CODEWITHSUNDEEP
pythonsocketsmulticast
Astronaut
Astronaut

Reputation: 1

Setting source address when sending UDP multicast messages

How to set the source IP address for UDP multicast packages to something else than the interface IP?

I am trying to write a small router that selectively routes UDP SSDP packages from one network to another. The plan is to do it in python, although I am flexible on that.

It seems fairly easy to route SSDP NOTIFY messages: I receive them on one interface and decide which interface to re-broadcast them on. However the protocol for M-SEARCH messages require that the source IP is set to the original source of the message as any service that chooses to respond will respond with a unicast message to the source IP and port.

Example (heavily simplified):

A client on network A with IP 192.168.10.10 sends an M-SEARCH message:

My "router application" on 192.168.10.2 receives the packet and would like to rebroadcast it on network B. However I cannot find any method in the socket API that allows me to set the source IP address. Only to pick the source interface.

Thus the rebroadcasted packet now looks like this:

And now the receiving service is unable to unicast back to the client as the original IP and port are lost.

Upvotes: 0

Views: 1076

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 596307

How to set the source IP address for UDP multicast packages to something else than the interface IP?

The only way I know of is to use a RAW socket and construct the IP headers manually. Note that use of RAW sockets is typically restricted to admin users on most platforms.

However I cannot find any method in the socket API that allows me to set the source IP address.

Because there isn't one.

And now the receiving service is unable to unicast back to the client as the original IP and port are lost.

Can't you just have your router remember the original source when it receives the M-SEARCH request, and when it receives the unicast reply then forward it to the original requester? That is how most routers usually work.

Upvotes: 3

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