In the TCP/IP protocol, the FTP standard command TCP port number is 21 and the Port method data port is 20. The task of the FTP protocol is to transfer files from one computer to another, which has nothing to do with where the two computers are, how they are connected, or even whether they are using the same operating system. Assuming that two computers talk through the ftp protocol and have access to the Internet, you can use the ftp command to transfer files. There are some nuances in use of each operating system, but the basic command structure of each protocol is the same.
FTP transmission method
There are two ways to transmit FTP: ASCII transmission mode and binary data transmission mode.
1. ASCII transmission method: Assuming that the file the user is copying contains simple ASCII text. If it is not UNIX running on the remote machine, ftp will usually automatically adjust the content of the file when the file is transferred so as to interpret the file into the format of the other computer storing the text file.
However, it is often the case that the files the user is transmitting do not contain text files, they may be programs, databases, word processing files or compressed files (although most of the word processing files contain text, which also contains non-print characters indicating page size, font library and other information). Before copying any non-text files, use the binary command to tell ftp to copy verbatim and not to process these files. This is also the binary transmission to be discussed below.
2. Binary transmission mode: In binary transmission, save the bit order of the file so that the original and copying correspond one by one. Even if a file containing a bit sequence on the destination machine is meaningless. For example, macintosh transmits executable files to Windows systems in binary mode, and on the other party's system, this file cannot be executed.
If you transfer binary files in ASCII mode, you will still translate them even if you don't need them. This will slow down the transfer slightly, and will also corrupt the data and make the file useless. (On most computers, the ASCII approach generally assumes that the first significant bit of each character is meaningless, because the ASCII character combination does not use it. If you transfer a binary file, all bits are important.)If you know that both machines are the same, binary method is valid for both text and data files.
How FTP works
FTP supports two modes, one is called Standard (that is, PORT method, active method), and the other is Passive (that is, PASV, passive method). Standard mode FTP client sends PORT command to the FTP server. Passive mode FTP client sends PASV commands to FTP Server.
The following is a two-way approachHow it works：
Port mode FTP client first establishes a connection with the TCP 21 port of the FTP server and sends commands through this channel. When the client needs to receive data, it sends PORT commands on this channel. The PORT command contains what port the client uses to receive data. When transmitting data, the server side connects to the client's designated port through its own TCP 20 port to send data. The FTP server must establish a new connection with the client to transfer data.(It can be seen that in this way, the client and the server establish a control connection, and the server establishes a data connection to the client. Among them, the port numbers of the control connection and data connection of the client are two port numbers (temporary ports) greater than 1024, while the data port of the FTP server is 20 and the control port is 21)
Passive mode is similar to Standard mode when establishing a control channel, but after establishing a connection, it is not the Port command, but the Pasv command. After the FTP server receives the Pasv command, it randomly opens a temporary port (also called a free port, with port number greater than 1023 and less than 65535) and notifies the client of the request to transmit data on this port. The client connects to the FTP server for this port, and the FTP server will then transmit data through this port. At this time, the FTP server no longer needs to establish a new connection to the client.(It can be seen that in this case, the connections are initiated by the client to the server. It corresponds to the following "In order to solve the problem of server-initiating connections to customers, people have developed a different FTP connection method. This is the so-called passive method", and the data port on the server side is a temporary port, not a regular 20)
Many firewalls do not allow externally initiated connections when set up, so many FTP servers located behind the firewall or intranet do not support PASV mode because clients cannot pass through the firewall to open the high-end port of the FTP server; while many intranet clients cannot log in to the FTP server in PORT mode, because TCP 20 from the server cannot establish a new connection with the client of the internal network, causing them to be unable to work.
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Active and Passive mode FTP has two usage modes: active and passive. Active mode requires both the client and the server to open and listen to a port at the same time to establish a connection. In this case, the client will have some problems due to the installation of a firewall. Therefore, a passive model was created. Passive mode only requires the server to generate a process that listens to the corresponding port, so that the client can bypass the problem of installing a firewall.
The following steps are followed for establishing an active mode FTP connection:
The client opens a random port (port number greater than 1024, here we call it x), and an FTP process connects to the server's command port 21. At this time, the source port is random port x, on the client, the remote port is 21, on the server.
The client starts listening to the port (x+1)(I personally feel that x+1 here is not x+1 in the numerical sense, otherwise I don’t have to tell the server that the port number), and also send a port command to the server (through the server's command port 21) which tells the server the port number that the server is listening and is ready to receive data from this port. This port is what we know as data port.
The server opens the source port 20 and establishes a connection to the client data port. At this time, the source port is 20 and the remote data port is (x+1).
The client establishes a connection to the server port 20 through the local data port, and then sends a reply to the server, telling the server that it has established a connection.
There are two modes of use in FTP: active and passive. Active mode requires both the client and the server to open and listen to a port at the same time to establish a connection. In this case, the client will have some problems due to the installation of a firewall. Therefore, a passive model was created. Passive mode only requires the server to generate a process that listens to the corresponding port, so that the client can bypass the problem of installing a firewall.
oneActive modeThe following steps are required to establish an FTP connection:
1. The client opens a random port (port number greater than 1024, here we call it x), and an FTP process connects to the server's command port No. 21. At this time, the source port is random port x, on the client, the remote port is 21, on the server.
2. The client starts listening to port (x+1) and sends a port command to the server (through the server's command port 21). This command tells the server the client the port number that is being listened to and is ready to receive data from this port. This port is what we know as data port.
3. The server opens the source port No. 20 and establishes a connection with the client data port. At this time, the source port is 20 and the remote data port is (x+1).
4. The client establishes a connection to the server port 20 through the local data port, and then sends a reply to the server, telling the server that it has established a connection.
Passive modeFTP：
To solve the problem of server-initiating connections to customers, people have developed a different FTP connection method. This is called a passive method, or PASV, which is enabled when the client notifies the server that it is in passive mode.
In passive FTP, both command connections and data connections are initiated by the client, which can solve the problem that the incoming connections from the server to the client are filtered out by the firewall.
When an FTP connection is enabled, the client opens two arbitrary unprivileged local ports (N > 1024 and N+1). The first port connects to port 21 of the server, but unlike FTP in the active way, the client does not submit a PORT command and allows the server to connect its data port back and forth, but submits a PASV command. The result of this is that the server will open an arbitrary unprivileged port (P > 1024) and send a PORT P command to the client. The client then initiates a connection from local port N+1 to port P of the server to transmit data.
For server-side firewalls, the following communication must be allowed to support passive FTP:
1. From any port greater than 1024 to port 21 of the server (initialized connection of the client)
2. Server port 21 to any port larger than 1024 (the server responds to the connection to the client's control port)
3. From any port greater than 1024 to the server (the client initializes the data to any port specified by the server)
4. The server's port greater than 1024 to the remote port greater than 1024 (the server sends ACK response and data to the client's data port)
Edit the protocol structure of this section
| Order | describe |
|---|---|
| ABOR | Interrupt data connection program |
ACCT<account>
|
System privileged account |
ALLO <bytes>
|
Assign bytes to file memory on the server |
APPE <filename>
|
Add file to the server file with the same name |
CDUP <dir path>
|
Change the parent directory on the server |
CWD <dir path>
|
Change the working directory on the server |
DELE <filename>
|
Delete the specified file on the server |
HELP <command>
|
Return the specified command information |
LIST <name>
|
If it is a file name, list the file information, if it is a directory, list the file list |
MODE <mode>
|
Transmission mode (S=stream mode, B=block mode, C=compression mode) |
MKD <directory>
|
Create a specified directory on the server |
NLST <directory>
|
List the contents of the specified directory |
| NOOP | No action, except recognition from the server |
PASS <password>
|
System login password |
| PASV | Request the server to wait for data connection |
PORT <address>
|
IP address and two-byte port ID |
| PWD | Show the current working directory |
| QUIT | Log out of the FTP server |
| REIN | Reinitialize the login status connection |
REST <offset>
|
Restart file delivery by a specific offset |
RETR <filename>
|
Retrieve (copy) files from the server |
RMD <directory>
|
Delete the specified directory on the server |
RNFR <old path>
|
Rename old paths |
RNTO <new path>
|
Rename the new path |
SITE <params>
|
Special parameters of the site provided by the server |
SMNT <pathname>
|
Mount the specified file structure |
STAT <directory>
|
Return information on the current program or directory |
STOR <filename>
|
Save (copy) files to the server |
STOU <filename>
|
Save the file to the server name |
STRU <type>
|
Data structure (F=file, R=record, P=page) |
| SYST | Return to the operating system used by the server |
TYPE <data type>
|
Data type (A=ASCII, E=EBCDIC, I=binary) |
USER <username>
|
The username of the system login |
The standard FTP information is as follows
| Response code | Explanation |
|---|---|
| 110 | Restart mark on new file indicator |
| 120 | Time for server ready (minutes) |
| 125 | Open the data connection and start the transmission |
| 150 | Open the connection |
| 200 | success |
| 202 | The command was not executed |
| 211 | System status reply |
| 212 | Directory status reply |
| 213 | File status reply |
| 214 | Help message reply |
| 215 | System type reply |
| 220 | Service ready |
| 221 | Exit the network |
| 225 | Open a data connection |
| 226 | End the data connection |
| 227 | Enter passive mode (IP address, ID port) |
| 230 | Log in to the Internet |
| 250 | File behavior completion |
| 257 | Path name creation |
| 331 | Require password |
| 332 | Require an account |
| 350 | File behavior paused |
| 421 | Service Closed |
| 425 | Unable to open data connection |
| 426 | End the connection |
| 450 | File not available |
| 451 | A local error encountered |
| 452 | Insufficient disk space |
| 500 | Invalid command |
| 501 | Error parameters |
| 502 | The command was not executed |
| 503 | Error command sequence |
| 504 | Invalid command parameters |
| 530 | Not logged into the network |
| 532 | Requires an account to store files |
| 550 | File not available |
| 551 | Unknown page types |
| 552 | More than storage allocation |
| 553 | File name not allowed |
Reprinted:/u010154760/article/details/45458219