A simple TFTP Server and Client Implementation in C. Only RRQ, DATA, ACK,
and ERROR packets have been implemented. The WRQ packet is to be
implemented.
The Project comes with a Makefile which can be used to compile everything:
$ make
Compiled src/common.c successfully.
Compiled src/tftp_server.c successfully.
Compiled src/tftp_client.c successfully.
Linking obj/tftp_server.o obj/common.o completed.
Linking obj/tftp_client.o obj/common.o completed.
No clean up is made after compilation. You can manually do it using:
make cleanup
TFTP |--base_dir/ Contains sample files for testing purposes
|--bin/ Contains executable files after linking
|--include/ Contains header files (.h)
|--obj/ Contains object files (.o) after compilation
|--references/ Contains reference PDF files
|--src/ Contains source code files (.c)
|--Makefile Project Makefile
Follows a brief introduction to the TFTP Protocol as introduced by RFC 1350 - THE TFTP PROTOCOL (REVISION 2) . TFTP is a very simple protocol used to transfer files. It is from this that its name comes, Trivial File Transfer Protocol or TFTP.
The protocol was originally designed by Noel Chiappa, and wasredesigned by him, Bob Baldwin and Dave Clark, with comments fromSteve Szymanski.
It has been implementedon top of the Internet User Datagram protocol (UDP or Datagram) so it may be used to move files between machines on different networks implementing UDP. It is designed to be small and easy to implement. Therefore, it lacks most of the features of a regular FTP. The only thing it can do is read and write files (or mail) from/to a remote server. It cannot list directories, and currently has no provisions for user authentication. In common with other Internet protocols, it passes 8 bit bytes of data.
Three modes of transfer are currently supported: netascii (This is ascii as defined in "USA Standard Code for Information Interchange" with the modifications specified in "Telnet Protocol Specification"). Note that it is 8 bit ascii. The term "netascii" will be used throughout this document to mean this particular version of ascii.); octet (This replaces the "binary" mode of previous versions of this document.) raw 8 bit bytes; mail,netascii characters sent to a user rather than a file. (The mail mode is obsolete and should not be implemented or used.) Additional modes can be defined by pairs of cooperating hosts.
Any transfer begins with a request to read or write a file, which also serves to request a connection. If the server grants the request, the connection is opened and the file is sent in fixed length blocks of 512 bytes. Each data packet contains one block of data, and must be acknowledged by an acknowledgment packet before the next packet can be sent. A data packet of less than 512 bytessignals termination of a transfer.
Most errors cause termination of the connection. An error is signalled by sending an error packet. This packet is not acknowledged, and not retransmitted (i.e., a TFTP server or user mayterminate after sending an error message), so the other end of the connection may not get it. Therefore timeouts are used to detect such a termination when the error packet has been lost. Errors are caused by three types of events: not being able to satisfy the request (e.g., file not found, access violation, or no such user), receiving a packet which cannot be explained by a delay or duplication in the network (e.g., an incorrectly formed packet), and losing access to a necessary resource (e.g., disk full or access denied during a transfer).
A transfer is established by sending a request (WRQ to write onto a foreign file system, or RRQ to read from it), and receiving apositive reply, an acknowledgment packet for write, or the first datapacket for read. In general an acknowledgment packet will contain the block number of the data packet being acknowledged. Each data packet has associated with it a block number; block numbers are consecutive and begin with one. Since the positive response to a write request is an acknowledgment packet, in this special case the block number will be zero. (Normally, since an acknowledgment packetis acknowledging a data packet, the acknowledgment packet will contain the block number of the data packet being acknowledged). If the reply is an error packet, then the request has been denied.
TFTP supports five types of packets, all of which have been mentioned above:
opcode operation
1 Read request (RRQ)
2 Write request (WRQ)
3 Data (DATA)
4 Acknowledgment (ACK)
5 Error (ERROR)
The TFTP header of a packet contains the opcode associated with that packet.
RRQ and WRQ packets (opcodes 1 and 2 respectively) have the formats hown below:
2 bytes string 1 byte string 1 byte
+--------+------------+------+------------+------+
| Opcode | Filename | 0 | Mode | 0 |
+--------+------------+------+------------+------+
The file name is a sequence of bytes in netascii terminated by a zero byte. The mode field contains the string "netascii", "octet", or "mail" (or any combination of upperand lower case, such as "NETASCII", "NetAscii", etc.) in netascii indicating the three modes defined in the protocol.
Data is actually transferred in DATA packets
2 bytes 2 bytes n bytes
+--------+------------+------------+
| Opcode | Block # | Data |
+--------+------------+------------+
DATA packets (opcode = 3) have a block number and data field. The block numbers on data packets begin with one and increase by one for each new block of data. This restriction allows the program to use a single number to discriminate between new packets and duplicates. The data field is from zero to 512 bytes long. If it is 512 bytes long, the block is not the last block of data; if it is from zero to 511 bytes long, it signals the end of the transfer.
ACK Packet:
2 bytes 2 bytes
+--------+------------+
| Opcode | Block # |
+--------+------------+
All packets other than duplicate ACK's and those used for termination are acknowledged unless a timeout occurs. Sending a DATA packet is an acknowledgment for the first ACK packet of the previous DATA packet.
ERROR packet:
2 bytes 2 bytes string 1 byte
+--------+------------+------------+------+
| Opcode | ErrorCode | ErrMsg | 0 |
+--------+------------+------------+------+
An ERROR packet can be the acknowledgment of any other type of packet. The error code is an integer indicating the nature of the error. The error message is intended for human consumption, and should be in netascii. Like all other strings, it is terminated witha zero byte.
Error Codes:
Value Meaning
--------------------------------------------------
0 Not defined, see error message (if any).
1 File not found.
2 Access violation.
3 Disk full or allocation exceeded.
4 Illegal TFTP operation.
5 Unknown transfer ID.
6 File already exists.
7 No such user.
--------------------------------------------------