`flannel.1` interface disappears when `--flannel-iface` is recreated

[NiklasRosenstein]

Environmental Info:

$ k3s -v
k3s version v1.32.3+k3s1 (079ffa8d)
go version go1.23.6

Node(s) CPU architecture, OS, and Version:

• Node 1: Linux kasoku 6.11.0-25-generic #25~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Apr 15 17:20:50 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
• Node 2: Linux bristle 6.8.0-59-generic #61-Ubuntu SMP PREEMPT_DYNAMIC Fri Apr 11 23:16:11 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
• Node 3: Linux blackview 6.14.0-15-generic #15-Ubuntu SMP PREEMPT_DYNAMIC Sun Apr 6 15:05:05 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
• Node 4: Linux raspberrypi 6.12.25+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.12.25-1+rpt1 (2025-04-30) aarch64 GNU/Linux

Cluster Configuration:

• 1 server, 3 agents
• Using a Tailscale VPN to connect the nodes to each other (--flannel-iface=tailscale0)

Describe the bug:

Nodes in my cluster sometimes experience a network connectivity issue that I usually first recognize by being unable to visit a particular service in the browser. Upon inspection, I've found that the flannel.1 interface is missing on at least one of the involved nodes.

This has occurred exactly once so far on nodes 1, 3 and 4 (at the same time for 3 and 4 though I could not tell when exactly it went missing, it might as well not be related to each other).

I believe this may be caused if there is in intermittent issue with Tailscale such that the tailscale0 interface is recreated (e.g. when it is restarted, either manually or by automatic upgrades), because I reproduce it by running service tailscaled restart).

Steps To Reproduce:

• Setup a device with Tailscale
• Install a k3s server or agent with --flannel-iface=tailscale0
• ip addr | grep flannel.1 should show have a result
• Run service tailscaled restart
• ip addr | grep flannel.1 should no longer show a result

Expected behavior:

The flannel interface is not lost or at least recovers in due time when the --flannel-iface is available again.

Actual behavior:

The flannel interface is gone until k3s restarts.

Additional notes:

The issue title is a bit of a guess; I don't actually know if restarting Tailscale causes the tailscale0 interface to be "recreated", but something is happening with it that breaks the flannel interface that depends on it.

[brandond]

Please report this to the flannel project: https://github.com/flannel-io/flannel

[NiklasRosenstein]

flannel-io/flannel#2247