[Bug 2142510] Re: Regression: ip route get intermittently returns broadcast … for public IPs on 6.8.0-100 (breaks outbound TCP/Docker pulls)

see also another report which is about the same thing -
https://www.dedoimedo.com/computers/ubuntu-kernel-6-8-100-buggy-network.html

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https://bugs.launchpad.net/bugs/2142510

Title:
Regression: ip route get intermittently returns broadcast …
<local,brd> for public IPs on 6.8.0-100 (breaks outbound TCP/Docker
pulls)

Status in linux package in Ubuntu:
New

Bug description:
Summary / Regression
This is a regression between Ubuntu kernels on an Ubuntu VM (Proxmox/QEMU/KVM, bridged networking, DHCP reservation). With 6.8.0-100-generic, ip -details route get <public-ip> intermittently returns an invalid cached result marking public IPs as broadcast/local (cache <local,brd>). When it enters this state, outbound TCP fails with "Network is unreachable" and Docker/Portainer pulls fail. Booting 6.8.0-90-generic on the same VM fixes it (route lookups remain unicast).

Environment

Ubuntu VM on Proxmox VE (QEMU/KVM), bridge vmbr0, DHCP reservation.

Reproduced across configuration changes: virtio/e1000 NIC, cpu
host/kvm64, cloud-init disabled; persists specifically with 6.8.0-100
and disappears with 6.8.0-90.

Evidence (6.8.0-100-generic)
Running repeatedly:
ip -details route get 1.1.1.1
shows flapping between correct unicast and incorrect broadcast classification:

unicast 1.1.1.1 via 192.168.1.1 dev ens18 table main src 192.168.1.72
cache
broadcast 1.1.1.1 via 192.168.1.1 dev ens18 table main src 192.168.1.72
cache <local,brd>
broadcast 1.1.1.1 via 192.168.1.1 dev ens18 table main src 192.168.1.72
cache <local,brd>

When the broadcast … <local,brd> state appears, outbound TCP
connections to public IPs fail ("Network is unreachable"), and ubuntu-
bug linux upload also sometimes fails with urlopen error [Errno 101]
Network is unreachable.

Expected
Public IPs should never be classified as broadcast … <local,brd> in route lookup; outbound TCP should stay stable.

Workaround
Boot kernel 6.8.0-90-generic (issue disappears). Flushing caches (ip route flush cache and ip neigh flush dev ens18) temporarily restores correct behavior on 6.8.0-100.

ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: linux-image-6.8.0-100-generic 6.8.0-100.100
ProcVersionSignature: Ubuntu 6.8.0-100.100-generic 6.8.12
Uname: Linux 6.8.0-100-generic x86_64
ApportVersion: 2.28.1-0ubuntu3.8
Architecture: amd64
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/by-path', '/dev/snd/controlC0', '/dev/snd/pcmC0D1c', '/dev/snd/pcmC0D1p', '/dev/snd/pcmC0D0c', '/dev/snd/pcmC0D0p', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CRDA: N/A
CasperMD5CheckResult: pass
Date: Mon Feb 23 19:39:48 2026
InstallationDate: Installed on 2025-03-13 (347 days ago)
InstallationMedia: Ubuntu-Server 24.04.1 LTS "Noble Numbat" - Release amd64 (20240827)
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lsusb:
Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 002: ID 0627:0001 Adomax Technology Co., Ltd QEMU Tablet
Lsusb-t:
/: Bus 001.Port 001: Dev 001, Class=root_hub, Driver=uhci_hcd/2p, 12M
|__ Port 001: Dev 002, If 0, Class=Human Interface Device, Driver=usbhid, 12M
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
ProcEnviron:
LANG=en_US.UTF-8
PATH=(custom, no user)
SHELL=/bin/bash
TERM=xterm
ProcFB: 0 bochs-drmdrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-6.8.0-100-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro usbcore.autosuspend=-1
RelatedPackageVersions:
linux-restricted-modules-6.8.0-100-generic N/A
linux-backports-modules-6.8.0-100-generic N/A
linux-firmware 20240318.git3b128b60-0ubuntu2.22
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/01/2014
dmi.bios.release: 0.0
dmi.bios.vendor: SeaBIOS
dmi.bios.version: rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-9.2
dmi.modalias: dmi:bvnSeaBIOS:bvrrel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org:bd04/01/2014:br0.0:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-9.2:cvnQEMU:ct1:cvrpc-i440fx-9.2:sku:
dmi.product.name: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-9.2
dmi.sys.vendor: QEMU

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