In a message written on Thu, Apr 04, 2013 at 02:57:11PM -0400, Jay Ashworth wrote:
Yes. In the fallout from the Cloudflare attack of last week it was
announced that several IXs were going to stop advertising the
address space of their peering lan, which properly does not need to
be advertised anyway.
Well, now that's a big maybe. I was a big advocate for the peering
exchanges each having their own ASN and announcing the peering block
back in the day, and it seems people may have forgotten some of the
issues with unadvertised peering exchange blocks.
It breaks traceroute for many people:
The ICMP TTL Unreachable will come from a non-routed network (the
exchange LAN). If it crosses another network boundary doing uRPF,
even in loose mode, those unreachables will be dropped.
It also reduces the utility of a tool like MTR. Without the ICMP
responese it won't know where to ping, and even if it receives
the ICMP it's likely packets towards the LAN IP's will be dropped
with no route to host.
It has the potential to break PMTU discovery for many people:
If a router is connected to the exchange and a lower MTU link a
packet coming in with DF set will get an ICMP would-fragment
reply. Most vendors source from the input interface, e.g. the
exchange IP. Like the traceorute case, if crosses another network
boundary doing uRPF, even in loose mode, those ICMP messages
will be lost, resulting in a PMTU black hole.
Some vendors have knobs to force the ICMP to be emitted from a
loopback, but not all. People would have to turn it on.
But hey, this is a good thing because a DDOS caused issues, right?
Well, not so much. Even if the exchange does not advertise the
exchange LAN, it's probably the case that it is in the IGP (or at
least IBGP) of everyone connected to it, and by extension all of
their customers with a default route pointed at them. For the most
popular exchanges (AMS-IX, for instance) I suspect the percentage
of end users who can reach the exchange LAN without it being
explicitly routed to be well over 80%, perhaps into the upper 90%
range. So when those boxes DDOS, they are going to all DDOS the
LAN anyway.
Security through obscurity does not work. This is going to annoy some
people just trying to do their day job, and not make a statistical
difference to the attackers trying to take out infrastructure.
How about we all properly implement BCP 38 instead?
