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Why client issue an active open for control connection ans a apsive open for data connection?
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What is the difference between passive and active FTP?
A client can ask an FTP server to establish either a PASV (passive-default) or a PORT (active) connection for data transfer. Some servers are limited to only one type of connection. The difference between a passive and an active FTP connection lies in whether the client or server initiates the data connection. In passive mode, the client initiates the connection to the server (port 20-default(; in active mode, the server initiates the connection. Neither is inherently more secure than the other. A Practical Guide to Fedora and Red Hat Enterprise Linux pg 644-Passive versus active connections
Is Token ring a passive or active network?
Token Ring is an active topology - each packet must be recreated as it goes from client to client.
What allows a remote user or client computer to control another computer ccross a LAN or WAN connection?
"Remore Access" must be enabled on the computer to be controlled.
Why must the server program be executed before the client program?
A server program must be executed before the client program because once a client is run, it will attempt to initiate a connection with the server. If the server is down, then the client will not be able to make a connection.
What ports need to be open for the client to use the FTP client software?
FTP is a TCP based service exclusively. There is no UDP component to FTP. FTP is an unusual service in that it utilizes two ports, a 'data' port and a 'command' port (also known as the control port). Traditionally these are port 21 for the command port and port 20 for the data port. The confusion begins however, when we find that depending on the mode, the data port is not always on port 20. In active mode FTP the client connects from a random unprivileged port (N > 1023) to the FTP server's command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20. From the server-side firewall's standpoint, to support active mode FTP the following communication channels need to be opened: * FTP server's port 21 from anywhere (Client initiates connection) * FTP server's port 21 to ports > 1023 (Server responds to client's control port) * FTP server's port 20 to ports > 1023 (Server initiates data connection to client's data port) * FTP server's port 20 from ports > 1023 (Client sends ACKs to server's data port) In order to resolve the issue of the server initiating the connection to the client a different method for FTP connections was developed. This was known as passive mode, or PASV, after the command used by the client to tell the server it is in passive mode. In passive mode FTP the client initiates both connections to the server, solving the problem of firewalls filtering the incoming data port connection to the client from the server. When opening an FTP connection, the client opens two random unprivileged ports locally (N > 1023 and N+1). The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command. The result of this is that the server then opens a random unprivileged port (P > 1023) and sends the PORT P command back to the client. The client then initiates the connection from port N+1 to port P on the server to transfer data. From the server-side firewall's standpoint, to support passive mode FTP the following communication channels need to be opened: * FTP server's port 21 from anywhere (Client initiates connection) * FTP server's port 21 to ports > 1023 (Server responds to client's control port) * FTP server's ports > 1023 from anywhere (Client initiates data connection to random port specified by server) * FTP server's ports > 1023 to remote ports > 1023 (Server sends ACKs (and data) to client's data port)
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