# DNS

Kuma ships with DNS resolver to provide service naming - a mapping of hostname to Virtual IPs (VIPs) of services registered in Kuma.

The usage of Kuma DNS is only relevant when transparent proxying is used.

# How it works

Kuma DNS server responds to type A and AAAA DNS requests, and answers with A or AAAAA records, for example redis.mesh. 60 IN A 240.0.0.100 or redis.mesh. 60 IN AAAAA fd00:fd00::100.

The virtual IPs are allocated by the control plane from the configured CIDR (by default 240.0.0.0/4) , by constantly scanning the services available in all Kuma meshes. When a service is removed, its VIP is also freed, and Kuma DNS does not respond for it with A and AAAA DNS record. Virtual IPs are stable (replicated) between instances of the control plane and data plane proxies.

Once a new VIP is allocated or an old VIP is freed, the control plane configures the data plane proxy with this change.

All name lookups are handled locally by the data plane proxy, not by the control plane. This approach allows for more robust handling of name resolution. For example, when the control plane is down, a data plane proxy can still resolve DNS.

The data plane proxy DNS consists of:

  • an Envoy DNS filter provides responses from the mesh for DNS records
  • a CoreDNS instance launched by kuma-dp that sends requests between the Envoy DNS filter and the original host DNS
  • iptable rules that will redirect the original DNS traffic to the local CoreDNS instance

As the DNS requests are sent to the Envoy DNS filter first, any DNS name that exists inside the mesh will always resolve to the mesh address. This in practice means that DNS name present in the mesh will "shadow" equivalent names that exist outside the mesh.

Kuma DNS is not a service discovery mechanism, it does not return real IP address of service instances. Instead, it always returns a single VIP that is assigned to the relevant service in the mesh. This makes for a unified view of all services within a single zone or across multiple zones.

The default TTL is 60 seconds, to ensure the client synchronizes with Kuma DNS and to account for any intervening changes.

# Installation

    # Special considerations

    This mode implements advanced networking techniques, so take special care for the following cases:

    # Overriding the CoreDNS configuration

    In some cases it might be useful for you to configure the default CoreDNS.

      This file is a CoreDNS configuration (opens new window) that is processed as a go-template. If you edit this configuration you should base yourself on the default existing configuration, which looks like the following

      .:{{ .CoreDNSPort }} {
          forward . 127.0.0.1:{{ .EnvoyDNSPort }}
          # We want all requests to be sent to the Envoy DNS Filter, unsuccessful responses should be forwarded to the original DNS server.
          # For example: requests other than A, AAAA and SRV will return NOTIMP when hitting the envoy filter and should be sent to the original DNS server.
          # Codes from: https://github.com/miekg/dns/blob/master/msg.go#L138
          alternate NOTIMP,FORMERR,NXDOMAIN,SERVFAIL,REFUSED . /etc/resolv.conf
          prometheus localhost:{{ .PrometheusPort }}
          errors
      }
      
      .:{{ .CoreDNSEmptyPort }} {
          template ANY ANY . {
            rcode NXDOMAIN
          }
      }
      
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      # Configuration

      You can configure Kuma DNS in kuma-cp:

      dnsServer:
        CIDR: "240.0.0.0/4" # ENV: KUMA_DNS_SERVER_CIDR
        domain: "mesh" # ENV: KUMA_DNS_SERVER_DOMAIN
        serviceVipEnabled: true # ENV: KUMA_DNS_SERVER_SERVICE_VIP_ENABLED
      
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      The CIDR field sets the IP range of virtual IPs. The default 240.0.0.0/4 is reserved for future IPv4 use and is guaranteed to be non-routable. We strongly recommend to not change this value unless you have a specific need for a different IP range.

      The domain field specifies the default .mesh DNS zone that Kuma DNS provides resolution for. It's only relevant when serviceVipEnabled is set to true.

      The serviceVipEnabled field defines if there should be a vip generated for each kuma.io/service. This can be disabled for performance reason and virtual-outbound provides a more flexible way to do this.

      # Usage

      Consuming a service handled by Kuma DNS, whether from Kuma-enabled Pod on Kubernetes or VM with kuma-dp, is based on the automatically generated kuma.io/service tag. The resulting domain name has the format {service tag}.mesh. For example:

      <kuma-enabled-pod>$ curl http://echo-server_echo-example_svc_1010.mesh:80
      <kuma-enabled-pod>$ curl http://echo-server_echo-example_svc_1010.mesh
      
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      A DNS standards compliant name is also available, where the underscores in the service name are replaced with dots. For example:

      <kuma-enabled-pod>$ curl http://echo-server.echo-example.svc.1010.mesh:80
      <kuma-enabled-pod>$ curl http://echo-server.echo-example.svc.1010.mesh
      
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      The default listeners created on the VIP default to port 80, so the port can be omitted with a standard HTTP client.

      Kuma DNS allocates a VIP for every service within a mesh. Then, it creates an outbound virtual listener for every VIP. If you inspect the result of curl localhost:9901/config_dump, you can see something similar to:

          {
           "name": "outbound:240.0.0.1:80",
           "active_state": {
            "version_info": "51adf4e6-287e-491a-9ae2-e6eeaec4e982",
            "listener": {
             "@type": "type.googleapis.com/envoy.api.v2.Listener",
             "name": "outbound:240.0.0.1:80",
             "address": {
              "socket_address": {
               "address": "240.0.0.1",
               "port_value": 80
              }
             },
             "filter_chains": [
              {
               "filters": [
                {
                 "name": "envoy.filters.network.tcp_proxy",
                 "typed_config": {
                  "@type": "type.googleapis.com/envoy.config.filter.network.tcp_proxy.v2.TcpProxy",
                  "stat_prefix": "echo-server_kuma-test_svc_80",
                  "cluster": "echo-server_kuma-test_svc_80"
                 }
                }
               ]
              }
             ],
             "deprecated_v1": {
              "bind_to_port": false
             },
             "traffic_direction": "OUTBOUND"
            },
            "last_updated": "2020-07-06T14:32:59.732Z"
           }
          }
      
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      The following setup will work when serviceVipEnabled=true which is a default value.

      The preferred way to define hostnames is using Virtual Outbounds. Virtual Outbounds also makes it possible to define dynamic hostnames using specific tags or to expose services on a different port.

      Last Updated: 5/6/2022, 8:14:08 AM