Return an ICMP unreachable to a packet. Scapy example.

scapy_icmp_unreachable.py

# Sending ICMP HOST UNREACHABLE USING SCAPY (PYTHON TOOLS)
# Will WORK for UDP/TCP, just give the "packet" (scapy's representation)

def send_icmp_unreachable (packet):
    global MYMAC
    global IPGATEWAY
    global INTERFACE

    p = Ether(src=MYMAC, dst=packet.src)/IP(src=IPGATEWAY, dst=packet.getlayer(IP).src)
    # ICMP type=3 code=1 Host Unreachable
    icmp = ICMP()
    icmp.type = 3
    icmp.code = 1
    try:
        sendp(p/icmp/packet.getlayer (IP)/packet.getlayer (UDP), iface=INTERFACE)
    except:
        sendp(p/icmp/packet.getlayer (IP)/packet.getlayer (TCP), iface=INTERFACE)
    return