Internet Security - Dang Thanh Binh's Page

50 INTERNET SECURITY
2.4 File Transfer
The file transfer application allows users to send or receive a copy of a data file. Access
to data on remote files takes two forms: whole-file copying and shared online access.
FTP is the major file transfer protocol in the TCP/IP suite. TFTP provides a small, simple
alternative to FTP for applications that need only file transfer. NFS provides online shared
file access.
2.4.1 File Transfer Protocol (FTP)
File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying
a file from one host to another. The FTP protocol is defined in RFC959. It is further
defined in RFC 2227, 2640, 2773 for updated documentation.
In transferring files from one system to another, two systems may have different ways
to represent text and data. Two systems may have different directory structures. All of
these problems have been solved by FTP in a very simple and elegant way.
FTP differs from other client–server applications in that it establishes two connections
between the hosts. One connection is used for data transfer (port 20), the other for control
information (port 21). The control connection port remains open during the entire FTP
session and is used to send control messages and client commands between the client and
server. A data connection is established using an ephemeral port. The data connection
is created each time a file is transferred between the client and server. Separation of
commands and data transfer makes FTP more efficient. FTP allows the client to specify
whether a file contains text (ASCII or EBCDIC character sets) or binary integers. FTP
requires clients to authorise themselves by sending a log name and password to the server
before requesting file transfers.
Since FTP is used only to send and receive files, it is very difficult for hackers to
exploit.
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2.4.2 Trivial File Transfer Protocol (TFTP)
Trivial File Transfer Protocol (TFTP) is designed to simply copy a file without the need
for all of the functionalities of the FTP protocol. TFTP is a protocol that quickly copies
files because it does not require all the sophistication provided in FTP. TFTP can read or
write a file for the client. Since TFTP restricts operations to simple file transfer and does
not provide authentication, TFTP software is much smaller than FTP.
2.4.3 Network File System (NFS)
The Network File System (NFS), developed by Sun Microsystems, provides online shared
file access that is transparent and integrated. The file access mechanism accepts the request
and automatically passes it to either the local file system software or to the NFS client,
depending on whether the file is on the local disk or on a remote machine. When it
receives a request, the client software uses the NFS protocol to contact the appropriate
server on a remote machine and performs the requested operation. When the remote server
replies, the client software returns the results to the application program.
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