For systems engineers and bare-metal cluster operators

Fix vSwitch MTU fragmentation and NCCL socket discovery on Hetzner bare metal.

Separate VLAN vSwitch jumbo frame drops, asymmetric NIC binding, and unconfigured RDMA probes when connecting multi-node GPU servers on Hetzner.

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Replay a resolved Hetzner bare-metal cluster incident

Upload your log file. Review the selected evidence and masking, then run diagnosis. Keep the original log for any follow-up questions.

Before uploading, review cloud data handling and local options.

training_logs.txt

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Up to 50 diagnoses a day for 30 days. Verify your work email to activate an eligible company evaluation, then replay a resolved incident and compare the evidence and next steps with your team's confirmed cause.

What a useful evaluation should establish

Verify that multi-node connection failures are traced to exact MTU mismatch or socket interface bindings rather than silent framework faults.

Use Denpex alongside your existing telemetry tools to investigate submitted incident evidence. With monitoring configured on a supported plan, detected failures can trigger alerts and an automatic diagnosis from the captured evidence.

Inside Denpex

See the evidence, not just the claim

Follow the causal chain, inspect the quoted evidence and prepare the next handoff. These actual product captures use founder-account examples and the VS Code extension, with the source identified under each image. Open a capture to inspect it at full size.

Denpex founder-account example: a causal graph connects an Xid 48 ECC event to a rank exit and downstream collective failures, with quoted evidence for each stage. — Live founder-account capture from the bundled NCCL/ECC example. Follow the hardware event, rank exit and downstream symptoms with a supporting log quote at each stage.Inspect original image

Denpex hardware handoff summary asks the operator to capture diagnostic evidence and verify RMA eligibility with the vendor. — Actual hardware handoff summary from the same founder-account example. Capture diagnostic evidence before an approved reset, then confirm RMA eligibility with the vendor.Inspect original image

Denpex historical example says a collective timeout does not establish the initiating rank, cause or failure domain and requests distinguishing evidence before a remedy. — Founder-account example restored from history. The original timeout remains unattributed, and the UI keeps the missing-evidence assessment visible.Inspect original image

Denpex VS Code extension displaying a bundled OOM-before-NCCL example, matched log evidence, verification guidance and an investigation handoff. — Actual VS Code extension capture of the bundled OOM-before-NCCL example. Review matched log evidence, verification guidance and the investigation handoff without leaving the editor.Inspect original image

Failure Signature Triage

Hetzner - Observed Runtime Evidence

Read Hetzner documentation

Incident pattern summary

Two Hetzner dedicated GPU servers connect over a private vSwitch VLAN on interface enp7s0.4000. Control-plane rendezvous succeeds via small TCP packets, but the first large tensor AllReduce hangs indefinitely because the host VLAN sub-interface is set to MTU 1500 while the vSwitch path requires MTU 1400.

Terminal traceback excerpt (Hetzner)

hetzner-gpu-01:2104:2140 [0] NCCL INFO Bootstrap : Using enp7s0.4000:10.0.40.11<0>
hetzner-gpu-01:2104:2140 [0] NCCL INFO NET/Plugin : No plugin found (libnccl-net.so), using internal implementation
hetzner-gpu-01:2104:2140 [0] include/socket.h:403 NCCL WARN socketProgress: Connection closed by remote peer 10.0.40.12<54102>

Recommended verification flags

Set these environment flags in your training container or launch script to capture high-resolution collective logs:

NCCL_IB_DISABLE=1NCCL_SOCKET_IFNAME=enp7s0.4000NCCL_DEBUG=INFONCCL_SOCKET_NTHREADS=4

Diagnostic Scope Boundary

Denpex cannot modify Hetzner Robot network configurations, VLAN tags, or host iptables rules. The diagnosis identifies exact socket transport and packet-size mismatches from your job logs.