>I'm haveing a problem where, in one app I have a number of sockets all
>IP_ADD_MEMBERSHIPed to individual meulticast groups, and i'm finding that
a
>local transmission to one of these groups is appearing on all the sockets,
>when according to teh sockets standard it shoudl only appear on the one
that
>was IP_ADD_MEMBERSHIPed to that group.
That isn't true. Multicast membership is for the entire interface. If you
want the socket to receive only a particular group, you need to bind to
that group address. All INADDR_ANY sockets will (and should) receive all
packets for any local address with the same port, including all multicast
groups that have been joined.
+-DLS
I create and attach a socket to s singel group using (roughly) this code:
[error checking removed for brevity]
this->socketID = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
localAddress.sin_port=6666;
localAddress.sin_addr.s_addr = htonl(INADDR_ANY);
on = 1;
setsockopt(this->socketID, SOL_SOCKET, SO_REUSEADDR,
(RMF_OPTVAL_TYPE)&on, (socklen_t)sizeof(on)) ;
setsockopt(this->socketID, SOL_SOCKET, SO_REUSEPORT,
(RMF_OPTVAL_TYPE)&on, (socklen_t)sizeof(on));
bind(this->socketID, (struct sockaddr*)&localAddress,
sizeof(localAddress));
localAddressSize = sizeof(localAddress);
getsockname(this->socketID, (struct sockaddr*)&localAddress,
(socklen_t*)&localAddressSize);
struct ip_mreq localGroup;
localGroup.imr_multiaddr.s_addr = htonl(mcastGroup);
localGroup.imr_interface.s_addr = htonl(mcastIF);
setsockopt(this->socketID, IPPROTO_IP, IP_ADD_MEMBERSHIP,
(RMF_OPTVAL_TYPE)&localGroup,
(socklen_t)sizeof(localGroup));
ifAddress.s_addr = htonl(mcastIF);
setsockopt(this->socketID, IPPROTO_IP, IP_MULTICAST_IF,
(RMF_OPTVAL_TYPE)&ifAddress,
(socklen_t)sizeof(ifAddress));
setsockopt(this->socketID, IPPROTO_IP, IP_MULTICAST_LOOP,
(RMF_OPTVAL_TYPE)&on, (socklen_t)sizeof(on));
and i'm finding that i can do a recvfrom on a socket subscribed in this way
to, say 239.255.48.42:6666 on interface 192.168.0.210, and receive a udp
packet sent from, say 192.168.0.210:34330 to 239.255.48.40:6666. I've
checked that this is what's on the wire using ethereal. Has anyone got any
idea why i see this?
Thanks,
Rob Taylor
Senior Developer robt at flyingpig dot com
Flying Pig Systems http://www.flyingpig.com
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