== The kernel should not generate messages at the behest of the Internet

Here is a kernel message that one of my machines logged recently:

.pn prewrap on
>  sit: Src spoofed 98.247.192.43/2002:4d4d:4d07::4d4d:4d07 -> 128.100.3.51/2002:8064:333::1

Did I say 'a message'? Actually, no, I meant 493 messages in a few
days (and it would be more if I had not used iptables to block these
packets). Perhaps you begin to see the problem here. This message
shows two failures.  The first is that [[it's not usefully ratelimited
RatelimitKernelMessages]]. This exact message was repeated periodically,
often in relatively close succession and with no intervening messages,
yet it was not effectively ratelimited and suppressed.

(The kernel code uses _``net_warn_ratelimited()''_ but this is
clearly not ratelimited enough.)

The second and more severe failure is ~~the kernel should not default
to logging messages at the behest of the Internet~~. If you have a
_sit_ tunnel up for [[6to4 ../tech/ThoughtsOn6to4]], anyone on the
Internet can flood your kernel logs with their own version of this
message; all they have to do is hand-craft a 6to4 packet with the
wrong IPv6 address.  As we've seen here, such packets can probably
even be generated by accident or misconfiguration or perhaps funny
routing. Allow me to be blunt: the kernel should not be handing
this power to people on the Internet. Doing so is a terrible idea
for all of the usual reasons that giving Internet strangers any
power over your machine is a bad idea.

These messages should not be generated by default (at any logging
level, because there is no logging level that means 'only log
messages that are terrible ideas'). If the kernel wants to generate
them, it can and should be controlled by a sysctl or a sysfs option
or the like that defaults to off. People who really really want to
know can then turn it on; the rest of us will leave it off in our
usual great indifference to yet another form of Internet badness.

(Since I haven't been this quite this harsh on kernel messages
[[earlier RatelimitKernelMessages]], I'll admit it: my attitude on
kernel messages has probably steadily gotten stricter and more
irritated over time. I should probably write down my thoughts on
good kernel messages sometime.)

=== Sidebar: what this message means

A 6to4 encapsulated packet has two addresses; the outside IPv4
address and the inner IPv6 address. The kernel insists that the
inner IPv6 address is [[the IPv4 address's 6to4 address
../tech/ManyIpv6Addresses]]. Here the outside source is 98.247.192.43
but the inner 6to4 address in 2002::/16 is for the IPv4 address
77.77.77.7. You can get a similar message if the destination address
has a mismatch between the IPv4 address and the 6to4 IPv6 address.

(To decode the 6to4 IPv6 address, take off the leading 2002: bit
and then convert the next four hex octets to decimal bytes; each
byte is one digit in the address. So the source is claimed to be
4d.4d.4d.07 aka 77.77.77.7. We can follow the same procedure for
the destination address, getting (hex) 80.64.03.33 aka decimal
128.100.3.51, which matches the outer IPv4 address.)