Because the modprobe command can add or remove extra more than one module, due to module dependencies, we need a method of specifying what options are to be used with those modules. /etc/modprobe.conf (or, if that does not exist, all files under the /etc/modprobe.d directory) specifies those options, as required. It can also be used to create convenient aliases: alternate names for a module. Finally, it can override the normal modprobe behavior altogether, for those with very special requirements (such as inserting more than one module).
Note that module and alias names (like other module names) can have - or _ in them: both are interchangable throughout all the module commands.
The format of modprobe.conf and files under modprobe.d is simple: one command per line, with blank lines and lines starting with # ignored (useful for adding comments). A \ at the end of a line causes it to continue on the next line, which makes the file a bit neater.
The syntax is a simplification of modules.conf, used in 2.4 kernels and earlier.
Note that modules can also contain their own aliases, which you can see using modinfo. These aliases are used as a last resort (ie. if there is no real module, install, remove, or alias command in the configuration).
All options are added together: they can come from an option for the module itself, for an alias, and on the command line.
You can also use install to make up modules which don't otherwise exist. For example: "install probe-ethernet /sbin/modprobe e100 || /sbin/modprobe eepro100", which will try first the e100 driver, then the eepro100 driver, when you do "modprobe probe-ethernet".
If you use the string "$CMDLINE_OPTS" in the command, it will be replaced by any options specified on the modprobe command line. This can be useful because users expect "modprobe fred opt=1" to pass the "opt=1" arg to the module, even if there's an install command in the configuration file. So our above example becomes "install fred /sbin/modprobe barney; /sbin/modprobe --ignore-install fred $CMDLINE_OPTS"
There is a generate_modprobe.conf program which should do a reasonable job of generating modprobe.conf from your current (2.4 or 2.2) modules setup.
Although the syntax is similar to the older /etc/modules.conf, there are many features missing. There are two reasons for this: firstly, install and remove commands can do just about anything, and secondly, the module-init-tools modprobe is designed to be simple enough that it can be easily replaced.
With the complexity of actual module insertion reduced to three system calls (open, read, init_module), and the modules.dep file being simple and open, producing a more powerful modprobe variant can be done independently if there is a need.
This manual page Copyright 2004, Rusty Russell, IBM Corporation.