dr 🛠️🛰️📡🎧Aug 19, 2024

#it #networking gurus!

I have a number of real-time systems that #multicast data. I want to forward that mcast data from one machine to another on the same physical network but a different mcast address (I guess...?)

It looks like #ncat, #socat and maybe even #ssh could do this but I can't figure out how to do it with multicast specifically

Show threadT_XAug 20, 2024
@davidr #IPv4 or #IPv6? If you want to do it the real IP routing way (and if your IP multicast addresses are routable?) you can use things like #smcroute, #pim6sd or #mrouted. Many of these have been kept alive and/or got a revamp/resurrection thanks to @troglobit. I like/use pim6sd personally (with MLDv1 / any-source-multicast only so far, not source-specific-multicast).
Show threadT_XAug 20, 2024
@davidr @troglobit Of course, you could also kind of try to route things on your own, manually some layers above, with #socat for instance (though it broke for IPv6 in the recent version again, the dev has a patch, but don't know why he hasn't made a new release with it yet... v1.7.3.2 worked for me, nor has he added it to his Git either last time I checked).
Show threaddr 🛠️🛰️📡🎧Aug 20, 2024

@T_X @troglobit #IPv4

I think a #socat or similar solution is what I need. Ideally user-space using #linux standard tools tools. If it's some custom download or requires lots of config (or network hardware mods) then I might as well go right to #it

It's a temp hack to show value before we make a permanent change

Show threadT_XAug 20, 2024
@davidr @troglobit oki doki. Do you want to receive some content in an IPv4 UDP multicast to some 224.0.0.0-239.255.255.255 address on one host and forward it to a single (or few) specific other hosts by repackaging this to IP UDP unicast packet(s)? What does "on the same physical network" mean? (if all three were on the same, then the desired destination host would just be able to receive it on its own?)
I guess you have found this IP multicast socat page already?
http://www.dest-unreach.org/socat/doc/socat-multicast.html
IP Multicasting with Socat

Show threaddr 🛠️🛰️📡🎧

@T_X @troglobit There are three multicasts, two of which are on the same multicast addr. I assume those are somehow being routed to not interfere with each other

The two non-overlapping ones I can already receive on machine A. I can get the third cast on machine B. A and B can reach each other

I'd like to forward B to A, ideally still as multicast but on another address (to simplify my code). Forwarding A to B would be better, just because I have more control over B (i.e. root, etc)

Aug 20, 2024 at 10:54amWeb
Show threaddr 🛠️🛰️📡🎧Aug 20, 2024

@T_X @troglobit Oh, I should probably also note that most of these machines have multiple NICs. There's some complicated subnet scheme in use that I don't understand but doesn't seem to affect the multicasts (i.e. apparently I can get the casts on any of the NICs)

It would probably also help a lot if I understood multicast better...

Show threadT_XAug 20, 2024

@davidr @troglobit I'm also still a bit confused myself how your setup looks like :D. A diagram would be nice with the involved hosts, their network cards, how these are connected and the packets are supposedly travel.

I think the two green boxes with in example 1 are actually a great point to start with, just try them out :-). One is the multicast sender and the other the multicast receiver side.

Show threadT_XAug 20, 2024
@davidr @troglobit I can also highly recommend looking at packet flows and what socat might spit out with #Wireshark. I find it more comprehensible / structured than a wall of text in many cases, too.