Check if the user passed a valid interface

create_ap

@@ -82,6 +82,11 @@ version_cmp() {
 
 USE_IWCONFIG=0
 
+is_interface() {
+    [[ -z "$1" ]] && return 1
+    ip link show $1 > /dev/null 2>&1
+}
+
 is_wifi_interface() {
     which iw > /dev/null 2>&1 && iw dev $1 info > /dev/null 2>&1 && return 0
     if which iwconfig > /dev/null 2>&1 && iwconfig $1 > /dev/null 2>&1; then
@@ -579,6 +584,11 @@ else
     fi
 fi
 
+if [[ "$SHARE_METHOD" != "none" ]] && ! is_interface $INTERNET_IFACE; then
+    echo "ERROR: '${INTERNET_IFACE}' is not an interface" >&2
+    exit 1
+fi
+
 if [[ "$SHARE_METHOD" == "bridge" ]]; then
     OLD_BRIDGE_IPTABLES=$(cat /proc/sys/net/bridge/bridge-nf-call-iptables)