add example icmp drop script

backend/example_scripts/icmp-drop-requirements.txt

@@ -0,0 +1,2 @@
+ netfilterqueue
+scapy

backend/example_scripts/icmp_drop.py

@@ -0,0 +1,38 @@
+ 
+#!/usr/bin/env python3
+# drop_icmp_v4.py
+# Requirements: NetfilterQueue, scapy
+
+import sys
+from netfilterqueue import NetfilterQueue
+from scapy.all import IP
+
+def on_packet(pkt):
+    data = pkt.get_payload()
+    try:
+        ip = IP(data)
+        # IPv4 ICMP protocol number == 1
+        if ip.proto == 1:
+            pkt.drop()
+            return
+    except Exception:
+        # parsing error -> accept (conservative choice)
+        pass
+    pkt.accept()
+
+def main():
+    if len(sys.argv) < 2:
+        print("Usage: drop_icmp_v4.py <QUEUE_NUM>", file=sys.stderr)
+        sys.exit(1)
+    qnum = int(sys.argv[1])
+    nfq = NetfilterQueue()
+    nfq.bind(qnum, on_packet)
+    try:
+        nfq.run()
+    except KeyboardInterrupt:
+        pass
+    finally:
+        nfq.unbind()
+
+if __name__ == "__main__":
+    main()