updated client anserver for week02 easy

week02/easy/server.py

@@ -0,0 +1,127 @@
+import hashlib
+import logging
+import subprocess
+
+import threading
+
+from scapy.config import conf
+from scapy.layers.inet import TCP, IP
+from scapy.packet import Packet
+from scapy.sendrecv import send, sniff
+
+log = logging.getLogger(__name__)
+TCP_CLIENTS = {}  # ((IP, port) -> [sent_packets])
+
+SERVER_IP = '192.168.1.4'  # TODO
+SERVER_PORT = 20102
+COOKIE_SECRET = 'TASTY_COOKIES123'
+INITIAL_SEQ = 1337
+
+
+# The cookie is calculated by first taking the sha256 hash of (clientIP || clientPort || serverSecret) and then
+# converting the hex digest to an integer
+# The cookie is then this result modulo 2^32 to fit the 32-bit field
+def generate_syn_cookie(client_ip: str, client_port: int, server_secret: str):
+    hash_input = f'{client_ip}{client_port}{server_secret}'.encode()
+    return int(hashlib.sha256(hash_input).hexdigest(), 16) % (2**32)
+
+
+def get_initial_syn(ip, port, ack) -> Packet:
+    ip = IP(dst=ip)
+    syn = TCP(sport=SERVER_PORT, dport=port, flags='SA', seq=INITIAL_SEQ, ack=ack)
+    return ip / syn
+
+
+def get_rst(ip, port, ack) -> Packet:
+    ip = IP(dst=ip)
+    syn = TCP(sport=SERVER_PORT, dport=port, flags='R', seq=INITIAL_SEQ, ack=ack)
+    return ip / syn
+
+
+def handle_packet(packet: Packet):
+    if packet.haslayer(TCP) and packet[TCP].dport == SERVER_PORT:
+        if 'F' in packet[TCP].flags or 'R' in packet[TCP].flags:
+            print('Received FIN or Reset packet:', packet.summary())
+            if (packet[IP].src, packet[TCP].sport) in TCP_CLIENTS:
+                del TCP_CLIENTS[(packet[IP].src, packet[TCP].sport)]
+            return
+        
+        print('Received packet:', packet.summary())
+        
+        # Extract the TCP layer
+        tcp_layer = packet[TCP]
+        src_ip = packet[IP].src
+        src_port = tcp_layer.sport
+        seq = packet[TCP].seq
+        ack = packet[TCP].ack
+        
+        expected_cookie = generate_syn_cookie(SERVER_IP, SERVER_PORT, COOKIE_SECRET)
+        
+        if (src_ip, src_port) not in TCP_CLIENTS:
+            print('New client:', src_ip, src_port, seq)
+            # first packet from client to initiate handshake
+            if (not 'S' in packet[TCP].flags) or (not packet[TCP].seq == expected_cookie):
+                print(f'Invalid cookie {seq}, expected {expected_cookie}')
+                rst = get_rst(src_ip, src_port, seq)
+                send(rst)
+            else:    
+                TCP_CLIENTS[(src_ip, src_port)] = 1
+                initial_syn = get_initial_syn(src_ip, src_port, seq)
+                
+                print(f'Cookie {expected_cookie} and packet is correct')
+                print(f'Sending packet: {initial_syn.summary()}')
+                send(initial_syn)
+        
+        else:
+            print("i am in this state")
+            if (0x02 in packet[TCP].flags) or (not seq == expected_cookie) or (not ack == INITIAL_SEQ):
+                print(f'Invalid cookie {seq}, expected {expected_cookie}')
+                rst = get_rst(src_ip, src_port, seq)
+                send(rst)
+            else:
+                print(f'Cookie {expected_cookie} and packet is again correct')
+                
+                flag = "hello world"
+                ip = IP(dst=src_ip)
+                syn_ack = TCP(
+                    sport=SERVER_PORT,
+                    dport=src_port,
+                    flags='A',
+                    seq=ack,
+                    ack=seq,
+                ) / flag
+                
+                send(ip / syn_ack)
+            
+            del TCP_CLIENTS[(src_ip, src_port)]
+
+
+# Function to start the packet sniffing
+def start_sniffing():
+    print('Starting TCP server on port:', SERVER_PORT)
+    sniff(
+        filter=f'tcp port {SERVER_PORT}',
+        prn=handle_packet,
+        store=False,
+        monitor=True,
+        iface='eth0',
+    )
+
+
+# Run the server in a separate thread
+def main():
+    conf.use_pcap = False
+    server_thread = threading.Thread(target=start_sniffing)
+    server_thread.start()
+
+
+if __name__ == '__main__':
+    logging.basicConfig(
+        level=logging.INFO,
+        format='%(asctime)s %(levelname)s [%(module)s:%(lineno)d] %(message)s',
+    )
+
+    # "INFO:asyncio:poll took 25.960 seconds" is annyoing
+    logging.getLogger('asyncio').setLevel(logging.WARNING)
+
+    main()