app: Start wpa_supplicant only when Ethernet has no DHCP lease

Probe the configured Ethernet interface (default eth0, overridable via
-ethernet-interface) at startup. If no DHCP-assigned IPv4 is present,
start the wpa_supplicant service so the WiFi backend has something to
talk to; otherwise leave it alone and rely on the wired uplink.

internal/app/app.go

@@ -57,6 +57,11 @@ func (a *App) Initialize(cfg *config.Config) error {
 	// Store config reference
 	a.Config = cfg
 
+	// If Ethernet uplink is not already providing connectivity (no DHCP lease
+	// on the configured interface), bring up wpa_supplicant so the WiFi
+	// backend has something to talk to.
+	ensureUplink(cfg.EthernetInterface)
+
 	// Initialize WiFi backend
 	if err := wifi.Initialize(cfg.WifiInterface, cfg.WifiBackend); err != nil {
 		return err

internal/app/network.go

@@ -0,0 +1,54 @@
+package app
+
+import (
+	"fmt"
+	"log"
+	"os/exec"
+	"strings"
+)
+
+// hasDHCPAddress reports whether iface holds an IPv4 address that ip(8) marks
+// as "dynamic" — the flag set on addresses with a finite valid_lft, which is
+// how DHCP-assigned leases appear (static addresses are "permanent").
+func hasDHCPAddress(iface string) (bool, error) {
+	out, err := exec.Command("ip", "-4", "-o", "addr", "show", "dev", iface).Output()
+	if err != nil {
+		return false, fmt.Errorf("ip addr show %s: %w", iface, err)
+	}
+	for _, line := range strings.Split(string(out), "\n") {
+		fields := strings.Fields(line)
+		hasInet := false
+		hasDynamic := false
+		for _, f := range fields {
+			switch f {
+			case "inet":
+				hasInet = true
+			case "dynamic":
+				hasDynamic = true
+			}
+		}
+		if hasInet && hasDynamic {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// ensureUplink boots the WiFi uplink only if Ethernet is not already providing
+// connectivity: when iface has no DHCP-assigned IPv4, start wpa_supplicant so
+// the WiFi backend can take over.
+func ensureUplink(iface string) {
+	has, err := hasDHCPAddress(iface)
+	if err != nil {
+		log.Printf("Could not probe %s for DHCP address (%v); starting wpa_supplicant as fallback", iface, err)
+	} else if has {
+		log.Printf("DHCP address present on %s, leaving wpa_supplicant alone", iface)
+		return
+	} else {
+		log.Printf("No DHCP address on %s, starting wpa_supplicant", iface)
+	}
+
+	if out, err := exec.Command("systemctl", "start", "wpa_supplicant").CombinedOutput(); err != nil {
+		log.Printf("Failed to start wpa_supplicant: %v: %s", err, strings.TrimSpace(string(out)))
+	}
+}

internal/config/cli.go

@@ -9,6 +9,7 @@ func declareFlags(o *Config) {
 	flag.StringVar(&o.Bind, "bind", "127.0.0.1:8080", "Bind address (host:port). Defaults to localhost; set to ':8080' to expose on the LAN — but note: there is no built-in authentication.")
 	flag.StringVar(&o.WifiInterface, "wifi-interface", "wlan0", "WiFi interface name")
 	flag.StringVar(&o.HotspotInterface, "hotspot-interface", "wlan1", "Hotspot WiFi interface name")
+	flag.StringVar(&o.EthernetInterface, "ethernet-interface", "eth0", "Ethernet interface to probe for a DHCP-assigned address at startup; if no DHCP address is present, wpa_supplicant is started")
 	flag.StringVar(&o.WifiBackend, "wifi-backend", "", "WiFi backend to use: 'iwd' or 'wpasupplicant' (required)")
 	flag.StringVar(&o.StationBackend, "station-backend", "hostapd", "Station discovery backend: 'arp', 'dhcp', or 'hostapd'")
 	flag.StringVar(&o.DHCPLeasesPath, "dhcp-leases-path", "/var/lib/dhcp/dhcpd.leases", "Path to DHCP leases file")

internal/config/config.go

@@ -9,17 +9,18 @@ import (
 )
 
 type Config struct {
-	Bind             string
-	WifiInterface    string
-	HotspotInterface string
-	WifiBackend      string
-	StationBackend   string // "arp", "dhcp", or "hostapd"
-	DHCPLeasesPath   string
-	ARPTablePath     string
-	SyslogEnabled    bool
-	SyslogPath       string
-	SyslogFilter     []string
-	SyslogSource     string
+	Bind              string
+	WifiInterface     string
+	HotspotInterface  string
+	EthernetInterface string
+	WifiBackend       string
+	StationBackend    string // "arp", "dhcp", or "hostapd"
+	DHCPLeasesPath    string
+	ARPTablePath      string
+	SyslogEnabled     bool
+	SyslogPath        string
+	SyslogFilter      []string
+	SyslogSource      string
 }
 
 // ConsolidateConfig fills an Options struct by reading configuration from
@@ -29,15 +30,16 @@ type Config struct {
 func ConsolidateConfig() (opts *Config, err error) {
 	// Define defaults options
 	opts = &Config{
-		Bind:             "127.0.0.1:8080",
-		WifiInterface:    "wlan0",
-		HotspotInterface: "wlan1",
-		DHCPLeasesPath:   "/var/lib/dhcp/dhcpd.leases",
-		ARPTablePath:     "/proc/net/arp",
-		SyslogEnabled:    false,
-		SyslogPath:       "/var/log/messages",
-		SyslogFilter:     []string{"daemon.info wpa_supplicant:", "daemon.info iwd:", "daemon.info hostapd:"},
-		SyslogSource:     "iwd",
+		Bind:              "127.0.0.1:8080",
+		WifiInterface:     "wlan0",
+		HotspotInterface:  "wlan1",
+		EthernetInterface: "eth0",
+		DHCPLeasesPath:    "/var/lib/dhcp/dhcpd.leases",
+		ARPTablePath:      "/proc/net/arp",
+		SyslogEnabled:     false,
+		SyslogPath:        "/var/log/messages",
+		SyslogFilter:      []string{"daemon.info wpa_supplicant:", "daemon.info iwd:", "daemon.info hostapd:"},
+		SyslogSource:      "iwd",
 	}
 
 	declareFlags(opts)