Kuma DNS

The Kuma control plane deploys its Domain Name Service resolver on UDP port 5653 (resembling the standard port 53). Its purpose is to allow for decoupling the service name resolving from the underlying infrastructure and thus make Kuma more flexible. When Kuma is deployed as a distributed control plane, the Kuma DNS enables cross-cluster service discovery.

Deployment

To enable the redirection of the DNS requests for the .mesh DNS zone (the default), within a Kubernetes, use kumactl install dns | kubectl apply -f -. This invocation of kumactl expects to find the environment variable KUBECONFIG set, so it can fetch the active Kubernetes DNS server configuration. Once this is done, kumactl install dns will output a patched resource ready to be applied through kubectl apply. Since this is a modification to system resources, it is strongly recommended that you first inspect the resulting configuration.

kumactl install dns is recognizing and supports the major flavors of CoreDNS as well as Kube DNS resources.

The typical environment where Kuma DNS will be used is Kubernetes. It leverages the transparent proxy by default, which is a strict requirement for utilizing the Kuma DNS virtual IPs (VIP). In the future, Kuma will provide DNS support in Universal mode too.

Configuration

Kuma DNS can be configured by the configuration file, or by the respective environment variables as follows:

  1. # DNS Server configuration
  2. dnsServer:
  3.   # The domain that the server will resolve the services for
  4.   domain: "mesh" # ENV: KUMA_DNS_SERVER_DOMAIN
  5.   # Port on which the server is exposed
  6.   port: 5653 # ENV: KUMA_DNS_SERVER_PORT
  7.   # The CIDR range used to allocate
  8.   CIDR: "240.0.0.0/4" # ENV: KUMA_DNS_SERVER_CIDR

The domain field can change the default .mesh DNS zone that Kuma DNS will resolve for. If this is changed, please check the output of kumactl install dns and change the zone accordingly, so that your Kube DNS or Core DNS server will redirect all the relevant DNS requests.

The port can set the port on which the Kuma DNS is accepting requests. Changing this value on Kubernetes shall be reflected in the respective port setting in the kuma-control-plane service.

The CIDR field sets the IP range of virtual IPs. The default 240.0.0.0/4 is reserved for future use IPv4 range and is guaranteed to be non-routable. We strongly recommend to not change this, unless it is needed.

Operation

The basic operation of Kuma DNS includes a couple of main components: DNS server, VIPs allocator, cross-replica persistence.

The DNS server listens on port 5653 and responds for type A DNS requests and answers with A record, e.g. <service>.mesh. 60 IN A 240.0.0.100. The default TTL is set to 60 seconds, to ensure the client will synchronize with Kuma DNS and account for any changes happening meanwhile.

Kuma DNS allocates the virtual IPs from the configured CIDR, by constantly scanning the services available in all Kuma meshes. When a service is removed its VIP is freed too and Kuma DNS will not respond for it with A DNS record.

Kuma DNS is not a service discovery mechanism, instead it returns a single VIP, mapped to the relevant service in the mesh. This makes for a unified view of all services within the zone or cross-zones.

Usage

Consuming a service handled by Kuma DNS from inside a Kubernetes container is based on the automatically generated kuma.io/service tag. The resulting domain name has the format {service tag}.mesh, for example:

  1. <kuma-enabled-pod>$ curl http://echo-server_echo-example_svc_1010.mesh:80
  2. <kuma-enabled-pod>$ curl http://echo-server_echo-example_svc_1010.mesh

Alternatively, a DNS standards compliant name is available, where the underscores in the service name are replaced with dots. The above example can be rewritten as follows:

  1. <kuma-enabled-pod>$ curl http://echo-server.echo-example.svc.1010.mesh:80
  2. <kuma-enabled-pod>$ curl http://echo-server.echo-example.svc.1010.mesh

Since the default VIP created listeners will default to port 80, it can be omitted when using a standard HTTP client.

Kuma DNS allocates a VIP for every Service within a mesh. Then, it creates outbound virtual listener for every VIP. However, by inspecting curl localhost:9901/config_dump, we can see sections similar to this one:

  1.     {
  2.      "name": "outbound:240.0.0.1:80",
  3.      "active_state": {
  4.       "version_info": "51adf4e6-287e-491a-9ae2-e6eeaec4e982",
  5.       "listener": {
  6.        "@type": "type.googleapis.com/envoy.api.v2.Listener",
  7.        "name": "outbound:240.0.0.1:80",
  8.        "address": {
  9.         "socket_address": {
  10.          "address": "240.0.0.1",
  11.          "port_value": 80
  12.         }
  13.        },
  14.        "filter_chains": [
  15.         {
  16.          "filters": [
  17.           {
  18.            "name": "envoy.filters.network.tcp_proxy",
  19.            "typed_config": {
  20.             "@type": "type.googleapis.com/envoy.config.filter.network.tcp_proxy.v2.TcpProxy",
  21.             "stat_prefix": "echo-server_kuma-test_svc_80",
  22.             "cluster": "echo-server_kuma-test_svc_80"
  23.            }
  24.           }
  25.          ]
  26.         }
  27.        ],
  28.        "deprecated_v1": {
  29.         "bind_to_port": false
  30.        },
  31.        "traffic_direction": "OUTBOUND"
  32.       },
  33.       "last_updated": "2020-07-06T14:32:59.732Z"
  34.      }
  35.     },