Re: [MBONED] draft-eitf-mboned-auto-multicast: Defending against DoS via Teardown

[Greg Bumgardner <[e-mail]>]

  Minor fix below...

On 8/4/2011 2:43 PM, Greg Bumgardner wrote:
>
>  All,
>
> The protocol described in the latest draft does not provide any 
> defense against a denial-of-service attack on a gateway in which an 
> eavesdropper forges a Teardown message to terminate a session and 
> thereby interrupt any active multicast streams.
>
> I believe we can make it much more difficult for another entity to 
> send a valid teardown message by changing how a request nonce is 
> generated and used in the Request, Membership Update and Teardown 
> messages.
>
> The solution involves sending a hashed value as the request nonce in 
> the request and update messages, but sending the un-hashed value in 
> the Teardown message. If both the relay and gateway use the same hash 
> function, the relay can obtain the hashed nonce value used in MAC 
> generation by applying the hash function to the nonce value sent in 
> the Teardown message. If a cryptographic hash is used (e.g. MD5), an 
> eavesdropper will not (easily) be able to guess what nonce value must 
> be sent in the Teardown message to produce a valid MAC value.
>
> What follows is some simplistic pseudo-code that describes the procedure:
>
> H(): Hash function used by relay to generate MAC values
> HKr = Secret hash key used by relay to generate MAC values
> MD5(): MD5 Hash function
> N: nonce (random value)
>
> ---- Gateway ----
> GatewayInterface gif;
> gif.last_address = null
> gif.last_port = null
> gif.last_mac = null
> gif.last_nonce = null;
> gif.nonce = rand();
> gif.hashed_nonce = MD5(nonce);
> Request request;
> request.hashed_nonce = gif.hashed_nonce;
> send(request);
>
> ---- Relay ----
> Message message = receive();
> if (message.type == REQUEST) {
>     Request request = (Request)message;
>     Query query;
This
>     query.nonce = request.nonce;
Should be:

query.hashed_nonce = request.hashed_nonce;
>     query.mac = H(HKr, request.src_addr, request.src_port, 
> request.hashed_nonce);
>     query.gateway_addr = request.src_addr;
>     query.gateway_port = request.src_port;
>     send(query);
> }
>
> ---- Gateway ----
> Query query = receive();
> if (last_addr != null && (query.gateway_addr != last_addr || 
> query.gateway_port != last_port)) {
>     // Send Teardown
>     Teardown teardown;
>     teardown.mac = gif.last_mac;
>     teardown.nonce = gif.last_nonce; // Not hashed!
>     teardown.orig_addr = gif.last_addr;
>     teardown.orig_port = gif.last_port;
>     send(teardown);
> }
> gif.last_nonce = nonce;
> gif.last_mac = query.mac;
> gif.last_addr = query.gateway_addr;
> gif.last_port = query.gateway_port;
> Update update;
> update.hashed_nonce = gif.hashed_nonce;
> update.mac = query.mac;
> update.report = <igmp/mld report>
> send(update);
>
> ---- Relay ----
> Message message = receive();
> if (message.type == UPDATE) {
>     Update update = (Update)message;
>     if (update.mac == H(HKr, update.src_addr, update.src_port, 
> update.hashed_nonce)) {
>         if (!session_exists(update.src_addr, update.src_port)) {
>             create_session(update.src_addr, update.src_port);
>         }
>         update_session(update.src_addr, update.src_port, update.report);
>     }
>     else {
>         // MAC validation failed - ignore message
>     }
> }
> else if (message.type == TEARDOWN) {
>     Teardown teardown = (Teardown)message;
>     if (teardown.mac == H(HKr, teardown.orig_addr, teardown.orig_port, 
> MD5(teardown.nonce))) {
>         if (session_exists(teardown.orig_addr, teardown.src_port)) {
>             destroy_session(update.src_address, update.src_port);
>         }
>     }
>     else {
>         // MAC validation failed - ignore message
>     }
> }
>
>
> ======================
>
>
> Thanks,
>
> -g.b.
>
> Greg Bumgardner
> Cisco Systems
>
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