genserv (a generic server program) : SuperFrink.Net

genserv : A Generic Network Server Program

by Chad Clark

The Beginning

Supose you have a program written in a language where you don't want to learn how to deal with network sockets or maybe a language that doesn't support sockets directly such as Bash. What if you want to make this program available on a network? Well you could write a web based front end. I chose to write genserv (from Generic or General Server).

The Idea

There is a C program that listens for a network connect.
When a connection is made a child process is forked to deal with the new connection.
The child process sets the standard file descriptors (STDIN, STDOUT and STDERR) to all point to the network file descriptor.
The child then executes the application we want to connect to the network.
The application runs and deals with all I/O on the three standard file descriptors as normal.

This works because of the way Linux handles network connections as files.
The application doesn't need to even know the network connection exists.

The Code

The code is all in one C file.
There is also a makefile but it's really simple and is only used because I like make.
Just put both files in one directory and type make.

Using genserv

From the program's usage description:

usage:
         ./genserv port_num program_name args
where:
  "port_num" is a valid TCP port number.
  "program_name" is a program to be run when a client connects.
  "args" is an optional list of arguments to the program to be executed.

eg: ./genserv 12345 /usr/bin/uptime
    will listen on port 12345 and print the uptime string then exit.

eg: ./genserv 12345 /usr/bin/uptime -V
    will listen on port 12345 and print the uptime version then exit.

In the future I want to add some sort of basic authentication scheme.
Most likely just a password that is set when genserv is started.
Currently there is no authentication build into genserv so any programs
which get run from genserv need to either do their own authentications
or be open to the network.

It may also be usefull/interesting to read the Secure Programming for Linux and Unix HOWTO
if you have to write programs which other people will be running.

Testing

All of my work was done a stock Slackware Linux 8.1 machine.

To test the program run ./genserv 12321 ls -l .
From another TTY telnet to port 12321 (eg telnet 127.0.0.1 12321 ).
You should see something like:

 
[frink@thepurplebuffalo ~/code/cxx/genserv]$telnet 127.0.0.1 12321
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
total 32
drwx------    2 frink    users        4096 May 17 18:00 CVS
-rw-r--r--    1 frink    users          94 May 17 21:46 Makefile
-rwx------    1 frink    users       16693 May 17 21:46 genserv
-rw-r--r--    1 frink    users        3887 May 17 21:46 genserv.c
-rw-------    1 frink    users           0 May 17 21:47 typescript
Connection closed by foreign host.
[frink@thepurplebuffalo ~/code/cxx/genserv]$

Known Bugs

ioctl() causes an issue on "who am i" but not on "who".
I belive this is because STDIN is mapped to a network file descriptor and not to a tty.

In particular ioctl sets errno for "Request or argp is not valid." who then exits with status zero.
```
        ioctl(0, SNDCTL_TMR_TIMEBASE, 0xbffff598) = -1 EINVAL (Invalid argument)
        _exit(0) 
        
```