sysklogd/man/syslog.conf.5

.\" syslog.conf - syslogd(8) configuration file                  -*- nroff -*-
.\" Copyright (c) 1995-2009  Martin Schulze <joey@infodrom.org>
.\" 
.\" This file is part of the sysklogd package, a kernel and system log daemon.
.\" 
.\" This program is free software; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published by
.\" the Free Software Foundation; either version 2 of the License, or
.\" (at your option) any later version.
.\" 
.\" This program is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\" 
.\" You should have received a copy of the GNU General Public License
.\" along with this program; if not, write to the Free Software
.\" Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111, USA.
.\"
.TH SYSLOG.CONF 5 "12 October 2019" "Version 2.0" "Linux System Administration"
.SH NAME
syslog.conf \- syslogd(8) configuration file
.SH DESCRIPTION
The
.I syslog.conf
file is the main configuration file for
.BR syslogd (8)
which logs system messages on *nix systems.  This file specifies rules
for logging.  For special features see the
.BR sysklogd (8)
manpage.

Every rule consists of two fields, a
.I selector
field an
.I action
and an optional
.I logrotate
field.  The 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.  The action details where or what to
do with the selected input.  The optional logrotate field is only for
files and details the max SIZE:COUNT a file can reach before it is
rotated, and later compressed.  The log rotated feature is mostly
intended for embedded systems that do not want to have cron and a
separate log rotate daemon.

Lines starting with a hash mark (``#'') and empty lines are ignored.

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

The
.I facility
is one of the following keywords:
.BR auth ", " authpriv ", " cron ", " daemon ", " ftp ", " kern ", " lpr ", "
.BR mail ", " mark ", " news ", " security " (same as " auth "), "
.BR syslog ", " user ", " uucp " and " local0 " through " local7 .
The keyword 
.B security
is deprecated and
.B mark
is only for internal use and therefore should not be used in
applications.  The
.I facility
specifies the subsystem that produced the message, e.g. all mail
programs log with the mail facility
.RB ( 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
.I priority
is one of the following keywords, in ascending order: 
.BR debug ", " info ", " notice ", " warning ", " warn " (same as "
.BR warning "), " err ", " error " (same as " err "), " crit ", "
.BR alert ", " emerg ", " panic " (same as " emerg ).
The keywords
.BR warn ", " error " and " panic
are deprecated and should not be used anymore.  The
.I 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
.BR syslogd (8)
behaves the same, but has some extensions.

In addition to the above mentioned names the
.BR 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
.B 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
.I 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 
.BR 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
.B 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
.B syslogd
to ignore this priority and all higher priorities.
You may even use both, the exclamation mark and the equation sign if
you want
.B syslogd
to ignore only this single priority.  If you use both extensions
than the exclamation mark must occur before the equation sign, just
use it intuitively.
.SH ACTIONS
The action field of a rule describes the abstract term
``logfile''.  A ``logfile'' need not to be a real file, btw.  The
.BR syslogd (8)
provides the following actions.

.SS 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.

.SS Named Pipes
This version of
.BR 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  
.BR mkfifo (1)
command  before
.BR syslogd (8)
is started.

.SS Terminal and Console
If the file you specified is a tty, special tty-handling is done, same
with
.IR /dev/console .

.SS Remote Machine
This 
.BR syslogd (8)
provides full remote logging, i.e. is able to send messages to a
remote host running 
.BR 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.

By default messages to remote remote hosts are formatted in the original
BSD style.  To enable new RFC5424 style formatting, append ``;RFC5424``
after the hostname.

.SS 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.

.SS 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
.IR wall (1)-feature
use an asterisk (``*'').
.SH EXAMPLES
Here are some examples, partially taken from a real existing site and
configuration.  Hopefully they answer all questions about
configuring this
.BR syslogd (8) .
If not, don't hesitate to contact the mailing list.
.IP
.nf
# Store critical stuff in critical
#
*.=crit;kern.none            /var/adm/critical
.fi
.LP
This will store all messages of priority
.B crit
in the file
.IR /var/adm/critical ,
with the exception of any kernel messages.

.IP
.nf
# 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;RFC5424
kern.crit                    /dev/console
kern.info;kern.!err          /var/adm/kernel-info
.fi
.LP
The first rule directs any message that has the kernel facility to the
file
.IR /var/adm/kernel .
(But recall that only the kernel itself can log to this facility.)

The second statement directs all kernel messages of priority
.B crit
and higher to the remote host finlandia in RFC5424 style formatting.
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
.BR info " up to " warning
in the file
.IR /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
.BR info
and higher.  The second selector filters out kernel messages of
priority
.BR error
and higher.  This leaves just priorities
.BR info ", " notice " and " warning
to get logged.

.IP
.nf
# The tcp wrapper logs with mail.info, we display
# all the connections on tty12
#
mail.=info                   /dev/tty12
.fi
.LP
This directs all messages that use 
.BR mail.info " (in source " LOG_MAIL " | " LOG_INFO )
to
.IR /dev/tty12 , 
the 12th console.  For example the tcpwrapper
.BR tcpd (8)
uses this as its default.

.IP
.nf
# Write all mail related logs to a file
#
mail.*;mail.!=info           /var/adm/mail
.fi
.LP
This pattern matches all messages that come with the
.B mail
facility, except for the
.B info
priority.  These will be stored in the file
.IR /var/adm/mail .

.IP
.nf
# Log all mail.info and news.info messages to info
#
mail,news.=info              /var/adm/info
.fi
.LP
This will extract all messages that come either with
.BR mail.info " or with " news.info 
and store them in the file
.IR /var/adm/info .

.IP
.nf
# Log info and notice messages to messages file
#
*.=info;*.=notice;\\
	mail.none  /var/log/messages
.fi
.LP
The following is almost the same but will also log rotate and compress
aged out messages.  The size argument takes the same modifiers as the
command line '-b' option.  Notice the leading '-' to ensure the file is
flushed to disk after each message.

.IP
.nf
# Log all messages, including kernel, to messages file
# rotated every 100 kB and keep up to 10 aged out and
# compressed files.
*.*;kern,kern.none   -/log/messages   100k:10

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

.IP
.nf
# Log info messages to messages file
#
*.=info;\\
	mail,news.none       /var/log/messages
.fi
.LP
This statement causes the
.B syslogd
to log all messages that come with the
.B info
priority to the file
.IR /var/log/messages .
But any message coming either with the
.BR mail " or the " news
facility will not be stored.

.IP
.nf
# Emergency messages will be displayed using wall
#
*.=emerg                     *
.fi
.LP
This rule tells the
.B syslogd
to write all emergency messages to all currently logged in users.  This
is the wall action.

.IP
.nf
# Messages of the priority alert will be directed
# to the operator
#
*.alert                      root,joey
.fi
.LP
This rule directs all messages of priority
.B alert
or higher to the terminals of the operator, i.e. of the users ``root''
and ``joey'' if they're logged in.

.IP
.nf
*.*                          @finlandia;RFC5424
.fi
.LP
This rule would redirect all messages to a remote host called finlandia
with RFC5424 style formatting.  This is useful especially in a cluster
of machines where all syslog messages will be stored on only one
machine.
.SH CONFIGURATION FILE SYNTAX DIFFERENCES
.B 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
.B 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.
.SH FILES
.PD 0
.TP
.I /etc/syslog.conf
Configuration file for
.B syslogd
.SH 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 isn't 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.

.SH SEE ALSO
.BR sysklogd (8),
.BR klogd (8), 
.BR logger (1),
.BR syslog (2),
.BR syslog (3).
.SH AUTHORS
The
.B syslogd
is taken from BSD sources, Greg Wettstein <greg@wind.enjellic.com>
performed the port to Linux, Martin Schulze <joey@infodrom.org>
fixed some bugs, added several new features and took over maintenance.