OpenWrt agent: get interface speed from sysfs if ethtool is missing

ethtool is not default on most OpenWrt targets and these devices are
typically found in networking environments, so missing interface speed
is not the desired behavior.

Adapt fallback from Linux agent and extract speed from sysfs if ethtool
is not available.

Author: stklcode

agents/check_mk_agent.openwrt

@@ -561,13 +561,19 @@ section_lnx_if() {
 
     echo '<<<lnx_if:sep(58)>>>'
     sed 1,2d /proc/net/dev
-    if inpath ethtool; then
-        sed -e 1,2d /proc/net/dev | cut -d':' -f1 | sort | while read -r eth; do
-            echo "[${eth}]"
-            ethtool "${eth}" | grep -E '(Speed|Duplex|Link detected|Auto-negotiation):'
-            echo "Address: $(cat "/sys/class/net/${eth}/address")"
-        done
-    fi
+    sed -e 1,2d /proc/net/dev | cut -d':' -f1 | sort | while read -r eth; do
+        echo "[${eth}]"
+            if inpath ethtool; then
+                ethtool "${eth}" | grep -E '(Speed|Duplex|Link detected|Auto-negotiation):'
+            else
+                speed=$(cat "/sys/class/net/${eth}/speed" 2>/dev/null)
+                if [ -n "${speed}" ] && [ "${speed}" -ge 0 ]; then
+                    echo "Speed: ${speed}Mb/s"
+                fi
+            fi
+        echo "Address: $(cat "/sys/class/net/${eth}/address")"
+    done
+
 }
 
 # Current state of bonding interfaces