rsyslog

The rocket-fast system for log processing

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sysklogd format

This is the format in use since the beginning of syslogging. It still is an excellent choice to do very simple things.

For more advanced things, use the advanced format.

DESCRIPTION

The syslog.conf file is the main configuration file for syslogd(8) which logs system messages on *nix systems. This file specifies rules for logging. For special features see the sysklogd(8) manpage.

Every rule consists of two fields, a selector field and an action field. These two fields are separated by one or more spaces or tabs. The selector field specifies a pattern of facilities and priorities belonging to the specified action.

Lines starting with a hash mark (“#”) and empty lines are ignored.

This variant of syslogd is able to understand a slightly extended syntax compared to the original BSD syslogd. One rule may be divided into several lines if the leading line is terminated with an backslash (“").

SELECTORS

The selector field consists of two parts, a facility and a priority, separated by a period (“.”). Both parts are case insensitive and can also be specified as decimal numbers corresponding to the definitions in /usr/include/syslog.h. It is safer to use symbolic names rather than decimal numbers. Both facilities and priorities are described in syslog(3). The names mentioned below correspond to the similar LOG_ values in /usr/include/syslog.h.

The facility is one of the following keywords: auth, authpriv, cron, daemon, ftp, kern, lpr, mail, mark, news, security (same as auth), syslog, user, uucp and local0 through local7. The keyword security is deprecated and mark is only for internal use and therefore should not be used in applications. The facility specifies the subsystem that produced the message, e.g. all mail programs log with the mail facility (LOG_MAIL) if they log using syslog.

In most cases anyone can log to any facility, so we rely on convention for the correct facility to be chosen. However, generally only the kernel can log to the “kern” facility. This is because the implementation of openlog() and syslog() in glibc does not allow logging to the “kern” facility. Klogd circumvents this restriction when logging to syslogd by reimplementing those functions itself.

The priority is one of the following keywords, in ascending order: debug, info, notice, warning, warn (same as warning), err, error (same as err), crit, alert, emerg, panic (same as emerg). The keywords warn, error and panic are deprecated and should not be used anymore. The priority defines the severity of the message.

The behavior of the original BSD syslogd is that all messages of the specified priority and higher are logged according to the given action. This syslogd(8) behaves the same, but has some extensions.

In addition to the above mentioned names the syslogd(8) understands the following extensions: An asterisk (“*”) stands for all facilities or all priorities, depending on where it is used (before or after the period). The keyword none stands for no priority of the given facility.

Multiple facilities may be specified for a single priority pattern in one statement using the comma (“,”) operator to separate the facilities. You may specify as many facilities as you want. Please note that only the facility part from such a statement is taken, a priority part would be ignored.

Multiple selectors may be specified for a single action using the semicolon (“;”) separator. Selectors are processed from left to right, with each selector being able to overwrite preceding ones. Using this behavior you are able to exclude some priorities from the pattern.

This syslogd(8) has a syntax extension to the original BSD source, which makes its use more intuitive. You may precede every priority with an equation sign (“=”) to specify that syslogd should only refer to this single priority and not this priority and all higher priorities.

You may also precide the priority with an exclamation mark (“!”) if you want syslogd to ignore this priority and all higher priorities. You may even use both, the exclamation mark and the equation sign if you want syslogd to ignore only this single priority. If you use both extensions then the exclamation mark must occur before the equation sign, just use it intuitively.

ACTIONS

The action field of a rule describes the abstract term “logfile”. A “logfile” need not to be a real file, btw. The syslogd(8) provides the following actions.

Regular File

Typically messages are logged to real files. The filename is specified with an absolute pathname.

You may prefix each entry with a minus sign (“-“) to avoid syncing the file after each log message. Note that you might lose information if the system crashes right after a write attempt. Nevertheless this might give you back some performance, especially if you run programs that use logging in a very verbose manner.

Named Pipes

This version of syslogd(8) has support for logging output to named pipes (fifos). A fifo or named pipe can be used as a destination for log messages by prepending a pipe symbol (“|”) to the name of the file. This is handy for debugging. Note that the fifo must be created with the mkfifo(1) command before syslogd(8) is started.

Terminal and Console

If the file you specified is a tty, special tty-handling is done, same with /dev/console.

Remote Machine

This syslogd(8) provides full remote logging, i.e. is able to send messages to a remote host running syslogd(8) and to receive messages from remote hosts. The remote host won’t forward the message again, it will just log them locally. To forward messages to another host, prepend the hostname with the at sign (“@”).

Using this feature you are able to collect all syslog messages on a central host, if all other machines log remotely to that one. This reduces administration needs.

Using a named pipe log method, messages from remote hosts can be sent to a log program. By reading log messages line by line such a program is able to sort log messages by host name or program name on the central log host. This way it is possible to split the log into separate files.

List of Users

Usually critical messages are also directed to “root” on that machine. You can specify a list of users that ought to receive the log message on the terminal by writing their usernames. You may specify more than one user by separating the usernames with commas (“,”). If they’re logged in they will receive the log messages.

Everyone logged on

Emergency messages often go to all users currently online to notify them that something strange is happening with the system. To specify this wall(1)-feature use an asterisk (“*”).

EXAMPLES

Here are some examples, partially taken from a real existing site and configuration. Hopefully they answer all questions about configuring this syslogd(8). If not, don’t hesitate to contact the mailing list.

# Store critical stuff in critical
#
\*.=crit;kern.none   /var/adm/critical

This will store all messages of priority crit in the file /var/adm/critical, with the exception of any kernel messages.

# Kernel messages are stored in the kernel file,
# critical messages and higher ones also go
# to another host and to the console
#
kern.\*      /var/adm/kernel
kern.crit     @finlandia
kern.crit     /dev/console
kern.info;kern.!err   /var/adm/kernel-info

The first rule directs any message that has the kernel facility to the file /var/adm/kernel. (But recall that only the kernel itself can log to this facility.)

The second statement directs all kernel messages of priority crit and higher to the remote host finlandia. This is useful, because if the host crashes and the disks get irreparable errors you might not be able to read the stored messages. If they’re on a remote host, too, you still can try to find out the reason for the crash.

The third rule directs kernel messages of priority crit and higher to the actual console, so the person who works on the machine will get them, too.

The fourth line tells the syslogd to save all kernel messages that come with priorities from info up to warning in the file /var/adm/kernel-info.

This is an example of the 2nd selector overwriting part of the first one. The first selector selects kernel messages of priority info and higher. The second selector filters out kernel messages of priority error and higher. This leaves just priorities info, notice and warning to get logged.

# The tcp wrapper logs with mail.info, we display
# all the connections on tty12
#
mail.=info     /dev/tty12

This directs all messages that use mail.info (in source LOG_MAIL | LOG_INFO) to /dev/tty12, the 12th console. For example the tcpwrapper tcpd(8) uses this as its default.

# Write all mail related logs to a file
#
mail.*;mail.!=info   /var/adm/mail

This pattern matches all messages that come with the mail facility, except for the info priority. These will be stored in the file /var/adm/mail.

# Log all mail.info and news.info messages to info
#
mail,news.=info    /var/adm/info

This will extract all messages that come either with mail.info or with news.info and store them in the file /var/adm/info.

# Log info and notice messages to messages file
#
*.=info;*.=notice;\
mail.none /var/log/messages

This lets the syslogd log all messages that come with either the info or the notice priority into the file /var/log/messages, except for all messages that use the mail facility.

# Log info messages to messages file
#
*.=info;\
mail,news.none  /var/log/messages

This statement causes the syslogd to log all messages that come with the info priority to the file /var/log/messages. But any message coming either with the mail or the news facility will not be stored.

# Emergency messages will be displayed using wall
#
*.=emerg      *

This rule tells the syslogd to write all emergency messages to all currently logged in users. This is the wall action.

# Messages of the priority alert will be directed
# to the operator
#
*.alert      root,joey

This rule directs all messages of priority alert or higher to the terminals of the operator, i.e. of the users “root” and “joey” if they’re logged in.

*.*       @finlandia

This rule would redirect all messages to a remote host called finlandia. This is useful especially in a cluster of machines where all syslog messages will be stored on only one machine.

CONFIGURATION FILE SYNTAX DIFFERENCES

Syslogd uses a slightly different syntax for its configuration file than the original BSD sources. Originally all messages of a specific priority and above were forwarded to the log file. The modifiers “=”, “!” and “-” were added to make the syslogd more flexible and to use it in a more intuitive manner.

The original BSD syslogd doesn’t understand spaces as separators between the selector and the action field.

BUGS

The effects of multiple selectors are sometimes not intuitive. For example “mail.crit,*.err” will select “mail” facility messages at the level of “err” or higher, not at the level of “crit” or higher.

Also, if you specify a selector with an exclamation mark in it which is not preceded by a corresponding selector without an exclamation mark, nothing will be logged. Intuitively, the selector “ftp.!alert” on its own will select all ftp messages with priorities less than alert. In fact it selects nothing. Similarly “ftp.!=alert” might reasonably be expected to select all ftp messages other than those with priority alert, but again it selects nothing. It seems the selectors with exclamation marks in them should only be used as “filters” following selectors without exclamation marks.

Finally, using a backslash to divide a line into two doesn’t work if the backslash is used immediately after the end of the selector, without intermediate whitespace.

See also

Help with configuring/using Rsyslog:

  • Mailing list - best route for general questions
  • GitHub: rsyslog source project - detailed questions, reporting issues that are believed to be bugs with Rsyslog
  • Stack Exchange (View, Ask) - experimental support from rsyslog community

See also

Contributing to Rsyslog:

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