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.
