CMSC 417 Programming Assignment #1

Due September 10, 2001 (10:00 AM)


The rest of the projects will require heavy use of the Berkeley UNIX networking facilities (often called The BSD Socket Interface.) The user interface to these facilities is rather complicated and confusing, so you are asked to do a simple exercise to gain familiarity with the interface before you have to start using it in earnest. You will use Internet-domain sockets and UDP (the User Datagram Protocol) for communication between a client and server, possibly running on different machines.

You should use the CSC Alpha cluster (see  for more information).  Your account is:

                name:  «name»

user:  «Account»

                password: «Password»

The Assignment

You will write two programs, a server and a client. The server creates a socket in the Internet domain bound to port SERVER_PORT (a constant you should define in a .h  file included in both programs), receives requests from it, acts on the requests, and returns the results to the requester. For this simple exercise, there are only two kinds of requests: an ADDR request and a FILESIZE request. Both kinds of requests have the same packet structure: a 2-byte (short integer) type field, a 4-byte (long) data field and a variable-length string. To make things easier, you can assume the string will never be longer than 256 bytes, including the string terminator. NOTE: integers (short or long) must be sent in network byte order, see the Hints section).

Ports are a shared resource (meaning that each person must have their own ports).  You should use ports in the range «lowPort» to «highPort» for your assignment.

When the server receives an ADDR request (type field is set to ADDR_REQ), it should construct a response packet and send it back to the requester immediately. The ADDR response packet has the same structure as the ADDR request packet. The type field of the ADDR response should be set to ADDR_ACK and the string should be the Internet Address (in dotted-decimal format) of the host whose name was specified in the string field for the ADDR_REQ packet. The 4-byte data field should be set to zero. Some hosts may have more than one address. In such cases, just return any one of them.

To satisfy the FILESIZE request (type field is set to FILESIZE_REQ), the server will again construct a response packet and send it back to the requester immediately. The type field of the FILESIZE response should be set to FILESIZE_ACK, the 4-byte data field should be the size of the file whose name was specified in the string field for the FILESIZE_REQ packet and the string field should contain the name of the file. If the file doesn't exist, return -1 for the size and the string “file not found” for the filename.

You should find a convenient way to define the necessary packet types.

Remember that integers must be sent in network order.

The client program takes three command-line arguments: the name of the host where the server is running, the request type (addr or filesize) and a string. In the ADDR request case, the string is the name of the host (e.g., for which the internet address is to be found. In the FILESIZE request case, it is the name of the file for which you would like the size. The requests should have the packet structure described above. The client sends the request to the server, waits for a reply, and prints the result. If it is an ADDR response, you should simply print the string echoed. For the FILESIZE response packet, print the file name and size (or an error if the file doesn't exist). For both requests, the 4-byte data field is zero.

The server must be able to serve multiple clients. Therefore, it cannot have the client's address wired in.


You should submit (via the submit program) the programs and a transcript (using script(1)) of them action. Your transcript should demonstrate that your client and server programs handle properly formed requests; also demonstrate that the client is able to deal with incorrect arguments, such as the name of a non-existent host or file. For the address request, use nslookup to verify the correctness of your output. The programs should be clearly written and well-commented. You will be building on these programs for subsequent stages of the term project, so it is in your own best interest to make them maintainable.


These programs will not be long (probably about 100-200 lines each including comments), but they may be difficult to write, and they will certainly be difficult to debug. First read by Lawrence Besaw very carefully. This document will have some information that you will not need to know for this assignment (e.g. UNIX domain sockets and the TCP protocol). Pay particular attention to the program example at the end. Note that this example uses TCP sockets. You must use UDP sockets, so you cannot copy the example exactly. In particular, you should not use the system calls connect, listen, or accept. Figure out which system calls will be needed by each of your programs and read the corresponding man pages.

You should use gcc as your compiler, rather than cc. Gcc supports prototypes and other ANSI extensions, which you should use in your programs. The option -Wall turns on most of the useful gcc warnings. Compiling with this option will find many simple mistakes in your programs. You should try to get your programs to compile with as few warnings as possible. Part of your grade will be based on how warning-free your program compiles. You may also add options -ansi. This will make the compiler reject non-ANSI programs. You should also link the network and socket libraries to your program. You should use the make program to compile your program.  Make(1) is program that reads a file (Makefile) that describes how to compile your program.

In your client program, use gethostbyname(3) to translate the host name to an internet address, and in the server program use gethostbyname(3) again to obtain the internet address. If the host has multiple Internet addresses, gethostbyname will return all of them. You should return only one of them. Also, gethostbyname() does not return the addresses in dotted-decimal form. You should use inet_ntoa(3) to convert the address returned by gethostbyname() to a dotted-decimal form. Be careful about the difference between host byte order and network byte order; be sure you know the meaning of htonl(3) and friends (htons(3), ntohl(3), and ntohs(3)) and use them in the appropriate places. The routines described in inet(3) are very helpful for debugging output. For handling the FILESIZE request, you could use the stat(2) system call to get the filesize.

You should be aware that the packet structure requires that an integer is 4 bytes long. For some architectures, this is not true. Keep this in mind, but your programs are not required to support this.

Use netstat(1) to see what sockets have been created by your programs (you may have put them in the background; once a program terminates, the sockets it created go away).

Clean up! Use ps(1) to see what processes you have left lying around, and use kill(1) to get rid of them.