Bug#1108430: Kernel Regression between 6.1.0-35 and 6.1.0-37: IPv6 link-local multicast over GRE tunnel is silently dropped
Hello,
Just a follow-up to report that the upstream kernel developers have
now merged a fix for this issue into the 'net' tree.
The official commit is:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=4e914ef063de40397e25a025c70d9737a9e45a8c
The patch submission cover letter on the mailing list can be found
here, providing additional context:
https://lore.kernel.org/netdev/cover.1752070620.git.gnault@redhat.com/T/
I will be looking forward to seeing this fix in a future Debian kernel update.
Best regards,
Aiden Yang
Salvatore Bonaccorso <carnil@debian.org> 于2025年6月29日周日 03:17写道：
>
> Control: tags -1 + moreinfo
>
> On Sat, Jun 28, 2025 at 08:49:17PM +0800, Aiden Yang wrote:
> > PACKAGE: linux SUBJECT: Kernel Regression between 6.1.0-35 and
> > 6.1.0-37: IPv6 link-local multicast over GRE tunnel is silently
> > dropped SEVERITY: normal
> >
> > ===================================================================
> >
> > Summary:
> > This report details a regression in the Linux kernel that prevents
> > IPv6 link-local all-nodes multicast packets (ff02::1) from being
> > transmitted over a GRE tunnel. The issue is confirmed to have been
> > introduced between kernel versions 6.1.0-35-cloud-amd64 (working) and
> > 6.1.0-37-cloud-amd64 (failing) on Debian 12 (Bookworm).
> > On affected kernels, the ping utility reports 100% packet loss, and a
> > tcpdump on the underlying physical interface confirms that the kernel
> > is silently dropping the encapsulated GRE packets instead of sending
> > them. The sendto() system call does not return an error to the
> > userspace application in the default namespace.
> >
> > ===================================================================
> >
> > Regression Point:
> > Last Known Good Version: 6.1.0-35-cloud-amd64
> > First Failing Version: 6.1.0-37-cloud-amd64
> > The regression is also present in later kernels tested, including
> > 6.12.33 and 6.15.x on Debian 13 (Trixie).
> >
> > ===================================================================
> >
> > Steps to Reproduce:
> > Use a Debian system with an affected kernel (e.g., >= 6.1.0-37).
> > Establish a GRE tunnel. Replace [PEER_IP] and [LOCAL_IP] with actual
> > endpoint addresses.
> > ip tunnel add tun_gre mode gre remote [PEER_IP] local [LOCAL_IP] ttl
> > 255 ip link set tun_gre up
> > In one terminal, start a tcpdump on the physical interface that
> > provides the local IP, to monitor for outgoing GRE packets (GRE is IP
> > protocol 47).
> > tcpdump -i [PHYSICAL_IFACE] -n 'proto gre'
> > In a second terminal, attempt to ping the link-local all-nodes
> > multicast address via the GRE tunnel interface.
> > ping ff02::1%tun_gre -c 4
> >
> > ===================================================================
> >
> > Observed Behavior (The Bug):
> > The ping command runs and reports "4 packets transmitted, 0 received,
> > 100% packet loss".
> > The tcpdump window on the physical interface shows NO outgoing GRE
> > packets. This proves the kernel is silently dropping the packets.
> >
> > ===================================================================
> >
> > Expected Behavior (as observed on kernel 6.1.0-35):
> > The ping command runs.
> > The tcpdump window shows outgoing GRE packets being sent from
> > [LOCAL_IP] to [PEER_IP] for each ICMPv6 echo request. (Receiving a
> > reply is dependent on the peer configuration, but the packets should
> > be transmitted).
> >
> > ===================================================================
> >
> > Additional Diagnostic Information:
> > VRF Context: When the failing GRE interface (tun_gre) is placed within
> > a VRF, the failure mode changes. The ping or sendto() system call
> > fails immediately with an ENETUNREACH (Network is unreachable) error.
> > This is likely because the VRF routing table does not have a route to
> > the tunnel's physical peer address, and the kernel correctly
> > identifies this dependency issue.
> > veth Control Test: The issue is specific to the gre tunnel interface
> > type. A control test using a veth pair inside a VRF works perfectly
> > for link-local multicast, proving the core VRF and multicast logic is
> > sound.
> > This detailed bracketing of the regression should provide a strong
> > starting point for identifying the specific commit that introduced
> > this behavior.
>
> since you can reproduce the regression on all newer upstream versions
> as well, can you please report in to upstream and report back here the
> upstream report so we can follow its status.
>
> Thanks already.
>
> Regards,
> Salvatore
>
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> Drop this when you send public bugreports, thanks.
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