Introduction
Sockets are interfaces that can "plug into" each other over a network. Once so "plugged in", the programs so connected communicate.This article discusses only simple aspects of stream inet sockets (don't worry about exactly what that is right now). For the purposes of this article, a "server" program is exposed via a socket connected to a certain /etc/services port number. A "client" program can then connect its own socket to the server's socket, at which time the client program's writes to the socket are read as stdin to the server program, and stdout from the server program are read from the client's socket reads. This is one subset of socket programming, but it's perhaps the easiest to master, so this is where you should start. |
Note that the client communicates by reading and writing the socket, but the server program communicates via stdin and stdout. |
This tutorial requires a Linux box. It hasn't been tested on other types of UNIX, but I think it might work. This tutorial is centered around a system using xinetd, but it would be simple enough to adapt it to older inetd systems. This tutorial will not work under Windows. I think it's important that this complex type of programming be learned on the most reliable, straightforward system possible, so Windows is out.
For the purposes of this tutorial, the server application will be at port 3333. Note that you can implement both the client and the server on a single computer, in which case the client is connected to a port on the computer containing both the client and the server. So if you have only one Linux box you can still do this tutorial.
Acknowledgments
This material was told to me by LEAP-CF members Mark Alexander and Nickolai Zeldovich. Other LEAPsters provided additional information.A big shout out to all my buddies at Linux Enthusiasts and Professionals of Central Florida (LEAP-CF). With mailing list and meetings, they (we) form a 70 brain parallel supercomputer.And of course, thanks to the help and support of Troubleshooters.Com's visitors.
Hello World
Our first socket implementation uses a simple write-only shellscript as a server application, and telnet as the client. This is the simplest possible implementation. I've tested this on a mandrake box, and I'd imagine it would work on any Linux box using xinetd. If your box still uses the older inetd system, the only difference is that you modify inetd.conf instead of a file in /etc/xinetd.d, and you restart inetd instead of xinetd. The following steps are involved:- Create the server application (a simple shellscript)
- Test the server application at the command line
- Decide on a port number and service name
- Declare the port and name in /etc/services
- Create a file for this service in /etc/xinetd.d
- Restart xinetd
- Telnet into the service, and see the server app's output
- Review what you learned
Create the server application (a simple shellscript)
We want the simplest possible script for this Hello World, so here it is:| #!/bin/bash /bin/echo -n "Hello World:" | /usr/bin/tee /tmp/log.log /bin/date | /usr/bin/tee -a /tmp/log.log |
Note several things. The top line, showing the program to read the script, is ABSOLUTELY ESSENTIAL. Although most environments run shellscripts without that top line, those same scripts will not output when called through sockets as certain users (user nobody, for instance). For a similar reason, make sure every executable is declared with a complete path, and is not a link. With a user like user nobody, you don't know what the path will look like. Similarly, the log file is completely specified so you know where to find it. If you don't specifiy it, it goes in the home directory of the user hooked to the service by xinetd. Better to specify it so you know exactly where to find it. /tmp is an excellent place, because on most systems it has universal read and write permissions.
This script writes a single line containing "Hello World:" followed by a timestamp, both to stdout and to a log file called /tmp/log.log. The log file will become essential in socket debugging, because when the program runs under a socket the program has no terminal.
For security reasons, I recommend you implement the client and server on the same computer, and that you temporarily disconnect that computer from your network. The last thing you want is an under-development service exposed to your LAN, and maybe the entire Internet.
Be sure to set the script as executable by all. Remember, you don't know what user will be executing it. Also, be sure the entire directory tree housing the script is executable by all, or else other users won't be able to traverse the tree to execute the file.
Test the server application at the command line
Run the program, and it should output the hello world phrase followed by a timestamp. When you look at /tmp/log.log it should contain the same information. See the session below:| [myuid@mydesk myuid]$ ./hello.sh Hello World:Tue Jan 23 12:27:08 EST 2001 [myuid@mydesk myuid]$ cat /tmp/log.log Hello World:Tue Jan 23 12:27:08 EST 2001 [myuid@mydesk myuid]$ |
Decide on a port number and service name
First, the port number needs to be unused. The port number you choose should not exist in /etc/services. Furthermore, for security reasons the port number should not be below 1024. For this exercise I use port 3333. You may wish to use a different port so someone knowing you've taken this tutorial doesn't know of an open port on your system. But in this tutorial I'll use 3333.The first thing to do is make sure the port number is not contained in the /etc/services file. Do that as follows:| [myuid@mydesk myuid]$ cat /etc/services | grep 3333 [myuid@mydesk myuid]$ |
If there's no output, there's no such port number. If there's output, investigate thoroughly, and you'll probably want to select a different number.
You also need to check for the existence of an xinetd declared service with a declared port number matching the new one. Do that with a directory wide grep:
| [myuid@mydesk myuid]$ grep -r 3333 /etc/xinetd.d/* [myuid@mydesk myuid]$ |
Once again, no output means no such port is declared. If there is output, investigate, and change your new port number if necessary.
As far as the service name, we're going to call the service "hello". Here's how we check for the existence of a current service called "hello":
| [myuid@mydesk myuid]$ /sbin/chkconfig -- list | grep -i hello [myuid@mydesk myuid]$ |
Once again, if there's no output, there's no such service yet, so you're free to use that service name.
Once you've decided on a port number and a service name (3333 and hello in this tutorial), continue on...
Declare the port and name in /etc/services
Add the following line to the /etc/services file:| hello 3333/tcp # hello tutorial, delete when finished |
Beware: The xinetd man page says an xinetd invoked service needn't be declared in /etc/services. That does not match my experience. I was unable to make this work without declaring it in /etc/services, even using the xinetd port= parameter. Make your life easy. Declare it in /etc/services for the purposes of this tutorial. Then, if you wish, go ahead and find a way to remove it from /etc/services while retaining the service.
Create a file for this service in /etc/xinetd.d
As user root, create the following file in /etc/xinetd.d:| # default: on # description: Hello World socket server service hello { port = 3333 socket_type = stream wait = no user = nobody server = /home/slitt/hello.sh log_on_success += USERID log_on_failure += USERID disable = no } |
| [root@mydesk xinetd.d]# ls -ldF hello -rw-r--r-- 1 root root 303 Jan 23 13:14 hello [root@mydesk xinetd.d]# |
Once you've saved the file properly, you're ready to "turn on" the server by restarting xinetd...
Restart xinetd
According to the xinetd man page there are many ways to "reconfigure" xinetd. In the xinetd man page search for SIGUSR2 and you'll find some of the less intrusive methods. But in my experience, there's no substitute for a known state. I prefer a complete restart. Here's how it's done on a Mandrake box:| [root@mydesk xinetd.d]# /etc/rc.d/init.d/xinetd restart Stopping xinetd: [ OK ] Starting xinetd: [ OK ] [root@mydesk xinetd.d]# |
Telnet into the service, and see the server app's output
Run the following command:telnet 192.168.100.2 3333The IP address is the address of the server containing the hello world server. The 3333 is the port number. Here's one example of what might happen:
| [myuid@mydesk myuid]$ telnet 192.168.100.2 3333 Trying 192.168.100.2... telnet: Unable to connect to remote host: Connection refused [myuid@mydesk myuid]$ |
- Make sure the service and name are listed in /etc/services.
- In /etc/xinetd.d/hello, make sure:
- The port= line's value matches the port number in /etc/services.
- The service name (just before the squiggly braces) is correct.
- If there's a disable=, its value must be no.
- If there's a default: line, its value must be on.
- The user= line names a valid user.
- The server= line must point to the script to be run (hello.sh). It must be fully pathed.
/etc/rc.d/init.d/xinetd restartIf none of the above helps, try TEMPORARILY changing the user= line to root. That should fix any permission problems. But make sure that before you change it to root, you disconnect from the Internet. Otherwise the script kiddies will be on you like squirrels on a tree.If none of the above helps, use the Universal Troubleshooting Process to diagnose the problem. Try to find ways to narrow the problems, and ways to divide and conquer.
Once you actually get the connection to work, you may be confronted with a no-output run, as shown below:
| [myuid@mydesk myuid]$ telnet 192.168.100.2 3333 Trying 192.168.100.2... Connected to 192.168.100.2. Escape character is '^]'. Connection closed by foreign host. [myuid@mydesk myuid]$ |
In the preceding, the connection was made, but no output was recieved. There's probably something wrong with the script itself. Most likely the top line declaring the script's interpreter (/bin/bash in this example) is missing or wrong. It could also be a pathing problem with the "run service as" user, which is why it's so important to fully path all files in the script. Note that the log file will not have been rewritten, so if one exists it has an old timestamp (this is why we put the timestamp in the server script).
Verify that the script's name is the same as named in /etc/xinetd.d/hello, that it's executable for all, that its directory is executable all the way up the path, and that it contains the #!/bin/bash line at the top, and that there really is a /bin/bash, and that it's a command interpreter. Verify that all files in the script are fully pathed. Beyond that, troubleshoot.
Once you get it running, you might get output like the following:
| [myuid@mydesk myuid]$ telnet 192.168.100.2 3333 Trying 192.168.100.2... Connected to 192.168.100.2. Escape character is '^]'. /usr/bin/tee: /tmp/log.log: Permission denied Hello World:/usr/bin/tee: /tmp/log.log: Permission denied Tue Jan 23 13:26:44 EST 2001 Connection closed by foreign host. [myuid@mydesk myuid]$ |
You can tell you're close. You're connecting, and you're getting output obviously originating from your script. But you have a problem creating /tmp/log.log. That's not surprising, because in a previous step you created it as user myuid, and user nobody hasn't the permissions to delete the old one. As root or another user permissioned to delete /tmp/log.log, delete the old one manually and you should clear the problem. Once all problems have been cleared, the telnet session should look as follows:
| [myuid@mydesk myuid]$ telnet 192.168.100.2 3333 Trying 192.168.100.2... Connected to 192.168.100.2. Escape character is '^]'. Hello World:Tue Jan 23 13:33:55 EST 2001 Connection closed by foreign host. [myuid@mydesk myuid]$ |
If you obtain the preceding output, congratulations -- you've just implemented a Hello World sockets setup. So let's review...
Review What You've Learned
Sockets are a software methodology to connect different processes (programs) on the same computer or on different computers. The name "socket" reminds us that once we "plug in" one process into another process's socket, they can talk to each other by reading and writing the socket.Sockets are a huge and diverse subject. This tutorial deals with the subset in which one of the two processes is a "server" by virtue of its configuration into xinetd (or inetd if you're running an older UNIX or Linux distro). In such a scenario, the "server" process, which in our example is a simple shellscript, is associated with a port number and a process name via a config file in the /etc/xinetd.d directory. In our case, the file is /etc/xinetd.d/hello. Additionally, the port number is associated with the service name and the tcp protocol via an entry in /etc/services. Once those associations are made, restarting xinetd makes the script available as a service at the specified port. Pictorially, it looks something like this: |
Note that the client communicates by reading and writing the socket, but the server program communicates via stdin and stdout. |
telnet 192.168.100.2 3333The server script's output then appears in the telnet program's output.This has been a simple implementation indeed. You didn't code the client, but instead used telnet. And communication goes only one way. It's output from the server app and read in telnet. Nevertheless, you're over the biggest hurdle.The next step is two way socket communication. Read on...
A Bidirectional Implementation with Telnet Client
Now let's make a bidirectional program. We'll simply attach a new script, called hellobidirectional.sh, to the hello service. First, let's make the hellobidirectional.sh script:| #!/bin/bash logfile="/tmp/log.log" firstname="initialization" lastname="initialization" /bin/echo "LOG: $(/bin/date)" > $logfile /bin/echo >> $logfile while test 1; do ###Forever, see the break in the case statement /bin/echo >> $logfile /bin/echo "Begin Cycle: $(/bin/date) **********" >> $logfile /bin/echo /bin/echo "Please type Linux celebs name below:" | /usr/bin/tee -a $logfile read firstname ##### NEXT STATEMENT REMOVES EXTRA CTRL+M PUT IN BY TELNET ##### ##### PROGRAMMER: MAKE SURE THE CHAR IS CTRL+M, NOT A CARAT AND AN M! ##### firstname=$(/bin/echo ${firstname}|/bin/sed -e 's/^M$//') /bin/echo ":$firstname, " | /usr/bin/tee -a $logfile case "$firstname" in (richard) lastname="stallman";; (linus) lastname="torvalds";; (maddog) lastname="hall";; (q*) lastname="quit"; break;; (*) lastname="Unknown celeb";; esac /bin/echo "$lastname" | /usr/bin/tee -a $logfile done /bin/echo | /usr/bin/tee -a $logfile /bin/echo "Finished" | /usr/bin/tee -a $logfile |
This script repeatedly queries for a first name, and for three Linux celebrities it returns their last name. For other people it returns the string "Unknown celeb", and for any first name beginning with q it terminates, as you can see in the case statement.
Make the script executable by all, and once again make sure its diirectory is executable all the way up the tree. Now run it from the command line. The results should look something like the following, keeping in mind that the bolded text is what you typed in:
| [myuid@mydesk myuid]$ ./hellobidirectional.sh Please type Linux celebs name below: richard :richard, stallman Please type Linux celebs name below: linus :linus, torvalds Please type Linux celebs name below: q :q, Finished [myuid@mydesk myuid]$ |
Now you know you have a working script. The next step is to attach this script to the hello service. Simply edit /etc/xinetd.d/hello, and replace the following line:
server = /home/slitt/hello.shWith the following line:
server = /home/slitt/hellobidirectional.shNow restart xinetd with the following command:
/etc/rc.d/init.d/xinetd restartAnd finally, use telnet as the client. The following shows the telnet session with text you type bolded:
| [myuid@mydesk myuid]$ telnet 192.168.100.2 3333 Trying 192.168.100.2... Connected to 192.168.100.2. Escape character is '^]'. Please type Linux celebs name below: richard :richard, stallman Please type Linux celebs name below: maddog :maddog, hall Please type Linux celebs name below: q :q, Finished Connection closed by foreign host. [myuid@mydesk myuid]$ |
Cygwin for xinetd service on windows
http://www.cygwin.com/
http://sourceware.org/cygwinports/
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