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

Public bug reported:

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

** Affects: linux (Ubuntu)
Importance: Undecided
Status: New

** Attachment added: "apport.linux-image-6.8.0-100-generic.nqq2zom5.apport"
https://bugs.launchpad.net/bugs/2142118/+attachment/5947014/+files/apport.linux-image-6.8.0-100-generic.nqq2zom5.apport

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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

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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