piholeDebug - Get default route robustly

Determine address and interface of default route by preceeding
    'via' and 'dev' fields in json output instead of plain text
    field position.

    Log if unable to determine default gateway

Signed-off-by: Rob Gill <rrobgill@protonmail.com>

advanced/Scripts/piholeDebug.sh

@@ -497,16 +497,25 @@ ping_gateway() {
     ping_ipv4_or_ipv6 "${protocol}"
     # Check if we are using IPv4 or IPv6
     # Find the default gateways using IPv4 or IPv6
-    local gateway gateway_addr gateway_iface
+    local gateway gateway_addr gateway_iface default_route
 
     log_write "${INFO} Default IPv${protocol} gateway(s):"
 
-    while IFS= read -r gateway; do
-        log_write "     $(cut -d ' ' -f 3 <<< "${gateway}")%$(cut -d ' ' -f 5 <<< "${gateway}")"
-    done < <(ip -"${protocol}" route | grep default)
+    while IFS= read -r default_route; do
+        gateway_addr=$(jq -r '.gateway' <<< "${default_route}")
+        gateway_iface=$(jq -r '.dev' <<< "${default_route}")
+        log_write "     ${gateway_addr}%${gateway_iface}"
+    done < <(ip -j -"${protocol}" route | jq -c '.[] | select(.dst == "default")')
+
+    # Find the first default route
+    default_route=$(ip -j -"${protocol}" route show default)
+    if echo "$default_route" | grep 'gateway' | grep -q 'dev'; then
+        gateway_addr=$(echo "$default_route" | jq -r -c '.[0].gateway')
+        gateway_iface=$(echo "$default_route" | jq -r -c '.[0].dev')
+    else
+        log_write "     Unable to determine gateway address for IPv${protocol}"
+    fi
 
-    gateway_addr=$(ip -"${protocol}" route | grep default | cut -d ' ' -f 3 | head -n 1)
-    gateway_iface=$(ip -"${protocol}" route | grep default | cut -d ' ' -f 5 | head -n 1)
     # If there was at least one gateway
     if [ -n "${gateway_addr}" ]; then
         # Append the interface to the gateway address if it is a link-local address