[96727] in RedHat Linux List
Re: Private IP address space - why an ISP might _want_ to use
daemon@ATHENA.MIT.EDU (William Stearns)
Thu Oct 29 00:38:50 1998
Date: Thu, 29 Oct 1998 00:42:03 -0500 (EST)
From: William Stearns <wstearns@pobox.com>
To: Kevin Myer <kevin_myer@elanco.k12.pa.us>
cc: ML-redhat <redhat-list@redhat.com>
In-Reply-To: <Pine.LNX.4.02.9810281927450.489-100000@marble.elanco.k12.pa.us>
Resent-From: redhat-list@redhat.com
Reply-To: redhat-list@redhat.com
Good day, Kevin,
On Wed, 28 Oct 1998, Kevin Myer wrote:
> Thanks for the response! I follow your statements without a problem but I
> guess in my mind, what a company does with their routers internally I
> should never know about. For example, consider the following traceroute
> (the one I was talking about in my previous post) to www.linuxtoday.com
>
> [myer@marble ~]$ /usr/sbin/traceroute www.linuxtoday.com
> traceroute to www.linuxtoday.com (24.4.53.58), 30 hops max, 40 byte
> packets
> 1 wellfleet (172.20.0.1) 2.956 ms 2.819 ms 2.774 ms
> 2 172.19.1.1 (172.19.1.1) 6.782 ms 6.787 ms 6.565 ms
> 3 n1-195-15.iu13.k12.pa.us (204.171.195.254) 7.918 ms 9.507 ms 7.463
> ms
> 4 mdt0-h5-0-0.hrbg.pa.verio.net (129.250.202.100) 10.739 ms 10.323 ms
> 10.193 ms
> 5 mdt0.phl0.verio.net (129.250.2.105) 16.815 ms 15.070 ms 14.022 ms
> 6 phl0-0.pne0-0.verio.net (129.250.2.85) 15.940 ms 17.936 ms 20.579
> ms
> 7 sprint-nap.home.net (192.157.69.85) 16.199 ms 15.173 ms 15.885 ms
> 8 172.16.1.149 (172.16.1.149) 99.262 ms 99.517 ms 99.941 ms
> 9 cr1.mckiny1.tx.home.net (24.4.48.57) 97.626 ms 98.338 ms 102.476 ms
> 10 c53565-a.mckiny1.tx.home.com (24.4.53.58) 110.793 ms 102.636 ms
> 110.105 ms
>
> Hops 1 and 2 are our internal routers and we use a pool of fictious class
> B and C addresses for our private network. Hop 3 is the box that is doing
> NAT and then all hops up to 7 are the routers in between. What I was
> puzzled about was hop 8 - to my mind if an ISP, especially tier one
> providers, uses fictious addresses, thats fine but I should never see
> anything about it. Yet I'm still two hops away from my final destination
<-----------Router 7----------------------Router 8----------------->
^ ^ ^ ^
| | | |
192.157.69.85 172.16.1.150 172.16.1.149 ???
(probably)
It actually makes some sense if you think about what traceroute is
doing. On line 8, your machine sends out a bogus packet with a time to
live of 8. Each packet decrements that time to live. When 8 receives the
packet, it decrements the TTL to 0 and discards the packet, but it sends
back a warning to your box that says I threw away your packet. This
response packet is a fresh packet with your IP in the destination and
router 8's .149 IP address as a source; this is how traceroute knows the
intermediate IP's.
With a traceroute you get to see half of the IP's between you and
the destination; specifically, you see all the IP's of the interfaces
_facing_ you. I'm guessing on the .150, but if you could convince someone
at linuxtoday to tracreroute to your NAT host, they could read off all the
IP's on the other sides of those routers.
> (www.linuxtoday.com). Somehow to me it seems like that hop 8 is actually
> in real address space and everything else (besides my own routers with
> fictious addresses) resolves to a real address. Have I missed the boat on
> this one and am I actually seeing inside an ISP's internal routing schema?
You _are_ seeing the internal routing scheme.
> And if I can see a fictious address on the other end, is it possible for
> that address to "pollute" real address space?
No, because (with the exception of traceroute and other low level
error messages) these routers only pass packets without touching their
source or destination addresses. Backbone routers on the Internet are
generally smart enough to _not_ advertise routes to any of the reserved
networks and to _not_ accept routes to these networks. That's why if
someone from linuxtoday tried to traceroute to 172.20.0.1 or the IP
address on your workstation, they might get a few hops in before some
router complained of "Network Unreachable".
Cheers,
- Bill
---------------------------------------------------------------------------
Unix _is_ user friendly. It's just very selective about who its friends
are. And sometimes even best friends have fights.
William Stearns (wstearns@pobox.com)
Mason, buildkernel, and named2hosts are at: http://www.pobox.com/~wstearns
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