs/xds_protocol).

Consul can configure Envoy sidecars to proxy traffic over the following protocols:

| Protocol                | Service network support |
| ----------------------- | ----------------------- |
| HTTP/1.1                | L7                      |
| HTTP2                   | L7                      |
| gRPC                    | L7                      |
| All TCP-based protocols | L4                      |

On Consul 1.5.0 and older, Envoy proxies can only proxy TCP traffic at L4.

Some [L7 features](/docs/connect/l7-traffic-management) can be configured using [configuration entries](/docs/agent/config-entries). You can add [custom Envoy configurations](#advanced-configuration) to the [proxy service definition](/docs/connect/registration/service-registration) to use Envoy features that are not currently exposed through configuration entries. Adding custom Envoy configurations to the service definition is an interim solution that enables you to use the more powerful features of Envoy.

~> **Note:** When using Envoy with Consul and not using the [`consul connect envoy` command](/commands/connect/envoy)
Envoy must be run with the `--max-obj-name-len` option set to `256` or greater for Envoy versions prior to 1.11.0.

## Supported Versions

Consul's Envoy support was added in version 1.3.0. The following table shows
compatible Envoy versions.

| Consul Version      | Compatible Envoy Versions                              |
| ------------------- | ------------------------------------------------------ |
| 1.11.x              | 1.20.0, 1.19.1, 1.18.4, 1.17.4                         |
| 1.10.x              | 1.18.4, 1.17.4, 1.16.5, 1.15.5                         |
| 1.9.x               | 1.16.5, 1.15.5, 1.14.7<sup>1</sup>, 1.13.7<sup>1</sup> |
| 1.8.x               | 1.14.7, 1.13.7, 1.12.7, 1.11.2                         |
| 1.7.x               | 1.13.7, 1.12.7, 1.11.2, 1.10.0<sup>2</sup>             |
| 1.6.x, 1.5.3, 1.5.2 | 1.11.1, 1.10.0, 1.9.1, 1.8.0<sup>3</sup>               |
| 1.5.1, 1.5.0        | 1.9.1, 1.8.0<sup>3</sup>                               |
| 1.4.x, 1.3.x        | 1.9.1, 1.8.0†, 1.7.0<sup>3</sup>                       |

1. Use Consul 1.9.0+ with Envoy 1.15.0+ to ensure that intention enforcement is updated as quickly as possible after any changes. [Additional information](https://github.com/envoyproxy/envoy/pull/10662).
1. Envoy 1.10.0 requires setting [`-envoy-version`](/commands/connect/envoy#envoy-version) in the `consul connect envoy` command. This was introduced in Consul 1.7.0.
1. Envoy 1.9.1 and older are vulnerable to [CVE-2019-9900](https://github.com/envoyproxy/envoy/issues/6434) and [CVE-2019-9901](https://github.com/envoyproxy/envoy/issues/6435). Both issues are related to parsing HTTP requests and only affect Consul Connect users if they have configured HTTP routing rules. We recommend that you use the most recent supported Envoy for your version of Consul when possible.

## Getting Started

To get started with Envoy and see a working example you can follow the [Using
Envoy with Connect](https://learn.hashicorp.com/tutorials/consul/service-mesh-with-envoy-proxy) tutorial.

## Configuration

Envoy proxies require two types of configuration: an initial _bootstrap
configuration_ and a _dynamic configuration_ that is discovered from a "management
server", in this case Consul.

The bootstrap configuration at a minimum needs to configure the proxy with an
identity (node id) and the location of its local Consul agent from which it
discovers all of its dynamic configuration. See [Bootstrap
Configuration](#bootstrap-configuration) for more details.

The dynamic configuration Consul Connect provides to each Envoy instance includes:

- TLS certificates and keys to enable mutual authentication and keep certificates
  rotating.
- [Intentions] to enforce service-to-service authorization rules.
- Service-discovery results for upstreams to enable each sidecar proxy to load-balance
  outgoing connections.
- L7 configuration including timeouts and protocol-specific options.
- Configuration to [expose specific HTTP paths](/docs/connect/registration/service-registration#expose-paths-configuration-reference).

For more information on the parts of the Envoy proxy runtime configuration
that are currently controllable via Consul Connect see [Dynamic
Configuration](#dynamic-configuration).

We plan to enable more and more of Envoy's features through
Connect's first-class configuration over time, however some advanced users will
need additional control to configure Envoy in specific ways. To enable this, we
provide several ["escape hatch"](#advanced-configuration) options that allow
users to provide low-level raw Envoy config syntax for some sub-components in each
Envoy instance. This allows operators to have full control over and
responsibility for correctly configuring Envoy and ensuring version support etc.

## Intention Enforcement

[Intentions] are enforced using Envoy's RBAC filters. Depending on the
configured [protocol] of the proxied service, intentions are either enforced
per-connection (L4) using a network filter, or per-request (L7) using an HTTP
filter.

-> **Note:** Prior to Consul 1.9.0 intentions were exclusively enforced
per-connection (L4) using an `ext_authz` network filter.

## Fetching Certificates

Envoy will use the [`CONSUL_HTTP_TOKEN`](/commands#consul_http_token) and [`CONSUL_HTTP_ADDR`](/commands#consul_http_addr) environment variables to contact Consul to fetch certificates if the following conditions are met:

- The `CONSUL_HTTP_TOKEN` environment variable contains a Consul ACL token.
- The Consul ACL token has the necessary permissions to read configuration for that service.

If TLS is enabled on Consul, you will also need to add the following environment variables _prior_ to starting Envoy:

- [`CONSUL_CACERT`](/commands#consul_cacert)
- [`CONSUL_CLIENT_CERT`](/commands#consul_client_cert)
- [`CONSUL_CLIENT_KEY`](/commands#consul_client_key)
- [`CONSUL_HTTP_SSL`](/commands#consul_http_ssl)

## Bootstrap Configuration

Envoy requires an initial bootstrap configuration file. The easiest way to
create this is using the [`consul connect envoy`
command](/commands/connect/envoy). The command can either output the
bootstrap configuration directly to stdout, or generate the configuration and issue an `exec` command
to the Envoy binary as a convenience wrapper.

Because some Envoy configuration options, such as metrics and tracing sinks, can only be
specified via the bootstrap configuration, Connect as of Consul 1.5.0 adds
the ability to control some parts of the bootstrap config via proxy configuration options.

Add the following configuration items to the [global `proxy-defaults`
configuration entry](/docs/connect/config-entries/proxy-defaults) or override them directly in the `proxy.config` field
of a [proxy service
definition](/docs/connect/registration/service-registration) or
[`sidecar_service`](/docs/connect/registration/sidecar-service) block.

- `envoy_statsd_url` - A URL in the form `udp://ip:port` identifying a UDP
  StatsD listener that Envoy should deliver metrics to. For example, this may be
  `udp://127.0.0.1:8125` if every host has a local StatsD listener. In this case
  users can configure this property once in the [global `proxy-defaults`
  configuration entry](/docs/connect/config-entries/proxy-defaults) for convenience. Currently, TCP is not supported.

  ~> **Note:** currently the url **must use an ip address** not a dns name due
  to the way Envoy is setup for StatsD.

  Expansion of the environment variable `HOST_IP` is supported, e.g.
  `udp://${HOST_IP}:8125`.

  Users can also specify the whole parameter in the form `$ENV_VAR_NAME`, which
  will cause the `consul connect envoy` command to resolve the actual URL from
  the named environment variable when it runs. This, for example, allows each
  pod in a Kubernetes cluster to learn of a pod-specific IP address for StatsD
  when the Envoy instance is bootstrapped while still allowing global
  configuration of all proxies to use StatsD in the [global `proxy-defaults`
  configuration entry](/docs/connect/config-entries/proxy-defaults). The env
  variable must contain a full valid URL value as specified above and nothing else.

- `envoy_dogstatsd_url` - The same as `envoy_statsd_url` with the following
  differences in behavior:

  - Envoy will use dogstatsd tags instead of statsd dot-separated metric names.
  - As well as `udp://`, a `unix://` URL may be specified if your agent can
    listen on a unix socket (e.g. the dogstatsd agent).

- `envoy_prometheus_bind_addr` - Specifies that the proxy should expose a Prometheus
  metrics endpoint to the _public_ network. It must be supplied in the form
  `ip:port` and port and the ip/port combination must be free within the network
  namespace the proxy runs. Typically the IP would be `0.0.0.0` to bind to all
  available interfaces or a pod IP address.

  -> **Note:** Envoy versions prior to 1.10 do not export timing histograms
  using the internal Prometheus endpoint.

- `envoy_stats_bind_addr` - Specifies that the proxy should expose the /stats prefix
  to the _public_ network. It must be supplied in the form `ip:port` and
  the ip/port combination must be free within the network namespace the proxy runs.
  Typically the IP would be `0.0.0.0` to bind to all available interfaces or a pod IP address.

- `envoy_stats_tags` - Specifies one or more static tags that will be added to
  all metrics produced by the proxy.

- `envoy_stats_flush_interval` - Configures Envoy's
  [`stats_flush_interval`](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/bootstrap/v3/bootstrap.proto#envoy-v3-api-field-config-bootstrap-v3-bootstrap-stats-flush-interval).

The [Advanced Configuration](#advanced-configuration) section describes additional configurations that allow incremental or complete control over the bootstrap configuration generated.

## Dynamic Configuration

Consul automatically generates Envoy's dynamic configuration based on its
knowledge of the cluster. Users may specify default configuration options for
each service such as which protocol they speak. Consul will use this information
to configure appropriate proxy settings for that service's proxies and also for
the upstream listeners of any downstream service.

One example is how users can define a service's protocol in a [`service-defaults` configuration
entry](/docs/connect/config-entries/service-defaults). Agents with
[`enable_central_service_config`](/docs/agent/options#enable_central_service_config)
set to true will automatically discover the protocol when configuring a proxy
for a service. The proxy will discover the main protocol of the service it
represents and use this to configure its main public listener. It will also
discover the protocols defined for any of its upstream services and
automatically configure its upstream listeners appropriately too as below.

This automated discovery results in Consul auto-populating the `proxy.config`
and `proxy.upstreams[*].config` fields of the [proxy service
definition](/docs/connect/registration/service-registration) that is
actually registered.

To learn about other options that can be configured centrally see the
[Configuration Entries](/docs/agent/config-entries) docs.

### Proxy Config Options

These fields may also be overridden explicitly in the [proxy service
definition](/docs/connect/registration/service-registration), or defined in
the [global `proxy-defaults` configuration
entry](/docs/connect/config-entries/proxy-defaults) to act as
defaults that are inherited by all services.

- `protocol` - The protocol the service speaks. Connect's Envoy integration
  currently supports the following `protocol` values:

  - `tcp` - Unless otherwise specified this is the default, which causes Envoy
    to proxy at L4. This provides all the security benefits of Connect's mTLS
    and works for any TCP-based protocol. Load-balancing and metrics are
    available at the connection level.
  - `http` - This specifies that the service speaks HTTP/1.x. Envoy will setup an
    `http_connection_manager` and will be able to load-balance requests
    individually to available upstream services. Envoy will also emit L7 metrics
    such as request rates broken down by HTTP response code family (2xx, 4xx, 5xx,
    etc).
  - `http2` - This specifies that the service speaks http2 (specifically h2c since
    Envoy will still only connect to the local service instance via plain TCP not
    TLS). This behaves much like `http` with L7 load-balancing and metrics but has
    additional settings that correctly enable end-to-end http2.
  - `grpc` - gRPC is a common RPC protocol based on http2. In addition to the
    http2 support above, Envoy listeners will be configured with a
    [gRPC bridge
    filter](https://www.envoyproxy.io/docs/envoy/v1.17.2/configuration/http/http_filters/grpc_http1_bridge_filter)
    that translates HTTP/1.1 calls into gRPC, and instruments
    metrics with `gRPC-status` trailer codes.

    ~> **Note:** The protocol of a service should ideally be configured via the
    [`protocol`](/docs/connect/config-entries/service-defaults#protocol)
    field of a
    [`service-defaults`](/docs/connect/config-entries/service-defaults)
    config entry for the service. Configuring it in a
    proxy config will not fully enable some [L7
    features](/docs/connect/l7-traffic-management).
    It is supported here for backwards compatibility with Consul versions prior to 1.6.0.

- `bind_address` - Override the address Envoy's public listener binds to. By
  default Envoy will bind to the service address or 0.0.0.0 if there is not explicit address on the service registration.

- `bind_port` - Override the port Envoy's public listener binds to. By default
  Envoy will bind to the service port.

- `local_connect_timeout_ms` - The number of milliseconds allowed to make
  connections to the local application instance before timing out. Defaults to 5000
  (5 seconds).

- `local_request_timeout_ms` - In milliseconds, the request timeout for HTTP requests
  to the local application instance. Applies to HTTP based protocols only. If not
  specified, inherits the Envoy default for route timeouts (15s). A value of 0 will
  disable request timeouts.

### Proxy Upstream Config Options

The following configuration items may be overridden directly in the
`proxy.upstreams[].config` field of a [proxy service
definition](/docs/connect/registration/service-registration) or
[`sidecar_service`](/docs/connect/registration/sidecar-service) block.

- `protocol` - Same as above in main config but affects the listener setup for
  the upstream.

  ~> **Note:** The protocol of a service should ideally be configured via the
  [`protocol`](/docs/connect/config-entries/service-defaults#protocol)
  field of a
  [`service-defaults`](/docs/connect/config-entries/service-defaults)
  config entry for the upstream destination service. Configuring it in a
  proxy upstream config will not fully enable some [L7
  features](/docs/connect/l7-traffic-management).
  It is supported here for backwards compatibility with Consul versions prior to 1.6.0.

- `connect_timeout_ms` - The number of milliseconds to allow when making upstream
  connections before timing out. Defaults to 5000
  (5 seconds).

  ~> **Note:** The connection timeout for a service should ideally be
  configured via the
  [`connect_timeout`](/docs/connect/config-entries/service-resolver#connecttimeout)
  field of a
  [`service-resolver`](/docs/connect/config-entries/service-resolver)
  config entry for the upstream destination service. Configuring it in a
  proxy upstream config will override any values defined in config entries.
  It is supported here for backwards compatibility with Consul versions prior to 1.6.0.

- `limits` - A set of limits to apply when connecting to the upstream service.
  These limits are applied on a per-service-instance basis. The following
  limits are respected:

  - `max_connections` - The maximum number of connections a service instance
    will be allowed to establish against the given upstream. Use this to limit
    HTTP/1.1 traffic, since HTTP/1.1 has a request per connection.
  - `max_pending_requests` - The maximum number of requests that will be queued
    while waiting for a connection to be established. For this configuration to
    be respected, a L7 protocol must be defined in the `protocol` field.
  - `max_concurrent_requests` - The maximum number of concurrent requests that
    will be allowed at a single point in time. Use this to limit HTTP/2 traffic,
    since HTTP/2 has many requests per connection. For this configuration to be
    respected, a L7 protocol must be defined in the `protocol` field.

- `passive_health_check` - Passive health checks are used to remove hosts from
  the upstream cluster which are unreachable or are returning errors.

  - `interval` - The time between checks. Each check will cause hosts which
    have exceeded `max_failures` to be removed from the load balancer, and
    any hosts which have passed their ejection time to be returned to the
    load balancer.
  - `max_failures` - The number of consecutive failures which cause a host to be
    removed from the load balancer.

### Gateway Options

These fields may also be overridden explicitly in the [proxy service
definition](/docs/connect/registration/service-registration), or defined in
the [global `proxy-defaults` configuration
entry](/docs/connect/config-entries/proxy-defaults) to act as
defaults that are inherited by all services.

Prior to 1.8.0 these settings were specific to Mesh Gateways. The deprecated
names such as `envoy_mesh_gateway_bind_addresses` and `envoy_mesh_gateway_no_default_bind`
will continue to be supported.

- `connect_timeout_ms` - The number of milliseconds to allow when making upstream
  connections before timing out. Defaults to 5000 (5 seconds). If the upstream
  service has the configuration option
  [`connect_timeout_ms`](/docs/connect/config-entries/service-resolver#connecttimeout)
  set for the `service-resolver`, that timeout value will take precedence over
  this gateway option.

- `envoy_gateway_bind_tagged_addresses` - Indicates that the gateway
  services tagged addresses should be bound to listeners in addition to the
  default listener address.

- `envoy_gateway_bind_addresses` - A map of additional addresses to be bound.
  This map's keys are the name of the listeners to be created and the values are
  a map with two keys, address and port, that combined make the address to bind the
  listener to. These are bound in addition to the default address.

- `envoy_gateway_no_default_bind` - Prevents binding to the default address
  of the gateway service. This should be used with one of the other options
  to configure the gateway's bind addresses.

- `envoy_dns_discovery_type` - Determines how Envoy will resolve hostnames. Defaults to `LOGICAL_DNS`.
  Must be one of `STRICT_DNS` or `LOGICAL_DNS`. Details for each type are available in
  the [Envoy documentation](https://www.envoyproxy.io/docs/envoy/v1.17.2/intro/arch_overview/upstream/service_discovery).
  This option applies to terminating gateways that route to services
  addressed by a hostname, such as a managed database. It also applies to mesh gateways,
  such as when gateways in other Consul datacenters are behind a load balancer that is addressed by a hostname.

## Advanced Configuration

To support more flexibility when configuring Envoy, several "lower-level" options exist
that require knowledge of Envoy's configuration format.
Many options allow configuring a subsection of either the bootstrap or
dynamic configuration using your own custom protobuf config.

We separate these into two sets, [Advanced Bootstrap
Options](#advanced-bootstrap-options) and [Escape Hatch
Overrides](#escape-hatch-overrides). Both require writing Envoy config in the
protobuf JSON encoding. Advanced options cover smaller chunks that might
commonly need to be set for tasks like configuring tracing. In contrast, escape hatches
give almost complete control over the proxy setup, but require operators to
manually code the entire configuration in protobuf JSON.

~> **Advanced Topic!** This section covers options that allow users to take almost
complete control of Envoy's configuration. We provide these options so users can
experiment or take advantage of features not yet fully supported in Consul Connect. We
plan to retain this ability in the future, but it should still be considered
experimental because it requires in-depth knowledge of Envoy's configuration format.
Users should consider Envoy version compatibility when using these features because they can configure Envoy in ways that
are outside of Consul's control. Incorrect configuration could prevent all
proxies in your mesh from functioning correctly, or bypass the security
guarantees Connect is designed to enforce.

### Configuration Formatting

All configurations are specified as strings containing the serialized proto3 JSON encoding
of the specified Envoy configuration type. They are full JSON types except where
noted.

The JSON supplied may describe a protobuf `types.Any` message with an `@type`
field set to the appropriate type (for example
`type.googleapis.com/envoy.config.listener.v3.Listener`).

For example, given a tracing config:

```json
{
  "http": {
    "name": "envoy.tracers.zipkin",
    "typedConfig": {
      "@type": "type.googleapis.com/envoy.config.trace.v3.ZipkinConfig",
      "collector_cluster": "zipkin",
      "collector_endpoint_version": "HTTP_JSON",
      "collector_endpoint": "/api/v1/spans",
      "shared_span_context": false
    }
  }
}
```

JSON escape the value of `tracing` into a string, for example using [https://codebeautify.org/json-escape-unescape](https://codebeautify.org/json-escape-unescape),
or using [jq](https://stedolan.github.io/jq/).

```shell
$ cat <<EOF | jq '. | @json'
{
  "http": {
    "name": "envoy.tracers.zipkin",
    "typedConfig": {
      "@type": "type.googleapis.com/envoy.config.trace.v3.ZipkinConfig",
      "collector_cluster": "zipkin",
      "collector_endpoint_version": "HTTP_JSON",
      "collector_endpoint": "/api/v1/spans",
      "shared_span_context": false
    }
  }
}
EOF
"{\"http\":{\"name\":\"envoy.tracers.zipkin\",\"typedConfig\":{\"@type\":\"type.googleapis.com/envoy.config.trace.v3.ZipkinConfig\",\"collector_cluster\":\"zipkin\",\"collector_endpoint_version\":\"HTTP_JSON\",\"collector_endpoint\":\"/api/v1/spans\",\"shared_span_context\":false}}}"
```

Then use that as the value for `envoy_tracing_json`:

```json
{
  "kind": "proxy-defaults",
  "name": "global",
  "config": {
    "envoy_tracing_json": "{\"http\":{\"name\":\"envoy.tracers.zipkin\",\"typedConfig\":{\"@type\":\"type.googleapis.com/envoy.config.trace.v3.ZipkinConfig\",\"collector_cluster\":\"zipkin\",\"collector_endpoint_version\":\"HTTP_JSON\",\"collector_endpoint\":\"/api/v1/spans\",\"shared_span_context\":false}}}\n"
  }
}
```

If using HCL, this escaping is done automatically:

```hcl
Kind = "proxy-defaults"
Name = "global"
Config {
  envoy_tracing_json = <<EOF
{
  "http": {
    "name": "envoy.tracers.zipkin",
    "typedConfig": {
      "@type": "type.googleapis.com/envoy.config.trace.v3.ZipkinConfig",
      "collector_cluster": "zipkin",
      "collector_endpoint_version": "HTTP_JSON",
      "collector_endpoint": "/api/v1/spans",
      "shared_span_context": false
    }
  }
}
EOF
}
```

### Advanced Bootstrap Options

Users may add the following configuration items to the [global `proxy-defaults`
configuration
entry](/docs/connect/config-entries/proxy-defaults) or
override them directly in the `proxy.config` field of a [proxy service
definition](/docs/connect/registration/service-registration) or
[`sidecar_service`](/docs/connect/registration/sidecar-service) block.

- `envoy_extra_static_clusters_json` - Specifies one or more [Envoy clusters][pb-cluster]
  that will be appended to the array of [static
  clusters](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/bootstrap/v3/bootstrap.proto#envoy-v3-api-field-config-bootstrap-v3-bootstrap-staticresources-clusters)
  in the bootstrap config. This allows adding custom clusters for tracing sinks
  for example. For a single cluster just encode a single object, for multiple,
  they should be comma separated with no trailing comma suitable for
  interpolating directly into a JSON array inside the braces.

- `envoy_extra_static_listeners_json` - Similar to
  `envoy_extra_static_clusters_json` but appends one or more [Envoy listeners][pb-listener] to the array of [static
  listener](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/bootstrap/v3/bootstrap.proto#envoy-v3-api-field-config-bootstrap-v3-bootstrap-staticresources-listeners) definitions.
  Can be used to setup limited access that bypasses Connect mTLS or
  authorization for health checks or metrics.

- `envoy_extra_stats_sinks_json` - Similar to `envoy_extra_static_clusters_json`
  but for [stats sinks](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/bootstrap/v3/bootstrap.proto#envoy-v3-api-field-config-bootstrap-v3-bootstrap-stats-sinks).
  These are appended to any sinks defined by use of the
  higher-level [`envoy_statsd_url`](#envoy_statsd_url) or
  [`envoy_dogstatsd_url`](#envoy_dogstatsd_url) config options.

- `envoy_stats_config_json` - The entire [stats
  config](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/bootstrap/v3/bootstrap.proto#envoy-v3-api-field-config-bootstrap-v3-bootstrap-stats-config).
  If provided this will override the higher-level
  [`envoy_stats_tags`](#envoy_stats_tags). It allows full control over dynamic
  tag replacements etc.

- `envoy_tracing_json` - The entire [tracing
  config](https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/bootstrap/v3/bootstrap.proto#envoy-v3-api-field-config-bootstrap-v3-bootstrap-tracing).
  Most tracing providers will also require adding static clusters to define the
  endpoints to send tracing data to.

### Escape-Hatch Overrides

Users may add the following configuration items to the [global `proxy-defaults`
configuration
entry](/docs/connect/config-entries/proxy-defaults) or
override them directly in the `proxy.config` field of a [proxy service
definition](/docs/connect/registration/service-registration) or
[`sidecar_service`](/docs/connect/registration/sidecar-service) block.

- `envoy_bootstrap_json_tpl` - Specifies a template in Go template syntax that
  is used in place of [the default
  template](https://github.com/hashicorp/consul/blob/71d45a34601423abdfc0a64d44c6a55cf88fa2fc/command/connect/envoy/bootstrap_tpl.go#L129)
  when generating bootstrap via [`consul connect envoy`
  command](/commands/connect/envoy). The variables that are available
  to be interpolated are [documented
  here](https://github.com/hashicorp/consul/blob/71d45a34601423abdfc0a64d44c6a55cf88fa2fc/command/connect/envoy/bootstrap_tpl.go#L5).
  This offers complete control of the proxy's bootstrap although major
  deviations from the default template may break Consul's ability to correctly
  manage the proxy or enforce its security model.

- `envoy_public_listener_json` - Specifies a complete [Envoy listener][pb-listener]
  to be delivered in place of the main public listener that the proxy used to
  accept inbound connections. This will be used verbatim with the following
  exceptions:

  - Every `FilterChain` added to the listener will have its `TlsContext`
    overridden by the Connect TLS certificates and validation context. This
    means there is no way to override Connect's mutual TLS for the public
    listener.
  - Every `FilterChain` will have the `envoy.filters.{network|http}.rbac` filter
    prepended to the filters array to ensure that all inbound connections are
    authorized by Connect. Before Consul 1.9.0 `envoy.ext_authz` was inserted instead.

- `envoy_local_cluster_json` - Specifies a complete [Envoy cluster][pb-cluster]
  to be delivered in place of the local application cluster. This allows
  customization of timeouts, rate limits, load balancing strategy etc.

The following configuration items may be overridden directly in the
`proxy.upstreams[].config` field of a [proxy service
definition](/docs/connect/registration/service-registration) or
[`sidecar_service`](/docs/connect/registration/sidecar-service) block.

~> **Note:** - When a
[`service-router`](/docs/connect/config-entries/service-router),
[`service-splitter`](/docs/connect/config-entries/service-splitter), or
[`service-resolver`](/docs/connect/config-entries/service-resolver) config
entry exists for a service the below escape hatches are ignored and will log a
warning.

- `envoy_listener_json` - Specifies a complete [Listener][pb-listener]
  to be delivered in place of the upstream listener that the proxy exposes to
  the application for outbound connections. This will be used verbatim with the
  following exceptions:

  - Every `FilterChain` added to the listener will have its `TlsContext`
    overridden by the Connect TLS certificates and validation context. This
    means there is no way to override Connect's mutual TLS for the public
    listener.

- `envoy_cluster_json` - Specifies a complete [Envoy cluster][pb-cluster]
  to be delivered in place of the discovered upstream cluster. This allows
  customization of timeouts, circuit breaking, rate limits, load balancing
  strategy etc.

[protocol]: /docs/connect/config-entries/service-defaults#protocol
[intentions]: /docs/connect/intentions
[intentions]: /docs/connect/intentions
[pb-cluster]: https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/cluster/v3/cluster.proto
[pb-listener]: https://www.envoyproxy.io/docs/envoy/v1.17.2/api-v3/config/listener/v3/listener.proto