Re: [magma] Question about IGMP host implementation

Hi,

Obviously there is no legitimate case where a query would come from 
another subnet.
The question is how to avoiding such queries from being processed...

IGMPv3 specs talks a bit about this aspect ;  RFC3376, section 9.1, 
Security Considerations, Query message:

  There are three measures necessary to defend against externally forged Queries:

    o Routers SHOULD NOT forward Queries.  This is easier for a router to
      accomplish if the Query carries the Router-Alert option.

    o Hosts SHOULD ignore v2 or v3 Queries without the Router-Alert
      option.

    o Hosts SHOULD ignore v1, v2 or v3 General Queries sent to a
      multicast address other than 224.0.0.1, the all-systems address.

If a host enforces these rules, AFAICT there is no case were it would 
process a Query from another subnet.
Even though RFC2236 is silent on this aspect, an RFC3376 host 
implementation in IGMPv2 compatibility mode would be expected to apply 
these checks.

On the other hand, RFC3376 also says, in 4.1.12. IP Destination 
Addresses for Queries:

    In IGMPv3, General Queries are sent with an IP destination address of
    224.0.0.1, the all-systems multicast address.  Group-Specific and
    Group-and-Source-Specific Queries are sent with an IP destination
    address equal to the  multicast address of interest.  *However*, a
    system MUST accept and  process any Query whose IP Destination Address
           ^^^^^^^^^^^^^^^
    field contains *any* of the addresses (unicast or multicast)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^
    assigned to the interface on which the Query arrives.

This rule contradicts the third rule above (which would in itself 
deserve a discussion), but the two first rules are possibly enough: if 
an attacker forges a Query, under the assumption that at least one 
Router on the path to the victim supports the Router Alert option and 
implements IGMP and enforces the first of the three rules above, if 
hosts apply the second rule, then no forged packet will be processed by 
hosts...  The issue is that it may not be reasonable to count on all 
this to be true (eg. there are IGMP Querier implementations in the wild 
that do not set the RA option in Query messages...).

The most reasonable thing to do, as suggested below, is to drop Queries 
whose source address is from another subnet.

A nicer solution would be to apply GTSM (RFC5082) to IGMP, but 
transitioning to it is absolutely not trivial.

-Thomas

Bharat Joshi a écrit :
> Kunal,
>
>          What I am suggesting is that though RFC does not explicitly suggest it, it might be better to do this for broadcast interfaces.
>
>          But yes, RFC does not suggest anything on this so a host can process a query message with a source address from any other subnet as well.
>
> Regards,
> Bharat
> ________________________________________
> From: Kunal Shah [kunal.shah@ericsson.com]
> Sent: Wednesday, October 12, 2011 9:17 PM
> To: Bharat Joshi;magma@ietf.org
> Subject: RE: Question about IGMP host implementation
>
> Hi Bharat,
>
> The security consideration addresses the processing of a report from a different subnet on the router. My question pertains to the processing of a Query from a different subnet on the host.
>
> Kunal
>
> -----Original Message-----
> From: Bharat Joshi [mailto:bharat_joshi@infosys.com]
> Sent: Wednesday, October 12, 2011 4:20 AM
> To: Kunal Shah;magma@ietf.org
> Subject: RE: Question about IGMP host implementation
>
> Hi Kunal,
>
>          I think to keep the security tight, it is better to not respond to queries received from a source address which does not fall on a subnet on that interface. Please note that this should be done only broadcast interfaces. It may not work on point-to-point links.
>
>          If you look at the security consideration in RFC 2236, it is mentioned that for reports, the above check should be done.
>
> Regards,
> Bharat
> ________________________________________
> From:magma-bounces@ietf.org  [magma-bounces@ietf.org] On Behalf Of Kunal Shah [kunal.shah@ericsson.com]
> Sent: Wednesday, October 12, 2011 6:08 AM
> To:magma@ietf.org
> Subject: [magma] Question about IGMP host implementation
>
> Hi all,
>
> Can an IGMPv2 host respond to a general query originated from a subnet other then its own?? RFC 2236 states:
>
> ""query received" occurs when the host receives either a valid
>       General Membership Query message, or a valid Group-Specific
>       Membership Query message.  To be valid, the Query message must be
>       at least 8 octets long, and have a correct IGMP checksum.  The
>       group address in the IGMP header must either be zero (a General
>       Query) or a valid multicast group address (a Group-Specific Query)"
>
> There is no requirement for the source address to be on the same subnet as the host.
>
> Thanks,
> Kunal
>
>
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