svn commit: r217592 - head/sys/netinet

Randall Stewart rrs at
Wed Mar 30 11:27:43 UTC 2011


The original complaint on this came from Daniel... I believe he
claimed that up until bms's multi-cast work.. you would NOT
get a packet sent to you if you did not join the multi-cast group.

I will also comment in-line below...

On Mar 29, 2011, at 2:01 PM, John Baldwin wrote:

> On Wednesday, January 19, 2011 2:07:16 pm Randall Stewart wrote:
>> Author: rrs
>> Date: Wed Jan 19 19:07:16 2011
>> New Revision: 217592
>> URL:
>> Log:
>>  Fix a bug where Multicast packets sent from a
>>  udp endpoint may end up echoing back to the sender
>>  even with OUT joining the multi-cast group.
>>  Reviewed by:	gnn, bms, bz?
>>  Obtained from:	deischen (with help from)
>> Modified:
>>  head/sys/netinet/udp_usrreq.c
>> Modified: head/sys/netinet/udp_usrreq.c
> ==============================================================================
>> --- head/sys/netinet/udp_usrreq.c	Wed Jan 19 18:20:11 2011	(r217591)
>> +++ head/sys/netinet/udp_usrreq.c	Wed Jan 19 19:07:16 2011	(r217592)
>> @@ -479,11 +479,13 @@ udp_input(struct mbuf *m, int off)
>> 			 * and source-specific multicast. [RFC3678]
>> 			 */
>> 			imo = inp->inp_moptions;
>> -			if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) &&
>> -			    imo != NULL) {
>> +			if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
>> 				struct sockaddr_in	 group;
>> 				int			 blocked;
>> -
>> +				if(imo == NULL) {
>> +					INP_RUNLOCK(inp);
>> +					continue;
>> +				}
>> 				bzero(&group, sizeof(struct sockaddr_in));
>> 				group.sin_len = sizeof(struct sockaddr_in);
>> 				group.sin_family = AF_INET;
> So it turns out that this is a feature, not a bug, and is how multicast has 
> always worked.  Specifically, if you bind a UDP socket with a wildcard 
> address, it should receive all traffic for the bound port, unicast or 
> multicast.  When you join a group, you have switched the socket into a mode 
> where it now has a whitelist of acceptable multicast groups, but if a socket 
> has no joined groups, it should receive all multicast traffic, not none.  This 
> change breaks that.
> I did not find this behavior intuitive at first, but it does seem to be 
> required.  Note the description of IP_ADD_MEMBERSHIP from RFC 3678 for 
> example:

I agree getting a packet that is coming to your port without joining the
multi-cast group is not intuitive to me... 

> 3.  Overview of APIs
>   There are a number of different APIs described in this document that
>   are appropriate for a number of different application types and IP
>   versions.  Before providing detailed descriptions, this section
>   provides a "taxonomy" with a brief description of each.
>   There are two categories of source-filter APIs, both of which are
>   designed to allow multicast receiver applications to designate the
>   unicast address(es) of sender(s) along with the multicast group
>   (destination address) to receive.
>      o  Basic (Delta-based): Some applications desire the simplicity of
>         a delta-based API in which each function call specifies a
>         single source address which should be added to or removed from
>         the existing filter for a given multicast group address on
>         which to listen.  Such applications typically fall into either
>         of two categories:
>         +  Any-Source Multicast: By default, all sources are accepted.
>            Individual sources may be turned off and back on as needed
>            over time.  This is also known as "exclude" mode, since the
>            source filter contains a list of excluded sources.
>         +  Source-Specific Multicast: Only sources in a given list are
>            allowed.  The list may change over time.  This is also known
>            as "include" mode, since the source filter contains a list
>            of included sources.
>            This API would be used, for example, by "single-source"
>            applications such as audio/video broadcasting.  It would
>            also be used for logical multi-source sessions where each
>            source independently allocates its own Source-Specific
>            Multicast group address.

Not the above document is talking about a receiver that as joined the
multicast group (or is joining it and wants some filtering)... I don't
see how that applies to a UDP socket that has NOT joined the M-cast group..

> .....
> 4.1.1.  IPv4 Any-Source Multicast API
>   The following socket options are defined in <netinet/in.h> for
>   applications in the Any-Source Multicast category:
>   Socket option             Argument type
>   IP_ADD_MEMBERSHIP         struct ip_mreq
>   IP_BLOCK_SOURCE           struct ip_mreq_source
>   IP_UNBLOCK_SOURCE         struct ip_mreq_source
>   IP_DROP_MEMBERSHIP        struct ip_mreq
>   IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP are already implemented on
>   most operating systems, and are used to join and leave an any-source
>   group.
>   IP_BLOCK_SOURCE can be used to block data from a given source to a
>   given group (e.g., if the user "mutes" that source), and
>   IP_UNBLOCK_SOURCE can be used to undo this (e.g., if the user then
>   "unmutes" the source).
> As to why the packets loop back to the receiver, I believe that is a separate 
> issue on the output side, not the receive side.

But that is what the commit fixes... and as to the above, it again is
talking about Multicast members.. AFAIKT... I am actually at the IETF
so if you would like I can gladly go talk to the authors of this RFC
(if they are here) and get their opinion on this.

One other thing.. note this is NOT a standard but a informational RFC. Informational
RFC are guidelines and NOT mandatory at all.. there will never be a MUST/SHOULD etc
within them.

I see that the authors are Dave Thaler, Bill Fenner and B Quinn

I don't know B Quinn.. not sure if Fenner is here (have not seen him).. but I have
seen Dave around.. I will look him up and ask him his opinion..


> -- 
> John Baldwin

Randall Stewart
803-317-4952 (cell)

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