From 92b6113d726c44881c89e0fb0fec54ea735b76a0 Mon Sep 17 00:00:00 2001
From: Pierre-Olivier Mercier <nemunaire@nemunai.re>
Date: Sat, 2 May 2026 11:37:28 +0800
Subject: [PATCH] app: Require link carrier to consider Ethernet uplink active

A static IPv4 (e.g. the hotspot gateway address on eth0) was wrongly
classified as a DHCP uplink on stacks where ip(8) omits the "dynamic"
flag, and a NO-CARRIER interface with any address was reported as up.
Gate probeEthernet on /sys/class/net/<iface>/carrier == 1, and accept
either the dynamic flag or a finite valid_lft as the DHCP signal so
BusyBox's ip output is handled too.
---
 internal/app/network.go | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/internal/app/network.go b/internal/app/network.go
index 557dffb..07502f1 100644
--- a/internal/app/network.go
+++ b/internal/app/network.go
@@ -3,18 +3,33 @@ package app
 import (
 	"fmt"
 	"log"
+	"os"
 	"os/exec"
 	"strings"
 
 	"github.com/nemunaire/repeater/internal/models"
 )
 
-// probeEthernet reports the wired uplink state of iface by parsing
-// `ip -4 -o addr show dev <iface>`. An IPv4 line carrying the "dynamic" flag
-// (set by ip(8) on addresses with a finite valid_lft) is treated as a
-// DHCP-assigned lease — that's the signal we use to decide whether the box
-// already has connectivity through Ethernet.
+// hasCarrier reports whether the kernel sees an active link on iface.
+// `/sys/class/net/<iface>/carrier` is "1" when the link layer is up; reading
+// it on an admin-down interface yields EINVAL, which we treat the same way.
+func hasCarrier(iface string) bool {
+	data, err := os.ReadFile("/sys/class/net/" + iface + "/carrier")
+	if err != nil {
+		return false
+	}
+	return strings.TrimSpace(string(data)) == "1"
+}
+
+// probeEthernet reports the wired uplink state of iface. Active requires both
+// a live link (`carrier == 1`) and a DHCP-assigned IPv4 — detected either by
+// the `dynamic` flag emitted by full iproute2, or by a finite `valid_lft`
+// (which BusyBox's `ip` does emit even when it omits the flag).
 func probeEthernet(iface string) (*models.EthernetStatus, error) {
+	if !hasCarrier(iface) {
+		return &models.EthernetStatus{Active: false, Interface: iface}, nil
+	}
+
 	out, err := exec.Command("ip", "-4", "-o", "addr", "show", "dev", iface).Output()
 	if err != nil {
 		return nil, fmt.Errorf("ip addr show %s: %w", iface, err)
@@ -23,7 +38,7 @@ func probeEthernet(iface string) (*models.EthernetStatus, error) {
 		fields := strings.Fields(line)
 		var addr string
 		hasInet := false
-		hasDynamic := false
+		isDHCP := false
 		for i, f := range fields {
 			switch f {
 			case "inet":
@@ -32,10 +47,14 @@ func probeEthernet(iface string) (*models.EthernetStatus, error) {
 					addr = fields[i+1]
 				}
 			case "dynamic":
-				hasDynamic = true
+				isDHCP = true
+			case "valid_lft":
+				if i+1 < len(fields) && fields[i+1] != "forever" {
+					isDHCP = true
+				}
 			}
 		}
-		if hasInet && hasDynamic {
+		if hasInet && isDHCP {
 			if slash := strings.IndexByte(addr, '/'); slash >= 0 {
 				addr = addr[:slash]
 			}
@@ -60,7 +79,7 @@ func ensureUplink(iface string) *models.EthernetStatus {
 		log.Printf("DHCP address %s on %s, leaving wpa_supplicant alone", eth.IPv4, iface)
 		return eth
 	}
-	log.Printf("No DHCP address on %s, starting wpa_supplicant", iface)
+	log.Printf("No usable DHCP uplink on %s (carrier or lease missing), starting wpa_supplicant", iface)
 	if out, err := exec.Command("service", "wpa_supplicant", "start").CombinedOutput(); err != nil {
 		log.Printf("Failed to start wpa_supplicant: %v: %s", err, strings.TrimSpace(string(out)))
 	}