svn commit: r217592 - head/sys/netinet

[Randall Stewart]

John:

So I caught up with Dave Thaler here at the IETF...

He said that NO UDP socket that has NOT joined a multicast
group should ever receive a packet sent to a multicast address.
He also said this was part of the POSIX API and the way
all Unix machines worked. 

So.. no it is a bug and the fix is correct.

R
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: http://svn.freebsd.org/changeset/base/217592
>> 
>> 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:
> 
> 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.
> 
> 
> .....
> 
> 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.
> 
> -- 
> John Baldwin
> 

------------------------------
Randall Stewart
803-317-4952 (cell)