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phân tích giao thức FTP

Dựng 1 FTP Server và cho client kết nối vào để download - Cấu hình Netflow trên Router 2691 và dùng phần mềm Netflow Analyzer để phân tích thông tin thu được. Vấn đề là khi truyền file bằng giao thức FTP, tại sao phần mềm lại thông báo dùng các Port khác để download, chứ không phải Port 21 như hình: | FTP Protocol Analysis http www.troubleshootingnetworks.com flpinfo.html Analysis of the File Transfer Protocol FTP by Priscilla Oppenheimer There are many myths about how FTP works. The goal of this white paper is to briefly describe FTP protocol processes to dispel some of the myths. More detailed information about FTP behavior is available in Chapter 9 of the Troubleshooting Campus Networks book by Priscilla Oppenheimer and Joseph Bardwell. FTP was one of the first Internet protocols. It was designed for use on the Internet when the Internet was still a closed system that connected universities government agencies and a few commercial companies involved in the development of the system. FTP s behavior on modern networks especially networks where security is a big concern is problematic. This paper provides the theoretical background you will need to get FTP working properly on a modern network that uses firewalls and Network Address Translation NAT . FTP was designed long before the term client server came in vogue but it does behave like a client server protocol nonetheless. FTP uses two TCP connections one for control information and one for the actual data. Analysis of an FTP session involves an examination of the FTP commands sent on the control connection and an assessment of the TCP segments sent on the data connection. With normal or active FTP the control connection is initiated by the client side and the data connection is initiated by the server side. Active mode is also sometimes called port mode The other option is passive FTP in which case the client side establishes the data connection. FTP and TCP Port Numbers FTP uses different TCP port numbers depending on whether active or passive FTP is in use. Before we cover FTP in more detail we ll briefly discuss some basic concepts regarding TCP port numbers. TCP uses port numbers to identify the sending and receiving application. A port number helps TCP demultiplex byte streams and deliver bytes to the .