In our instance, our problem was solved by sysctl parameters, one different from Maciej.
Please note that I do not speak for the OP (buecking), I came on this post due to the problem being related by the basic detail (no multicast traffic in userland).
We have an application that reads data sent to four multicast addresses, and a unique port per multicast address, from an appliance that is (usually) connected directly to an interface on the receiving server.
We were attempting to deploy this software on a customer site when it mysteriously failed with no known reason. Attempts at debugging this software resulted in inspecting every system call, ultimately they all told us the same thing:
Our software asks for data, and the OS never provides any.
The multicast packet counter incremented, tcpdump showed the traffic reaching the box/specific interface, yet we couldn't do anything with it. SELinux was disabled, iptables was running but had no rules in any of the tables.
Stumped, we were.
In randomly poking around, we started thinking about the kernel parameters that sysctl handles, but none of the documented features was either particularly relevant, or if they had to do with multicast traffic, they were enabled. Oh, and ifconfig did list "MULTICAST" in the feature line (up, broadcast, running, multicast). Out of curiosity we looked at /etc/sysctl.conf. 'lo and behold, this customer's base image had a couple of extra lines added to it at the bottom.
In our case, the customer had set net.ipv4.all.rp_filter = 1. rp_filter is the Route Path filter, which (as I understand it) rejects all traffic that could not have possibly reached this box. Network subnet hopping, the thought being that the source IP is being spoofed.
Well, this server was on a 192.168.1/24 subnet and the appliance's source IP address for the multicast traffic was somewhere in the 10.* network. Thus, the filter was preventing the server from doing anything meaningful with the traffic.
A couple of tweaks approved by the customer; net.ipv4.eth0.rp_filter = 1 and net.ipv4.eth1.rp_filter = 0 and we were running happily.
Ok, I solved this.
First, I switched to a bridged network. I created one bridge device, and two tap devices which are attached (I don't have an ethernet device on the bridge). The UDP server listens on 192.168.192.1 [and on tap0], the TCP on 192.168.192.2 [and on tap1]. The bridge itself gets 192.168.192.1, but I don't think this is that important. Both have the same ifconfig-pool-persist file.
This worked, but I couldn't connect from clients connected via UDP to clients connected via TCP or vice versa (clients with the same method could talk to each other). I added a firewall rule:
iptables -A FORWARD -i br0 -o br0 -j ACCEPT
and now everything works.
Best Answer
You can use netcat - just start it, and type something inside, and pres the return key.
And on the other side you can listen with netcat too (you should see the written text), or just start a tcpdump, and see packets coming in.