The Selectors function are encoded as a facility a period (``.''), and a level with no intervening white-space. Both the facility and the level are case insensitive.
The facility describes the part of the system generating the message, and is one of the following keywords: auth, authpriv, cron, daemon, kern, lpr, mail, mark, news, syslog, user, uucp and local0 through local7. These keywords (with the exception of mark) correspond to the similar ``LOG_ '' values specified to the openlog(3) and syslog(3) library routines.
The level describes the severity of the message, and is a keyword from the following ordered list (higher to lower): emerg, alert, crit, err, warning, notice, info and debug. These keywords correspond to the similar (LOG_ ) values specified to the syslog library routine.
Each block of lines is separated from the previous block by a tag. The tag is a line beginning with #!prog or !prog (the former is for compatibility with the previous syslogd, if one is sharing syslog.conf files, for example) and each block will be associated with calls to syslog from that specific program.
See syslog(3) for a further descriptions of both the facility and level keywords and their significance. It's preferred that selections be made on facility rather than program since the latter can easily vary in a networked environment. In some cases, though, an appropriate facility simply doesn't exist (for example, ftpd logs under LOG_DAEMON along with a myriad other programs).
If a received message matches the specified facility and is of the specified level (or a higher level) and the first word in the message after the date matches the program the action specified in the action field will be taken.
Multiple selectors may be specified for a single action by separating them with semicolon (``;'') characters. It is important to note, however, that each selector can modify the ones preceding it.
Multiple facilities may be specified for a single level by separating them with comma (``,'') characters.
An asterisk (``*'') can be used to specify all facilities all levels or all programs
The special facility ``mark'' receives a message at priority ``info'' every 20 minutes (see syslogd(8)). This is not enabled by a facility field containing an asterisk.
The special level ``none'' disables a particular facility
The action field of each line specifies the action to be taken when the selector field selects a message. There are five forms:
The command will only be started once data arrives that should be piped to it. If it exited later, it will be restarted as necessary. So if it is desired that the subprocess should get exactly one line of input only (which can be very resource-consuming if there are a lot of messages flowing quickly), this can be achieved by exiting after just one line of input. If necessary, a script wrapper can be written to this effect.
Unless the command is a full pipeline, it's probably useful to start the command with exec so that the invoking shell process does not wait for the command to complete. Warning: the process is started under the UID invoking syslogd(8), normally the superuser.
Blank lines and lines whose first non-blank character is a hash (``#'') character are ignored.
A configuration file might appear as follows:
# Log all kernel messages, authentication messages of # level notice or higher and anything of level err or # higher to the console. # Don't log private authentication messages! *.err;kern.*;auth.notice;authpriv.none /dev/console # Log anything (except mail) of level info or higher. # Don't log private authentication messages! *.info;mail.none;authpriv.none /var/log/messages # The authpriv file has restricted access. authpriv.* /var/log/secure # Log all the mail messages in one place. mail.* /var/log/maillog # Everybody gets emergency messages, plus log them on another # machine. *.emerg * *.emerg @arpa.berkeley.edu # Root and Eric get alert and higher messages. *.alert root,eric # Save mail and news errors of level err and higher in a # special file. uucp,news.crit /var/log/spoolerr # Pipe all authentication messages to a filter. auth.* |exec /usr/local/sbin/authfilter # Save ftpd transactions along with mail and news !ftpd *.* /var/log/spoolerr