first commit

Task pwn02/week02/easy/server.py

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+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 = "127.0.0.1"  # 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:
+            if (
+                ("S" 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 = subprocess.check_output("flag").decode()
+                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="lo",
+    )
+
+
+# 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()