internal/router: support openwrt 24.10

openwrt 24.10 changes the dnsmasq default config path, causing breaking
changes to softwares which depends on old behavior.

This commit adds a workaround for the issue, by querying the actual
config directory from ubus service list, instead of relying on the
default hardcode one.

internal/router/openwrt/openwrt_test.go

@@ -0,0 +1,58 @@
+package openwrt
+
+import (
+	"io"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// Sample output from https://github.com/openwrt/openwrt/pull/16806#issuecomment-2448255734
+const ubusDnsmasqBefore2410 = `{
+	"dnsmasq": {
+		"instances": {
+			"guest_dns": {
+				"mount": {
+					"/tmp/dnsmasq.d": "0",
+					"/var/run/dnsmasq/": "1"
+				}
+			}
+		}
+	}
+}`
+
+const ubusDnsmasq2410 = `{
+	"dnsmasq": {
+		"instances": {
+			"guest_dns": {
+				"mount": {
+					"/tmp/dnsmasq.guest_dns.d": "0",
+					"/var/run/dnsmasq/": "1"
+				}
+			}
+		}
+	}
+}`
+
+func Test_dnsmasqConfPath(t *testing.T) {
+	var dnsmasq2410expected = filepath.Join("/tmp/dnsmasq.guest_dns.d", openwrtDNSMasqConfigName)
+	tests := []struct {
+		name     string
+		in       io.Reader
+		expected string
+	}{
+		{"empty", strings.NewReader(""), openwrtDnsmasqDefaultConfigPath},
+		{"invalid", strings.NewReader("}}"), openwrtDnsmasqDefaultConfigPath},
+		{"before 24.10", strings.NewReader(ubusDnsmasqBefore2410), openwrtDnsmasqDefaultConfigPath},
+		{"24.10", strings.NewReader(ubusDnsmasq2410), dnsmasq2410expected},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if got := dnsmasqConfPath(tc.in); got != tc.expected {
+				t.Errorf("dnsmasqConfPath() = %v, want %v", got, tc.expected)
+			}
+		})
+	}
+}