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

[Iljitsch van Beijnum <[e-mail]>]

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
> [e-mail] or [e-mail] 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.

_______________________________________________
Ietf mailing list
[e-mail]
https://www1.ietf.org/mailman/listinfo/ietf