Before pppd parses its command-line arguments, it scans several files for default options. These files may contain any valid command-line arguments spread out across an arbitrary number of lines. Hash signs introduce comments.
The first options file is /etc/ppp/options, which is always scanned when pppd starts up. Using it to set some global defaults is a good idea, because it allows you to keep your users from doing several things that may compromise security. For instance, to make pppd require some kind of authentication (either PAP or CHAP) from the peer, you add the auth option to this file. This option cannot be overridden by the user, so it becomes impossible to establish a PPP connection with any system that is not in your authentication databases. Note, however, that some options can be overridden; the connect string is a good example.
The other options file, which is read after /etc/ppp/options, is .ppprc in the user's home directory. It allows each user to specify her own set of default options.
A sample /etc/ppp/options file might look like this:
# Global options for pppd running on vlager.vbrew.com lock # use UUCP-style device locking auth # require authentication usehostname # use local hostname for CHAP domain vbrew.com # our domain name
The lock keyword makes pppd comply to the standard UUCP method of device locking. With this convention, each process that accesses a serial device, say /dev/ttyS3, creates a lock file with a name like LCK..ttyS3 in a special lock-file directory to signal that the device is in use. This is necessary to prevent signal other programs, such as minicom or uucico, from opening the serial device while it is used by PPP.
The next three options relate to authentication and, therefore, to system security. The authentication options are best placed in the global configuration file because they are “privileged” and cannot be overridden by users' ~/.ppprc options files.