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The Host CRD, ACME Support, and External Load Balancer Configuration

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

  • The hostname by which Ambassador will be reachable
  • How Ambassador should handle TLS certificates
  • How Ambassador should handle secure and insecure requests
  • Which resources to examine for further configuration

A minimal Host resource, using Let’s Encrypt to handle TLS, would be:

apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: minimal-host
spec:
  hostname: host.example.com
  acmeProvider:
    email: julian@example.com

This Host tells Ambassador to expect to be reached at host.example.com, and to manage TLS certificates using Let’s Encrypt, registering as julian@example.com. Since it doesn’t specify otherwise, requests using cleartext will be automatically redirected to use HTTPS, and Ambassador will not search for any specific further configuration resources related to this Host.

ACME and TLS Settings

The Host is responsible for high-level TLS configuration in Ambassador.

The are two settings in the Host that are responsible for TLS configuration:

  • acmeProvider defines how Ambassador should handle TLS certificates
  • tlsSecret tells Ambassador which secret to look for the certificate in

In combination, these settings tell Ambassador how it should manage TLS certificates.

ACME Support

The Ambassador Edge Stack comes with built in support for automatic certificate management using the ACME protocol.

It does this by using the hostname of a Host to request a certificate from the acmeProvider.authority using the HTTP-01 challenge. After requesting a certificate, Ambassador will then manage the renewal process automatically.

The acmeProvider element of the Host configures the Certificate Authority Ambassador will request the certificate from and the email address that the CA will use to notify about any lifecycle events of the certificate.

acmeProvider:
  authority: url-to-provider
  email: email-of-registrant

Notes on ACME Support:

  • If the authority is not supplied, the Let’s Encrypt production environment is assumed.

  • In general, email-of-registrant is mandatory when using ACME: it should be a valid email address that will reach someone responsible for certificate management.

  • ACME stores certificates in Kubernetes secrets. The name of the secret can be set using the tlsSecret element:

    acmeProvider:
      email: user@example.com
    tlsSecret:
      name: tls-cert

    if not supplied, a name will be automatically generated from the hostname and email.

  • Ambassador uses the HTTP-01 challenge for ACME support:

    • Does not require permission to edit DNS records
    • The hostname must be reachable from the internet so the CA can check POST to an endpoint in Ambassador.
    • Wildcard domains are not supported.

TLS Configuration

Regardless of if you are using the built in ACME support in the Ambassador Edge Stack, the Host is responsible for reading TLS certificates from Kubernetes Secrets and configuring Ambassador to terminate TLS using those certificates.

If you are using the Open-Source Ambassador API Gateway or choosing to not use the provided ACME support, you need to tell Ambassador to not request a certificate with ACME by setting acmeProvider.authority: none in the Host.

After that you simply point Ambassador at the secret you are using to store your certificates with the tlsSecret field.

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

apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: example-host
spec:
  hostname: host.example.com
  acmeProvider:
    authority: none
  tlsSecret:
    name: tls-cert

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:

requestPolicy:
  insecure:
    action: insecure-action
    additionalPort: insecure-port

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

The additionalPort element tells Ambassador to listen on the specified insecure-port and treat any request arriving on that port as insecure. By default, additionalPort will be set to 8080 for any Host using TLS. To disable this redirection entirely, set additionalPort explicitly to -1:

requestPolicy:
  insecure:
    additionalPort: -1   # This is how to disable the default redirection from 8080.

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 will automatically arrange for insecure ACME challenges to be handled correctly. If you are handling ACME yourself - as you must when running Ambassador Open Source - you will need to supply appropriate Host resources and Mappings to correctly direct ACME challenges to your ACME challenge handler.

Load Balancers, the Host Resource, and X-Forwarded-Proto

In a typical installation, Ambassador runs behind a load balancer. The configuration of the load balancer can affect how Ambassador sees requests arriving from the outside world, which can in turn can affect whether Ambassador 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.
  • Ambassador 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 xff_num_trusted_hops element in the ambassador module must be set to the number of layer 7 load balancers the request passes through to reach Ambassador (in the typical case, where the client speaks to the load balancer, which then speaks to Ambassador, you would set xff_num_trusted_hops to 1). If xff_num_trusted_hops 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 xff_num_trusted_hops element, although its name reflects X-Forwarded-For, 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 Ambassador is behind only layer 4 load balancers; it should need to be changed only when layer 7 load balancers are involved.

Use Cases and Examples

In the definitions below, "L4 LB" refers to a layer 4 load balancer, while "L7 LB" refers to a layer 7 load balancer.

HTTPS-only, TLS terminated at Ambassador, not redirecting cleartext

This example is the same with a L4 LB, or without a load balancer. It also covers an L4 LB that terminates TLS, then re-originates TLS from the load balancer to Ambassador.

In this situation, Ambassador does everything on its own, and insecure requests are flatly rejected.

apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: minimal-host
spec:
  hostname: host.example.com
  acmeProvider: <as needed>
  requestPolicy:
    insecure:
      action: Reject

The acmeProvider must be set appropriately for your certificate-management needs; by default, it is set to allow the Ambassador Edge Stack to manage certificates for you. Or, you could set acmeProvider.authority to none if you want to manage certificates by hand.

An example using the default acmeProvider, ACME with Let's Encrypt:

apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: acme-lets-encrypt-host
spec:
  hostname: host.example.com
  requestPolicy:
    insecure:
      action: Reject

An example managing certificates by hand:

---
apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: manual-tls-only-host
spec:
  hostname: foo.example.com
  # Specifying acmeProvider.authority none with a manual tlsSecret.name
  # turns off the ACME client, but leaves TLS enabled.
  acmeProvider:
    authority: none
  tlsSecret:
    name: manual-secret-for-foo
  # The default insecure action is Redirect, which is not what we want.
  requestPolicy:
    insecure:
      action: Reject

With the configuration above, the system will look for a TLS secret in manual-secret-for-foo, but it will not run ACME for it.

HTTPS-only, TLS terminated at Ambassador, redirecting cleartext from port 8080

This example is the same for an L4 LB, or without a load balancer at all.

apiVersion: getambassador.io/v2
 kind: Host
 metadata:
   name: minimal-host
 spec:
   hostname: host.example.com
   acmeProvider: <as needed>
   requestPolicy:
     insecure:
       action: Redirect
       additionalPort: 8080

The default for insecure.action is Redirect, so that line could be removed.

If you do not set insecure.additionalPort, Ambassador won't listen on port 8080 at all. However, with the Redirect action still in place, Ambassador will still redirect requests that arrive on port 8443 with an X-Forwarded-Proto of http.

HTTP-only

This example is the same with an L4 LB, or without a load balancer at all.

apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: minimal-host
spec:
  hostname: host.example.com
  acmeProvider:
    authority: none
  requestPolicy:
    insecure:
      action: Route

In this case, the Host resource explicitly requests no ACME handling and no TLS, then states that insecure requests must be routed instead of redirected.

L4 LB, HTTPS-only, TLS terminated at Ambassador, not redirecting cleartext

Configure this exactly like case 1. Leave xff_num_trusted_hops in the ambassador module at its default of 0.

L4 LB, HTTPS-only, TLS terminated at Ambassador, redirecting cleartext from port 8080

This use case is not supported by Ambassador 1.1.1. It will be supported in a forthcoming release.

L4 LB, HTTP-only

Configure this exactly like case 3. Leave xff_num_trusted_hops in the ambassador module at its default of 0.

L4 LB, TLS terminated at LB, LB speaks cleartext to Ambassador

Configure this exactly like case 3, since by the time the connection arrives at Ambassador, it will appear to be insecure. Leave xff_num_trusted_hops in the ambassador module at its default of 0.

L4 LB, TLS terminated at LB, LB speaks TLS to Ambassador

Configure this exactly like case 1. Leave xff_num_trusted_hops in the ambassador module at its default of 0.

Note that since Ambassador is terminating TLS, managing Ambassador's TLS certificate will be important.

L4 split LB, TLS terminated at Ambassador

In this scenario, an L4 load balancer terminates TLS on port 443 and relays that traffic as cleartext to Ambassador on port 8443, but the load balancer also relays cleartext traffic on port 80 to Ambassador on port 8080. (This could also be two L4 load balancers working in concert.)

Configure this exactly like case 2. Leave xff_num_trusted_hops in the ambassador module at its default of 0.

L4 split LB, TLS terminated at LB

In this scenario, an L4 load balancer terminates TLS on port 443 and relays that traffic as cleartext to Ambassador on port 8443, but the load balancer also relays cleartext traffic on port 80 to Ambassador on port 8080. (This could also be two L4 load balancers working in concert.)

This case is not supported in Ambassador 1.1.1. It will be supported in a forthcoming release.

L7 LB

In general, L7 load balancers will be expected to provide a correct X-Forwarded-Proto header, and will require xff_num_trusted_hops set to the depth of the L7 LB stack in front of Ambassador.

  • client -> L7 LB -> Ambassador would require xff_num_trusted_hops: 1

  • client -> L7 LB -> L7 LB -> Ambassador would require xff_num_trusted_hops: 2

  • etc.

    If using an L7 LB, we recommend that the LB handle TLS termination and redirection of cleartext. For this use case, you can use a Host without TLS, but still turn on redirection as a failsafe:

    ---
    apiVersion: getambassador.io/v2
    kind: Host
    metadata:
      name: l7-redirection-host
    spec:
      hostname: foo.example.com
      # TLS happens at the LB, so disable it here.
      acmeProvider:
        authority: none
      # The default insecure action is Redirect, which is fine.

    However, as long as the L7 LB is properly supplying X-Forwarded-Proto and xff_num_trusted_hops is set correctly, it should be possible to configure Ambassador to handle TLS and redirection of cleartext, by configuring Ambassador as if the L7 LB was not present (cases 1 - 3 above).

    Again, it is critical that the load balancer correctly supplies X-Forwarded-Proto, and that xff_num_trusted_hops is set correctly.

Host Specification

The Ambassador Edge Stack automates the creation of TLS certificates via the Edge Policy Console, which provides HTTPS for your hosts. Note that in order to have TLS and automatic HTTPS, your host must be an FQDN.

The Host CRD defines how Ambassador will be visible to the outside world. A minimal Host defines a hostname by which the Ambassador will be reachable, but a Host can also tell an Ambassador how to manage TLS, and which resources to examine for further configuration.

CRD Specification

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

Questions?

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