Auto-detect network interface when not configured

If no interface is set in the config, the ARP scanner now picks the
first non-loopback, up, IPv4-enabled interface automatically. The
setting is optional in the TOML config, sample config, and Nix module.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TODO.md

@@ -4,7 +4,7 @@
 
 ## Backend
 
-- [ ] Use first available network interface when not configured
+- [x] Use first available network interface when not configured
 - [x] Add Swagger and API docs (OpenAPI)
 - [x] Record an activity log in the database
   - [x] One event for each device appearance

backend/src/arp_scanner.rs

@@ -13,7 +13,7 @@ pub async fn scan() -> Result<(), Box<dyn std::error::Error>> {
 
         // Find online devices via ARP
         let devices =
-            device_finders::arp::find(get_settings().networking.interface.to_string()).await?;
+            device_finders::arp::find(get_settings().networking.interface.clone()).await?;
 
         info!("Done with ARP probes");
         info!("Found {} online devices", devices.len());

backend/src/device_finders/arp.rs

@@ -14,18 +14,36 @@ use pnet::{
 };
 use tokio::time::{Duration, timeout};
 
-pub async fn find(interface: String) -> Result<Vec<Device>, Box<dyn std::error::Error>> {
-    debug!("Looking up devices via ARP using interface {}", interface);
+fn select_interface<'a>(
+    interfaces: &'a [NetworkInterface],
+    configured: &Option<String>,
+) -> Option<&'a NetworkInterface> {
+    match configured {
+        Some(name) => interfaces
+            .iter()
+            .filter(|el| el.is_up())
+            .find(|el| &el.name == name),
+        None => interfaces
+            .iter()
+            .find(|el| el.is_up() && !el.is_loopback() && el.ips.iter().any(|ip| ip.is_ipv4())),
+    }
+}
+
+pub async fn find(interface: Option<String>) -> Result<Vec<Device>, Box<dyn std::error::Error>> {
+    debug!("Looking up devices via ARP, configured interface: {:?}", interface);
 
     // Get the network device to use
-    let network_interface: NetworkInterface = match datalink::interfaces()
-        .iter()
-        .filter(|el| el.is_up())
-        .find(|el| el.name == interface)
-    {
-        Some(value) => value.clone(),
+    let all_interfaces = datalink::interfaces();
+    let network_interface: NetworkInterface = match select_interface(&all_interfaces, &interface) {
+        Some(value) => {
+            info!("Using network interface: {}", value.name);
+            value.clone()
+        }
         None => {
-            error!("Interface ({interface}) not found or not active.");
+            match &interface {
+                Some(name) => error!("Interface ({name}) not found or not active."),
+                None => error!("No suitable non-loopback interface with an IPv4 address found."),
+            }
             return Err(InvalidDeviceError.into());
         }
     };
@@ -42,7 +60,7 @@ pub async fn find(interface: String) -> Result<Vec<Device>, Box<dyn std::error::
     {
         Some(value) => value,
         None => {
-            error!("No IP address found for selected interface ({interface}).");
+            error!("No IP address found for selected interface ({}).", network_interface.name);
             return Err(NoIPAddressError.into());
         }
     };
@@ -51,7 +69,7 @@ pub async fn find(interface: String) -> Result<Vec<Device>, Box<dyn std::error::
     let mac = match network_interface.mac {
         Some(mac) => mac,
         None => {
-            error!("Could not get MAC address for selected interface ({interface}).");
+            error!("Could not get MAC address for selected interface ({}).", network_interface.name);
             return Err(NoMACAddressError.into());
         }
     };
@@ -120,3 +138,91 @@ pub async fn find(interface: String) -> Result<Vec<Device>, Box<dyn std::error::
 
     Ok(result_receive.unwrap_or(Vec::new()))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pnet::datalink::NetworkInterface;
+    use pnet::ipnetwork::IpNetwork;
+
+    // Raw Linux IFF flag values used by pnet's is_up() / is_loopback()
+    const IFF_UP: u32 = 0x1;
+    const IFF_LOOPBACK: u32 = 0x8;
+
+    fn make_interface(name: &str, up: bool, loopback: bool, ipv4: bool) -> NetworkInterface {
+        let mut flags: u32 = 0;
+        if up {
+            flags |= IFF_UP;
+        }
+        if loopback {
+            flags |= IFF_LOOPBACK;
+        }
+        let ips = if ipv4 {
+            vec![IpNetwork::V4("192.168.1.1/24".parse().unwrap())]
+        } else {
+            vec![]
+        };
+        NetworkInterface {
+            name: name.to_string(),
+            description: String::new(),
+            index: 0,
+            mac: None,
+            ips,
+            flags,
+        }
+    }
+
+    #[test]
+    fn test_select_configured_interface_found() {
+        let ifaces = vec![
+            make_interface("eth0", true, false, true),
+            make_interface("wlan0", true, false, true),
+        ];
+        let result = select_interface(&ifaces, &Some("wlan0".to_string()));
+        assert!(result.is_some());
+        assert_eq!(result.unwrap().name, "wlan0");
+    }
+
+    #[test]
+    fn test_select_configured_interface_not_found() {
+        let ifaces = vec![make_interface("eth0", true, false, true)];
+        let result = select_interface(&ifaces, &Some("missing0".to_string()));
+        assert!(result.is_none());
+    }
+
+    #[test]
+    fn test_select_configured_interface_down_not_found() {
+        let ifaces = vec![make_interface("eth0", false, false, true)];
+        let result = select_interface(&ifaces, &Some("eth0".to_string()));
+        assert!(result.is_none());
+    }
+
+    #[test]
+    fn test_auto_select_skips_loopback() {
+        let ifaces = vec![
+            make_interface("lo", true, true, true),
+            make_interface("eth0", true, false, true),
+        ];
+        let result = select_interface(&ifaces, &None);
+        assert!(result.is_some());
+        assert_eq!(result.unwrap().name, "eth0");
+    }
+
+    #[test]
+    fn test_auto_select_only_loopback_returns_none() {
+        let ifaces = vec![make_interface("lo", true, true, true)];
+        let result = select_interface(&ifaces, &None);
+        assert!(result.is_none());
+    }
+
+    #[test]
+    fn test_auto_select_skips_interface_without_ipv4() {
+        let ifaces = vec![
+            make_interface("eth0", true, false, false),
+            make_interface("wlan0", true, false, true),
+        ];
+        let result = select_interface(&ifaces, &None);
+        assert!(result.is_some());
+        assert_eq!(result.unwrap().name, "wlan0");
+    }
+}

backend/src/settings.rs

@@ -13,7 +13,7 @@ pub struct Database {
 
 #[derive(Debug, Deserialize, Clone)]
 pub struct Networking {
-    pub interface: String,
+    pub interface: Option<String>,
 }
 
 #[derive(Debug, Deserialize, Clone)]

examples/sample_oott.toml

@@ -2,7 +2,7 @@
 path = "./oott.db" # Database path. Make sure it's writeable for the user running this process
 
 [networking]
-interface = "eno1" # Network interface to use for scans
+# interface = "eno1" # Optional: network interface to use for scans. If not set, the first non-loopback connected interface is used.
 
 [log]
 level = "info" # off, error, warn, info, debug, trace

nix/modules/oott-service.nix

@@ -22,9 +22,9 @@ in {
       default = "/var/lib/oott.db";
     };
     networking.interface = mkOption {
-      type = types.str;
-      description = "Network interface to use for scans";
-      default = "eno1";
+      type = types.nullOr types.str;
+      description = "Network interface to use for scans. If not set, the first non-loopback connected interface is used automatically.";
+      default = null;
     };
     log.level = mkOption {
       type = types.str;