Forwarding mDNS on second OpenWRT device

Installing and Using OpenWrt
1

I have the following setup:

2 Networks configured on an Archer C7 running OpenWrt 22.03.5: LAN and IOT.
Within IOT there is a hub running Homebridge that advertises mDNS.

I want to access my Smart Devices from LAN, so the Archer is running avahi-daemon using default config except enable-reflector=yes.

When I'm connected to the Archer directly everything works as expected and I get mDNS advertisements

I run another OpenWrt 22.03.5 on a Raspi 4 that handles WAN and LAN. The Archers LAN interface is a Client in Raspi4 LAN.
The Raspi handles DHCP for LAN and has a static route to the Archer for the IOT Network.
Also on the Raspi there are three interfaces: eth0 -> Going to Archer, eth1 -> WAN, eth2 -> going to Unifi AP in another part of my house. (both eth0 and eth2 are connected to br_lan)

All IOT devices can be accessed from either side, however when I'm connected to the AP that's connected to the Raspi I do not get mDNS advertisements from IOT lan.

I would have considered LAN a single network and therefore expected the reflected mDNS ads to hop over to the other side of the Raspi, but that doesn't seem to be the case.

Do I need to re-reflect, or where are my advertisement lost?

2

To have mDNS between subnets try with reflector enabled

3

But wouldn't all of LAN be the same subnet?

4

Normally with the default configuration yes.

But you have options which work very well:

Add in /etc/avahi/avahi-daemon.conf (if the config don't exist it is the global config there I forgot its name because I'm on phone)

deny-interfaces=br-device,br-anotherone
allow-interfaces=br-bla,br-bla2

I guess only using allow-interfaces would be a better suite.

5

I set it up with allow interfaces and it seems to be working.
Still don't really understand why I need to do this to be honest...

1 Like
6

Normally if the reflector is set to yes it will use mdns on each local interface, mostly fine in alot of use cases.

However this on a larger scale with bigger networks could create issues with load and latency, but it could also conflict for example if you ran a instance with iptv and igmpproxy.

These options give you more a fine control over it as per interface mdns control, its optional :+1:

7

Thank you for the explanation.
I understand why allow-interfaces makes sense.
What I do not understand is why i need to enable reflection on the second router at all, as I'm under the impression that the broadcast should reach the whole Network (192.168.0.0/24) without reflection.

8

That depends how router 2 is setup.

Usually I think the best setup you want for router 2 is a dumb ap.

Meaning that it has no firewall, and the network and interfaces it gets are from router 1, it basicly only sents out the traffic from router 1 through the router 2 antennas this can be accomplished via vlans.

See:

However if router 2 hosts its own nat/firewall and is not a dumbap, you have to allow multicast from wan and I think you can just remove the wan firewall zone from interface wan, and then aswell I think use avahi there on the lan on router 2.

Router 2 is basicly just a dhcp client on router 1 in that regards, so the avahi instance on router 2 automaticly sents multicast traffic to router 1.

Also note if there is a switch in between that the switch needs to be able to do multicast otherwise it strips out multicast packets.