IPv6 filtering (was Re: IPv6 internet broken, cogent/telia/hurricane not peering)

Tue Oct 13 18:22:23 UTC 2009

On Mon, Oct 12, 2009 at 2:44 PM, Seth Mattinen <sethm at rollernet.us> wrote:

> Marco Hogewoning wrote:
> >
> > As this thread has drifted off topic any way, would it for instance be a
> > good idea to simply not accept mail from hosts that clearly use
> > autoconfig ie reject all smtp from EUI-64 addresses. Of course not a
> > wise idea for your own outbound relays which should handle mail from
> > your customers but on the incoming side it might as well save a lot of
> > headache and there is no need to keep track of which /64 are access
> > networks.
> >
>
> That would be really, really bad. My 3750's won't accept arbitrary
> /128's in an ACL unless it's EUI-64 or I make up something similar that
> it will like. I'm sure I'm not the only person who owns a 3750. As such,
> my mail servers are using EUI-64 addresses.
>
> ~Seth
>
>
As I understand it, (and Cisco's documentation seems to support this,
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2ZY/command/reference/M1.html#wpxref54198
as an example), if you put a /128 in an ACL, you cannot specify any L4 port
information for the address due to the limited width of the TCAM; in
order to specify L4 information for the ACL, Cisco stuffs it into bits 24
through 39, losing what information was originally stored in those bits.
It just so happens those are the fixed FFFE bits in an EUI-64 address,
so if you're using EUI-64, no "real" information is lost.  You can do your
own non-EUI-64 addressing and still use ACLs with layer 4 port information
as long as you don't put any addressing information into bits 24 through 39.

Or, if you want to be *really* clever, you can address blocks of hosts with
identical functions and identical security rules by assigning them addresses
that differ *only* in bits 24 through 39; then, a single L4 /128 rule in you
v6
ACL will actually apply to the entire equivalence class of servers, since
from
the router's perspective, it doesn't distinguish one server from the next as
far
as applying the ACL rule.  However, if you opt to do this, make sure you
document it *really* carefully, so the poor engineer who has to pick up
after
you will understand why the router is treating all of the servers
identically,
in spite of having what looks to be a single /128 listed in its ACL.  ^_^;

Matt