On Mon, Feb 22, 2021 at 14:31:38 -0800, Jakub Kicinski wrote: > On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote: > > >> This will not be sufficient for my usecase: To stay compatible with older > > >> versions of fastd, I can't set the T flag in the first packet of the > > >> handshake, as it won't be known whether the peer has a new enough fastd > > >> version to understand packets that have this bit set. Luckily, the second > > >> handshake byte is always 0 in fastd's protocol, so these packets fail the > > >> tunnel version check and are passed to userspace regardless. > > >> > > >> I'm aware that this usecase is far outside of the original intentions of > > >> the > > >> code and can only be described as a hack, but I still consider this a > > >> regression in the kernel, as it was working fine in the past, without > > >> visible warnings. > > >> > > > > > > I'm sorry, but for the reasons stated above I disagree about it being > > > a regression. > > > > Hmm, is it common for protocol implementations in the kernel to warn about > > invalid packets they receive? While L2TP uses connected sockets and thus > > usually no unrelated packets end up in the socket, a simple UDP port scan > > originating from the configured remote address/port will trigger the "short > > packet" warning now (nmap uses a zero-length payload for UDP scans by > > default). Log spam caused by a malicous party might also be a concern. > > Indeed, seems like appropriate counters would be a good fit here? > The prints are both potentially problematic for security and lossy.
Yes, I agree with this argument.
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