[matbun]

Summary

Part of #548.

With ShadowMode: ssh an offloaded pod can open connections back into the cluster, without WireGuard and without a network namespace. The site enables it with Network.SSH.Reverse.Enabled: true; the pod lists what it needs:

annotations:
  interlink.eu/reverse-forwards: "minio.user.svc.cluster.local:9000 6443:kubernetes.default.svc.cluster.local:443"

The shadow's ssh-reverse container opens a session to the compute node's own sshd through the login node, the same hop as the forward of #580, and asks it for one ssh -R per endpoint, each ending in a unix socket in the pod's socket directory. A relay started by the job holds a loopback address derived from the pod UID for the whole job and passes on only connections from the job's own user, and the job's /etc/hosts maps the Service names to it, so the workload uses the names and ports unchanged. If a site's node sshd refuses forwarding, Reverse.Sshd: userspace starts an sshd inside the job instead, and Reverse.NodeExec: srun carries the session through Slurm where ssh to the node is not possible, at lower throughput.

interlink.eu/reverse-socks-port adds a SOCKS5 proxy for undeclared endpoints only where the virtual kubelet sets Reverse.AllowSocks. It listens on 127.0.0.1 of the node, open to every user there, so a pod that asks for it gets a Warning event, the virtual kubelet and the shadow log a warning, and without AllowSocks the pod is refused. The virtual kubelet's Events now reach the API server.

Tested on three Slurm sites, with the node's sshd and, on one, with the user-space fallback: S3 and Redis by Service name, the Kubernetes API by FQDN with TLS verified, sockets srw------- in a drwx------ directory, SOCKS refused by default and working with its warnings when allowed. Unit tests cover the annotations, the rendered scripts in every mode (sh -n, bash -n) and the relay.

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Related issue : #548

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[Copilot]

🟡 Changes recommended

Unresolved critical defects can skip requested reverse-forward setup, discard existing bind mounts, and generate invalid Deployments for UDP-only pods.

6 open issues

• Reject reverse annotations with pod VPN annotation · New
• Preserve legacy bindings when setting APPTAINER_BIND · New
• Skip empty tunnel shadows for UDP-only pods · New
• Check IPv4-mapped clients in the tcp6 ownership table · New
• Pin carrier steps to the allocation batch host · New
• Exit relay when forwarded sockets disappear · New

What changed in this PR

Adds opt-in SSH reverse tunnels so offloaded pods can reach cluster endpoints without WireGuard or a network namespace, extending the existing SSH shadow networking.

Changes:

• Adds reverse-forward annotations, configuration validation, and workload setup.
• Introduces an owner-only relay, node/userspace SSH forwarding, and Slurm carrier support.
• Adds gated SOCKS access with warnings, Kubernetes Event delivery, and reverse-tunnel tests.

File	Description
pkg/​virtualkubelet/​virtualkubelet.go	Integrates reverse tunnels into shadow creation and pod validation.
pkg/​virtualkubelet/​templates/​shadow-ssh-template.yaml	Renders reverse SSH sessions and optional forwarding containers.
pkg/​virtualkubelet/​templates/​reverse-relay.py	Relays same-user loopback connections to Unix sockets.
pkg/​virtualkubelet/​shadow_ssh.go	Normalizes reverse configuration and host-key requirements.
pkg/​virtualkubelet/​shadow_ssh_reverse.go	Parses annotations and generates workload tunnel setup.
pkg/​virtualkubelet/​shadow_ssh_reverse_test.go	Tests validation, rendered scripts, warnings, and relay behavior.
pkg/​virtualkubelet/​config.go	Defines SSH reverse-tunnel settings.
cmd/​virtual-kubelet/​main.go	Connects event recording to the Kubernetes API.

_{Review effort: Balanced}