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External Service

This policy allows services running inside the mesh to consume services that are not part of the mesh. The ExternalService resource allows you to declare specific external resources by name within the mesh, instead of implementing the default passthrough mode. Passthrough mode allows access to any non-mesh host by specifying its domain name or IP address, without the ability to apply any traffic policies. The ExternalService resource enables the same observability, security, and traffic manipulation for external traffic as for services entirely inside the mesh

When you enable this policy, you should also disable passthrough mode for the mesh and enable the data plane proxy builtin DNS name resolution.

Usage

Simple configuration of external service requires name of the resource, kuma.io/service: service-name, and address. By default, a protocol used for communication is TCP. It’s possible to change that by configuring kuma.io/protocol tag. Apart from that, it’s possible to define TLS configuration used for communication with external services. More information about configuration options can be found here.

Below is an example of simple HTTPS external service:

apiVersion: kuma.io/v1alpha1
kind: ExternalService
mesh: default
metadata:
  name: httpbin
spec:
  tags:
    kuma.io/service: httpbin
    kuma.io/protocol: http # optional, one of http, http2, tcp, grpc, kafka
  networking:
    address: httpbin.org:443
    tls: # optional
      enabled: true
      allowRenegotiation: false
      serverName: httpbin.org # optional
      caCert: # one of inline, inlineString, secret
        inline: LS0tLS1CRUdJTiBDRVJUSUZJQ0FURS0tLS0t... # Base64 encoded cert
      clientCert: # one of inline, inlineString, secret
        secret: clientCert
      clientKey: # one of inline, inlineString, secret
        secret: clientKey

Then apply the configuration with kubectl apply -f [..].

Accessing the External Service

Consuming the defined service from within the mesh for both Kubernetes and Universal deployments (assuming transparent proxy) can be done:

  • With the .mesh naming of the service curl httpbin.mesh. With this approach, specify port 80.
  • With the real name and port, in this case curl httpbin.org:443. This approach works only with the data plane proxy builtin DNS name resolution.

It’s possible to define TLS origination and validation at 2 different layers:

  • Envoy is responsible for originating and verifying TLS.
  • Application itself is responsible for originating and verifying TLS and Envoy is just passing the connection to a proper destination.

In the first case, the external service is defined as HTTPS, but it’s consumed as plain HTTP. This is possible because when networking.tls.enabled is set to true then Envoy is responsible for originating and verifying TLS.

The second approach allows consuming the service using HTTPS. It’s possible when kuma.io/protocol: tcp and networking.tls.enabled=false are set in the configuration of the external service.

The first approach has an advantage that we can apply HTTP based policies, because Envoy is aware of HTTP protocol and can apply request modifications before the request is encrypted. Additionally, we can modify TLS certificates without restarting applications.

Available policy fields

  • tags the external service can include an arbitrary number of tags, where:
    • kuma.io/service is mandatory.
    • kuma.io/protocol tag is also taken into account and supports the standard Kuma protocol values. It designates the specific protocol for the service (one of: http, tcp, grpc, kafka, default: tcp).
    • kuma.io/zone tag is taken into account when locality aware load balancing is enabled or external service should be accessible only from the specific zone.
  • ` networking` describes the networking configuration of the external service:
    • address the address of the external service. It has to be a valid IP address or a domain name, and must include a port.
    • tls is the section to configure the TLS originator when consuming the external service:
      • enabled turns on and off the TLS origination.
      • allowRenegotiation turns on and off TLS renegotiation. It’s not recommended enabling this for security reasons. However, some servers require this setting to fetch client certificate after TLS handshake. TLS renegotiation is not available in TLS v1.3.
      • serverName overrides the default Server Name Indication. Set this value to empty string to disable SNI.
      • caCert the CA certificate for the external service TLS verification.
      • clientCert the client certificate for mTLS.
      • clientKey the client key for mTLS.

As with other services, avoid duplicating service names under kuma.io/service with already existing ones. A good practice is to derive the tag value from the domain name or IP of the actual external service.

External Services and Locality Aware Load Balancing

There are might be scenarios when a particular external service should be accessible only from the particular zone. In order to make it work we should use kuma.io/zone tag for external service. When this tag is set and locality-aware load balancing is enabled then the traffic from the zone will be redirected only to external services associated with the zone using kuma.io/zone tag.

Example:

type: ExternalService
mesh: default
name: httpbin-for-zone-1
tags:
  kuma.io/service: httpbin
  kuma.io/protocol: http
  kuma.io/zone: zone-1
networking:
  address: zone-1.httpbin.org:80
---
type: ExternalService
mesh: default
name: httpbin-for-zone-2
tags:
  kuma.io/service: httpbin
  kuma.io/protocol: http
  kuma.io/zone: zone-2
networking:
  address: zone-2.httpbin.org:80

In this example, when locality-aware load balancing is enabled, if the service in the zone-1 is trying to set connection with httpbin.mesh it will be redirected to zone-1.httpbin.org:80. Whereas the same request from the zone-2 will be redirected to zone-2.httpbin.org:80.

If ZoneEgress is enabled, there is a limitation that prevents the behavior described above from working. The control-plane replaces the external service’s address in the remote zone with the IP address of ZoneEgress. This causes a problem because Envoy does not support a cluster that use both DNS and IP addresses as endpoints definition.

External Services and ZoneEgress

In scenarios when traffic to external services needs to be sent through a unique set of hosts you will configure ZoneEgress.

For example when there is:

  • disabled passthrough mode
  • ZoneEgress deployed
  • ExternalService configuration that allows communicating with https://example.com. 
    type: ExternalService
mesh: default
name: example
tags:
kuma.io/service: example
kuma.io/protocol: tcp
networking:
address: example.com:443
tls:
  enabled: false

When application makes a request to https://example.com, it will be first routed to ZoneEgress and then to https://example.com. You can completely block your instances to communicate to things outside the mesh by disabling passthrough mode. In this setup, applications will only be able to communicate with other applications in the mesh or external-services via the ZoneEgress.

The ExternalService with the same kuma.io/service name cannot mix dns names and IP addresses of the endpoint.

Example:

---
type: ExternalService
mesh: default
name: example-1
tags:
  kuma.io/service: example
  kuma.io/protocol: tcp
networking:
  address: example.com:443
---
type: ExternalService
mesh: default
name: example-2
tags:
  kuma.io/service: example
  kuma.io/protocol: tcp
networking:
  address: 192.168.0.1:443

The above configuration is incorrect and configuration generation will fail.

External Services accessible from specific zone through ZoneEgress

There are might be scenarios when a specific ExternalService might be accessible only through the specific zone. To make it work we should use the kuma.io/zone tag for external service. In order to make it work, we need a multi-zone setup with ZoneIngress and ZoneEgress deployed. Also, zone egress needs to be enabled.

Example:

type: ExternalService
mesh: default
name: httpbin-only-in-zone-2
tags:
 kuma.io/service: httpbin
 kuma.io/protocol: http
 kuma.io/zone: zone-2
networking:
 address: httpbin.org:80

In this example, when all the conditions mentioned above are fulfilled if the service in zone-1 is trying to set a connection with httpbin.mesh it will be redirected to the ZoneEgress instance within the zone-1. Next, this request goes to the ZoneIngress instance in zone-2 which redirects it to the ZoneEgress cluster instance from where it goes outside to the ExternalService.

Builtin Gateway support

Kuma Gateway fully supports external services. Note that mesh Dataplanes can be configured with the same kuma.io/service tag as an external service resource. In this scenario, Kuma Gateway will prefer the ExternalService and not route any traffic to the Dataplanes. Note that before gateway becomes generally available this behaviour will change to be the same as for any other dataplanes.

All options