I’m sharing this in case it helps someone else, because this issue was extremely hard to diagnose and looked like a Proxmox or kernel problem at first.
→ Switch (VLANs)
→ Proxmox VE nodes
This made it look like:
…but none of those were the cause.
At this point it was clear the problem was not Proxmox, but somewhere in the network path.
On the router, the following were enabled:
With:
VLANs
IGMP Proxy / Snooping caused silent drops of long-lived TCP connections, while ICMP kept working.
This resulted in:
SSH freezing
On Router:
After disabling both:
This was a network-layer issue, not a Proxmox bug.
IGMP Proxy / Snooping can break TCP sessions in VLAN + bridge + routing setups, even when multicast is not explicitly used.
Posting this because it’s a very “ghost-like” issue and easy to misdiagnose.
Hope this helps someone else avoid losing days on the same problem.
Topology
ISP → ONT → Router → ZimaBoard 2 (pfSense)→ Switch (VLANs)
→ Proxmox VE nodes
- Proxmox VE cluster (2 nodes + qdevice)
- Multiple VLANs
- pfSense used as router/firewall
- Accessing Proxmox via Web UI shell and SSH
Symptoms
- Proxmox Web UI works, but shell disconnects after ~10–15 seconds
- SSH sessions from macOS/Linux also freeze and then timeout
- Error in browser console: WebSocket close code 1006
- ICMP (ping) works perfectly and never drops
- TCP connections remain in ESTABLISHED state but stop transferring data
- Issue happens even with no VMs or containers running
- Happens both from Proxmox GUI shell and external SSH clients
This made it look like:
- Proxmox bug
- kernel / driver issue
- cluster or corosync instability
- NIC offloading problem
…but none of those were the cause.
Investigation
- Verified corosync, quorum, HA → OK
- Disabled GPU passthrough, VMs, containers → no change
- Checked NIC offloading, MTU, VLAN config → no change
- Packet captures showed TCP packets being silently dropped, not reset
At this point it was clear the problem was not Proxmox, but somewhere in the network path.
Root Cause
On the router, the following were enabled:
- IGMP Proxy
- IGMP Snooping
With:
VLANs
- bridge interfaces
- inter-VLAN routing
IGMP Proxy / Snooping caused silent drops of long-lived TCP connections, while ICMP kept working.
This resulted in:
SSH freezing
- Proxmox Web UI shell disconnecting (code 1006)
- “ghost” timeouts without clear errors
Solution
On Router:
- Disable IGMP Proxy
- Disable IGMP Snooping
After disabling both:
- SSH sessions remain stable
- Proxmox Web UI shell works indefinitely
- No more WebSocket 1006 errors
- No packet loss observed
Conclusion
This was a network-layer issue, not a Proxmox bug.
IGMP Proxy / Snooping can break TCP sessions in VLAN + bridge + routing setups, even when multicast is not explicitly used.
Posting this because it’s a very “ghost-like” issue and easy to misdiagnose.
Hope this helps someone else avoid losing days on the same problem.
