Clarify network policy behaviour for hostNetwork pods

Signed-off-by: Nadia Pinaeva <[email protected]>

Author: npinaeva

content/en/docs/concepts/services-networking/network-policies.md

@@ -354,6 +354,46 @@ For example, a newly-created pod that is supposed to be able to reach both Pod A
 on Node 1 and Pod B on Node 2 may find that it can reach Pod A immediately,
 but cannot reach Pod B until a few seconds later.
 
+## NetworkPolicy and `hostNetwork` pods
+
+NetworkPolicy behaviour for `hostNetwork` pods is undefined, but it should be limited to 2 possibilities:
+- The network plugin can distinguish `hostNetwork` pod traffic from all other traffic
+  (including being able to distinguish traffic from different `hostNetwork` pods on
+  the same node), and will apply NetworkPolicy to `hostNetwork` pods just like it does
+  to pod-network pods.
+- The network plugin cannot properly distinguish `hostNetwork` pod traffic, 
+  and so it ignores `hostNetwork` pods when matching `podSelector` and `namespaceSelector`. 
+  Traffic to/from `hostNetwork` pods is treated the same as all other traffic to/from the node IP. 
+  (This is the most common implementation.)
+
+This applies when
+1. a `hostNetwork` pod is selected by `spec.podSelector`.
+   
+   ```yaml
+     ...
+     spec:
+       podSelector:
+         matchLabels:
+           role: client
+     ...
+   ```
+ 
+2. a `hostNetwork` pod is selected by a `podSelector` or `namespaceSelector` in an `ingress` or `egress` rule.
+
+   ```yaml
+     ...
+     ingress:
+       - from:
+         - podSelector:
+             matchLabels:
+               role: client
+     ...
+   ```
+
+At the same time, since `hostNetwork` pods have the same IP addresses as the nodes they reside on,
+their connections will be treated as node connections. For example, you can allow traffic
+from a `hostNetwork` Pod using an `ipBlock` rule.
+
 ## What you can't do with network policies (at least, not yet)
 
 As of Kubernetes {{< skew currentVersion >}}, the following functionality does not exist in the