Receive specific multicast message on a client connected over VPN

Question

Case: [ Subnet A , 192.168.2.0/24, Padavan firmware based internet gw ]

[ Subnet B , 192.168.1.0/24, Padavan firmware based internet gw ]

Host from subnet A (2.155) is connected via VPN (possible options: PPTP, OpenVPN, L2TP w/o ipsec) to subnet B, and receives address, saying 1.245/32

In subnet B exists host (1.10/32) which sends multicast datagramms to 224.0.0.50:9898 ; On router I see them with

tcpdump -i br0 -c 10 dst host 224.0.0.50 and port 9898 and multicast

13:46:54.345369 IP 192.168.1.10.4321 > 224.0.0.50.9898: UDP, length 135

I am looking for solutions, to receive/forward those broadcast messages, so they could be seen by hosts, connected via VPN

On router B, which is Padavan firmware based, I have, and limited to udpxy, igmproxy utilities, if needed.

On client host, I am debian based, and generally not limited in tools.

Datagrams are proprietary protocol, i.e. not a iptv or video stream.

Any ideas are welcomed.

[UPD] Additional info - per discussion in comments

That's a very specific hardware device, which is not very chatty in ethernet terms (saying max 1-2 datagramms in 5 seconds), thus for sure should be pretty forwardable. Unfortunately, It sends status updates purely via broadcasting. in Subnet A do exist similar device + control software. Thus I am looking for a way datagramms broadcasted to 224.0.0.50:9898 in subnet B to re-appear in subnet A. May be with help of some tool. May be smcroute, may be udpxy, maybe igmproxy

Answer 1

As I don't like to leave resolved questions unanswered, here is currently working solution

In subnet B I have installed openVPN server endpoint, configured as L2.

In subnet A, on a control host I have installed openvpn client, that connects to subnet B, assigned interface is tapz

20: tapz: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 100
    link/ether 0a:da:be:96:78:d9 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.245/24 brd 192.168.1.255 scope global noprefixroute tapz
       valid_lft forever preferred_lft forever
    inet6 fe80::8da:beff:fe96:78d9/64 scope link 
       valid_lft forever preferred_lft forever

So now on a control host I have:

broadcasting from local device on physical ethernet enp5s0

sudo tcpdump -i enp5s0 -c 10 dst host 224.0.0.50 and port 9898 and multicast
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp5s0, link-type EN10MB (Ethernet), capture size 262144 bytes
13:55:05.642963 IP lumi-gateway-v3_miio56591509.4321 > 224.0.0.50.9898: UDP, 
length 136

and now I also receive broadcasts from remote network device on tapz

sudo tcpdump -i tapz -c 10 dst host 224.0.0.50 and port 9898 and multicast
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tapz, link-type EN10MB (Ethernet), capture size 262144 bytes
13:53:49.141751 IP 192.168.1.10.4321 > 224.0.0.50.9898: UDP, length 135

So far that it what I was looking for I am getting necessary datagrams on a VPN client. OpenVPN on remote side can be also optimized on filter of information forwarded for multicasts.

Answer 2

For those who come here, with the same question.

When you will have necessary multicast on tap0,

you can create bridge from, saying, eth0 and tap0

For notes of everyone interested, who would came here.

ip link add br0 type bridge
ip link set tap0 master br0
ip link set eth0 master br0

POC - both multicasts on single interface

sudo tcpdump -i br0 dst host 224.0.0.50 and port 9898
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br0, link-type EN10MB (Ethernet), capture size 262144 bytes
21:09:51.823632 IP 192.168.1.10.4321 > 224.0.0.50.9898: UDP, length 135
21:09:55.045138 IP 192.168.2.214.4321 > 224.0.0.50.9898: UDP, length 136