North American Network Operators Group

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Re: private ip addresses from ISP

  • From: Daniel Senie
  • Date: Tue May 23 09:39:08 2006

At 09:22 AM 5/23/2006, Robert Bonomi wrote:

> Date: Tue, 23 May 2006 03:33:34 -0400
> From: Richard A Steenbergen <[email protected]>
> To: [email protected]
> Subject: Re: private ip addresses from ISP
> On Mon, May 22, 2006 at 04:30:37PM -0400, Andrew Kirch wrote:
> >
> > >   3) You are seeing packets with source IPs inside private space
> > > arriving at
> > > your interface from your ISP?
> ...
> > Sorry to dig this up from last week but I have to strongly disagree with
> > point #3.
> > >From RFC 1918
> >    Because private addresses have no global meaning, routing information
> >    about private networks shall not be propagated on inter-enterprise
> >    links, and packets with private source or destination addresses
> >    should not be forwarded across such links. Routers in networks not
> >    using private address space, especially those of Internet service
> >    providers, are expected to be configured to reject (filter out)
> >    routing information about private networks.
> >
> > The ISP shouldn't be "leaving" anything to the end-user, these packets
> > should be dropped as a matter of course, along with any routing
> > advertisements for RFC 1918 space(From #1). ISP's who leak 1918 space
> > into my network piss me off, and get irate phone calls for their
> > trouble.
> The section you quoted from RFC1918 specifically addresses routes, not
> packets.

I quote, from the material cited above:
      "  ..., and packets with private source or destination addresses
       should not be forwarded across such links.  ...  "

There are some  types of packets that can legitimately have RFC1918 source
addresses --  'TTL exceeded' for example -- that one should legitimately
allow across network boundaries.
Really? You really want TTL-E messages with RFC1918 source addr? Even if they're used as part of a denial of service attack? Even though you can't tell where they actually came from?

> If you're receiving RFC1918 *routes* from anyone, you need to
> thwack them over the head with a cluebat a couple of times until the cluey
> filling oozes out. If you're receiving RFC1918 sourced packets, for the
> most part you really shouldn't care.

*I* care.

When those packets contain 'malicious' content, for example.

When the provider =cannot= tell me which of _their_own_customers_ originated
that attack, for example. (This provider has inbound source-filtering on
their Internet 'gateway' routers, but *not* on their customer-facing equipment
(either inbound or outbound.)
So you really don't want ANY packets with RFC 1918 source addresses then, not even ICMP TTL-E messages, since they could be used in a malicious fashion, and you would not be able to determine the true origin.

It's even more comical when the NSP uses RFC1918 space internally, and does
*not* filter those source addresses from their customers.
You mean like Comcast using Cisco routers in their head-ends and having the 10/8 address show up in traceroutes and so forth? Not sure to what degree it's the NSP's fault vs. the router vendors', but yes.

>                                      There are semi-legitimate reasons for
> packets with those sources addresses to float around the Internet, and
> they don't hurt anything.

I guess you don't mind paying for transit of packets that _cannot_possibly_
have any legitimate purpose on your network.
Along with this goes the usual flamewar over RFC 2827, ingress filtering (of which URPF is a subset implementation).

Some of us, on the other hand, _do_ object.
And some of us pay for bandwidth, care about getting congestion problems from useless traffic, etc. Perhaps it makes the case a lot clearer for selling "better than equal" service to the highest bidder if your network is overrun with undesired traffic.