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Question

Setup:

The user has two applications - one sender one receiver - running on the same host/server. The user sets it up such that the sender sends messages to its own IP address not 127.0.0.1. Lets say its IP and port is x:y for simplicity. The user then sets up the receiver to receiver messages on x:y. Again this is on the same host/server.

Questions:

1. From my understanding this is not possible since the port will already be reserved. Therefore I cannot use the same port to try and send packets out to myself. Can I have a port used for a sender and receiver on the same node?
2. Is this resolved if I use SO_REUSEADDR or does this only resolve the IP conflict and not the port reuse?
3. If the program is not setup with IP_MULTICAST_LOOP the host will not multicast the message to itself, correct?
4. With IP_MULITCAST_LOOP set, if I only wanted to send the message to myself can I use 127.0.0.1 or must I use another address? Additionally, how do the ports get resolved?
5. If I am not seeing messages on the same node, would the first best guess be that IP_MULITCAST_LOOP is not set?

Answer 1

Let's take it step by step:

1. The sending port does not matter at all. So you can choose an arbitrary port for the sender, and use the specific port number for your service just for the receiver.

2. No, SO_REUSEADDR/PORT does not solve this problem. Even if you manage to achieve it: Do not start multiple listeners on the same port. This will cause strange effects. The main purpose of SO_REUSEADDR/PORT is to allow servers to create a TCP (not UDP) socket when the previous server process just died, without waiting for a timeout of the TCP state machine of the stale socket.

3. Corrects, assuming you mean multicast rather than broadcast ,

4. Yes and no: If you only want to send messages to yourself you can send the packets to 127.0.0.1, and then you message will be a normal unicast packet and no longer a multicast packet, and IP_MULTICAST_LOOP does not matter at all. Multicast packets are normal UDP packets which have a destination address in the multicast address range (ie 224.0.0.0-239.255.255.255). The receiving socket cannot easily tell whether a packet was sent via unicast or multicast.

5. IP routing on the same host between interfaces is far from trivial. There are a lot of mechanisms and routing rules involved which are not shown in the normal routing table, which is just for outgoing traffic. It also depends on by which means you try to observe the messages. There is not a single point where you can see all messages going through a node (unfortunately). This is usually all attached to interfaces, and there also to an ingress and egress side, and the latter is usually not documented and not configurable. Monitoring local traffic can be tricky and may require virtual network interfaces. Really messy.

In summary: You are trying to send messages from one process to another process on the same host. Use unicast UDP for this and you are done. No multicast involved.