Merge pull request #33550 from rtreffer/kubelet-allow-multiple-dns-server

Automatic merge from submit-queue

Allow multipe DNS servers as comma-seperated argument for kubelet --dns

This PR explores how kubectls "--dns" could be extended to specify multiple DNS servers for in-cluster PODs. Testing on the local libvirt-coreos cluster shows that multiple DNS server are injected without issues.

Specifying multiple DNS servers increases resilience against
- Packet drops
- Single server failure

I am debugging services that do 50+ DNS requests for a single incoming interactive request, thus highly increase the chance of a slowdown (+5s) due to a single packet drop. Switching to two DNS servers will reduce the impact of the issues (roughly +1s on glibc, 0s on musl, error-rate goes down to error-rate^2).

Note that there is no need to change any runtime related code as far as I know. In the case of "default" dns the /etc/resolv.conf is parsed and multiple DNS server are send to the backend anyway. This only adds the same capability for the clusterFirst case.

I've heard from @thockin that multiple DNS entries are somehow considered. I've no idea what was considered, though. This is what I would like to see for our production use, though.

```release-note
NONE
```

pkg/generated/openapi/zz_generated.openapi.go

@@ -10710,9 +10710,16 @@ func GetOpenAPIDefinitions(ref openapi.ReferenceCallback) map[string]openapi.Ope
 						},
 						"clusterDNS": {
 							SchemaProps: spec.SchemaProps{
-								Description: "clusterDNS is the IP address for a cluster DNS server.  If set, kubelet will configure all containers to use this for DNS resolution in addition to the host's DNS servers",
-								Type:        []string{"string"},
-								Format:      "",
+								Description: "clusterDNS is a list of IP address for the cluster DNS server.  If set, kubelet will configure all containers to use this for DNS resolution instead of the host's DNS servers",
+								Type:        []string{"array"},
+								Items: &spec.SchemaOrArray{
+									Schema: &spec.Schema{
+										SchemaProps: spec.SchemaProps{
+											Type:   []string{"string"},
+											Format: "",
+										},
+									},
+								},
 							},
 						},
 						"streamingConnectionIdleTimeout": {