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Bug#1127823: ip6_tunnel, ip6tnl, module fails to encapsulate/transmit packets in kernel 6.1.0-43

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  Bug#1127823: ip6_tunnel, ip6tnl, module fails to encapsulate/transmit packets in kernel 6.1.0-43 Salvatore Bonaccorso <carnil@debian.org> - 2026-02-13 18:20 +0100
    Bug#1127823: ip6_tunnel, ip6tnl, module fails to encapsulate/transmit packets in kernel 6.1.0-43 Salvatore Bonaccorso <carnil@debian.org> - 2026-02-17 11:40 +0100

#91204 — Bug#1127823: ip6_tunnel, ip6tnl, module fails to encapsulate/transmit packets in kernel 6.1.0-43

Hi,

On Fri, Feb 13, 2026 at 05:12:07PM +0200, Hostinger NOC wrote:
> Package: linux-image-6.1.0-43-amd64
> Version: 6.1.162-1
> Severity: important
> Tags: upstream
> 
>   Hello,
> 
>   The ip6_tunnel kernel module is broken in linux-image-6.1.0-43-amd64
>   (upstream 6.1.162). IPv6 tunnels (mode "any") fail to encapsulate or
>   transmit any packets.
> 
>   WORKING CONFIGURATION:
>   - Kernel: 6.1.0-37-amd64 (upstream 6.1.140)
>   - Tunnel works correctly with identical configuration
> 
>   BROKEN CONFIGURATION:
>   - Kernel: 6.1.0-43-amd64 (upstream 6.1.162)
>   - Same tunnel configuration fails completely
> 
>   REPRODUCTION STEPS:
>   1. Create IPv6 tunnel with mode "any":
>      ip link add tun0 type ip6tnl mode any \
>        remote 2001:db8::2 \
>        local 2001:db8::1 \
>        encaplimit 4
>      ip addr add 192.0.2.1/30 dev tun0
>      ip addr add 2001:db8:1::1/126 dev tun0
>      ip link set tun0 up mtu 1500
> 
>   2. Send traffic through tunnel:
>      ping -c 3 -I tun0 192.0.2.2
> 
>   3. Monitor physical interface:
>      tcpdump -i eth0 -nn 'ip6 and host 2001:db8::2'
> 
>   EXPECTED BEHAVIOR (kernel 6.1.0-37):
>   - tcpdump shows encapsulated packets with protocol 60 (DSTOPT) or 4 (IPIP)
>   - Tunnel traffic flows successfully
> 
>   ACTUAL BEHAVIOR (kernel 6.1.0-43):
>   - tcpdump captures ZERO packets
>   - TX counter increments on tunnel interface but packets disappear in
> kernel
>   - RX packets show errors
> 
>   EVIDENCE - Side-by-side comparison:
> 
>   Kernel 6.1.0-37 (WORKING):
>     Protocol 60 (DSTOPT) packets visible on wire
>     Encapsulation functioning correctly
> 
>   Kernel 6.1.0-43 (BROKEN):
>     0 packets captured
>     No packets transmitted to physical interface
> 
>   SYSTEM INFORMATION:
>   - Debian version: 12 (bookworm)
>   - Architecture: amd64
>   - Broken kernel: 6.1.0-43-amd64 (6.1.162-1)
>   - Working kernel: 6.1.0-37-amd64 (6.1.140-1)
>   - Boot parameters tested: default and mitigations=off (both fail)
> 
>   IMPACT:
>   - All IPv6 tunnels using ip6tnl module are affected
>   - Workaround: downgrade to 6.1.0-37
> 
>   ADDITIONAL NOTES:
>   - Issue persists regardless of tunnel parameters (MTU, encaplimit,
> offloads)
>   - Module parameter changes have no effect
>   - Issue affects both encapsulation (TX) and decapsulation (RX)
>   - No relevant errors in dmesg or kernel logs
> 
>   Could you please investigate this regression? Is there any additional
>   debugging information or testing I can provide to help identify the root
> cause?
>   Would additional debugging output be helpful? I can provide kernel
>   logs, strace output, or test patches if needed.

See #1127597.

Can you confirm that applying 9990ddf47d41 ("net: tunnel: make
skb_vlan_inet_prepare() return drop reasons") on top as well of
6.1.162-1 (or 6.1.163 upstream) fixes the issue and report back
please?

Regards,
Salvatore

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#91254

Hi,

On Tue, Feb 17, 2026 at 11:32:30AM +0200, Hostinger NOC wrote:
> Hello,
> 
> We can confirm that applying 9990ddf47d41 on top off 6.1.162-1 resolves the
> issue.

Thanks for the confirmation, I asked for a 6.1.y inclusion in
https://lore.kernel.org/stable/177132401902.2893171.1371685164011289024@eldamar.lan

Regards,
Salvatore

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