Subject: I-D ACTION:draft-ietf-dnsext-mdns-21.txt

From: [email-masked]
Date: Wed, 11 Jun 2003 07:54:04 -0400

A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the DNS Extensions Working Group of the IETF.

	Title		: Linklocal Multicast Name Resolution (LLMNR)
	Author(s)	: L. Esibov, B. Aboba, D. Thaler
	Filename	: draft-ietf-dnsext-mdns-21.txt
	Pages		: 21
	Date		: 2003-6-10
	
Today, with the rise of home networking, there are an increasing number
of ad-hoc networks operating without a Domain Name Service (DNS) server.
In order to allow name resolution in such environments, Link-Local
Multicast Name Resolution (LLMNR) is proposed.  LLMNR supports all
current and future DNS formats, types and classes, while operating on a
separate port from DNS, and with a distinct resolver cache.

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---

From: Alain Durand <[email-masked]>
Date: Fri, 13 Jun 2003 16:07:29 -0700

This might have been previously discussed, but:

 > In composing LLMNR queries, the sender MUST set the Hop Limit field in
 > the IPv6 header and the TTL field in IPv4 header of the response to 
one
 > (1).  Even when LLMNR queries are sent to a link-scope multicast
 > address, it is possible that some routers may not properly implement
 > link-scope multicast, or that link-scope multicast addresses may leak
 > into the multicast routing system.  Therefore setting the IPv6 Hop 
Limit
 > or IPv4 TTL field to one provides an additional precaution against
 > leakage of LLMNR queries.
 >
 > In composing a response to an LLMNR query, the responder MUST set the
 > Hop Limit field in the IPv6 header and the TTL field in IPv4 header of
 > the response to one (1).  This is done so as to prevent the use of 
LLMNR
 > for denial of service attacks across the Internet.

==> this is different from Neighbor Discovery which require to set the
hop-count  to 255. Is there a good enough reason not to use the same 
algorithm here?

Also, I'm a bit confused on 
http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html
where issue #2 says to use TTL=255 and issue #35 says to use TTL=1.....

	- Alain.

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---

From: Bernard Aboba <[email-masked]>
Date: Fri, 13 Jun 2003 21:44:38 -0700 (PDT)

> hop-count  to 255. Is there a good enough reason not to use the same

The IPv4 LL draft now uses TTL=1. Where TCP is used for a query
(recommended for unicast), setting TTL=1 makes it difficult to
spoof a response, due to the three way handshake. Setting TTL=1 in
responses (UDP or TCP) ensures that a multicast LLMNR query can't be
used for a DoS attack against a victim on another LAN.
Setting TTL=255 doesn't accomplish this. The remaining case is a
multicast query answered with a UDP response. Here it would be possible
for the response to be spoofed with TTL=1 but not TTL=255 (with check).
But the DoS prevention benefits were judged to overide this.

> where issue #2 says to use TTL=255 and issue #35 says to use TTL=1.....

Different Issues filed by different individuals with different points of
view :)