hotspot ARP behavior ... ? [SOLVED]

miguelos · Tue Oct 10, 2023 2:12 pm

Hi folks

I have a bit weird setup, possibly could be done other way, but I wanted to use hotspot VLAN to have also access to broadcasted radio (even without internet access)

┌─────────────┐          ┌──────────────┐             ┌──────────┐
│Streaming PC │  access10│ OpenWRT      │ trunk 10,90 │ MT AP    │
│   VLAN 10   ├──────────┤  NAT 10->90  ├─────────────┤  VL90    │
└─────────────┘          └──────┬───────┘             └──────────┘
                                │
                                │
                                │ trunk 10,90
                                │
                         ┌──────┴────────┐
                         │  Mirotik AC2  │
                         │  default GW   │
                         └───────────────┘

Mikrotik HAP AC2 is the main router, vlans 10 (normal use, DHCP), 90 (main hotspot network, shared among other devices/wifi)
MT AP - got vlans 10 and 90, dummy AP with WIFI for vlans
OpenWRT - kind of trunk concentrator, one PC via ethernet, 3 vlans trunks (static IPs in vlans)
Streaming PC - running VBAN Spot software (https://vb-audio.com/Voicemeeter/vban.htm) - it broadcasts audio to configured IP range, devices are first found using ICMP. If there is reply - UDP stream is sent there.

I didn't want to play with routing that much on main router. Instead I made static route on Streaming PC pointing to Openwrt, it does NAT from vlan10 to vlan90. So - PC can reach VL90 as it's being translated to OpenWRT IP from this VLAN.

The problem is - it all works intermittenly on MT AP -
First I could see proper translation on Openwrt (pcap on vlan90 interface), then running torch on MT AP... I can see no traffic received.
Sometimes it suddenly starts working.. then stops without reason.

Is it because of hotspot configuration on main MT ? My guess is that since OpenWrt does not know where 10.1.90.0/24 endpoints are it sends ARP on both trunks, then Main MT (with hotspot configured) blocks the traffic.
I've already added Openwrt VL90 IP to walled garden on main router, still nothing.

miguelos · Tue Oct 10, 2023 2:31 pm

As expected I found that Main MT replies for ARP requests from Openwrt (whole range that is checked by VBAN spot app),
ie. ARP table contains entries all pointing at main MT, even for non-existing IPs (no device with this IP in vlan90)
Even devices that reside in vlan90 (like MT AP vlan90 ip address) is MT main router MAC...

tcpdump -i br-lan.90
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on br-lan.90, link-type EN10MB (Ethernet), snapshot length 262144 bytes
13:41:47.858796 ARP, Request who-has 10.1.90.5 tell 10.1.90.2, length 28
13:41:47.859368 ARP, Reply 10.1.90.5 is-at b8:69:f4:18:b0:e2 (oui Unknown), length 42
13:41:47.859475 IP 10.1.90.2 > 10.1.90.5: ICMP echo request, id 1, seq 6143, length 40

and for non-hotspot vlan there are no replies :

13:51:01.304214 ARP, Request who-has 10.1.10.111 tell 10.1.10.10, length 28
13:51:02.309059 ARP, Request who-has 10.1.10.111 tell 10.1.10.10, length 28
13:51:03.304533 ARP, Request who-has 10.1.10.111 tell 10.1.10.10, length 28
13:51:04.304301 ARP, Request who-has 10.1.10.111 tell 10.1.10.10, length 28

miguelos · Tue Oct 10, 2023 3:00 pm

changed ARP settings on VLAN90 on main router to reply-only
so far it works...

miguelos · Wed Oct 18, 2023 2:28 pm

seems that it works in RouterOS6, but not RouterOS7
same issue as reported here viewtopic.php?t=187802

hotspot ARP behavior ... ? [SOLVED]

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