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# Subnets Configuration

You need to specify the network interface and the network mask. You can also configure multiple subnets and specify VLANs (see VLAN exceptions below).

`ARPSCAN` can scan multiple networks if the network allows it. To scan networks directly, the subnets must be accessible from the network where NetAlertX is running. This means NetAlertX needs to have access to the interface attached to that subnet.

Warning

If you don't see all expected devices run the following command in the NetAlertX container (replace the interface and ip mask): `sudo arp-scan --interface=eth0 192.168.1.0/24`

If this command returns no results, the network is not accessible due to your network or firewall restrictions (Wi-Fi Extenders, VPNs and inaccessible networks). If direct scans are not possible, check the remote networks documentation for workarounds.

## Example Values

Note

Please use the UI to configure settings as it ensures the config file is in the correct format. Edit `app.conf` directly only when really necessary.

- Examples for one and two subnets:
- One subnet: `SCAN_SUBNETS = ['192.168.1.0/24 --interface=eth0']`
- Two subnets: `SCAN_SUBNETS = ['192.168.1.0/24 --interface=eth0','192.168.1.0/24 --interface=eth1 --vlan=107']`

Tip

When adding more subnets, you may need to increase both the scan interval (`ARPSCAN_RUN_SCHD`) and the timeout (`ARPSCAN_RUN_TIMEOUT`)—as well as similar settings for related plugins.

If the timeout is too short, you may see timeout errors in the log. To prevent the application from hanging due to unresponsive plugins, scans are canceled when they exceed the timeout limit.

To fix this:
- Reduce the subnet size (e.g., change `/16` to `/24`).
- Increase the timeout (e.g., set `ARPSCAN_RUN_TIMEOUT` to `300` for a 5-minute timeout).
- Extend the scan interval (e.g., set `ARPSCAN_RUN_SCHD` to `*/10 * * * *` to scan every 10 minutes).

For more troubleshooting tips, see Debugging Plugins.

## Explanation

### Network Mask

Example value: `192.168.1.0/24`

The `arp-scan` time itself depends on the number of IP addresses to check.

> The number of IPs to check depends on the network mask you set in the `SCAN_SUBNETS` setting.
> For example, a `/24` mask results in 256 IPs to check, whereas a `/16` mask checks around 65,536 IPs. Each IP takes a couple of seconds, so an incorrect configuration could make `arp-scan` take hours instead of seconds.

Specify the network filter, which significantly speeds up the scan process. For example, the filter `192.168.1.0/24` covers IP ranges from `192.168.1.0` to `192.168.1.255`.

### Network Interface (Adapter)

Example value: `--interface=eth0`

The adapter will probably be `eth0` or `eth1`. (Check `System Info` > `Network Hardware`, or run `iwconfig` in the container to find your interface name(s)).

Tip

As an alternative to `iwconfig`, run `ip -o link show | awk -F': ' '!/lo|vir|docker/ {print $2}'` in your container to find your interface name(s) (e.g.: `eth0`, `eth1`): `bash Synology-NAS:/# ip -o link show | awk -F': ' '!/lo|vir|docker/ {print $2}' sit0@NONE eth1 eth0`

### VLANs

Example value: `--vlan=107`

- Append `--vlan=107` to the `SCAN_SUBNETS` field (e.g.: `192.168.1.0/24 --interface=vmbr0 --vlan=107`) for multiple VLANs.

#### VLANs on a Hyper-V Setup

> Community-sourced content by mscreations from this discussion.

Tested Setup: Bare Metal → Hyper-V on Win Server 2019 → Ubuntu 22.04 VM → Docker → NetAlertX.

Approach 1 (may cause issues):
Configure multiple network adapters in Hyper-V with distinct VLANs connected to each one using Hyper-V's network setup. However, this action can potentially lead to the Docker host's inability to handle network traffic correctly. This might interfere with other applications such as Authentik.

Approach 2 (working example):

Network connections to switches are configured as trunk and allow all VLANs access to the server.

By default, Hyper-V only allows untagged packets through to the VM interface, blocking VLAN-tagged packets. To fix this, follow these steps:

1. Run the following command in PowerShell on the Hyper-V machine:

`powershell Set-VMNetworkAdapterVlan -VMName <Docker VM Name> -Trunk -NativeVlanId 0 -AllowedVlanIdList "<comma separated list of vlans>"`

1. Within the VM, set up sub-interfaces for each VLAN to enable scanning. On Ubuntu 22.04, Netplan can be used. In /etc/netplan/00-installer-config.yaml, add VLAN definitions:

`yaml network: ethernets: eth0: dhcp4: yes vlans: eth0.2: id: 2 link: eth0 addresses: [ "192.168.2.2/24" ] routes: - to: 192.168.2.0/24 via: 192.168.1.1`

1. Run `sudo netplan apply` to activate the interfaces for scanning in NetAlertX.

In this case, use `192.168.2.0/24 --interface=eth0.2` in NetAlertX.

#### VLAN Support & Exceptions

Please note the accessibility of macvlans when configured on the same computer. This is a general networking behavior, but feel free to clarify via a PR/issue.

- NetAlertX does not detect the macvlan container when it is running on the same computer.
- NetAlertX recognizes the macvlan container when it is running on a different computer.