SFTP (simple): Simple File Transfer Protocol
A middle-ground file transfer protocol between TFTP and FTP
Lottor, M., "Simple File Transfer Protocol", RFC 913, DOI 10.17487/RFC0913, September 1984, <https://www.rfc-editor.org/info/rfc913>.
@misc{rfc913,
series = {Request for Comments},
number = 913,
howpublished = {RFC 913},
publisher = {RFC Editor},
doi = {10.17487/RFC0913},
url = {https://www.rfc-editor.org/info/rfc913},
author = {M. Lottor},
title = {{Simple File Transfer Protocol}},
pagetotal = 15,
year = 1984,
month = sep,
} RFC text
Contents
Network Working Group Mark K. Lottor
Request for Comments: 913 MIT
September 1984
Simple File Transfer Protocol
STATUS OF THIS MEMO
This RFC suggests a proposed protocol for the ARPA-Internet
community, and requests discussion and suggestions for improvements.
Distribution of this memo is unlimited.
INTRODUCTION
SFTP is a simple file transfer protocol. It fills the need of people
wanting a protocol that is more useful than TFTP but easier to
implement (and less powerful) than FTP. SFTP supports user access
control, file transfers, directory listing, directory changing, file
renaming and deleting.
SFTP can be implemented with any reliable 8-bit byte stream oriented
protocol, this document describes its TCP specification. SFTP uses
only one TCP connection; whereas TFTP implements a connection over
UDP, and FTP uses two TCP connections (one using the TELNET
protocol).
THE PROTOCOL
SFTP is used by opening a TCP connection to the remote hosts' SFTP
port (115 decimal). You then send SFTP commands and wait for
replies. SFTP commands sent to the remote server are always 4 ASCII
letters (of any case) followed by a space, the argument(s), and a
<NULL>. The argument can sometimes be null in which case the command
is just 4 characters followed by <NULL>. Replies from the server are
always a response character followed immediately by an ASCII message
string terminated by a <NULL>. A reply can also be just a response
character and a <NULL>.
<command> : = <cmd> [<SPACE> <args>] <NULL>
<cmd> : = USER ! ACCT ! PASS ! TYPE ! LIST ! CDIR
KILL ! NAME ! DONE ! RETR ! STOR
<response> : = <response-code> [<message>] <NULL>
<response-code> : = + | - | | !
<message> can contain <CRLF>
Commands that can be sent to the server are listed below. The server
Simple File Transfer Protocol
replies to each command with one of the possible response codes
listed under each message. Along with the response, the server
should optionally return a message explaining the error in more
detail. Example message texts are listed but do not have to be
followed. All characters used in messages are ASCII 7-bit with the
high-order bit zero, in an 8 bit field.
The response codes and their meanings:
+ Success.
- Error.
An error occurred while processing your command.
Number.
The number-sign is followed immediately by ASCII digits
representing a decimal number.
! Logged in.
You have sent enough information to be able to log yourself in.
This is also used to mean you have sent enough information to
connect to a directory.
To use SFTP you first open a connection to the remote SFTP server.
The server replies by sending either a positive or negative greeting,
such as:
+MIT-XX SFTP Service
(the first word should be the host name)
-MIT-XX Out to Lunch
Simple File Transfer Protocol
If the server send back a '-' response it will also close the
connection, otherwise you must now send a USER command.
USER user-id
Your userid on the remote system.
The reply to this command will be one of:
!<user-id> logged in
Meaning you don't need an account or password or you
specified a user-id not needing them.
+User-id valid, send account and password
-Invalid user-id, try again
If the remote system does not have user-id's then you should
send an identification such as your personal name or host name
as the argument, and the remote system would reply with '+'.
ACCT account
The account you want to use (usually used for billing) on the
remote system.
Valid replies are:
! Account valid, logged-in
Account was ok or not needed. Skip the password.
+Account valid, send password
Account ok or not needed. Send your password next.
-Invalid account, try again
Simple File Transfer Protocol
PASS password
Your password on the remote system.
Valid replies are:
! Logged in
Password is ok and you can begin file transfers.
+Send account
Password ok but you haven't specified the account.
-Wrong password, try again
Simple File Transfer Protocol
You cannot specify any of the following commands until you receive a
'!' response from the remote system.
TYPE { A | B | C }
The mapping of the stored file to the transmission byte stream
is controlled by the type. The default is binary if the type
is not specified.
A - ASCII
The ASCII bytes are taken from the file in the source
system, transmitted over the connection, and stored in the
file in the destination system.
The data is the 7-bit ASCII codes, transmitted in the
low-order 7 bits of 8-bit bytes. The high-order bit of the
transmission byte must be zero, and need not be stored in
the file.
The data is "NETASCII" and is to follow the same rules as
data sent on Telnet connections. The key requirement here
is that the local end of line is to be converted to the pair
of ASCII characters CR and LF when transmitted on the
connection.
For example, TOPS-20 machines have 36-bit words. On TOPS-20
machines, The standard way of labeling the bits is 0 through
35 from high-order to low-order. On TOPS-20 the normal way
of storing ASCII data is to use 5 7-bit bytes per word. In
ASCII mode, the bytes transmitted would be [0-6], [7-13],
[14-20], [21-27], [28-34], (bit 35 would not be
transmitted), each of these 7-bit quantities would be
transmitted as the low-order 7 bits of an 8-bit byte (with
the high-order bit zero).
For example, one disk page of a TOPS-20 file is 512 36-bit
words. But using only 35 bits per word for 7-bit bytes, a
page is 17920 bits or 2560 bytes.
Simple File Transfer Protocol
B - BINARY
The 8-bit bytes are taken from the file in the source
system, transmitted over the connection, and stored in the
file in the destination system.
The data is in 8-bit units. In systems with word sizes
which are not a multiple of 8, some bits of the word will
not be transmitted.
For example, TOPS-20 machines have 36-bit words. In binary
mode, the bytes transmitted would be [0-7], [8-15], [16-23],
[24-31], (bits 32-35 would not be transmitted).
For example, one disk page of a TOPS-20 file is 512 36-bit
words. But using only 32 bits per word for 8-bit bytes, a
page is 16384 bits or 2048 bytes.
C - CONTINUOUS
The bits are taken from the file in the source system
continuously, ignoring word boundaries, and sent over the
connection packed into 8-bit bytes. The destination system
stores the bits received into the file continuously,
ignoring word boundaries.
For systems on machines with a word size that is a multiple
of 8 bits, the implementation of binary and continuous modes
should be identical.
For example, TOPS-20 machines have 36-bit words. In
continuous mode, the bytes transmitted would be [first word,
bits 0-7], [first word, bits 8-15], [first word, bits
16-23], [first word, bits 24-31], [first word, bits 32-35 +
second word, bits 0-3], [second word, bits 4-11], [second
word, bits 12-19], [second word, bits 20-27], [second word,
bits 28-35], then the pattern repeats.
For example, one disk page of a TOPS-20 file is 512 36-bit
words. This is 18432 bits or 2304 8-bit bytes.
Replies are:
+Using { Ascii | Binary | Continuous } mode
-Type not valid
Simple File Transfer Protocol
LIST { F | V } directory-path
A null directory-path will return the current connected
directory listing.
F specifies a standard formatted directory listing.
An error reply should be a '-' followed by the error message
from the remote systems directory command. A directory
listing is a '+' followed immediately by the current
directory path specification and a <CRLF>. Following the
directory path is a single line for each file in the
directory. Each line is just the file name followed by
<CRLF>. The listing is terminated with a <NULL> after the
last <CRLF>.
V specifies a verbose directory listing.
An error returns '-' as above. A verbose directory listing
is a '+' followed immediately by the current directory path
specification and a <CRLF>. It is then followed by one line
per file in the directory (a line ending in <CRLF>). The
line returned for each file can be of any format. Possible
information to return would be the file name, size,
protection, last write date, and name of last writer.
Simple File Transfer Protocol
CDIR new-directory
This will change the current working directory on the remote
host to the argument passed.
Replies are:
!Changed working dir to <new-directory>
-Can't connect to directory because: (reason)
+directory ok, send account/password
If the server replies with '+' you should then send an ACCT or
PASS command. The server will wait for ACCT or PASS commands
until it returns a '-' or '!' response.
Replies to ACCT could be:
!Changed working dir to <new-directory>
+account ok, send password
-invalid account
Replies to PASS could be:
!Changed working dir to <new-directory>
+password ok, send account
-invalid password
KILL file-spec
This will delete the file from the remote system.
Replies are:
+<file-spec> deleted
-Not deleted because (reason)
Simple File Transfer Protocol
NAME old-file-spec
Renames the old-file-spec to be new-file-spec on the remote
system.
Replies:
+File exists
-Can't find <old-file-spec>
NAME command is aborted, don't send TOBE.
If you receive a '+' you then send:
TOBE new-file-spec
The server replies with:
+<old-file-spec> renamed to <new-file-spec>
-File wasn't renamed because (reason)
DONE
Tells the remote system you are done.
The remote system replies:
+(the message may be charge/accounting info)
and then both systems close the connection.
Simple File Transfer Protocol
RETR file-spec
Requests that the remote system send the specified file.
Receiving a '-' from the server should abort the RETR command
and the server will wait for another command.
The reply from the remote system is:
<number-of-bytes-that-will-be-sent> (as ascii digits)
-File doesn't exist
You then reply to the remote system with:
SEND (ok, waiting for file)
The file is then sent as a stream of exactly the number
of 8-bit bytes specified. When all bytes are received
control passes back to you (the remote system is waiting
for the next command). If you don't receive a byte
within a reasonable amount of time you should abort the
file transfer by closing the connection.
STOP (You don't have enough space to store file)
Replies could be:
+ok, RETR aborted
You are then ready to send another command to the remote host.
Simple File Transfer Protocol
STOR { NEW | OLD | APP } file-spec
Tells the remote system to receive the following file and save
it under that name.
Receiving a '-' should abort the STOR command sequence and the
server should wait for the next command.
NEW specifies it should create a new generation of the file and
not delete the existing one.
Replies could be:
+File exists, will create new generation of file
+File does not exist, will create new file
-File exists, but system doesn't support generations
OLD specifies it should write over the existing file, if any,
or else create a new file with the specified name.
Replies could be:
+Will write over old file
+Will create new file
(OLD should always return a '+')
APP specifies that what you send should be appended to the file
on the remote site. If the file doesn't exist it will be
created.
Replies could be:
+Will append to file
+Will create file
(APP should always return a '+')
Simple File Transfer Protocol
You then send:
SIZE <number-of-bytes-in-file> (as ASCII digits)
where number-of-bytes-in-file
is the exact number of 8-bit bytes you will be
sending.
The remote system replies:
+ok, waiting for file
You then send the file as exactly the number of bytes
specified above.
When you are done the remote system should reply:
+Saved <file-spec>
-Couldn't save because (reason)
-Not enough room, don't send it
This aborts the STOR sequence, the server is waiting for
your next command.
You are then ready to send another command to the remote host.
Simple File Transfer Protocol
AN EXAMPLE
An example file transfer. 'S' is the sender, the user process. 'R'
is the reply from the remote server. Remember all server replies are
terminated with <NULL>. If the reply is more than one line each line
ends with a <CRLF>.
R: (listening for connection)
S: (opens connection to R)
R: +MIT-XX SFTP Service
S: USER MKL
R: +MKL ok, send password
S: PASS foo
R: ! MKL logged in
S: LIST F PS: <MKL>
R: +PS: <MKL>
Small.File
Large.File
S: LIST V
R: +PS: <MKL>
Small.File 1 69(7) P775240 2-Aug-84 20:08 MKL
Large.File 100 255999(8) P770000 9-Dec-84 06:04 MKL
S: RETR SMALL.FILE
R: 69
S: SEND
R: This is a small file, the file is sent without
a terminating null.
S: DONE
R: +MIT-XX closing connection
Simple File Transfer Protocol
EDITORS NOTE
Mark Lotter receives full credit for all the good ideas in this memo.
As RFC editor, i have made an number of format changes, a few wording
changes, and one or two technical changes (mostly in the TYPEs). I
accept full responsibility for any flaws i may have introduced.
A draft form of this memo was circulated for comments. I will
attempt to list the issues raised and summarize the pros and cons,
and resolution for each.
ASCII Commands vs Binary Operation Codes
The ASCII command style is easier to debug, the extra
programming cost in minimal, the extra transmission cost is
trivial.
Binary operation codes are more efficient, and a few days of
debugging should not out weigh years of use.
Resolution: I have kept the ASCII Commands.
Additional Modes
Pro: For some machines you can't send all the bits in a word
using this protocol. There should be some additional mode to
allow it.
Con: Forget it, this is supposed to be SIMPLE file transfer.
If you need those complex modes use real FTP.
Resolution: I have added the Continuous mode.
Simple File Transfer Protocol
CRLF Conversion
Pro: In ASCII type, convert the local end of line indicator to
CRLF on the way out of the host and onto the network.
Con: If you require that you have to look at the bytes as you
send them, otherwise you can just send them. Most of the time
both sides will have the same end of line convention anyway.
If someone needs a conversion it can be done with a TECO macro
separately.
Resolution: I have required CRLF conversion in ASCII type. If
you have the same kind of machines and the same end of line
convention you can avoid the extra cost of conversion by using
the binary or continuous type.
TCP Urgent
Pro: Use TCP Urgent to abort a transfer, instead of aborting
the connection. Then one could retry the file, or try a
different file without having to login again.
Con: That would couple SFTP to TCP too much. SFTP is supposed
to be able to be work over any reliable 8-bit data stream.
Resolution: I have not made use of TCP Urgent.
Random Access
Pro: Wouldn't it be nice if (WIBNIF) SFTP had a way of
accessing parts of a file?
Con: Forget it, this is supposed to be SIMPLE file transfer.
If you need random access use real FTP (oops, real FTP doesn't
have random access either -- invent another protocol?).
Resolution: I have not made any provision for Random Access.
-- jon postel.
AI ELI5
On this page
§1The SFTP Protocol
Simple File Transfer Protocol (SFTP) fills the gap between TFTP and FTP. TFTP is extremely minimal—no user authentication, no directory operations, and only the simplest command set. FTP is powerful but complex to implement. SFTP adds user access control and richer operations compared to TFTP, while staying simpler than FTP.
SFTP can be implemented over any reliable 8-bit byte stream protocol; RFC 913 specifies the TCP version. Unlike FTP, which uses two TCP connections (one for control, one for data transfer via a separate data connection) and TFTP (which uses UDP), SFTP uses only a single TCP connection. Clients connect to port 115 and send commands and receive replies over the same connection.
SFTP is a way to send files that is not as basic as TFTP but not as fancy as FTP.
§2Commands and Responses
SFTP commands are sent as four ASCII letters (case insensitive) followed by a space, arguments, and a null terminator. The simplest form uses just four letters and null when no arguments are needed. The server replies with a response code character (immediately, with no space) followed by an optional ASCII message string and a null terminator.
Response codes include: plus sign for success, minus sign for error, exclamation mark for state changes like successful login, and hash mark followed by decimal digits for numeric responses. An error response starts with the minus sign followed by an explanation message. Commands accepted are USER, ACCT, PASS, TYPE, LIST, CDIR, KILL, NAME, TOBE, DONE, RETR, STOR and SEND.
Commands are four letters like USER or STOR, and the server answers with a success sign for yes or a minus sign for no, plus a short message.
§3Session Flow and Authentication
A session starts when the client connects to port 115. The server sends a greeting (either +hostname SFTP Service for success or –hostname message if it is not accepting connections). If the server sends the error code, it closes the connection. Otherwise, the client sends USER userid. The server may reply with ! (user is already logged in), + (user valid, send account/password), or the error code (invalid user, try again).
The client then sends ACCT account and/or PASS password as needed, receiving replies that may require the other one or confirm login with !. Only after receiving a ! response from the login sequence can the client send other commands: TYPE to set transfer mode, LIST to list directories, CDIR to change directory, KILL to delete files, NAME/TOBE to rename, RETR to retrieve files, STOR to store files, and DONE to finish. This flow ensures user authentication before allowing file operations.
First you connect and say who you are, then you prove your password, and only then can you ask for files or list directories.
§4File Transfer Modes and Operations
The TYPE command sets how file data is encoded. TYPE A (ASCII) converts the local end-of-line convention to CRLF on transmission using NETASCII rules, the same as Telnet. TYPE B (Binary) sends 8-bit bytes as is. TYPE C (Continuous) sends bits continuously without regard to word boundaries, useful for systems like TOPS-20 where the word size is not a multiple of 8 bits. Example: a TOPS-20 page of 512 36-bit words contains 18432 bits or 2304 8-bit bytes in continuous mode.
For RETR file (retrieve), the server replies with the byte count if successful (as ASCII digits) or an error response if the file doesn't exist. The client then sends SEND to receive the file stream or STOP if it cannot accept the data. For STOR (store), the client specifies NEW (create a new generation), OLD (overwrite), or APP (append). The server confirms, the client sends SIZE bytes, then the file data. For LIST, F specifies formatted (file names only) and V specifies verbose (names plus sizes, dates, and other metadata).
You can send files in text or binary mode, and you can list directories, rename files, or delete files, all using simple commands and replies.
§5Example Session
An example session shows the message sequence. When a client connects, the server sends +MIT-XX SFTP Service. The client sends USER MKL. The server replies +MKL ok, send password. The client sends PASS foo. The server replies ! MKL logged in. Now the client can operate. It sends LIST F PS:<MKL> and receives a formatted directory listing. Then LIST V for verbose. Then RETR SMALL.FILE and the server replies with the byte count (69). The client sends SEND, receives the file data (69 bytes, no null terminator), and sends DONE to close the connection.
The design aims for clarity in debugging: commands are ASCII text, not binary codes. The RFC editor notes in the appendix that while binary operation codes would be more efficient, ASCII commands were chosen because the extra programming cost is minimal and the transmission cost is trivial, while the benefit of easier debugging is significant.
The client and server talk back and forth in plain words, not binary numbers, so if something goes wrong it is easy to see what happened.