rfc 1918?

Fri Feb 23 14:00:00 UTC 2001

Agreed Valdis,

Our upstream's use 1918 addresses internally  so that 1918 addresses are
constantly bouncing off our filters
we have an aggressive egress filter which makes sure no 1918's leak and
pollute the internet ;-} and filtering on core routers is a suboptimal
solution RFC 1819 addresses (10 points to the person who knows the
predecessor)  NEED to be filtered at the border IMHO.

Scott

Valdis.Kletnieks at vt.edu@merit.edu on 02/22/2001 10:14:35 PM

Sent by:  owner-nanog at merit.edu

To:   Mark Radabaugh <mark at amplex.net>
cc:   North America Network Operators Group Mailing List <nanog at merit.edu>

Subject:  Re: rfc 1918?

On Thu, 22 Feb 2001 19:12:14 EST, Mark Radabaugh <mark at amplex.net>  said:
> At our egress points the filters are fairly short -- they allow only
traffic
> with our IP source addresses to leave.  This was my interpretation of the
RFC's.

Thank you.  The rest of the network appreciates you doing your part.

> Some in this discussion seem to be saying that we should also filter for
RFC1918
> destinations.  Am I reading this correctly?

That's probably optional.  But if you have the router resources to do it.
every little bit helps.  You probably should filter and log it, to find out
why one of your hosts is trying to send to a 1918 address outside your
site - if you're not using 1918 space, it shouldn't happen, and if you ARE
using it, the packet should have ended up inside your net, not on your
border router.

In either case, if a packet is trying to leave your net bound for a 1918
destination, something is probably seriously wrong(*).

(*) I'll leave ICMP replies from 1918-addressed P2P links out for the
moment ;)

>  I can see that packets destined for RFC1918 addresses will leave our
network
> (due to default routes) but are promptly dropped at the first BGP
speaking
> router they encounter.  Is it worth the extra router processing time to
check
> all outgoing packet destinations as well?  I can't see where this extra
> filtering is worth the trouble.

There's 2 main classes of "next router":

1) You don't filter, but it goes to the OTHER end of the link and promptly
gets stomped by a fascist filter that refuses to accept any source address
that's outside the adddress block that's supposed to be at your end.

2) You don't filter, they don't filter either, because they actually USE
1918 space for their own stuff, so your packets wtih 1918 source addresses
and real destination addresses manage to go a LONG way before hitting
anything that will stop them (as the original poster showed, the packets
could actually *arrive* at the destination, with no way to reply).

Remember - this filtering often can't be done on core routers due to
performance issues, so if it doesn't get done at the border routers
it probably won't happen...

                    Valdis Kletnieks
                    Operating Systems Analyst
                    Virginia Tech