NCCL IPv6 Issue
NCCL IPv6 issues occur when NCCL tries to use IPv6 but the network only supports IPv4, or vice versa.
NCCL IPv6 issues occur when NCCL tries to use IPv6 but the network only supports IPv4, or vice versa.
What this failure is
NCCL IPv6 Issue is a Communication failure seen during ML training runs. NCCL IPv6 issues occur when NCCL tries to use IPv6 but the network only supports IPv4, or vice versa. Common tags: Nccl, Ipv6, Ipv4, Network.
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Why it happens (the mechanism)
NCCL defaults to IPv6 but only IPv4 available. DNS resolves to IPv6 first. Cluster doesn't have IPv6 configured. Firewall blocks IPv6 traffic. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- NCCL hangs at initialization
- NCCL uses wrong IP family
- Inter-node communication fails
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL WARN Net: No IPv6 interface found | NCCL defaults to IPv6 but only IPv4 available |
| NCCL slow to initialize | DNS resolves to IPv6 first |
| Connection refused on IPv6 | Cluster doesn't have IPv6 configured |
Which systems are affected
- Multi-node training on IPv4-only networks
- Training on mixed IPv4/IPv6 networks
- Older clusters without IPv6
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: NCCL WARN Net: No IPv6 interface found
- ✓Verified signal present: NCCL slow to initialize
- ✓Verified signal present: Connection refused on IPv6
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
The root cause is on this page and stays free. A free account adds the exact remediation steps, saved history, and the fix on every entry in the encyclopedia.
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Diagnose this failure in VS Code
Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.
Install the free VS Code extensionNCCL errors in context
NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.
Compare every nccl error side by sideRelated failures to investigate next
Root cause
- NCCL defaults to IPv6 but only IPv4 available
- DNS resolves to IPv6 first
- Cluster doesn't have IPv6 configured
- Firewall blocks IPv6 traffic
The fix and how to prevent it
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