Find a way to allow xds_cluster to be defined as a DNS name, not an IP address

[davecheney]

Using contour bootstrap --xds-address=contour.heptio-contour.svc.cluster.local causes the following bootstrap config to be written.

dynamic_resources:
  lds_config:
    api_config_source:
      api_type: GRPC
      cluster_names: [xds_cluster]
      grpc_services:
      - envoy_grpc:
          cluster_name: xds_cluster
  cds_config:
    api_config_source:
      api_type: GRPC
      cluster_names: [xds_cluster]
      grpc_services:
      - envoy_grpc:
          cluster_name: xds_cluster
static_resources:
  clusters:
  - name: xds_cluster
    connect_timeout: { seconds: 5 }
    type: STATIC
    hosts:
    - socket_address:
        address: contour.heptio-contour.svc.cluster.local
        port_value: 8001
    lb_policy: ROUND_ROBIN
    http2_protocol_options: {}
admin:
  access_log_path: /dev/null
  address:
    socket_address:
      address: 127.0.0.1
      port_value: 9001

Which envoy rejects

[2018-02-08 00:47:01.269][1][info][main] source/server/server.cc:184] initializing epoch 0 (hot restart version=9.200.16384.127.options=capacity=16384, num_slots=8209 hash=228984379728933363)
[2018-02-08 00:47:01.292][1][critical][main] source/server/server.cc:71] error initializing configuration '/config/contour.yaml': malformed IP address: contour.heptio-contour.svc.cluster.local

Figure out if it is possible to use a DNS name for the xds_cluster static resource.

[sevein]

It seems that STATIC clusters use IpResolver, that explains the error malformed IP address.

SocketAddress has a resolver_name field that we could use.

// The name of the resolver. This must have been registered with Envoy. If this is
// empty, a context dependent default applies. If address is a hostname this
// should be set for resolution other than DNS. If the address is a concrete
// IP address, no resolution will occur.
string resolver_name = 5;

What do you think they mean by registering it with Envoy?