sysklogd/man/syslog.conf.5

.\" syslog.conf - syslogd(8) configuration file                  -*- nroff -*-
.\" Copyright (c) 1995-2009  Martin Schulze <joey@infodrom.org>
.\" Copyright (c) 2018-2019  Joachim Nilsson <troglobit@gmail.com>
.\" 
.\" This file is part of the sysklogd package, a kernel and system log daemon.
.\" 
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.\"
.Dd Oct 30, 2019
.Dt syslog.conf 5
.Os "sysklogd (2.0)"
.Sh NAME
.Nm syslog.conf
.Nd configuration file for syslogd
.Sh DESCRIPTION
The
.Nm
file is the main configuration file for
.Xr syslogd 8
which logs system messages on UNIX like systems.  This file specifies
rules for logging.  For special features see the
.Xr syslogd 8
man page.
.Pp
Every rule has at least two fields, a
.Em selector
field and an
.Em action .
They may also have an
.Em option
field for a setting that applies only to that rule.  Fields are
separated by one or more spaces or tabs.  A rule may be divided into
several lines if the leading line ends with a single backslash ('\\')
character.
.Bd -literal -offset indent
RULE     := SELECTOR  ACTION  [;OPTION]
SELECTOR := [SELECTOR;]facility[,facility].[!=]severity
ACTION   := /path/to/file
         |= |/path/to/named/pipe
	 |= @remote[.host.tld]
OPTION   := [OPTION,]
	 |= RFC3164
	 |= RFC5424
         |= rotate=SIZE:COUNT
.Ed
.Pp
The
.Em selector
field specifies a pattern of facilities and priorities belonging to the
specified action.  The
.Em action
details where or what to do with the selected input.  The
.Em option
field currently supports log formattaing and log rotation.  The default
log format is the traditional RFC3164 (included here for completeness),
RFC5424 has a new format with RFC3339 time stamps, msgid, structured
data, and more.  The log rotation, which is only relevant for files,
details the max SIZE:COUNT a file can reach before it is rotated, and
later compressed.  This feature is mostly intended for embedded systems
that do not want to have cron or a separate log rotate daemon.
.Pp
Comments, lines starting with a hash mark ('#'), and empty lines are
ignored.  If an error occurs during parsing the whole line is ignored.
The configuration file can also include other files.  The example
.Pa /etc/syslog.conf
has the following at the end:
.Bd -literal -offset indent
#
# Drop your subsystem .conf file in /etc/syslog.d/
#
include /etc/syslog.d/*.conf
.Ed
.Sh SELECTORS
The selector field consists of two parts, a
.Em facility
and a 
.Em priority ,
separated by a period ('.').  Both parts are case insensitive and can
also be specified as decimal numbers corresponding to the definitions in
.Pa /usr/include/syslog.h .
It is safer to use symbolic names rather than decimal numbers.  Both
facilities and priorities are described in
.Xr syslog 3 .
The names mentioned below correspond to the similar 
.Ql LOG_FOO
values in
.Pa /usr/include/syslog.h .
.Pp
The
.Em facility
is one of the following keywords:
.Bl -column "Code" "Facility" "Description" -offset indent
.It Sy "Code" Ta Sy "Facility" Ta Sy "Description"
.It        0  Ta         kern  Ta Kernel log messages
.It        1  Ta         user  Ta User-level messages
.It        2  Ta         mail  Ta Mail system
.It        3  Ta       daemon  Ta General system daemons
.It        4  Ta         auth  Ta Security/authorization messages
.It        5  Ta       syslog  Ta Messages generated by syslogd
.It        6  Ta          lpr  Ta Line printer subystem
.It        7  Ta         news  Ta Network news subsystem
.It        8  Ta         uucp  Ta UNIX-to-UNIX copy
.It        9  Ta         cron  Ta Clock/cron daemon (BSD, Linux)
.It       10  Ta     authpriv  Ta Security/authorization messages (private)
.It       11  Ta          ftp  Ta FTP daemon
.It       12  Ta          ntp  Ta NTP subsystem
.It       13  Ta     security  Ta Log audit
.It       14  Ta      console  Ta Log alert
.It       15  Ta       unused  Ta Clock/cron daemon (Solaris)
.It       16  Ta       local0  Ta Reserved for local/system use
.It       17  Ta       local1  Ta Reserved for local/system use
.It       18  Ta       local2  Ta Reserved for local/system use
.It       19  Ta       local3  Ta Reserved for local/system use
.It       20  Ta       local4  Ta Reserved for local/system use
.It       21  Ta       local5  Ta Reserved for local/system use
.It       22  Ta       local6  Ta Reserved for local/system use
.It       23  Ta       local7  Ta Reserved for local/system use
.El
.Pp
Notice, several of the above listed facilities are not supported
by the standard C library (GLIBC, musl libc, or uClibc) on Linux.
The
.Lb libsyslog
shipped with
.Nm sysklogd ,
however, supports all the above facilities in full.  Also, the keyword
.Ql mark
is only for internal use and should therefore not be used in
applications.  The
.Em facility
specifies the subsystem that produced the message, e.g. all mail
programs log with the mail facility,
.Ql LOG_MAIL ,
if they log using syslog.
.Pp
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
.Ql kern
facility.  This because the implementation of
.Xr openlog 3
and
.Xr syslog 3
in GLIBC does not allow logging to the
.Ql kern
facility.
.Xr klogd 8
circumvents this restriction when logging to
.Xr syslogd 8
by using the
.Lb libsyslog
.Pp
The
.I priority
is one of the following keywords, in ascending order:
.Bl -column "Code" "Facility" "Description" -offset indent
.It Sy "Value" Ta Sy "Severity" Ta Sy "Description"
.It         0  Ta    emergency  Ta System is unusable
.It         1  Ta        alert  Ta Action must be taken immediately
.It         2  Ta     critical  Ta Critical condtions
.It         3  Ta        error  Ta Error conditions
.It         4  Ta      warning  Ta Warning conditions
.It         5  Ta       notice  Ta Normal but significal conditions
.It         6  Ta         info  Ta Informational messages
.It         7  Ta        debug  Ta Debug-level messages
.El
.Pp
The default log level of most applications is
.Ql notice ,
meaning only
.Ql notice
and above are forwarded to
.Nm syslogd .
See
.Xr setlogmask 3
for more information on how to change the default log level of your
application.
.Pp
In addition to the above mentioned facility and priority names,
.Xr syslogd 8
understands the following extensions:
.Pp
.Bl -tag -compact -width "'none'"
.It *
An asterisk ('*') matches all facilities or all priorities, depending on
where it is used (before or after the period).
.It none
The keyword
.Ql none
stands for no priority of the given facility.
.It ,
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.
.It ;
Multiple selectors may be specified for a single
.Em 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.
.It =
This version of
.Xr 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 only this single priority should be matched,
instead of the default: this priority and all higher priorities.
.It !
You may also precide the priority with an exclamation mark ('!') if you
want to ignore this priority and all higher priorities.  You may even
use both the exclamation mark and the equation sign if you want to
ignore a single priority.  If both extensions are used, the exclamation
mark must occur before the equation sign.
.El
.Sh ACTIONS
The action field of a rule is the destination or target for a match.  It
can be a file, a UNIX named pipe, the console, or a remote machine.
.Ss Regular File
Typically messages are logged to real files.  The filename is specified
with an absolute pathname.
.Pp
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
.Xr syslogd 8
supports logging 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 can be very handy for debugging.  Note
that the FIFO must be created with the
.Xr mkfifo 1
command before
.Nm syslogd
is started.
.Ss Terminal and Console
If the file you specified is a tty, special tty-handling is done, same
with
.Pa /dev/console .
.Ss Remote Machine
Full remote logging support is available in
.Nm syslogd ,
i.e. to send messages to a remote syslog server, and and to receive
messages from remote hosts.  To forward messages to another host,
prepend the hostname with the at sign ('@').
.Pp
This feature makes it possible to collect all syslog messages in a
network on a central host.  This reduces administration needs and
can be really helpful when debugging distributed systems.
.Pp
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.
.Pp
By default messages to remote remote hosts were formatted in the original
BSD style, without timestamp or hostname.  As of
.Nm syslogd
v2.0 the default includes timstamp and hostname.  It is also possible to
enable the new RFC5424 style formatting, append ';RFC5424' after the
hostname.
.Ss List of Users
Usually critical messages are also directed to
.Ql root
on that machine.  You can specify a list of users that ought to receive
the log message on their terminal by writing their usernames.  You may
specify more than one user by separating the usernames with commas
(',').  Only logged in users 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
.Xr wall 1
feature use an asterisk ('*').
.Sh EXAMPLES
This section lists some examples, partially from actual site setups.
.Ss Catch Everything
This example matches all facilities and priorities and stores everything
in the file
.Pa /var/log/syslog
in RFC5424 format.  Every time the file reaches 10 MiB it is rotated and
five files in total are kept, including the non-rotated file.
.Bd -literal -offset indent
# Match all log messages, store in RC5424 format and rotate every 10 MiB
#
*.*                          /var/log/critical    ;rotate=10M:5,RFC5424
.Ed
.Ss Critical
This stores all messages of priority
.Ql crit
in the file
.Pa /var/log/critical ,
with the exception of any kernel messages.
.Bd -literal -offset indent
# Store critical stuff in critical
#
*.=crit;kern.none            /var/log/critical
.Ed
.Ss Kernel
This is an example of the 2nd selector overwriting part of the first
one.  The first selector selects kernel messages of priority
.Ql info
and higher.  The second selector filters out kernel messages of priority
.Ql error
and higher.  This leaves just priorities
.Ql info ,
.Ql notice ,
and
.Ql warning
to get logged.
.Bd -literal -offset indent
# Kernel messages are stored in the kernel file, critical messages and
# higher ones also go to another host and to the console
#
kern.*                       /var/log/kernel
kern.crit                    @finlandia           ;RFC5424
kern.crit                    /dev/console
kern.info;kern.!err          /var/log/kernel.info
.Ed
.Pp
The first rule directs any message that has the kernel facility to the
file
.Pa /var/log/kernel .
Recall that only the kernel itself can log to this facility.
.Pp
The second statement directs all kernel messages of priority
.Ql crit
and higher to the remote host
.Ql 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.
.Pp
The third rule directs kernel messages of priority
.Ql crit
and higher to the actual console, so the person who works on the machine
will get them, too.
.Pp
The fourth line tells
.Nm syslogd
to save all kernel messages that come with priorities from
.Ql info
up to
.Ql warning
in the file
.Pa /var/log/kernel.info .
.Ss Redirecting to a TTY
This directs all messages that use 
.Ql mail.info
(in source
.Ql LOG_MAIL | LOG_INFO )
to
.IR /dev/tty12 , 
the 12th console.  For example the tcpwrapper
.BR tcpd (8)
uses this as its default.
.Bd -literal -offset indent
# The tcp wrapper logs with mail.info, we display
# all the connections on tty12
#
mail.=info                   /dev/tty12
.Ed
.Ss Redirecting to a file
This pattern matches all messages that come with the
.Ql mail
facility, except for the
.Ql info
priority.  These will be stored in the file
.Pa /var/log/mail .
.Bd -literal -offset indent
# Write all mail related logs to a file
#
mail.*;mail.!=info           /var/log/mail
.Ed
.Ss Single Priority from Two Facilities
This will extract all messages that come either with
.Ql mail.info
or with
.Ql news.info
and store them in the file
.Pa /var/log/info .
.Bd -literal -offset indent
# Log all mail.info and news.info messages to info
#
mail,news.=info              /var/log/info
.Ed
.Ss Advanced Filtering, part 1
This logs all messages that come with either the
.Ql info
or the
.Ql notice
priority into the file
.Pa /var/log/messages ,
except for all messages that use the
.Ql mail
facility.
.Bd -literal -offset indent
# Log info and notice messages to messages file
#
*.=info;*.=notice;\\
	mail.none            /var/log/messages
.Ed
.Ss Advanced Filtering, part 2
This statement logs all messages that come with the
.Ql info
priority to the file
.Pa /var/log/messages .
But any message with either
.Ql mail
or the
.Ql news
facility  are not logged.
.Bd -literal -offset indent
# Log info messages to messages file
#
*.=info;\\
	mail,news.none       /var/log/messages
.Ed
.Ss Wall Messages
This rule tells
.Nm syslogd
to write all emergency messages to all currently logged in users.  This
is the wall action.
.Bd -literal -offset indent
# Emergency messages will be displayed using wall
#
*.=emerg                     *
.Ed
.Ss Alerting Users
This rule directs all messages of priority
.Ql alert
or higher to the terminals of the operator, i.e. of the users 'root'
and 'joey', if they're logged in.
.Bd -literal -offset indent
# Messages of the priority alert will be directed
# to the operator
#
*.alert                      root,joey
.Ed
.Ss Log Rotation
This example logs all messages except kernel messages to the file
.Pa /var/log/messages
without syncing ('-') the file after each log message.  When the file
reaches 100 kiB it is rotated.  In total are only 10 rotated files,
including the main file itself and compressed files kept.  The size
argument takes the same modifiers as the
.Xr syslogd 8
command line option,
.Fl R .
.Bd -literal -offset indent
# Log all messages, including kernel, to messages file rotated
# every 100 kB and keep up to 10 aged out and compressed files
#
*.*;kern.none               -/var/log/messages    ;rotate=100k:10
.Ed
.Ss Logging to Remote Syslog Server
This rule redirects all messages to a remote host called
.Ql 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.
.Bd -literal -offset indent
*.*                          @finlandia           ;RFC5424
.Ed
.Sh FILES
.Bl -tag -compact -width /etc/syslog.d/*.conf
.It /etc/syslog.conf
Main configuration file for
.Xr syslogd 8
.It /etc/syslog.d/*.conf
Recommended directory for .conf snippets (per subsystem)
.El
.Sh BUGS
The effects of multiple selectors are sometimes not intuitive.  For
example
.Ql mail.crit,*.err
will select
.Ql mail
facility messages at the level of
.Ql err
or higher, not at the level of
.Ql crit or higher.
.Pp
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
.Ql ftp.!alert
on its own will select all
.Ql ftp
messages with priorities less than
.Ql alert .
In fact it selects nothing.  Similarly,
.Ql ftp.!=alert
might reasonably be expected to select all
.Ql ftp
messages other than those with priority
.Ql 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.
.Pp
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
.Xr mkfifo 1 ,
.Xr sysklogd 8 ,
.Xr klogd 8 , 
.Xr logger 1 ,
.Xr syslog 2 ,
.Xr syslog 3 .
.Sh AUTHORS
The system log daemon
.Nm syslogd
is originally taken from BSD sources and later updated with new
funcitonality from
.Fx
and
.Nx .
.An -nosplit
.An Greg Wettstein Aq Mt greg@wind.enjellic.com
performed the initial port to Linux.
.An Martin Schulze Aq Mt joey@infodrom.org
fixed some bugs, added several new features and took over maintenance.
.An Joachim Nilsson Aq Mt troglobit@gmail.com
later picked up the aging
.Nm sysklogd
and gave it a home at GitHub with new features imported from
.Fx
and
.Nx .