[Bug 2142118] Re: Kernel 6.8.0-100: route cache corrupts external IPs as broadcast on systems with VLAN interfaces

Hi Matthew,

Tested Ubuntu 24.04.4 LTS (noble) with 6.8.0-103-generic from -proposed.

Before testing, I reverted my netplan configuration to its original
state (removed the explicit metric on the VLAN default route) and
disabled my cron-based workaround that was flushing the route cache.
This restores the original dual-default configuration that was
triggering the regression.

On 6.8.0-100-generic the issue reproduced: ip route get 8.8.8.8
intermittently returned "broadcast … cache <local,brd>", and outbound
UDP/ICMP failed with "Permission denied". Flushing the route cache
temporarily restored connectivity.

On 6.8.0-103-generic the issue is not reproducing. Routing behaves
correctly, ip route get returns a proper unicast route, and connectivity
is stable after reboot.

However, since this regression was intermittent in nature, I would like
to let the system run for 24–48 hours without any workaround in place to
confirm long-term stability before marking verification-done. I will
report back with final confirmation after that observation period.

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https://bugs.launchpad.net/bugs/2142118

Title:
Kernel 6.8.0-100: route cache corrupts external IPs as broadcast on
systems with VLAN interfaces

Status in linux package in Ubuntu:
New
Status in linux source package in Noble:
New

Bug description:
Description:
- Regression from: 6.8.0-94 (works) to 6.8.0-100 (broken)
- Setup: Dual default routes — physical interface (enp0s31f6, DHCP, metric 100) + VLAN (enp0s31f6.3, static, metric 200)
- Symptom: ip route get 8.8.8.8 returns broadcast 8.8.8.8 ... cache <local,brd> — kernel classifies external unicast IPs as
broadcast, blocking all outbound UDP/ICMP
- Impact: All outbound DNS, NTP, ping fail with "Permission denied"
- Workaround: sudo ip route flush cache temporarily fixes it, but it recurs
- Confirmed: Booting into 6.8.0-94 resolves the issue

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