(Security) implications of binding server to `127.0.0.1` vs `0.0.0.0` vs `$POD_IP`?

General Discussions
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Hi,

I’m trying to understand the (security) implications of these various bindings.

To my understanding

  1. 127.0.0.1 Would effectively prevent traffic to the pod
  2. 0.0.0.0 Would allow traffic to the pod (on all interfaces)
  3. $POD_IP Would allow “normal” traffic to the pod but would prevent kubectl port-forward to work (see also
    https://github.com/kubernetes/kubernetes/issues/72597 )

It seems to me that 0.0.0.0 is the most convenient option to open a server as it allows for easy (network) debugging via kubectl port-forward

Binding on $POD_IP seems to me the “correct” way but I don’t have any strong arguments. It feels that it would help reduce the attack surface but I’m far from being a Kubernetes/Networking expert.

Does anyone have some insights into this topic? Any “opinion” pieces that go beyond the usual "127.0.0.1 vs 0.0.0.0" discussion? Any material I can read? Any other forum/website more appropriate for this discussion?

Thank you,
Oliver

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Binding to a loopback addr is clearly intending to be a “private” interface, for use within the pod only.

Binding to anything else other than 0 is “unusual”, and if you don’t know exactly WHY you are doing it, you are either likely wrong or you are acheiving the same result as 0, but doing more work to get there.