design-proposal: dns probe name customize

At some users customizing globally the dns probe name to resolve is a
must, this change add a design doc to do it at NMState CR.

Signed-off-by: Enrique Llorente <[email protected]>

design-proposals/customize-dns-probe-globally.md

@@ -0,0 +1,72 @@
+# Globally customize DNS cluster health probe.
+
+## Summary
+
+Support customize kubernetes-nmstate DNS resolving probes at NMState CR.
+
+## Motivation
+
+At some cluster their network configuration is not compatible with DNS health
+probe the kubernetes-nmstate cluster health probes, so the NNCPs will fail.
+
+### User Stories
+
+- As a cluster administrator, I want to customize globally the kubernetes-nmstate DNS health probes so they work for my cluster.
+
+### Goals
+
+- Allow users to customize cluster health probes DNS globally using the NMState CR
+
+### Non-Goals
+
+- Allow user to disable DNS probe
+- Allow user to customize cluster health probes per NNCP
+
+## Proposal
+
+### User roles
+
+**cluster admin** is a user responsible for managing the cluster node
+networking and install operators
+
+### Workflow Description (customize DNS probe name to resolve)
+
+1. The cluster admin configures DNS probe name to resolve at NMState CR.
+2. The cluster admin creates an NNCP and kubernetes-nmstate will use the custom DNS probe name to check cluster health
+
+### Workflow Description (go back to default DNS probe)
+
+1. The cluster admin remove the DNS probe customization 
+2. The cluster admin creates an NNCP and the default name resolution is use for the DNS probe
+
+### Alternatives
+
+#### Add a DNS probe fallback using global name
+
+A possible alternative is the DNS probe in case of current implementation 
+failing doing a fallback to use a global name like "redhat.com" or the like and
+the golang call `LookupNetIP` this can be done before apply the NNCP to select
+the proper mechanism to check after apply NNCP, it can be done at installation 
+time too.
+
+### API Extensions
+
+This proposal add a new `probes` field under the `NMState` CR to configure the 
+DNS probes name resolution, this way if the other probes need to be customize
+in the future more fields can be added there
+
+Following is the `NMState` CR example with custom DNS probe name resolution:
+
+```yaml
+apiVersion: nmstate.io/v1
+kind: NMState
+metadata:
+  name: nmstate
+spec:
+  probes:
+    DNS: 
+      name: "redhat.com"
+```
+
+After doing this the probe will use golang `LookupNetIP` function instead of 
+current `LookupNS` to resolve the specified `spec.probes.DNS.name` "redhat.com".