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(libc.info)Overview of Syslog


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Overview of Syslog
==================

   System administrators have to deal with lots of different kinds of
messages from a plethora of subsystems within each system, and usually
lots of systems as well.  For example, an FTP server might report every
connection it gets.  The kernel might report hardware failures on a disk
drive.  A DNS server might report usage statistics at regular intervals.

   Some of these messages need to be brought to a system administrator's
attention immediately.  And it may not be just any system administrator
- there may be a particular system administrator who deals with a
particular kind of message.  Other messages just need to be recorded for
future reference if there is a problem.  Still others may need to have
information extracted from them by an automated process that generates
monthly reports.

   To deal with these messages, most Unix systems have a facility called
"Syslog."  It is generally based on a daemon called "Syslogd" Syslogd
listens for messages on a Unix domain socket named `/dev/log'.  Based
on classification information in the messages and its configuration
file (usually `/etc/syslog.conf'), Syslogd routes them in various ways.
Some of the popular routings are:

   * Write to the system console

   * Mail to a specific user

   * Write to a log file

   * Pass to another daemon

   * Discard

   Syslogd can also handle messages from other systems.  It listens on
the `syslog' UDP port as well as the local socket for messages.

   Syslog can handle messages from the kernel itself.  But the kernel
doesn't write to `/dev/log'; rather, another daemon (sometimes called
"Klogd") extracts messages from the kernel and passes them on to Syslog
as any other process would (and it properly identifies them as messages
from the kernel).

   Syslog can even handle messages that the kernel issued before
Syslogd or Klogd was running.  A Linux kernel, for example, stores
startup messages in a kernel message ring and they are normally still
there when Klogd later starts up.  Assuming Syslogd is running by the
time Klogd starts, Klogd then passes everything in the message ring to
it.

   In order to classify messages for disposition, Syslog requires any
process that submits a message to it to provide two pieces of
classification information with it:

facility
     This identifies who submitted the message.  There are a small
     number of facilities defined.  The kernel, the mail subsystem, and
     an FTP server are examples of recognized facilities.  For the
     complete list, Note: syslog; vsyslog.  Keep in mind that these
     are essentially arbitrary classifications.  "Mail subsystem"
     doesn't have any more meaning than the system administrator gives
     to it.

priority
     This tells how important the content of the message is.  Examples
     of defined priority values are: debug, informational, warning,
     critical.  For the complete list, Note: syslog; vsyslog.  Except
     for the fact that the priorities have a defined order, the meaning
     of each of these priorities is entirely determined by the system
     administrator.

   A "facility/priority" is a number that indicates both the facility
and the priority.

   *Warning:* This terminology is not universal.  Some people use
"level" to refer to the priority and "priority" to refer to the
combination of facility and priority.  A Linux kernel has a concept of a
message "level," which corresponds both to a Syslog priority and to a
Syslog facility/priority (It can be both because the facility code for
the kernel is zero, and that makes priority and facility/priority the
same value).

   The GNU C library provides functions to submit messages to Syslog.
They do it by writing to the `/dev/log' socket.  Note: Submitting
Syslog Messages.

   The GNU C library functions only work to submit messages to the
Syslog facility on the same system.  To submit a message to the Syslog
facility on another system, use the socket I/O functions to write a UDP
datagram to the `syslog' UDP port on that system.  Note: Sockets.


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