Subject: Re: LLMNR and Rendezvous TTL disagreement, a modest proposal

From: Stuart Cheshire <[email-masked]>
Date: Fri, 20 Feb 2004 10:12:23 -0800

>what's the point of the TTL==255 stuff (for multicast
>packets anyway) ?

The immediate value is primarily for unicast packets, and less so for 
multicast packets.

When Rendezvous sends a multicast query, and the peers on the local link 
respond with unicast responses back to the querier, then checking TTL=255 
on reception verifies that no off-net interloper has managed to slip a 
few remote unicast responses in with the legitimate local unicast 
responses.

The value for multicast packets is an indirect code hygiene benefit: 
suppose a router vendor were to make a future multicast router that 
incorrectly forwards 224.0.0/24 packets. This could expose hosts to 
off-net attack. By setting and checking TTL=255, hosts are protected from 
this vulnerability, and because of the resultant traffic forwarding, the 
problem is detected and the router is fixed before it even gets through 
QA testing. Suppose we used TTL=1 instead: Now the router bug might go 
unnoticed for months or years, until the product is deployed in millions 
of homes around the world, and then some hacker finds it and exploits it.

Stuart Cheshire <[email-masked]>
 * Wizard Without Portfolio, Apple Computer, Inc.
 * www.stuartcheshire.org

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From: Masataka Ohta <[email-masked]>
Date: Sat, 21 Feb 2004 07:54:19 +0900

Stuart Cheshire;

> few remote unicast responses in with the legitimate local unicast 

It is unwise to use not DNS but Rendesvous, when LAN is connected
to the Internet.

> of homes around the world, and then some hacker finds it and exploits it.

Any protocol which relies LAN, which is connected to the Internet,
is secure is insecure.

Note that the LAN may be connected to other LAN with Ethernet
over IP.

Never bother to improve quality of illusion of security.

							Masataka Ohta