k8s_crd
A CoreDNS plugin that is very similar to k8s_external but supporting DNSEndpoint external resource.
This project is a modification of k8s_gateway plugin, adopted with DNSEndpoint client.
This plugin relies on it's own connection to the k8s API server and doesn't share any code with the existing kubernetes plugin. The assumption is that this plugin can now be deployed as a separate instance (alongside the internal kube-dns) and act as a single external DNS interface into your Kubernetes cluster(s).
Description
k8s_crd
resolves Kubernetes resources with their external IP addresses based on zones specified in the configuration. This plugin will resolve the following type of resources:
Kind |
Matching Against |
External IPs are from |
DNSEndponit |
all FQDNs from spec.endpoints.dnszone matching configured zones |
.spec.endpoints.dnszone.targets |
Currently only supports A-type queries, all other queries result in NODATA responses.
This plugin is NOT supposed to be used for intra-cluster DNS resolution and does not contain the default upstream kubernetes plugin.
Install
The recommended installation method is using the helm chart provided in the repo:
helm install exdns ./charts/coredns
k8s_crd [ZONE...]
Optionally, you can specify what kind of resources to watch, default TTL to return in response and a default name to use for zone apex, e.g.
k8s_crd example.com {
ttl 10
apex dns1
}
Resolving order
GeoIP
In case dnsEndpoint object's target has a label of strategy: geoip
CoreDNS k8s_crd
plugin will respond in a special way:
- Assuming record has multiple IPs associated with it, and DNS message comes with edns0
CLIENT-SUBNET
option.
- CoreDNS will compare
DC
tag for IP extracted from CLIENT-SUBNET
option against available Endpoint.Targets
- Return only IPs where tags match
- If IP has no common tag, all entries are returned.
- CoreDNS must be supplied with a specially crafted GeoIP database in MaxMind DB format and mounted as
/geoip.mmdb
Refer to ./terratest/geogen for examples.
Weight Round Robin
To enable the weight round robin you have to set the configuration to weight load-balancer:
k8s_crd example.com {
loadbalance weight
...
}
The dnsEndpoint must also contain information about the percentage distribution per region
and their IP addresses. Thanks to this, the weight round-robin module will know in which
order to return IP addresses. Addresses with high probability will often be at the top of
DNS responses, while those with low probability will be at the bottom.
labels:
strategy: roundrobin
weight-eu-0-50: 10.0.0.1
weight-eu-1-50: 10.0.0.2
weight-za-0-0: 10.10.0.1
weight-us-0-50: 10.20.0.1
For more information about balancing, please visit our go-weight-shuffling module.
Build
With compile-time configuration file
$ git clone https://github.com/coredns/coredns
$ cd coredns
$ vim plugin.cfg
# Replace lines with kubernetes and k8s_external with k8s_crd:github.com/absaoss/k8s_crd
$ go generate
$ go build
$ ./coredns -plugins | grep k8s_crd
With external golang source code
$ git clone https://github.com/absaoss/k8s_crd.git
$ cd k8s_crd
$ go build cmd/coredns.go
$ ./coredns -plugins | grep k8s_crd
For more details refer to this CoreDNS doc
Notes regarding Zone Apex and NS server resolution
Due to the fact that there is not nice way to discover NS server's own IP to respond to A queries, as a wokaround, it's possible to pass the name of the LoadBalancer service used to expose the CoreDNS instance as an environment variable EXTERNAL_SVC
. If not set, the default fallback value of external-dns.kube-system
will be used to look up the external IP of the CoreDNS service.