Advanced Linux Networking

Most Linux distributions start the majority of their standard servers through either SysV startup scripts or a super server. One notable exception to this rule is the X server, which is often started through a line near the end of /etc/inittab , which launches the X server only in a particular runlevel. Slackware also does things a bit differently, using /etc/rc.d/rc.inet2 to start most of its servers. Most distributions provide a "local" startup script, which is intended to be modified by the system administrator to handle starting unusual system-specific servers, running initialization utilities, and so on. Table 4.1 lists the local startup scripts for several common Linux distributions.

The usual reason to use a local startup script is that you're adding a server that you don't want to run via a super server and that doesn't come with an appropriate SysV startup script. Because SysV startup scripts are closely tied to specific distributions, you might not have an appropriate SysV startup script if you obtained a server from a source other than your own distribution. For instance, if you're running Mandrake but install a server intended for SuSE, the SuSE SysV startup script may not work on your Mandrake system. You may also run into this problem if you obtain the server in source code form from the server's author; an original source such as this is unlikely to include customizations for specific Linux distributions. The code may compile and run just fine, but you'll need to start the server yourself.

Of course, it's possible to write your own SysV startup scripts for such servers. You can do this by modifying a working SysV startup script. You might try modifying the startup script for an equivalent tool (such as an earlier version of the server you're installing, if you've resorted to a third-party source because your distribution's official tool is out of date), or some randomly selected startup script. This process can be tricky, though, particularly if you're not familiar with your distribution's startup script format or with shell scripting in general. (SysV startup scripts are written in the bash shell scripting language.) If you run into problems, or if you're in a hurry and don't want to put forth the effort to create or modify a SysV startup script, you can start it in the local startup script.

You can modify the local startup script using your favorite text editor. To start a server, simply include lines in the file that are the same as the commands you'd type at a command prompt to launch the program. For instance, the following line starts a Telnet server:

/usr/sbin/in.telnetd

If the server doesn't start up in daemon mode by default (that is, if it doesn't run in the background, relinquishing control of your shell if you launch it directly), you should use an ampersand ( & ) at the end of the command line to tell the server to run in the background. Failure to do this will cause the execution of the startup script to halt at the call to the server. This may be acceptable if it's the last line in the script, but if you want to start additional servers, the subsequent servers won't start if you omit the ampersand.

You may, of course, do something more complex than launching a server using a single line. You could use the bash shell's conditional expressions to test that the server file exists, or launch it only under certain circumstances. These are the sorts of tasks that are usually performed in SysV startup scripts, though, so if you want to go to that sort of effort, you might prefer writing your own SysV startup script.

One important point to keep in mind is that different distributions' local startup scripts aren't exactly equivalent to one another. For instance, SuSE runs its boot.local script earlier in the boot process than Red Hat runs its rc.local . Therefore, SuSE's local startup script is more appropriate for bringing up interfaces or doing other early startup tasks, whereas Red Hat's script is better for launching servers that rely on an already-up network connection. If the tasks you want to perform in the startup script are very dependent upon the presence or absence of other servers, you may be forced to create a SysV startup script with a sequence number that's appropriate for the tasks you want to perform.

The usual reason for using a local startup script is to create a quick-and-dirty method of launching a server or running some other program. Once launched, the local startup script provides no easy way to shut down the server (as does the stop parameter to most SysV startup scripts); you'll have to use kill , killall , or a similar tool to stop the server, if you need to do so.