Re: Last Call: 'Linklocal Multicast Name Resolution (LLMNR)' to Proposed Standard

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On 10-aug-2005, at 20:47, The IESG wrote:

The IESG has received a request from the DNS Extensions WG to consider the
following document:

- 'Linklocal Multicast Name Resolution (LLMNR) '
   <draft-ietf-dnsext-mdns-42.txt> as a Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits
final comments on this action.  Please send any comments to the
iesg@xxxxxxxx or ietf@xxxxxxxx mailing lists by 2005-08-24.

Sorry for the delay.

After the discussion on the IETF list I thought it prudent to do a more thorough review of the draft.

This statement:

[a]  Responders MUST listen on UDP port 5355 on the link-scope multicast
     address(es) defined in Section 2, and on UDP and TCP port 5355 on
the unicast address(es) that could be set as the source address (es)
     when the responder responds to the LLMNR query.

Seems to be in conflict with:

   Unicast LLMNR queries MUST be done using TCP and the responses MUST
   be sent using the same TCP connection as the query.  Senders MUST
   support sending TCP queries, and responders MUST support listening
   for TCP queries. If the sender of a TCP query receives a response to
   that query not using TCP, the response MUST be silently discarded.

   Unicast UDP queries MUST be silently discarded.

Why would a responder be required to listen on unicast UDP and then have to silently discard anything that comes in?

Section 2.4 discusses use of TCP for LLMNR queries and responses. In composing an LLMNR query using TCP, the sender MUST set the Hop Limit
   field in the IPv6 header and the TTL field in the IPv4 header of the
   response to one (1).  The responder SHOULD set the TTL or Hop Limit
   settings on the TCP listen socket to one (1) so that SYN-ACK packets
   will have TTL (IPv4) or Hop Limit (IPv6) set to one (1). This
   prevents an incoming connection from off-link since the sender will
   not receive a SYN-ACK from the responder.

For UDP queries and responses, the Hop Limit field in the IPv6 header
   and the TTL field in the IPV4 header MAY be set to any value.
   However, it is RECOMMENDED that the value 255 be used for
   compatibility with Apple Bonjour [Bonjour].

Why not REQUIRE that UDP queries/responses have a TTL of 255. Then the receiver of a packet can determine with almost complete certainty that the packet didn't traverse any routers by checking if the received packet has a TTL of 255. (See (amongst others) ICMPv6 for how this works.)

Since LLMNR queries can be sent when DNS server(s) do not respond, an
   attacker can execute a denial of service attack on the DNS server(s)
   and then poison the LLMNR cache by responding to an LLMNR query with
   incorrect information.

Couldn't have said it better myself.

LLMNR takes the approach of intermingling the locally resolved namespace with the globally resolved namespace. This will lead to security problems such as the one mentioned above (the security considerations section is way too cavalier), but also unduly puts strain on both the local and the global mechanisms in the expected common case where a host will first try to resolve a name globally and failing that, try to resolve the name locally.

For instance, when a user configures a name that doesn't exist in the global namespace on a locally reachable device, and then references that device by name, this will involve the global DNS unnecessarily. Since the intended result does materialize, the user doesn't see a failure condition and the situation persists.

Alternatively, whenever there is a failure in the global DNS, for instance because of lack of reachability, unavailability of a DNS server or the user typing an incorrect name, this will result in a local multicast query. On some links this is very undesirable. Low- bandwidth links such as cell phone data services are an obvious example. Another one is IEEE 802.11x. Due to its one-to-many nature broadcasts and multicasts must be sent at an artificial low bitrate on these links, using up an inordinate amount of link bandwidth relative to the packet size.

I suggest that the IESG either send back the draft to the wg to fix these problems or at most approves publication as an experimental RFC and NOT a standards track RFC.

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