Program 4 • Client/Server and Sockets

Program 4 • Client/Server and Sockets 

1. Overview
This project will implement a client/server application using sockets. A daemon
(cixd) listens on a socket for client connection requests. Each connection will cause
the server to fork a child process to serve the client. The daemon runs in an infinite
loop listening. The server exits when the client disconnects. A client (cix) connects
to a server and can send files, receive files, and get a summary listing of all files
present.
A socket is a two-way means of communication between processes, not necessarily
running on the same host. An IPv4 host is know by a 4-octet sequence such as
128.114.108.152, and a port is an unsigned 16-bit number (0 to 65535). Communication will be done via TCP/IP over IPv4 sockets.
2. Programs
In this project, two main programs are to be written for the three parts of the
project : the daemon and server, and the client. There will also be several library
files written to be used by the programs. The general function is similar to sftp(1).
cixd
Usage : cixd [port]
Creates a server socket and goes into an infinite loop : When it accepts a client
socket, it uses fork(2) to create a child process, which functions as the server to
communicate with the client.
The daemon listens for connections on the given port, if specified. If not, the
environment variable CIX_SERVER_PORT is used to determine the port. There is
no default port.
The server is forked with an open socket communicating with the client. Its
loop repeatedly reads commands and information from the client and acts on
those commands, returning information back to the client. Its loop will be a
receive followed by a send, responding to client requests. It exits when the
client closes the socket. It does no terminal I/O except possibly for debugging
purposes.
cix
Usage : cix [host] [port]
The client interacts with the user. Commands are read from the terminal (or
redirect), each of which is executed one at a time by communicating with the
server. Results are then displayed at the terminal.
The client connects to the given host, if specified. If not specified, the environment variable CIX_SERVER_HOST is used as the host to connect to. If not specified, localhost is used. It tries to connect to the given port, if specified. If not,
the environment variable CIX_SERVER_PORT is used to determine the port.
There is no default port.
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3. Interactive Commands
The cix-client responds to commands read from the standard output and writes
output to the standard output and error and accesses files. In the syntax below,
Courier Bold are literal characters actually typed in, while Roman Italic stands for
appropriate substitutions.
exit
Quit the program. An end of file marker or Control/D is equivalent.
get filename
Copy the file named filename on the remote server and create or overwrite a
file of the same name in the current directory.
help
A summary of available commands is printed.
ls
Causes the remote server to execute the command ls -l and prints the output
to the user’s terminal.
put filename
Copies a local file into the socket and causes the remote server to create that
file in its directory.
rm filename
Causes the remote server to remove the file.
4. Protocol used by the cix* programs
In order for the client and server to communicate,aprotocol needs to be established.
This means that each message needs to be framed in terms of a header and a payload. The header always consists of a struct of size 64 bytes. All messages between
client and server consist of these 64 bytes, possibly followed by a payload. For alignment purposes, the nbytes field is first. Before filling in the fields, use memset(3) to
clear the struct.
enum cix_command {CIX_ERROR = 0, CIX_EXIT,
CIX_GET, CIX_HELP, CIX_LS, CIX_PUT, CIX_RM,
CIX_FILE, CIX_LSOUT, CIX_ACK, CIX_NAK};
struct cix_header {
uint32_t cix_nbytes;
uint8_t cix_command;
char cix_filename[59];
};
The purposes of the fields are as follows :
uint32_t cix_nbytes;
The number of bytes in the payload if there is any payload. Otherwise it must
be zero (MBZ). This field is sent in network byte order and so must use the
functions ntohl(3) and htonl(3) when loading and storing data.
uint8_t cix_command;
A single byte containing one of the cix_command constants.
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char cix_filename[59];
The name of the file being transferred or removed. The filename may not have
any slash (’/’) characters in it and must be null-terminated (with ’\0’). All
bytes following the null must also be null. Pathnames with slashes and filenames longer than 58 characters are prohibited.
Following are the meanings of each of the cix_command values. Each is either client
to server (C→S) or server to client (S→C), but never both.
CIX_ERROR
An error flag to indicate an invalid header. Used internally.
CIX_EXIT
Internal to cix-client, not used in communication.
CIX_GET (C→S)
Request a file from the server. The filename is used both remotely and locally.
The payload length is 0.
CIX_HELP
Internal to cix-client, not used in communication.
CIX_LS (C→S)
Request file (ls) information. The payload length and filename are zeroed.
CIX_PUT (C→S)
The length of the payload is the number of bytes in the file. The contents of
the file immediately follow the header. The bytes of the payload are unstructured and may contain null bytes. Binary files are acceptable.
CIX_RM (C→S)
Request to remove a file. The payload length is 0.
CIX_FILE (S→C)
Response to a CIX_GET. The filename is the same as in the request and the
payload length reflects the number of bytes in the file. The payload consists of
the bytes of the file.
CIX_LSOUT (S→C)
Response to a CIX_LS. The filename is zeroed and the payload length is the
number of bytes sent in the payload. The payload is the output of the command ls -l.
CIX_ACK (S→C)
Response to either a CIX_PUT or a CIX_RM indicating that the request was successfully completed.
CIX_NAK (S→C)
Response to any request that fails. There is no payload. The filename field is
the same as was in the original request. The payload field is set to the value of
errno in the server’s attempt to preform a task.
5. Procedures
Each of the above commands requires procedures for accessing files, including reading files from disk and writing files to disk, as well as accessing directories. When
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any of the system calls fails in the server, the server immediately terminates the
operation and sends the value of errno back to the client in a CIX_NAK message.
(a) For the client or server to send a file it must first be read into a buffer. Binary
files must be properly handled, so protocols which assume text files won’t
work. To load a file from disk, use istream::read(), collecting characters into
a buffer. Read the entire file into a buffer then close it. After that, it may be
sent down the socket. Alternatively, stat(2) the file to see how large it is, and
send the file down the socket piecemeal.
(b) When receiving a file from the socket, Receive the header and determine the
size of the file. Create an ostream and use ostream::write() to write the parts
of the file as they are received from the socket. A C++ stream is closed when
the variable goes out of scope, or you can call close.
(c) To delete a file for the CIX_RM command, use unlink(2) :
rc = unlink (filename);
(d) To execute the CIX_LS command use popen(2) and pclose(2) to create a pipe
stream from the ls(1) command
FILE* pipe = popen (“ls -l”, “r”);
Then read the characters from the pipe in the easiest way, probably by using
fgets(3). Finally, pclose(pipe). Then send the output back the client in a CIX_
LSOUT message.
6. Modules
There will need to be several modules in this suite of programs. Each of the programs, of course, will have its own source file with a main function in it.
The sockets module will be a useful inclusion into the program as its own module.
There should also be a cix_protocol module to implement the protocols and contain
code for accessing files and sockets, since these will be used by both the client and
the server.
7. Use of ports
If your daemon listens on a port that has been bound by another process, you will
get the message ‘‘Address already in use’’ because only one process at any given time
is allowed to listen on a particular process. To avoid this, choose a port number not
being used by anyone else on the same server.
To avoid having to type in a port number every time you start your server, put the
following line in your .bashrc or .bash_profile file :
export CIX_CLIENT_PORT 60000
except that each person in class should use a different number. You should pick a
dynamic or private port number from the range 49152 through 65535 (0xC000
through 0xFFFF).
8. Runaway Processes
Be careful in using fork(2) so that you don’t accidentally create a fork-bomb. The
command pkill(1) can be used to kill all processes matching a particular pattern.
So the command
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pkill cix
will kill all of your processes whose executables contain the string ‘‘cix’’. A really
quick way to log out is to use kill(1) :
kill -9 -1
kill -s KILL -1
will send SIGKILL to all of your processes, thus logging you out.
9. What to Submit
Submit Makefile which builds the three programs, all necessary C++ header and
implementation files. And if doing pair programming, the PARTNER file. When the
grader uses the command make in the submit directory, the three binaries should be
built.