FTP protocol

be based on RFC959 Of FTP Sort out the agreement

Catalog

  Working mode ：
  Passive mode （PASV）：
  Active mode （PORT）：
  The data transfer ：（ Put the concept in front of it ）
  data structure ：
  Establish data connection
  Data connection management
  transmission mode
  Flow mode ：
  Block mode ：
  Compression mode ：
 FTP command
 1. Access control commands
  Return to the upper directory （CDUP）
 2. Transfer parameter command
 3.FTP The service command
 4.FTP Respond to
  The meaning of the first digit in the response code ：
 5. Caught analysis ：
  Packet capture analysis in passive mode
  Packet capture analysis in active mode

Introduce ：FTP yes FileTransferProtocol（

File transfer protocol

） Short for . be used for Internet Two way transfer of control files on . meanwhile , It's also an application （Application）. There are different based on different operating systems FTP Applications , And all of these applications follow the same protocol to transfer files . stay FTP In use , Users often encounter two concepts ：" download "（

Download

） and " Upload "（Upload）." download " A file is a copy of a file from a remote host to its own computer ;" Upload " A file is a copy of a file from your computer to a remote host . use Internet language , The user can use the client program to （ from ） Remote host upload （ download ） file .

By default FTP Agreement to use TCP In port 20 and 21 Two ports , among ,20 The port is used to transmit data （ Data port ）,21 The port is used to transmit control information （ The command port ）. Is it used 20 Port as the port domain for data transmission FTP The transmission mode used ：

If active mode is used , So the data transfer port is 20;
If passive mode is used , The specific final port to use needs to be decided by the server and the client through negotiation .

Working mode ：

FTP Two modes are supported ： Active mode （PORT） And passive mode （PASV）.

Passive mode （PASV）：

1、FTP The client uses a random port N（ Generally greater than 1024） towards FTP Server's 21 Port send connection request ;
2、ftp After the server main process receives the request , Derive a server-side control process , Establish a control connection with the client control process , Start session ;
3、ftp Client and ftp The server performs user name and password verification and other operations through their respective control processes ;
4、 When ftp When the client wants to initiate data file transmission ,ftp The client control process initiates to the server control process pasv command ;
5、ftp The server control process received pasv After the request , Derived from a data transmission process ,< And inform the client of the port number > Control process , return 227 entering passive mode (h1,h2,h3,h4,p1,p2), Where the port number is  p1*256+p2,ip  The address is h1.h2.h3.h4;《 The server informs the client 》
6、 ftp After the client control process receives the response , Derived from a client-side data transmission process , along with   Client data transmission port , The data transmission port number of the connection server （ That is ip+ Port number ）, Then transfer the data file ;
7、 When the data is transferred , The data transmission process of the server returns that the transmission is completed , The data connection channel will be closed , Keep the control connection channel ;
8、 If there is no other file transfer at this time , The control channels will also be closed one after another .

Active mode （PORT）：

1、FTP The client uses a random port N（ Generally greater than 1024） towards FTP Server's 21 Port send connection request ;
2、ftp After the server main process receives the request , Derive a server-side control process , Establish a control connection with the client control process , Start session ;
3、ftp Client and ftp The server performs user name and password verification and other operations through their respective control processes ;
4、 When ftp When the client wants to initiate data file transmission , The client derives a data transmission process , And connect the data through the client control process port Tell the server to control the process .< The client tells the server >
5、 When ftp After the server receives the request , Derive a server-side data transmission process , Generate a service port number , Then according to the request ftp The port number of the client data transmission program establishes the data transmission connection .
6、ftp After the client receives the response from the server , Transfer data files .
7、 When the data is transferred , The data transmission process of the server returns that the transmission is completed , The data connection channel will be closed , Keep the control connection channel ;
8、 If there is no other file transfer at this time , The control channels will also be closed one after another .

Similarities and differences between active mode and passive mode ： The two are the same in the process of controlling the connection , It's different when connecting data ： In active mode, the server actively connects and the client has established a data connection ; In passive mode, the server side is passive , The client connects to the server to establish a data connection .

The data transfer ：（ Put the concept in front of it ）

File by

Data connection

transmitted .

Control connection

It is used to send operation commands and corresponding response commands .

Data transmission commands include ： A mode that specifies how data bits are transmitted （MODE） command , And the structure used to define the data representation （STRU）、 type （TYPE） command .“ streaming ” The transfer mode depends on the file structure parameters , And when you use “ Compress ” In transfer mode , The representation of padding bytes depends on the representation type .

data structure ：

Due to different representation types ,FTP Allow files to have a specified structure . There are three kinds of ：

**$$ File structure ：** If the structure command is not used （STRU）, The file structure is used by default . There is no internal structure in the file structure , The file is treated as a continuous stream of bytes .

Record structure ：

Of all the FTP In the implementation , Must support “ Text ” file , That is to use TYPE　ASCII or EBCDIC Record structure , In the record structure file , Documents consist of continuous records .

Page structure ：

For transmission

Discontinuous

The file of ,FTP Defines the page structure . Generally speaking “ Random access file ” or “ Many holes ” The file belongs to the page structure type . For these files , Generally, there is another corresponding information of the whole file （ Such as file descriptor ）, Or some information of the corresponding document （ Such as page access control ）, Or both . stay FTP in , The part of the document is called a page .$$

To provide different page sizes and related information , Each page will be transmitted with an additional header . The header has the following defined fields ：
Head length ： The logical length of the header including this byte . The minimum head length is 4.
Page index ： Logical page number of the file area . Not the serial number of the transmission , It is the index number that identifies this page .
Data length ： Number of logical bytes of data in the page . The minimum data length is 0.
Page type ： Identifies the type of page . As follows ：
 0 =  Last page
  Used to identify the end of page structure transmission . The header length must be 4, The data length must be 0.
 1 =  Separate page
  This is a common type for individual pages without page related control information . The header length must be 4.
 2 =  Description page
  This type is used to transfer the description information of the whole file
 3 =  Access control page
  This type includes an additional header field that specifies page access information . The head length must be 5.
  Optional fields ：
  Other header fields may be used to provide control information for each page , Such as access control of each page .
  All fields are a logical byte . The length of logical bytes is determined by TYPE Command to specify .

Establish data connection

The process of data transmission includes establishing a data connection at the specified port and selecting transmission parameters . Users and servers DTP There are all default ports . The default data port of the user process is the same as the control connection port , The default port of the server process is adjacent to the port controlling the connection .

The transmission byte length is 8 Bits and bytes long . The byte length is related to the actual transmission data , It has nothing to do with the data representation of the host file system .

Passive data transmission process （PASV？） It should be sent FTP Listen to the data port before the request .FTP The request command determines the direction of data transmission . The server will establish a connection to the specified port after receiving the transmission request . When the connection is established , Data will be on both ends DTP Inter transmission , At the same time, the server PI To the user PI Send a confirmation reply .

Every FTP The implementation must support the default data port , Only users PI Variable non default ports can be used .

Users may use PORT The command specifies a different data port . If users want to download from a third-party host , user PI It will be established to two servers at the same time PI Control connection . One server （ use FTP command ） Waiting for the connection , Connect to another server . user PI To a server PI send out PORT The command is just the data port of another server . Last , Send appropriate transmission commands to both ends .

The server will close the data connection under the following conditions ：

1. The server completes data transmission in transmission mode , You need to close the connection , End of file .

2. The server received a message from the user ABORT command .

3. The user command changed the port setting

4. The control connection is closed legally or for other reasons .

5. An irreparable mistake has occurred .

Data connection management

Default data connection port ： all FTP The implementation must support using the default data connection port ,

Only users PI Non default ports may be used .

Negotiate non default ports ： user PI May use

PORT

Command specifies a non default user port . user PI The server may be required to use PASV Command specifies a non default port . The connection is specified with a stack of addresses , One of the above two actions will get a different connection , It is still allowed to use two commands at the same time to specify new ports at both ends .

Data connection multiplexing ： When using streaming mode to transmit data , The connection must be closed after the file transfer . At this time , If there are multiple file transfers , The possible problem is TCP To ensure transmission reliability , Keep the connection record for a period of time . Therefore, you cannot reconnect immediately . In this case , There are two solutions , One is to negotiate a non default port , The other is to use other transmission modes .

For transmission mode , The loss transmission mode is inherently unreliable , It is not sure whether the connection is closed prematurely . Other transmission modes （ block 、 Compress ） Do not close the connection to indicate the end of the file . They use FTP Code to determine the end of the file . Therefore, using these modes can keep using the same data connection when transferring multiple files .

transmission mode

FTP Three transmission modes are defined ：

Flow mode ：

Data is transmitted in byte stream . There are no restrictions on the representation type , You can use a record structure .

Block mode ：

Files are transmitted in consecutive blocks with headers . The data header includes a count field and a description code .

Compression mode ：

In this mode , There are three kinds of messages to send ： General data , Send as a byte string ; compressed data ; Control information , Transmitted in two byte escape characters . If sent N>0（ most 127） Regular data of bytes , this N There should be a header byte before bytes , The highest bit of this byte is 0, rest 7 Number of bits N.

FTP command

1. Access control commands

user name （USER）—— Used to identify users Telnet character string .

password （PASS）

Account （ACCT）

Some sites may need an account to log in , Other sites are only for special access , Such as storing files .

When you have to fill in the account information in the login process ,PASS The successful response code of the command is 332; If the login process does not require account information ,PASS The successful response code of the command 230; If the account information needs to be given in the subsequent dialog command , The server should be reserved according to the time （ wait for ACCT command ） Or give up the command and return accordingly 332 perhaps 532.、


Change the working directory （CWD）—— Enable the user to change the working directory or data set without changing the login user and account information . The transmission parameters remain unchanged ,

CWD The parameter of is a pathname

, Used to specify the corresponding directory or group name on other systems .

Return to the upper directory （CDUP）

Structural equipment （SMNT）—— This command allows users to equip a different file system data structure without changing user and account information . Parameter is a pathname used to identify a directory or other system dependent filegroup .

Reinitialize （REIN）—— This command allows the completion of the transmission process currently in progress , Terminate a user , Refresh all I/O And account information . All parameters are reset to default values , And keep the control connection . This is equivalent to the state in which the control connection has just been established .

This command may be followed by USER command .

Cancellation （QUIT）

—— Log off a user , And when no files are being transferred , The server will close the control connection . If there are currently files being transferred , The connection will remain and wait for a response , Then the server will close the connection . If the user process wants to transfer files with different users , Instead of closing and then reconnecting , You should use REIN instead of QUIT.

The accidental closing of the control connection will cause the server to produce the equivalent of abandonment （ABOR） And write off （QUIT） action .

2. Transfer parameter command

All data transmission parameters have default values , Only when the default value needs to be changed, the command is needed to specify the transmission data transmission parameters .

The default value is the last specified value

, Or if not specified , Is the standard default .

The following command is used to specify data transmission parameters ：

Data port （PORT）

—— Used to specify the data connection

Host data port

. There are default data port values for both users and servers , And generally, this command and its response do not need . If you use this command , Then its parameter is 32 Bit Internet host address and a 16 position TCP Port number . The address information is decomposed into each 8 Bit by bit , Each segment is treated as a decimal number （ Identify with string ） delivery . It's good to separate sections , One PORT Examples of commands are as follows ：

PORT h1,h2,h3,h4,p1,p2

h1 It is the height of Internet host 8 position


passive （PASV）

—— This command requests the server DTP On a data port （ Not its default port ） Listen on and wait for connection , Instead of actively initiating the connection after receiving the transmission command . The response of this command includes the address and port number of the server listening .

Indicates the type （TYPE）

File structure （STRU）

transmission mode （MODE）

3.FTP The service command

get （RETR）—— This command causes the server DTP Send a copy of the file specified by the path to the server at the other end of the data connection
Or user DTP. The status and content of the server file should not be affected .

preservation （STOP）—— This command causes the server DTP Accept the data transmitted through the data connection and store these data as the server
A document of . If the file specified in the path parameter already exists on the server , Then this file will be transferred
Data coverage from . If the specified file does not exist, a new file will be created on the server  .

Only save （STOU）

Additional （ Including the creation of ）（APPE）

Distribute （ALLO）

restart （REST）—— Specifies the location mark of the file that needs to restart the transfer . This command will not cause file transfer , Just ignore the data before the specified mark point in the file .

Rename start （RNFR）—— Specifies the original pathname of the file to be renamed . It must be followed by ” Rename it to “ command , To specify a new file path .

Rename it to （RNTO）

give up （ABOR）—— This command tells the server to abandon the previous FTP Service commands and related transmitted data .


$$ The current command （ Including data transmission ） When finished , There will be no action . The server will not close the control connection , But the data connection must be closed .

When the server receives this command, the pit can be in two states ：（1）FTP The service order has been completed ;（2）FTP The service order is still being executed .

For the first case , The server closes the data connection （ If the data connection is open ）, Respond 226 Code , Indicates that the abort command has been successfully processed ;

The second case , The server abandoned what was in progress FTP service , Close data connection , return 426 Code , Indicates that the request for service terminated abnormally . Then the server sends 226 Response code , Indicates that the abort command was successfully processed .$$

Delete （DELE）—— Delete the specified file on the server .

Delete directory （RMD）

new directory （MKD）

Print working directory （PWD）

list （LIST）

List of names （NLST）

Site parameters （SITE）—— The server uses this command to provide special services that the system may be helpful for transmission .

System （SYST)—— This command is used to get the type of server-side operating system .

state （STAT）—— This command returns status information in the form of response code through control connection .

help （HELP）

Empty operation （NOOP）—— Do not specify any action , Just ask the server to return OK Respond to .



$$FTP All communications on the control connection follow Telnet agreement .$$

$$ The current command （ Including data transmission ） When finished , There will be no action . The server will not close the control connection , But the data connection must be closed .

When the server receives this command, the pit can be in two states ：（1）FTP The service order has been completed ;（2）FTP The service order is still being executed .

For the first case , The server closes the data connection （ If the data connection is open ）, Respond 226 Code , Indicates that the abort command has been successfully processed ;

The second case , The server abandoned what was in progress FTP service , Close data connection , return 426 Code , Indicates that the request for service terminated abnormally . Then the server sends 226 Response code , Indicates that the abort command was successfully processed .$$

Delete （DELE）—— Delete the specified file on the server .

Delete directory （RMD）

new directory （MKD）

Print working directory （PWD）

list （LIST）

List of names （NLST）

Site parameters （SITE）—— The server uses this command to provide special services that the system may be helpful for transmission .

System （SYST)—— This command is used to get the type of server-side operating system .

state （STAT）—— This command returns status information in the form of response code through control connection .

help （HELP）

Empty operation （NOOP）—— Do not specify any action , Just ask the server to return OK Respond to .



$$FTP All communications on the control connection follow Telnet agreement .$$

4.FTP Respond to

The meaning of the first digit in the response code ：

1xx： Ready state —— The requested action has been initiated , Before the next new command , Expect a response . This type of response is used to indicate that the command has been accepted .

2xx： Completion status —— The request action was successfully completed . A new request can start .

3xx： In the middle —— The order was accepted , But the request action has not been executed for the time being , Waiting for further information . The user should send another command to specify this information . This response is used in command combinations .

4xx： Temporarily reject status —— The order was not accepted , The request action did not occur . But this error state is temporary , Action can be requested again . The user should return to the beginning of the command queue .

5xx： Permanent rejection status —— Orders are not accepted , The request action will not happen . The user process cannot repeat the same request .

Distinguish which response should belong to 4xx still 5xx One rule of is ： See if this command can be unmodified , And in the same user 、 Repeat again in the server state .

The meaning of the second digit in the response code ：
x0x： Grammar mistakes —— The command given does not exist 、 Not implemented or redundant .

x1x： Information —— Response to request information , Such as a request for status or help .

x2x： Connect —— About the response of control connection and data connection .

x3x： Authentication and accounts —— Response to login process and account processing .

x4x： Not currently used .

x5x： file system —— The status of the server file system or other file system actions when requesting transmission .

5. Caught analysis ：

Packet capture analysis in passive mode

The server IP：192.168.106.9

client IP：192.168.106.7

1. stay TCP After three handshakes , Successfully established the control connection ,ftp Server response 220 Indicates that the server is ready . You can see , At this time ftp The port number of the server control connection is 21, The port number of the client control connection is 53004;

2. The client is connected through control （192.168.106.7:53004→192.168.106.9:21） send out auth tls  and auth ssl Authentication order , Because it's not sftp Therefore, the authentication order was not executed ;

3. Sent by customer service terminal USER anonymous, Use anonymous login , Server return 230, Landing successful ;

4. The client sends TYPE A command , Server return 200, Set up ASCII Transmission format succeeded ;

4. The client sends through the control connection PASV command , Server response 227, Indicates that the server has successfully entered passive mode ,IP by 192.168.106.9, The port number is 1039.（ Combined with the port calculation formula mentioned above ：port = p1x256+p2, namely 4x256+15=1039）.

5. The client sends RETR Request a download named 123test.txt The file of ;

6. The client and server go through TCP3 The second handshake , Establish data connection （1039<===>53009), Start transmitting data ;

7. After transmission , The server 21 Client side 53008 send out 226 Respond to , Close data connection .

Packet capture analysis in active mode

Here's the picture ：