Talking to C socket with Scapy

c, localserversocket, networking, scapy, sockets

Solution

Scapy needs to be configured slightly differently to work on the Loopback interface, see http://www.secdev.org/projects/scapy/doc/troubleshooting.html under the heading "I can’t ping 127.0.0.1. Scapy does not work with 127.0.0.1 or on the loopback interface"

I used the code given there and sent a scapy packet which was received by a C Socket, this was specifically:

from scapy.all import *
conf.L3socket=L3RawSocket
packet=IP()/UDP(dport=32000)/"HELLO WORLD"
send(packet)

This was then received on a UDP C Socket bound to lo on port 32000 (Scapy defaults to sending IP packets over the loopback interface).

Problem

I have a UDP connection up and listening on a port (localhost) and I am trying to send a Scapy packet from localhost as well. For some reason, my C code never actually captures the packet, however I can see the packet show up in Wireshark just fine. It's been awhile since I've used sockets, but is there some special socket options I have to set or why would I be able to see the packet in Wireshark just fine but not by the C socket? Note: I was able to successfully catch a packet when I wrote corresponding socket code to send out packets (from localhost) however I am still unable to get the listening code to catch the packet when sent from another computer. I have found a similar question but when I tried their approach (using UDP instead of TCP), I still couldn't get netcat to catch the Scapy packet. C Code (condensed for clarity sake) ``` int main() { int sock, dataLen, inLen; struct sockaddr_in inAddr; short listen_port = 8080; char buffer[2048]; if (sock = socket(AF_INET,SOCK_DGRAM,0) < 0) { printf("ERROR: unable to establish socket\n"); return -1; } // zero out address structure memset(&inAddr, 0, sizeof(inAddr)); inAddr.sin_family = AF_INET; inAddr.sin_addr.s_addr = htonl(INADDR_ANY); inAddr.sin_port = htons(listen_port); if (bind(sock, (struct sockaddr*)&inAddr, sizeof(inAddr)) < 0) { printf("ERROR: unable to bind\n"); return -1; } inLen = sizeof(inAddr); printf("Now listening on port %d\n", listen_port); while(1) { dataLen = recvfrom(sock, buffer, 1500, 0, (struct sockaddr*)&inAddr, &inLen); if (dataLen < 0) printf("Error receiving datagram\n"); else printf("Received packet of length %d\n", dataLen); } return 0; } ``` Scapy Script ``` # set interface conf.iface="lo0" # create IP packet ip_pkt = IP()/UDP() ip_pkt.payload = "payload test message" ip_pkt.dport = 8080 ip_pkt.dst = "127.0.0.1" ip_pkt.src = "127.0.0.1" # send out packet send(ip_pkt) ```

Original source