CODEWITHSUNDEEP
cmulticastraspberry-piraspbian
Afonso Tsukamoto
Afonso Tsukamoto

Reputation: 1214

Read from UDP multicast raspberry pi

I've already started a question here (link) about this project I'm doing, and I keep having lots of problems with this.

In my earlier question I wanted to read from a same machine udp multicast and I find out how to do it, but not with ipmidi.(If you didn't read the previous question linked above, ipmidi is a tool for mac/win which allows me to send midi events through an UDP multicast).

I've been working around with wireshark and managed to see that ipmidi is sending datagrams to the ip 225.0.0.37, port 21928, wich tells me that is, in fact, a multicast. Then I used this code (the listener) to try to echo the datagrams sent by ipmidi with no success (I changed the multicast ip and the port). After this I decided to try to run this code from the raspberry pi, since I only wanted it to run in my mac for testing purposes, I went to the real thing.

In my raspi I compiled the code again and gcc claimed that setsockopt() had an invalid argument. perror() was useful enough to tell me that the problem was here:

setsockopt(fd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq))

So I removed the part where it asks for a multicast address to the kernel and made it a simple listening socket at port 21928, but it still won't work. I have my mac sending midi through ethernet to the raspi.

Now, the actual questions:

  1. I'm now doubting my multicast, so called, knowledge: if my mac is multicasting ipmidi's datagrams, it means the datagrams are being sended through all the possible places in my mac, right? I mean, the ip 225.0.0.37 means multicast through wifi and ethernet or I have to route it to the network interface of my choice?

  2. Since my raspi is, allegedly, receiving the datagrams through the ethernet, making my program listen at the ipmidi's datagram destination port would be enough or do I have to tell it explicitly 'where' those datagrams came from?

  3. Is there an easier way to make a multicast receiving app, like an api for C/C++? I'm currently using C style sockets, as you could see in the code I linked, but this is not the main purpose for the program I'm doing and I find myself spending lots of time just learning about networking instead of focusing on the real thing.

Sorry for the long question, lots of doubts and I wanted to be in context.

Thanks for the help!

Upvotes: 0

Views: 3556

Answers (2)

Afonso Tsukamoto
Afonso Tsukamoto

Reputation: 1214

So, after some research and trial/error my problem was the raspberry pi being hidden behind my mac. Since my mac was giving internet to the raspberry pi, the multicast wasn't being caught by the raspi, hence the reason why it failed to join it.

About the setsockopt, I made the same mistake has I had originally in the question: 255 instead of 225 in the multicast ip. Because of this, the options IPPROTO_IP and IP_ADD_MEMBERSHIP were wrong, and the program would block there.

Is working now and I already started to code the midi specification.

Thanks for the help, specially to @caf.

Upvotes: 1

caf
caf

Reputation: 239251

Just listening on the port isn't enough - you need to join the multicast group if you want to see datagrams sent to that group.

The "Invalid Argument" error from your setsockopt() call probably means that your mreq argument was bad. For the multicast address you've given, you should be doing:

struct ip_mreqn mreq;

inet_aton("225.0.0.37", &mreq.imr_multiaddr);
mreq.imr_address.s_addr = htonl(INADDR_ANY);
mreq.imr_ifindex = 0;

Upvotes: 2

Related Questions