XP Firewall und Port Range freigeben (FTP)
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Re: XP Firewall und Port Range freigeben (FTP)
20.09.2006, 10:41:45
Active Mode FTP Inspection
Active mode FTP uses two channels for communications. When a client starts an FTP connection, it opens a TCP channel from one of its high-order ports to port 21 on the server. This is referred to as the command channel. When the client requests data from the server, it tells the server to send the data to a given high-order port. The server acknowledges the request and initiates a connection from its own port 20 to the high-order port that the client requested. This is referred to as the data channel.

Because the server initiates the connection to the requested port on the client, it was difficult in the past to have firewalls allow this data channel to the client without permanently opening port 20 connections from outside servers to inside clients for outbound FTP connections. This created a potential vulnerability by exposing clients on the inside of the firewall. Protocol inspections have resolved this problem.


Passive Mode FTP Inspection
Passive mode FTP (PFTP) also uses two channels for communications. The command channel works the same as in a active mode FTP connection, but the data channel setup works differently. When the client requests data from the server, it asks the server if it accepts PFTP connections. If the server accepts PFTP connections, it sends the client a high-order port number to use for the data channel. The client then initiates the data connection from its own high-order port to the port that the server sent.

Because the client initiates both the command and data connections, early firewalls could easily support outbound connections without exposing inside clients to attack. Inbound connections, however, proved more of a challenge. The FTP inspection protocol resolved this issue.

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