Of what the service of FTP consists?
FTP (File Transfer Protocol) is a file transfer protocol between systems connected to a network TCP based on the architecture client-servant, so that from an equipment client we can be connected a servant to unload archives from him or to send our own archives to him independent of the used operating system in each equipment.
Service FTP is offered by the application layer of the model of layers of network TCP/IP to the user, having used normally the port of network 20 and the 21. A basic problem of FTP is that it is thought to offer the maximum speed in the connection, but not it maximum security, since all the exchange of information, from login and password of the user in the servant to the transference of any file, is realised in flat text without no type of coding, and so a possible attacker can capture this traffic, accede to the servant, or take control of the transferred archives.
When a navigator is not equipped with function FTP, or if he is wanted to load archives in a remote computer, he will be needed to use a program client FTP. A client FTP is a program that settles in the computer of the user, and that uses protocol FTP to connect itself to a servant FTP and to transfer archives, or to unload them or to raise them.
In order to use a client FTP, he is needed to know the name the file, the computer in which reside (servant, in the case of unloading of archives), the computer to which he is wanted to transfer the file (in case of wanting to raise it we the servant), and the folder in which one is.
Some basic clients of FTP in way console come integrated in the operating systems, including Windows, TWO, Linux and Unix. Nevertheless, there are available clients with added options and graphical interface. Although many navigators have already integrated FTP, is more reliable at the time of connecting themselves with nonanonymous servers FTP to use a program client.
Ways of connection of client FTP
FTP admits two ways of connection of the client. These ways denominate Assets (or Standard, or PORT, because the client sends commandos type PORT to the servant by the channel of control when settling down the connection) and Liabilities (or PASV, because in this case it sends commandos type PASV). As much in the Active way as in the Passive way, the client settles down a connection with the servant by means of the port 21, that establishes the control channel.
In Active way, the servant always creates the channel of data in his port 20, whereas in the side of the client the channel of data is associated to a random port greater than the 1024. For it, the client sends to a commando PORT to the servant by the control channel indicating to him that number of port, so that the servant can open a connection to him of data by where the archives and the listings will be transferred, in the specified port.
The previous thing has a serious problem of security, and is that the machine client must be arranged to accept any connection of enter port superior to the 1024, with the problems that it implies if we have the equipment connected to an uncertain network as Internet. In fact, the fire-guards that settle in the equipment to avoid attacks surely will reject those random connections. In order to solve this the Passive way was developed.
Passive way (recommended)
When the client sends a commando PASV on the control channel, servant FTP opens an ephemeral port (anyone between the 1024 and the 5000) and informs into it to client FTP so that, this way, she is the client whom he connects with that port of the servant and thus it is not necessary to accept uncertain random connections to realise the data transfer.
Before each new transference, as much in the Active way as in the Liabilities, the client must send a control commando again (PORT or PASV, according to the way in which she has connected), and the servant will receive that connection of data in a new random port (if he is in passive way) or by port 20 (if he is in active way).
Types of transference of archives in FTP
It is important to know how we must transport a file throughout the network. If we did not use the suitable options we can destroy the information of the file. For that reason, when executing application FTP, we must decide to us to use one of these commandos (or to put the corresponding option in a program with graphical interface):
* type ASCII
Adapted to transfer archives that only contain printable characters (ASCII files, resulting nonarchives of a text processor), for example pages HTML, but not them images that can contain.
* type binary
This type is used when he is compressed files, feasible for PC, images, archives of audio…
Examples of how to transfer some types of file following its extension:
EXTENSION OF THE FILE TYPE OF TRANSFERENCE
txt (text) ASCII
HTML (webpage) ASCII
doc (document) binary
ps (poscript) ASCII
hqx (compressed) ASCII
Z (compressed) binary
ZIP (compressed) binary
ZOO (compressed) binary
Sit (compressed) binary
pit (comrpimido) binary
to shar (compressed) binary
uu (compressed) binary
ARC (compressed) binary
to tar (packaging) binary
Guide of commandos FTP
COMMANDO AND ARGUMENTS ACTION THAT REALISES
open servant Initiates a connection with a servant FTP
close or disconnect Finaliza a connection FTP without closing the program client
bye or quit Finaliza a connection FTP and the session of work with the program client
directory CD Changes the working directory in the servant
delete file Flock a file in the servant
mdelete landlord Flock multiple archives on the basis of a landlord that is applied to the name
to dir Muestra the content of the directory in whom we are in the servant
get file Obtains a file
mget archives Obtains multiple archives
hash Active the impression of characters # as archives are transferred, as a progress bar
directory LCD Changes the local working directory
ls Sample the content of the directory in the servant
prompt Active/deactivates the confirmation on the part of the user of the execution of commandos. For example when erasing multiple archives
putt file Sends a file to the directory assets of the servant
mput archives Sends multiple archives
pwd Muestra the directory assets in the servant
rename file Changes the name to a file in the servant
to rmdir directory Eliminates a directory in the servant if that directory this emptiness
status Sample the present state of the connection
bin or binary Active the binary way of transference
Active ASCII the way of transference in way text ASCII
It allows to leave temporarily to line of commandos without cutting the connection. In order to return, to key exit in the line of commandos
commando name Sample the information regarding the commando
or help Muestra a list of the commandos available
append name of the Continuous file one unloading that has been cut previously
Active Bell/deactivates the reproduction of a sound when any process of transference of archives has finished
glob Active/deactivates the visualization of long names of our PC
directory LCD Changes the directory assets of our hard disk. Here the chosen archives of the servant will unload
literal With this order commandos of the servant of remote form can be executed. In order to know the available ones he is used: literal help
to mkdir Crea the indicated directory of remote form
quote Hace the same literal function that
send name of the file Sends the indicated file to the directory assets of the servant
to return above
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