Re: [magma] Question about IGMP host implementation

Bharat Joshi <> Fri, 14 October 2011 01:48 UTC

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From: Bharat Joshi <>
To: Stig Venaas <>, Thomas Morin <>
Date: Fri, 14 Oct 2011 07:14:34 +0530
Thread-Topic: [magma] Question about IGMP host implementation
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Subject: Re: [magma] Question about IGMP host implementation
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       I think GTSM does solve the issue of forwarding IGMP messages but as you mentioned its only for unicast.

       Till now, I have not seen any implementation using Unicast destination address for IGMP messages. So I am not sure if someone will really be interested in this work.

       May be there are some legacy or customized implementation which I am not aware of and we need to see if these people would be interested in this.,

From: [] On Behalf Of Stig Venaas []
Sent: Thursday, October 13, 2011 10:40 PM
To: Thomas Morin
Subject: Re: [magma] Question about IGMP host implementation

On 10/13/2011 1:26 AM, Thomas Morin wrote:
> 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, 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
>, 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.

Yes, I have been thinking whether GTSM should be used for unicast IGMPv3
messages. I don't think transitioning to that is all that hard. It
depends a bit how common it is to do unicast IGMP today. Checking GTSM
on the receiving end would have to be something that can be configured
until one can expect all senders to do it.

If there is some support for GTSM, we could try a draft updating the
IGMPv3 RFC perhaps...


> -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 []
>> Sent: Wednesday, October 12, 2011 9:17 PM
>> To: Bharat Joshi;
>> 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 []
>> Sent: Wednesday, October 12, 2011 4:20 AM
>> To: Kunal Shah;
>> 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
>> ________________________________________
>>  [] On Behalf Of Kunal Shah []
>> Sent: Wednesday, October 12, 2011 6:08 AM
>> 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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