9 min • read

The Host CRD

The custom Host resource defines how Emissary-ingress will be visible to the outside world. It collects all the following information in a single configuration resource:

  • The hostname by which Emissary-ingress will be reachable
  • How Emissary-ingress should handle TLS certificates
  • How Emissary-ingress should handle secure and insecure requests
  • Which Mappings should be associated with this Host

A minimal Host resource, assuming no TLS configuration, would be:

This Host tells Emissary-ingress to expect to be reached at host.example.com, with no TLS termination, and only associating with Mappings that also set a hostname that matches host.example.com.

Remember that a Listener will also be required for this example to be functional. Many examples of setting up Host and Listener are available in the Configuring Emissary-ingress Communications document.

Setting the hostname

The hostname element tells Emissary-ingress which hostnames to expect. hostname is a DNS glob, so all of the following are valid:

  • host.example.com
  • *.example.com
  • host.example.*

The following are not valid:

  • host.*.com -- Envoy supports only prefix and suffix globs
  • *host.example.com -- the wildcard must be its own element in the DNS name

In all cases, the hostname is used to match the :authority header for HTTP routing. When TLS termination is active, the hostname is also used for SNI matching.

Controlling Association with Mappings

A Mapping will not be associated with a Host unless at least one of the following is true:

  • The Mapping specifies a hostname attribute that matches the Host in question.
  • The Host specifies a mappingSelector that matches the Mapping's Kubernetes labels.

Note: The mappingSelector field is only configurable on v3alpha1 CRDs. In the v2 CRDs the equivalent field is selector. either selector or mappingSelector may be configured in the v3alpha1 CRDs, but selector has been deprecated in favour of mappingSelector.

If neither of the above is true, the Mapping will not be associated with the Host in question. This is intended to help manage memory consumption with large numbers of Hosts and large numbers of Mappings.

If the Host specifies mappingSelector and the Mapping specifies hostname, both must match for the association to happen.

The mappingSelector is a Kubernetes label selector. For a Mapping to be associated with a Host that uses mappingSelector, then all labels required by the mappingSelector must be present on the Mapping in order for it to be associated with the Host. A Mapping may have additional labels other than those required by the mappingSelector so long as the required labels are present.

in 2.0, only matchLabels is supported, for example:

The above Host will associate with these Mappings:

It will not associate with any of these:

Future versions of Emissary-ingress will support matchExpressions as well.

Note: In Emissary-ingress version 3.2, a bug with how Hosts are associated with Mappings was fixed. The mappingSelector field in Hosts was not properly being enforced in prior versions. If any single label from the selector was matched then the Host would be associated with the Mapping instead of requiring all labels in the selector to be present. Additonally, if the hostname of the Mapping matched the hostname of the Host then they would be associated regardless of the configuration of mappingSelector. In version 3.2 this bug was fixed and a Host will only be associated with a Mapping if all labels required by the selector are present. This brings the mappingSelector field in-line with how label selectors are used throughout Kubernetes. To avoid unexpected behavior after the upgrade, add all labels that Hosts have in their mappingSelector to Mappings you want to associate with the Host. You can opt-out of this fix and return to the old Mapping/Host association behavior by setting the environment variable DISABLE_STRICT_LABEL_SELECTORS to "true" (default: "false"). A future version of Emissary-ingress may remove the ability to opt-out of this bugfix.

Secure and insecure requests

A secure request arrives via HTTPS; an insecure request does not. By default, secure requests will be routed and insecure requests will be redirected (using an HTTP 301 response) to HTTPS. The behavior of insecure requests can be overridden using the requestPolicy element of a Host:

The insecure-action can be one of:

  • Redirect (the default): redirect to HTTPS
  • Route: go ahead and route as normal; this will allow handling HTTP requests normally
  • Reject: reject the request with a 400 response

Some special cases to be aware of here:

  • Case matters in the actions: you must use e.g. Reject, not reject.
  • The X-Forwarded-Proto header is honored when determining whether a request is secure or insecure. For more information, see "Load Balancers, the Host Resource, and X-Forwarded-Proto" below.
  • ACME challenges with prefix /.well-known/acme-challenge/ are always forced to be considered insecure, since they are not supposed to arrive over HTTPS.
  • Ambassador Edge Stack provides native handling of ACME challenges. If you are using this support, Ambassador Edge Stack will automatically arrange for insecure ACME challenges to be handled correctly. If you are handling ACME yourself - as you must when running Emissary-ingress - you will need to supply appropriate Host resources and Mappings to correctly direct ACME challenges to your ACME challenge handler.

TLS settings

The Host is responsible for high-level TLS configuration in Emissary-ingress. There are several settings covering TLS:

tlsSecret enables TLS termination

tlsSecret specifies a Kubernetes Secret is required for any TLS termination to occur. No matter what other TLS configuration is present, TLS termination will not occur if tlsSecret is not specified.

The following Host will configure Emissary-ingress to read a Secret named tls-cert for a certificate to use when terminating TLS.

tlsContext specifies a TLSContext to use for additional TLS information. Note that you must still define tlsSecret for TLS termination to happen. It is an error to supply both tlsContext and tls.

See the TLS discussion for more details.

tls allows manually providing additional configuration

tls allows specifying most of the things a TLSContext can, inline in the Host. Note that you must still define tlsSecret for TLS termination to happen. It is an error to supply both tlsContext and tls.

See the TLS discussion for more details.

Load balancers, the Host resource, and X-Forwarded-Proto

In a typical installation, Emissary-ingress runs behind a load balancer. The configuration of the load balancer can affect how Emissary-ingress sees requests arriving from the outside world, which can in turn can affect whether Emissary-ingress considers the request secure or insecure. As such:

  • We recommend layer 4 load balancers unless your workload includes long-lived connections with multiple requests arriving over the same connection. For example, a workload with many requests carried over a small number of long-lived gRPC connections.
  • Emissary-ingress fully supports TLS termination at the load balancer with a single exception, listed below.
  • If you are using a layer 7 load balancer, it is critical that the system be configured correctly:
    • The load balancer must correctly handle X-Forwarded-For and X-Forwarded-Proto.
    • The l7Depth element in the Listener CRD must be set to the number of layer 7 load balancers the request passes through to reach Emissary-ingress (in the typical case, where the client speaks to the load balancer, which then speaks to Emissary-ingress, you would set l7Depth to 1). If l7Depth remains at its default of 0, the system might route correctly, but upstream services will see the load balancer's IP address instead of the actual client's IP address.

It's important to realize that Envoy manages the X-Forwarded-Proto header such that it always reflects the most trustworthy information Envoy has about whether the request arrived encrypted or unencrypted. If no X-Forwarded-Proto is received from downstream, or if it is considered untrustworthy, Envoy will supply an X-Forwarded-Proto that reflects the protocol used for the connection to Envoy itself. The l7Depth element is also used when determining trust for X-Forwarded-For, and it is therefore important to set it correctly. Its default of 0 should always be correct when Emissary-ingress is behind only layer 4 load balancers; it should need to be changed only when layer 7 load balancers are involved.

CRD specification

The Host CRD is formally described by its protobuf specification. Developers who need access to the specification can find it here.