Hostname not registered in the namespace - kubernetes

Question

I have set Kuberbetes cluster via Rancher. All looks fine till I have decided to deploy Helm Chart app to it (bitnami/wordpress).
Pods and services in the wordpress namespace:
> kubectl get pods,svc -owide --namespace=wordpress
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
pod/wordpress-6647794f9b-4mmxd 0/1 Running 20 104m 10.42.0.19 dev-app <none> <none>
pod/wordpress-mariadb-0 1/1 Running 1 26h 10.42.0.14 dev-app <none> <none>
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
service/wordpress LoadBalancer 10.43.91.13 <pending> 80:30158/TCP,443:30453/TCP 26h app.kubernetes.io/instance=wordpress,app.kubernetes.io/name=wordpress,io.cattle.field/appId=wordpress
service/wordpress-mariadb ClusterIP 10.43.178.123 <none> 3306/TCP 26h app=mariadb,component=master,io.cattle.field/appId=wordpress,release=wordpress
Then I have tried to verify connections from the Wordpress CMS to the MariaDB:
On the MariaDB pod:
I have no name!#wordpress-mariadb-0:/$ cat /etc/hosts
# Kubernetes-managed hosts file.
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
fe00::0 ip6-mcastprefix
fe00::1 ip6-allnodes
fe00::2 ip6-allrouters
10.42.0.14 wordpress-mariadb-0.wordpress-mariadb.wordpress.svc.cluster.local wordpress-mariadb-0
No clue why -0 was added to the host name but
I have no name!#wordpress-mariadb-0:/$ mysql -h wordpress-mariadb-0.wordpress-mariadb.wordpress.svc.cluster.local -u root -p
Enter password:
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 48253
Server version: 10.3.22-MariaDB Source distribution
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]>
works fine from the MariaDB pod.
Than on the Wordpress pod:
I have no name!#wordpress-6647794f9b-4mmxd:/$ mysql -h wordpress-mariadb-0.wordpress-mariadb.wordpress.svc.cluster.local -u root -p
Enter password:
ERROR 2005 (HY000): Unknown MySQL server host 'wordpress-mariadb-0.wordpress-mariadb.wordpress.svc.cluster.local' (-3)
for some reason hostname is not registered for that namespace.
Any clue?
PS
From the wordpress pod:
I have no name!#wordpress-6647794f9b-4mmxd:/$ cat /etc/hosts
# Kubernetes-managed hosts file.
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
fe00::0 ip6-mcastprefix
fe00::1 ip6-allnodes
fe00::2 ip6-allrouters
10.42.0.19 wordpress-6647794f9b-4mmxd
# Entries added by HostAliases.
127.0.0.1 status.localhost
and
I have no name!#wordpress-6647794f9b-4mmxd:/$ cat /etc/resolv.conf
nameserver 10.43.0.10
search wordpress.svc.cluster.local svc.cluster.local cluster.local
options ndots:5
PS2
> kubectl get pod -n kube-system
NAME READY STATUS RESTARTS AGE
canal-8bf2l 2/2 Running 0 16d
canal-g782s 2/2 Running 2 16d
canal-vq474 2/2 Running 0 16d
coredns-849545576b-gcf7p 1/1 Running 0 16d
coredns-849545576b-vtqpw 1/1 Running 0 3d20h
coredns-autoscaler-84bf756579-v594n 1/1 Running 0 3d20h
metrics-server-697746ff48-rtw2h 1/1 Running 1 16d
rke-coredns-addon-deploy-job-2sjlv 0/1 Completed 0 3d20h
rke-ingress-controller-deploy-job-9q4c2 0/1 Completed 0 3d20h
rke-metrics-addon-deploy-job-cv42h 0/1 Completed 0 3d20h
rke-network-plugin-deploy-job-4pddn 0/1 Completed 0 3d20h

Answer 1

Mariadb is deployed as StatefulSet.Each Pod in a StatefulSet derives its hostname from the name of the StatefulSet and the ordinal of the Pod. The pattern for the constructed hostname is $(statefulset name)-$(ordinal). This explains why -0 in podname and hostname.
Ideally you should be using the Headless service exposing the mysql statefulset to access mysql from wordpress. Below is an example of service, you don't need to create it because it already exists with name
wordpress-mariadb
apiVersion: v1
kind: Service
metadata:
name: wordpress-mariadb
namespace: wordpress
labels:
app: wordpress
spec:
ports:
- port: 3306
selector:
app: wordpress
tier: mysql
clusterIP: None
In wordpress pod running in same namespace use wordpress-mariadb to access mariadb
Official guide on deploying wordpress with mysql

Answer 2

For some reason one of the nodes with CoreDNS pod (I had two of them) had a problem with providing data for the kube-dns service.
Rebuilding that node solved my problem.