How do I get individual pod hostnames in a Deployment registered and looked up in Kubernetes?
CoreDNS creates A and SRV records only for Services. It doesn't generate pods' A records as you may expect after reading the documentation:
The
pods insecure
option is provided for backward compatibility with kube-dns. You can use thepods verified
option, which returns an A record only if there exists a pod in same namespace with matching IP. Thepods disabled
option can be used if you don’t use pod records.
with the one exception: if you create a Headless service (when you specify ClusterIP: None
in the Service spec)
So, here is my example of Headless Service based on your YAML:
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGEdefault-subdomain ClusterIP None <none> 1234/TCP 50s
Here is the list of pods created by the above deployment on my cluster:
NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATESdefault busybox1-76745fcdbf-4ppsf 1/1 Running 0 18s 10.244.1.22 kube-node2-1 <none> <none>default busybox1-76745fcdbf-d76q5 1/1 Running 0 18s 10.244.1.23 kube-node2-1 <none> <none>
In this case, instead of one A and one SRV record for Service's ClusterIP, we have two A and two SRV records with the same name, and IP addresses of Pods which are Endpoints for the Headless Service:
default-subdomain.default.svc.cluster.local. 5 IN A 10.244.1.22_foo._tcp.default-subdomain.default.svc.cluster.local. 5 IN SRV 0 50 1234 10-244-1-22.default-subdomain.default.svc.cluster.local.default-subdomain.default.svc.cluster.local. 5 IN A 10.244.1.23_foo._tcp.default-subdomain.default.svc.cluster.local. 5 IN SRV 0 50 1234 10-244-1-23.default-subdomain.default.svc.cluster.local.
To resolve SRV records, A records also has been created for both Headless Service endpoints.
If you don't specify specify hostname
and subdomain
for pods, A records will be created with IP addresses as a hostnames:
10-244-1-22.default-subdomain.default.svc.cluster.local. 5 IN A 10.244.1.2210-244-1-23.default-subdomain.default.svc.cluster.local. 5 IN A 10.244.1.23
But if you are specify both of them you will get these record as follows:
dummy.default-subdomain.default.svc.cluster.local. 5 IN A 10.244.1.22dummy.default-subdomain.default.svc.cluster.local. 5 IN A 10.244.1.23
SRV records will look as follows in this case (yes, there are still two of them and they are the same):
_foo._tcp.default-subdomain.default.svc.cluster.local. 5 IN SRV 0 50 1234 dummy.default-subdomain.default.svc.cluster.local._foo._tcp.default-subdomain.default.svc.cluster.local. 5 IN SRV 0 50 1234 dummy.default-subdomain.default.svc.cluster.local.
CoreDNS server resolve such records in "random" way (IP addresses is changing):
root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.27) 56(84) bytes of data.root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.27) 56(84) bytes of data.root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.26) 56(84) bytes of data.root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.27) 56(84) bytes of data.root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.26) 56(84) bytes of data.root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.26) 56(84) bytes of data.root@ubuntu:/# ping dummy.default-subdomain.default.svc.cluster.local -c 1 | grep PINGPING dummy.default-subdomain.default.svc.cluster.local (10.244.1.27) 56(84) bytes of data.
To debug it, I've used zone transfer feature that CoreDNS supports. To enable it you should add transfer to *
line to coredns ConfigMap. You can replace * with specific IP for security. Example:
$ kubectl get cm coredns -n kube-system -o yamlapiVersion: v1data: Corefile: | .:53 { errors health kubernetes cluster.local in-addr.arpa ip6.arpa { transfer to * <---- enable zone transfer to anyone(don't use in production) pods insecure upstream fallthrough in-addr.arpa ip6.arpa } prometheus :9153 forward . /etc/resolv.conf cache 30 loop reload loadbalance }kind: ConfigMapmetadata: creationTimestamp: "2019-05-07T15:44:02Z" name: coredns namespace: kube-system resourceVersion: "9166" selfLink: /api/v1/namespaces/kube-system/configmaps/coredns uid: f0646569-70de-11e9-9af0-42010a9c0015
After that you'll be able to list all DNS records from cluster.local
zone using the following command:
dig -t AXFR cluster.local any
More information can be found here:
if you want to do that consider using instead an Stateful Set where you can go to the pod like that:
podname-{replica-index}.{serviceName}.default.svc.cluster.local
here an example https://kubernetes.io/docs/tutorials/stateful-application/cassandra/