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fsp/doc/PROTOCOL
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FILE SERVICE PROTOCOL VERSION 2
Document version 0.11
Last updated 24 Oct 2004
Also known as
File Slurping Protocol,
Flaky Stream Protocol,
FTP's Sexier Partner,
File Sharing Protocol or
Fucking Slow Protocol.
`FSP is what anonymous FTP *should* be'
This document was created by Radim Kolar, because there is no RFC for
FSP. It was planed, but never comes out. Based on sources of FSP 2.8.1 Beta 13.
This document is not copyrighted and is placed into public domain.
Data formats used in this document
byte - unsigned 1 byte integer 0-255
word - unsigned 2 byte integer 0-65535
long - unsigned 4 byte integer 0-4294967295
bits - counted from right to left. bit 0 is the lowest.
NULL - byte 00
ASCIIZ - ASCII string terminated with one NULL (the same as used in C
language)
numbers are stored in network byte order (high byte first).
File or directory names uses '/' as directory separator, they do not
need to start or end with it. There are in ASCIIZ format.
FSP servers starting from version 2.8 can have optional password
protection. To get password protected file, append '\n' followed
by password to filename.
Numbers starting with 0x are in hexadecimal (base 16) notation.
Transport
FSP uses UDP datagrams as transport medium. Minimum UDP packet size (not
including size of UDP, IP and link layer headers) is 12 bytes, maximum UDP
packet size is 1024+12 bytes.
Security
Design
FSP protocol was not designed to transfer secret data. It was designed as
alternative protocol for providing lightweight access to collection of public
files. FSP has the same user level of security as the anonymous FTP file
server. FSP has better network level security, because it was designed
to ressist various denial-of-service attacks. FSP protocol v3 will be
designed to be fully secure. FSP3 will not be backward compatible with
FSP2.
Passwords
Recently, password support was added to protocol, because just
too many people wants it and there were hacked version of FSP with
password support floating around. Passwords are transmited in clear
text over network which makes them a weak protection for determined
intruder.
FSP Packet format:
HEADER - size = Fixed size 12 bytes. Always present.
DATA - size = defined in header (DATA_LENGTH)
XTRA DATA - size = packet_size - header_size (12) - DATA_LENGTH
Maximal data size DATA_LENGTH + XTRA_DATA length is 1024. Clients and servers
are not required to support XTRA DATA (but in current FSP implementation does).
If XTRA DATA are provided, there must be also contained in MESSAGE_CHECKSUM.
HEADER FORMAT (12 bytes)
byte FSP_COMMAND
byte MESSAGE_CHECKSUM
word KEY
word SEQUENCE
word DATA_LENGTH
long FILE_POSITION
MESSAGE_CHECKSUM
Entire packet (HEADER + DATA + XTRA DATA) is checksumed. When computing a
checksum use zero in place of MESSAGE_CHECKSUM header field.
Due to some unknown reason, method of computing checksums is different in each
direction. For packets travelling from server to client initial checksum
value is zero, otherwise it is HEADER + DATA + XTRA DATA size.
Checksums in server->client direction are computed as follows:
/* assume that we have already zeroed checksum in packet */
unsigned int sum,checksum;
for(t = packet_start, sum = 0; t < packet_end; sum += *t++);
checksum= sum + (sum >> 8);
KEY
Client's message to server contain a KEY value that is the same as the KEY
value of the previous message received from the server. KEY is choosen random
by server.
TIMEOUTS
Resends:
Server will accept resent message from client with old KEY after 3 seconds.
Client MUST wait at least 1 second before resending a message.
It is recommended to use initial delay of 1.34 second and after each
unsuccessfull resend multiply delay time by 1.5. Maximum recommended delay
time is 300 seconds.
Session:
Server will accept message with bad key after 60 seconds. Clients should
sent CC_BYE at end of their session, CC_BYE terminates a session.
After session is terminated, sever will accept any next key.
SEQUENCE
Similarly, the server's message to client contains a SEQUENCE value
that is the same as the SEQUENCE value of the previous message from the client.
Client can choose any SEQUENCE number and can use it for detection of lost
packets (increase sequence number on message resend).
DATA_LENGTH
Size of DATA field in packet. Packet can also contain XTRA DATA field but
size of this field is not included in header and must be computed from
received packet size.
FILE POSITION
When transfering files, this field shows current position of requested
data.
FSP COMMANDS
============
REQUIRED COMMANDS
FSP File servers MUST supports following commands:
- sending error messages back to client with CC_ERR
- directory listings CC_GET_DIR
- file transfer CC_GET_FILE
- file status CC_STAT
- information about directory flags CC_GET_PRO
- terminate session CC_BYE
CC_VERSION 0x10 - Get server version string and setup
request
file position: ignored
data: not used
xtra data: not used
reply
file position: size of optional extra version data
data: ASCIIZ Server version string
xtra data: optional extra version data
byte - FLAGS
bit 0 set - server does logging
bit 1 set - server is read only
bit 2 set - reverse lookup required
bit 3 set - server is in private mode
bit 4 set - thruput control
if bit 4 is set thruput info follows
long - max_thruput allowed (in bytes/sec)
word - max. packet size supported by server
Compatibility
Max. packet size supported is reported only by fspd 2.8.1 b20
or newer.
CC_ERR 0x40 - error response from server
If you want to get a error from server, send
any unknown client command (for example CC_ERR).
CC_ERRs are normally sent only by server on
errors conditions.
request (not used)
file position: not used
data: not used
xtra data: not used
reply
file position: not used
data: ASCIIZ Error string
xtra data: not used
CC_GET_DIR 0x41 - get a directory listing
request
file position: position in directory
data: ASCIIZ directory name
xtra data: (not required)
word - prefered size of directory block
reply
file position: same as in request
data: directory listing (format follows)
xtra data: not used
Directory listing is transfered in similar way as file transfer. Directory
listing is divided into blocks of equal size, only exception is last block
which can be shorter. Default and maximum size of directory listing block is
1024 bytes. Size of directory block can be changed by server using extra
data information sent by client.
Directory blocks can't be split accross message boundary and client
can't do seeking to any arbitary offset, which can broke dirblock
into 2 messages. In short: Every message can contain only one
unsplited directory block.
RDIRENT is the structure of a directory entry contained in a directory listing.
Each entry contains a HEADER, which has 4 bytes quantity 'time' in Unix
standard format, a 4 bytes quantity 'size', and 1 byte of 'type'. Header is
followed by ASCIIZ encoded 'name'. RDIRENT is followed by enough number of
padding to fill to an 4-byte boundary.
At this point, if the next RDIRENT entry to follow will spread across 1k
boundary, then two possible things will happen:
1) if the HEADER fits between this entry and the 1k boundary,
a complete header will be filled in with a 'type' set to RDTYPE_SKIP
and no name followed - just pad to 1k boundary. Clients which sees
RDTYPE_SKIP header skips over next data in packet.
2) if the HEADER does not fit, then simply pad to the 1k boundary.
This will make sure that messages carrying directory information carry only
complete directory entries and no fragmented entries.
The last entry is type RDTYPE_END.
struct RDIRENT {
struct HEADER {
long time;
long size;
byte type;
}
ASCIIZ name;
}
RDIRENT.HEADER types:
RDTYPE_END 0x00
RDTYPE_FILE 0x01
RDTYPE_DIR 0x02
RDTYPE_SKIP 0x2A
CC_GET_FILE 0x42 - get a file
request
file position: offset in file
data: ASCIIZ filename
xtra data: (not required)
word - size of reply's optional data block
reply
file position: same as in request
data: binary file data
xtra data: not used
CC_UP_LOAD 0x43 - open a file for writing
request
file position: offset in file
data: binary file data
xtra data: not used
reply
file position: same as in request
data: not used
xtra data: not used
CC_INSTALL 0x44 - close and install file opened for writing
request
file position: not used
data: ASCIIZ filename
xtra data: (not required)
long - timestamp in Unix format
reply
file position: not used
data: not used
xtra data: not used
CC_DEL_FILE 0x45 - delete a file
request
file position: not used
data: ASCIIZ filename
xtra data: not used
reply
file position: not used
data: not used
xtra data: not used
CC_DEL_DIR 0x46 - delete a directory
request
file position: not used
data: ASCIIZ directory
xtra data: not used
reply
file position: not used
data: not used
xtra data: not used
CC_GET_PRO 0x47 - get directory protection
request
file position: not used
data: ASCIIZ directory
xtra data: (not required)
word - size of reply's optional data block
reply
file position: number of extra protection bytes (now 1)
data: ASCIIZ directory readme
xtra data: extra protection data (format follows)
Protection bits:
0 - caller owns the directory
1 - files can be deleted from this dir
2 - files can be added to this dir
3 - new subdirectories can be created
4 - files are NOT readable by non-owners
5 - directory contain an readme file
6 - directory can be listed
7 - files can be renamed in this directory
Compatibility
Versions older than 2.8.1b6 do not uses bits 6 and 7. This
causes that directory can be listable even it do not have
6th bit set.
CC_SET_PRO 0x48 - set directory protection
request
file position: not used
data: ASCIIZ directory
xtra data: 2 bytes of protection change command
1st byte:
<'+'|'-'> set or remove protection
2nd byte:
<'c'|'d'|'g'|'m'|'l'|'r'>
c public can create files
d public can delete files
g public can get files
m public can create directories here
l public can list directory
r public can rename files
reply
same as CC_GET_PRO
Compatibility
FSP versions older than 2.8.1 beta15 used p flag instead
g flag. +p = -g
CC_MAKE_DIR 0x49 - create a directory
request
file position: not used
data: ASCIIZ directory name
xtra data: not used
reply
same as CC_GET_PRO
CC_BYE 0x4A - finish a session
request
file position: not used
data: not used
xtra data: not used
reply
file position: not used
optional data: not used
xtra data: not used
You should send this packet when you are done with
talking to server. This causes that server will
accept next packet from your IP with any key.
Commands starting from FSP version 2.4 ( released March 27, 1992 )
CC_GRAB_FILE 0x4B - atomic get+delete a file
same format as CC_GET_FILE, but file is deleted after
sucessfull transfer is done. If there are multiple
grabs for the same file, only one will succeed.
CC_GRAB_DONE 0x4C - atomic get+delete a file done
same format as CC_INSTALL. File is not
installed, but deleted.
Commands starting from FSP 2.8.1 Beta 11
CC_STAT 0x4D - get information about file/directory
request
file position: not used
data: ASCIIZ directory or file name
xtra data: not used
reply
file position: not used
data: file stat info (format follows)
xtra data: not used
data format is the same as in directory listing with exception
that there is no file name appended. If file do not exists or
there is other problem (no access rights) return type of file is
0.
struct STAT {
long time;
long size;
byte type;
}
Compatibility
CC_ERR message is NEVER returned as reply to CC_STAT command
by server supporting CC_STAT command. If you have got CC_ERR reply,
you are talking to old server, which do not supports this
command.
CC_RENAME 0x4E - rename file or directory
request
file position: not used
data: ASCIIZ source file or directory
ASCIIZ destination file or directory
xtra data: not used
Note: It is possible to do cross-directory rename. In this
case you must have rights to DELETE in source directory and
to CREATE in target directory.
reply
file position: not used
data: not used
xtra data: not used
CC_CH_PASSW 0x4F - change password
not yet specified
Reserved commands:
CC_LIMIT 0x80 - commands > 0x7F will have extended
header. No such extensions or commands
which uses that are known today. This
header will be used in protocol version 3.
CC_TEST 0x81 - reserved for testing of new header
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