Add filter for non lorawan packet

[steph10200]

Hi,

Following this post on my chirpstack support question

It would be interesting to be able to add a filter on the mqtt forwarder to eliminate at the source (on the gateway) packets that do not correspond to lorawan frames.

eg :

kerlink lorafwd sent to mqtt-forwarder invalid packets :

2025-10-21T18:48:28.313205+00:00 klk-wiis-0840F3 lorafwd[1554]: <4> Invalid PHY payload size 
2025-10-21T18:48:28.314194+00:00 klk-wiis-0840F3 lorafwd[1554]: <6> Uplink message (C14E) sent 
2025-10-21T18:48:28.317337+00:00 klk-wiis-0840F3 lorafwd[1554]: <6> Uplink message (C14E) acknowledged in 3.79914 ms 

and mqtt forward this packet "invalid PHY" ( but also another )

2025-10-21T18:48:31.412347+00:00 klk-wiis-0840F3 chirpstack-mqtt-forwarder[1459]: PUSH_DATA received, random_token: 20417, remote: 127.0.0.1:55334 
2025-10-21T18:48:31.412422+00:00 klk-wiis-0840F3 chirpstack-mqtt-forwarder[1459]: Sending PUSH_ACK, random_token: 20417 remote: 127.0.0.1:55334 
2025-10-21T18:48:31.412754+00:00 klk-wiis-0840F3 chirpstack-mqtt-forwarder[1459]: Sending uplink event, uplink_id: 3565471015, topic: eu868/gateway/7076ff00560840f3/event/up 
2025-10-21T18:48:31.414452+00:00 klk-wiis-0840F3 chirpstack-mqtt-forwarder[1459]: Publish. Topic = eu868/gateway/7076ff00560840f3/event/up, Pkid = 0, Payload Size = 97

The filter on the devaddr is not sufficient because the packets still pass. This creates huge MQTT traffic and unnecessary processing load on the central ChirpStack server and also a significant volume of data on the mobile network connection (since the gateway is connected via LTE).

-- Steph --

[brocaar]

I've already implemented this in the filter crate:
chirpstack/chirpstack@775e440

Once that has been published, I will integrate here.

[1rabbit]

I’m seeing the same issue with "proprietary" packets being forwarded by the MQTT forwarder, which creates unnecessary traffic and load on our ChirpStack server as well as on LTE connections. A filter at the gateway to drop non-LoRaWAN frames before forwarding would be really useful.

Also, I’ve been overcharged by my SIM provider because of this extra traffic—so a fix would help avoid that as well.

It would be great if this could be added as an optional configuration in the forwarder so we can reduce processing, network usage, and costs.

Thanks!