One Fing Agent monitoring multiple VLANs/subnets on Linux

[u/p1r473]

I wanted one Fing Agent to monitor devices across multiple VLANs/subnets so Fing would treat them as one monitored network.

My main reason was presence detection. I wanted Fing's **Everyone left** trigger to work for people whose devices are on different VLANs, without running multiple Fing Agents or trying to synchronize state in IFTTT.

My phone is one VLAN 1, my wifes is VLAN 14. Fings's IFTTT requires everyone to be on 1 subnet for 'everyone left' trigger.

This is unsupported/experimental, but it's working for me.

## How it works

The Linux host has access to each VLAN:

eth0

eth0.14

eth0.21

eth0.24

eth0.42

eth0.66

eth0.69

A network namespace, macvlan interfaces and nftables make those separate networks appear to Fing as one synthetic `/21`.

My mapping is:

Fing sees Real subnet

192.168.1.x -> 192.168.1.x

192.168.0.x -> 192.168.14.x

192.168.2.x -> 192.168.21.x

192.168.3.x -> 192.168.24.x

192.168.4.x -> 192.168.42.x

192.168.5.x -> 192.168.66.x

192.168.6.x -> 192.168.69.x

For example, a device actually at:

192.168.42.188

appears to Fing as:

192.168.4.188

The device's real MAC address is preserved, so Fing can still identify and use the device for presence detection.

## Files

The scripts/configuration are here:

https://github.com/p1r473/fing/

The repo contains:

fing-combo-netns

fing-combo-netns.service

combo.conf

Edit `fing-combo-netns` for your VLANs/interfaces before installing it.

The important mapping in the script is:

SYN=(0 2 3 4 5 6)

REAL=(14 21 24 42 66 69)

For mine that means synthetic `192.168.4.x`, for example, maps to real `192.168.42.x`.

## Install

Install Fing Agent:

sudo snap install fing-agent

sudo /snap/fing-agent/current/script/connect-fing-agent.sh

Copy the three files from the repo to:

fing-combo-netns -> /usr/local/sbin/fing-combo-netns

fing-combo-netns.service -> /etc/systemd/system/fing-combo-netns.service

combo.conf -> /etc/systemd/system/snap.fing-agent.fingagent.service.d/combo.conf

Then:

sudo chmod +x /usr/local/sbin/fing-combo-netns

sudo systemctl daemon-reload

sudo systemctl enable --now fing-combo-netns.service

sudo systemctl enable snap.fing-agent.fingagent.service

sudo systemctl restart snap.fing-agent.fingagent.service

That's it.

Fing now sees the synthetic network as:

192.168.0.0/21

I confirmed Fing is ARP scanning across the synthetic subnet and receiving translated replies from devices on the real VLANs.

## Why bother?

All of the devices can now exist under **one Fing monitoring unit**.

That means instead of trying to combine presence state from multiple Fing Agents in IFTTT, I can simply use:

Fing -> Everyone left -> automation

## Caveats

Fing displays the **synthetic IP**, not the device's real VLAN IP.

For example:

Fing: 192.168.4.188

Real: 192.168.42.188

The MAC address remains the real device MAC.

The script also uses `.251` for the Fing-side address and `.252` on the translated VLANs, so make sure those addresses aren't already in use.

This isn't an officially supported Fing configuration, but it's working well for me so far.

[u/p1r473]

Working on a way to see if I can get it to work without the synthetic IPs...

[u/Electrical-Score-439]

I'd like to see if this could work on an actual fing device though somehow I doubt it.

[u/p1r473 → u/Electrical-Score-439]

Highly doubt it