Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!tut.cis.ohio-state.edu!ucbvax!TWG.COM!ljm
From: ljm@TWG.COM (Leo J McLaughlin)
Newsgroups: comp.protocols.tcp-ip
Subject: default routes in IP gateways
Message-ID: <8912211854.aa21721@Obelix.TWG.COM>
Date: 22 Dec 89 02:51:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 65


This note arises from a private discussion about installation of
routers.  The response seemed sufficiently useful to IP novices
to warrant distribution.

>>(paraphrased) defalt routes are bad.

>I am curious about your
>comment re the use of default routes. I am not a TCP guru, we are
>just getting into it here, but it seems to me that default routes
>are necessary, otherwise a router needs to know about ALL networks
>to which it can connect.

The primary problem is one of routing loops:

I set up

                      |
   A -- B -- C -- D --| great big wide world
                      |

with router B using C using D as its default gateway and you set up

					 |
                   great big wide world  |-- E -- F -- G -- H
					 |

with router G using F using E as its default gateway

If I send a packet from A to H and any of E, F, or G doesn't know that
H is behind it, the packet bounces back and forth over the Internet
until the TTL expires.

In practice this is a very easy topology to create.

  1) E and G, but not F under stand RIP. (The classic WIN/ROUTE example).

  2) 'Someone else' added G/H after you installed E and F. 

  3) 'Someone else' 'fixed' F's routing tables.


Or perhaps a simpler (common WIN/ROUTE customer) example:

  Novell -- A -- small -- B -- SLIP -- C -- small --D-- Novell
network 1      ethernet        link       ethernet     network 2

A's default is B,
B explicitly knows about A and has default of C
C explicitly knows about D and has default of B
D's default is C.

User on Novell network #1 mis-enters an internet address.
Just sit back and watch the phone bills.


Lastly, keep in mind that the errors in both of these examples are fairly
easy to spot and debug.  Much more complex and devious traps can be created
by adding additional adminstrative entities.


enjoy,
leo j mclaughlin iii
The Wollongong Group
ljm@twg.com