[Bug 2142776] Re: [Noble 6.8.0-101-generic] Route cache corruption: external IPs intermittently marked as broadcast, breaking all TCP connections

*** This bug is a duplicate of bug 2141531 ***
https://bugs.launchpad.net/bugs/2141531

This for me was most noticeable during audio calls. Sometimes i had to
refresh a page.

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https://bugs.launchpad.net/bugs/2142776

Title:
[Noble 6.8.0-101-generic] Route cache corruption: external IPs
intermittently marked as broadcast, breaking all TCP connections

Status in linux package in Ubuntu:
Confirmed

Bug description:
After upgrading from 6.8.0-90-generic to 6.8.0-101-generic, all outbound
TCP connections fail intermittently with "Network is unreachable". The
kernel's route cache randomly marks external (routable) IP addresses as
"broadcast" with the <local,brd> flag, causing connect() to fail.

ICMP ping and UDP (DNS resolution) continue to work because they bypass
the route cache check that rejects TCP to broadcast-flagged destinations.

The issue does NOT occur on 6.8.0-90-generic with identical hardware,
network configuration, and workload. Booting back into 6.8.0-90 on the
same machine resolves the problem completely.

STEPS TO REPRODUCE:

1. Boot Ubuntu 24.04.4 LTS with kernel 6.8.0-101-generic
2. Machine must be on a LAN with normal broadcast traffic (ARP, mDNS,
service discovery beacons, etc.)
3. Run: ip route get 8.8.8.8
4. Repeat every 2 seconds for 30 seconds
5. Observe that the output intermittently changes from:
8.8.8.8 via 10.0.0.1 dev eno1 src 10.0.0.50 cache
to:
broadcast 8.8.8.8 via 10.0.0.1 dev eno1 src 10.0.0.50 cache <local,brd>
6. During the "broadcast" periods, any TCP connection attempt (curl, wget,
Go net.Dial, etc.) fails with: "connect: network is unreachable"

EXPECTED RESULT:

External IP addresses should never be flagged as broadcast in the route
cache. Route lookups for routable IPs should always return a normal
unicast route via the default gateway.

ACTUAL RESULT:

The route cache intermittently corrupts entries for external IPs, marking
them as broadcast. The corruption correlates with UDP broadcast packets
on the LAN (every ~8-10 seconds, matching service discovery beacon
intervals). The broadcast flag from legitimate LAN broadcast traffic
appears to leak into route cache entries for unrelated destination IPs.

WHAT WAS TRIED (none of these fix the issue on 6.8.0-101):

- Flushing route cache (sudo ip route flush cache): temporarily clears
the corruption but it returns within seconds
- Disabling ICMP redirects (net.ipv4.conf.all.accept_redirects=0 and
secure_redirects=0 on all interfaces): no effect
- Disabling the application generating broadcast beacons: no effect,
other LAN broadcast traffic (ARP, UniFi discovery) still triggers it
- Verifying routing table: routing table is correct at all times, only
the route cache is affected

WHAT FIXES THE ISSUE:

- Booting into 6.8.0-90-generic on the same machine with identical
configuration: route cache remains clean indefinitely under the same
network conditions

REGRESSION:

Yes. Works on 6.8.0-90-generic (6.8.0-90.91), broken on 6.8.0-101-generic
(6.8.0-101.101).

SYSTEM INFORMATION:

Distribution: Ubuntu 24.04.4 LTS (Noble Numbat)
Architecture: x86_64
Broken kernel: 6.8.0-101-generic (6.8.0-101.101)
Working kernel: 6.8.0-90-generic (6.8.0-90.91)
Hardware: Micro Computer (HK) UM690
NIC: Intel I226-V (igc driver), firmware 2017:888d
Network: 10.0.0.50/24, gateway 10.0.0.1 (pfSense 2.8.1)
Network config: systemd-networkd via netplan (cloud-init generated)
Interface: eno1, 2Gbps full duplex, MTU 1500

LAN characteristics (relevant because broadcast traffic is the trigger):
- ARP requests from multiple devices (~1/second)
- UDP broadcast to 255.255.255.255:10001 (UniFi discovery)
- UDP broadcast to 255.255.255.255:50052 (application service discovery)
- Normal residential/small-office LAN traffic levels

MONITORING SCRIPT (to reproduce):

# Run on affected kernel, observe broadcast entries appearing
for i in 1
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30; do
sleep 2
echo 17:43:44
done

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