Date: Mon, 18 Jun 2007 13:48:20 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: "Bruce M. Simpson" <bms@incunabulum.net> Cc: Ian FREISLICH <ianf@clue.co.za>, current@freebsd.org Subject: Re: Multicast problems Message-ID: <Pine.GSO.4.64.0706181344060.24865@sea.ntplx.net> In-Reply-To: <4676C30E.7040300@incunabulum.net> References: <E1I0E3b-0000kk-Ky@clue.co.za> <46765CB9.9020105@incunabulum.net> <Pine.GSO.4.64.0706180833080.23884@sea.ntplx.net> <4676C30E.7040300@incunabulum.net>
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On Mon, 18 Jun 2007, Bruce M. Simpson wrote: > Daniel Eischen wrote: >> On Mon, 18 Jun 2007, Bruce M. Simpson wrote: >>> >>> Other folks reported issues with the new code. As you're probably aware, >>> ipv4 multicast group memberships must be bound to an interface. When they >>> are not, the interface selected as the source for the IGMP join uses a >>> routing table lookup on the group. >>> The newer code changed to perform this lookup by interface index as well >>> as by address, as interfaces used for ipv4 multicast traffic are generally >>> assumed to have a protocol-level address. >> >> What happens when you join a group without an interface (INADDR_ANY)? >> Assume there is no route for the multicast group. Does the new code >> select the interface that the default route is on? > Yes. > > The new code is written in terms of the RFC 3678 API. The old IPv4 ASM API is > a shell around it. > > If no interface address is provided, either via an interface index or > interface protocol address, it will perform a route lookup on the multicast > group address to determine which interface to use when the group is being > joined. Obviously a default route will satisfy this lookup; the BSD route > lookup matches most specific match first. > > I believe that the problems which folks have been seeing is that some old > behaviour hasn't been captured in the new code. > This behaviour is that if the route lookup fails, the code would select the > first interface in the system with IFF_MULTICAST set on it (usually the > loopback address). This used to be contained in the ip_multicast_if() > function which was phased out. > > Joining a multicast group on INADDR_ANY is non-specific. It's a bit like > sending a datagram to 255.255.255.255 -- the group address alone is not a > sufficient key without additional information from the routing table. > > It seems reasonable that the code should use the interface of the default > route if no interface address is provided. However this doesn't cover the > case where no default route exists during system bringup. I think in that case, the first non-loopback interface with IFF_MULTICAST should be chosen. I think the loopback interface should be chosen in the absence of any other interface with IFF_MULTICAST set. Our code does rely on this as well. -- DE
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