FTP 服务器未使用 sockets 将数据发送到 Python 中的代理

Question

I'm building a proxy in Python between a ftp client and a ftp server. All the control phase and data phase is well done (currently on active mode), but when I send the request (LIST, for example), server does not send the information back. I don't know what is causing this fault...

Here is the Wireshark capture:

And the corresponding code:

print(f"Waiting for a message from the server to the PORT message")
answer = fw_proxy_server.recv(BUFFER_FTP)
print(answer) # 200 PORT command successful
fw_proxy_client.send(answer) # forward to client

# Create the socket to listen on 192.168.30.80:port (IP_PROXY:port)
server_socket2 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_socket2.setsockopt(socket.SOL_SOCKET, 25, str(INTERFACE_PROXY_SERVER + '\0').encode('utf-8'))
server_socket2.bind((IP_PROXY, port))
server_socket2.listen()

print(f"Waiting for a message from the client")
message = fw_proxy_client.recv(BUFFER_FTP) # REQUEST: LIST

print(f"Forwarding message from the server")
message = send(fw_proxy_server, "LIST") # REQUEST: LIST (sent to port 21)

# Accept an incoming connection from the Server with data

server_socket2, data_address = server_socket2.accept() # HERE PROGRAM HANG
print(f"Data connection from {data_address} has been established!")

print(f"Waiting for a message from the data from server")
message = fw_proxy_server.recv(BUFFER_FTP) # never executed

The IPs are the following:
Client: 192.168.40.50
Proxy-client: 192.168.40.80
Proxy-server: 192.168.30.80
Server: 192.168.30.90

Thanks in advance.

EDIT:

Changed order of listening to port, so that the port is listened before sending the PORT command and the LIST request. Same result, connection refused:

start = str(message).find("(")
end = str(message).find(")")
tuple = str(message)[start+1:end].split(',')
port = int(tuple[4])*256 + int(list(filter(str.isdigit, tuple[5]))[0])
#Create the socket to listen on 192.168.40.80:port
server_socket2 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_socket2.setsockopt(socket.SOL_SOCKET, 25, str(INTERFACE_PROXY_SERVER + '\0').encode('utf-8'))
server_socket2.bind(("192.168.40.80", port)) # With 192.168.30.80 it doesn't work too
server_socket2.listen()

answer = send(fw_proxy_server, "PORT 192,168,30,80," + tuple[4] + "," + tuple[5])
print(f"Waiting for a message from the server to the PORT message")
print(answer) # 200 PORT command successful
fw_proxy_client.send(answer)

#Create the socket to forward the data to the server

print(f"Waiting for a request from the client") # REQUEST: LIST
message = fw_proxy_client.recv(BUFFER_FTP)
print(message)

fw_proxy_server.send(message)
print(f"Waiting for an answer from the server") # Response 425: Unable to build data connection: Connection refused
answer = fw_proxy_server.recv(BUFFER_FTP)
print(answer)


#Accept an incoming connection from the Client
server_socket2, data_address = server_socket2.accept() #Program hang
print(f"Data connection from {data_address} has been established!")

print(f"Waiting for a message from the server")
print(answer)

Answer 1

From the Wireshark capture session, it sounds like the port you are sending to the FTP server and the port you are actually listening for data-connection on the proxy-server are not same hence FTP server reporting connection refused. Make sure the port server_socket2 is binding ie, port is equal to 256 * tuple[4] + tuple[5] . I believe the socket_option 25 refers to SO_BINDTODEVICE , in that case when you are binding the socket using bind() then there is no need to explicitly bind the socket to interface (in general use case). Can you remove that line and check if it is still working? It will be a learning for me too. Thanks.