What does it mean to bind a multicast UDP socket

Question

I am using multicast UDP between hosts that have multiple network interfaces  I am using boost  asio  and am confused by the 2 operations receivers have to make  bind  then join-group   Why do you need to specify the local address of an interface  during bind  when you do that with every multicast group that you join   The sister-question regards the multicast port  Since during sending  you send to a multicast address  amp  port  why  during subscription to a multicast group  you only specify the address  not the port - the port being specified in the confusing call to bind   Note  the  join-group  is a wrapper over setsockopt IP ADD MEMBERSHIP   which as documented  may be called multiple times on the same socket to subscribe to different groups  over different networks    It would therefore make perfect sense to ditch the bind call and specify the port every time I subscribe to a group   From what I see  always binding to  0 0 0 0  and specifying the interface address when joining the group  works very well  Confused

User · Accepted Answer

To bind a UDP socket when receiving multicast means to specify an address and port from which to receive data (NOT a local interface, as is the case for TCP acceptor bind). The address specified in this case has a filtering role, i.e. the socket will only receive datagrams sent to that multicast address & port, no matter what groups are subsequently joined by the socket. This explains why when binding to INADDR_ANY (0.0.0.0) I received datagrams sent to my multicast group, whereas when binding to any of the local interfaces I did not receive anything, even though the datagrams were being sent on the network to which that interface corresponded.

Quoting from UNIX® Network Programming Volume 1, Third Edition: The Sockets Networking API by W.R Stevens. 21.10. Sending and Receiving

[...] We want the receiving socket to bind the multicast group and port, say 239.255.1.2 port 8888. (Recall that we could just bind the wildcard IP address and port 8888, but binding the multicast address prevents the socket from receiving any other datagrams that might arrive destined for port 8888.) We then want the receiving socket to join the multicast group. The sending socket will send datagrams to this same multicast address and port, say 239.255.1.2 port 8888.

User · Answer

Correction for What does it mean to bind a multicast  udp  socket  as long as it partially true at the following quote      The  bind  operation is basically saying   use this local UDP port for sending and receiving data  In other words  it allocates that UDP port for exclusive use for your application   There is one exception  Multiple applications can share the same port for listening  usually it has practical value for multicast datagrams   if the SO REUSEADDR option applied  For example  int sock   socket AF INET  SOCK DGRAM  IPPROTO UDP      create UDP socket somehow     int set option on   1     it is important to do  reuse address  before bind  not after int res   setsockopt sock  SOL SOCKET  SO REUSEADDR   char    amp set option on       sizeof set option on    res   bind sock  src addr  len     If several processes did such  reuse binding   then every UDP datagram received on that shared port will be delivered to each of the processes  providing natural joint with multicasts traffic     Here are further details regarding what happens in a few cases    attempt of any bind   exclusive  or  reuse   to free port will be successful attempt to  exclusive binding  will fail if the port is already  reuse-binded  attempt to  reuse binding  will fail if some process keeps  exclusive binding

User · Answer

The  bind  operation is basically saying   use this local UDP port for sending and receiving data   In other words  it allocates that UDP port for exclusive use for your application    Same holds true for TCP sockets    When you bind to  0 0 0 0   INADDR ANY   you are basically telling the TCP IP layer to use all available adapters for listening and to choose the best adapter for sending   This is standard practice for most socket code   The only time you wouldn t specify 0 for the IP address is when you want to send receive on a specific network adapter   Similarly if you specify a port value of 0 during bind  the OS will assign a randomly available port number for that socket   So I would expect for UDP multicast  you bind to INADDR ANY on a specific port number where multicast traffic is expected to be sent to   The  join multicast group  operation  IP ADD MEMBERSHIP  is needed because it basically tells your network adapter to listen not only for ethernet frames where the destination MAC address is your own  it also tells the ethernet adapter  NIC  to listen for IP multicast traffic as well for the corresponding multicast ethernet address   Each multicast IP maps to a multicast ethernet address   When you use a socket to send to a specific multicast IP  the destination MAC address on the ethernet frame is set to the corresponding multicast MAC address for the multicast IP   When you join a multicast group  you are configuring the NIC to listen for traffic sent to that same MAC address  in addition to its own    Without the hardware support  multicast wouldn t be any more efficient than plain broadcast IP messages   The join operation also tells your router gateway to forward multicast traffic from other networks   Anyone remember MBONE    If you join a multicast group  all the multicast traffic for all ports on that IP address will be received by the NIC   Only the traffic destined for your binded listening port will get passed up the TCP IP stack to your app   In regards to why ports are specified during a multicast subscription - it s because multicast IP is just that - IP only    ports  are a property of the upper protocols  UDP and TCP    You can read more about how multicast IP addresses map to multicast ethernet addresses at various sites   The Wikipedia article is about as good as it gets      The IANA owns the OUI MAC address 01 00 5e  therefore multicast   packets are delivered by using the Ethernet MAC address range   01 00 5e 00 00 00 - 01 00 5e 7f ff ff  This is 23 bits of available   address space  The first octet  01  includes the broadcast multicast   bit  The lower 23 bits of the 28-bit multicast IP address are mapped   into the 23 bits of available Ethernet address space

User · Answer

It is also very important to distinguish a SENDING multicast socket from a RECEIVING multicast socket   I agree with all the answers above regarding RECEIVING multicast sockets  The OP noted that binding a RECEIVING socket to an interface did not help  However  it is necessary to bind a multicast SENDING socket to an interface   For a SENDING multicast socket on a multi-homed server  it is very important to create a separate socket for each interface you want to send to   A bound SENDING socket should be created for each interface        This is a fix for that bug that causes Servers to pop offline online       Servers will intermittently pop offline online for 10 seconds or so       The bug only happens if the machine had a DHCP gateway  and the gateway is no longer accessible       After several minutes  the route to the DHCP gateway may timeout  at which      point the pingponging stops       You need 3 machines  Client machine  server A  and server B      Client has both ethernets connected  and both ethernets receiving CITP pings  machine A pinging to en0  machine B pinging to en1       Now turn off the ping from machine B  en1   but leave the network connected       You will notice that the machine transmitting on the interface with      the DHCP gateway will fail sendto   with errno  No route to host    if   theErr    0               inspired by  ping -b  option in man page                     -b boundif                     Bind the socket to interface boundif for sending       struct sockaddr in bindInterfaceAddr       bzero  amp bindInterfaceAddr  sizeof bindInterfaceAddr         bindInterfaceAddr sin len   sizeof bindInterfaceAddr        bindInterfaceAddr sin family   AF INET       bindInterfaceAddr sin addr s addr   htonl interfaceipaddr        bindInterfaceAddr sin port   0     Allow the kernel to choose a random port number by passing in 0 for the port       theErr   bind mSendSocketID   struct sockaddr    amp bindInterfaceAddr  sizeof bindInterfaceAddr         struct sockaddr in serverAddress       int namelen   sizeof serverAddress          if  getsockname mSendSocketID   struct sockaddr    amp serverAddress   socklen t    amp namelen   lt  0            DLogErr   ERROR Publishing service    getsockname err                else                DLog    socket  d bind     port  d   mSendSocketID   NSString stringFromIPAddress htonl serverAddress sin addr s addr    htons serverAddress sin port              Without this fix  multicast sending will intermittently get sendto   errno  No route to host   If anyone can shed light on why unplugging a DHCP gateway causes Mac OS X multicast SENDING sockets to get confused  I would love to hear it

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