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Syslog Tutorial

May 22nd, 2014 Go to comments

As an administrator of a network, you have just completed all the configuration and they are working nicely. Now maybe the next thing you want to do is to set up something that can alert you when something goes wrong or down in your network. Syslog is an excellent tool for system monitoring and is almost always included in your distribution.

Places to store and display syslog messages

There are some places we can send syslog messages to:

Place to store syslog messages Command to use
Internal buffer (inside a switch or router) logging buffered [size]
Syslog server logging
Flash memory logging file flash:filename
Nonconsole terminal (VTY connection…) terminal monitor
Console line logging console

Note: If sent to a syslog server, messages are sent on UDP port 514.

By default, Cisco routers and switches send log messages to the console. We should use a syslog server to contain our logging messages with the logging command. Syslog server is the most popular place to store logging messages and administrators can easily monitor the wealth of their networks based on the received information.

Syslog syntax

A syslog message has the following format:

seq no:timestamp%FACILTY-SEVERITY-MNEMONIC: message text

Each portion of a syslog message has a specific meaning:
+ Seq no: a sequence number only if the service sequence-numbers global configuration command is configured
+ Timestamp: Date and time of the message or event. This information appears only if the service timestamps global configuration command is configured.
+ FACILITY: This tells the protocol, module, or process that generated the message. Some examples are SYS for the operating system, IF for an interface…
+ SEVERITY: A number from 0 to 7 designating the importance of the action reported.

The Syslog levels are:

Level Keyword Description
0 emergencies System is unusable
1 alerts Immediate action is needed
2 critical Critical conditions exist
3 errors Error conditions exist
4 warnings Warning conditions exist
5 notification Normal, but significant, conditions exist
6 informational Informational messages
7 debugging Debugging messages

Note: You can remember the order above with the sentence: “Eventually All Critical Errors Will Not Involve Damage”.

The highest level is level 0 (emergencies). The lowest level is level 7. To change the minimum severity level that is sent to syslog, use the logging trap level configuration command. If you specify a level, that level and all the higher levels will be displayed. For example, by using the logging console warnings command, all the logging of emergencies, alerts, critical, errors, warnings will be displayed. Levels 0 through 4 are for events that could seriously impact the device, whereas levels 5 through 7 are for less-important events. By default, syslog servers receive informational messages (level 6).

+ MNEMONIC: A code that identifies the action reported.
+ message text: A plain-text description of the event that triggered the syslog message.

Let’s see an example of the syslog message:

39345: May 22 13:56:35.811: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/1, changed state to down

+ seq no: 39345
+ Timestamp: May 22 13:56:35.811
+ SEVERITY level: 5 (notification)
+ message text: Line protocol on Interface Serial0/0/1, changed state to down

Note: Facility levels and syslog levels are different. The facility represents the machine process that created the syslog event. Therefore the Facility value is a way of determining which process of the machine created the message. For example, is the event created by the kernel, by the mail system, by security/authorization processes, etc.

Facility Description
Auth Authorization system
Cron Cron/at facility
Daemon System daemons
Kern Kernel
local0 to local7 Local use
Lpr Line printer system
Mail Mail system
News USENET news
sys9 to sys14 System use
Syslog Syslog itself
User User process
Uucp Unix-to-Unix copy system

The default syslog facility setting is local7.

Syslog Configuration

The following example tells the device to store syslog messages to a server on and limit the messages for levels 4 and higher (0 through 4):

Router(config)#logging trap 4

Of course on the server we have to use a syslog software to capture the syslog messages sent to this server.

Comments (1) Comments
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  1. Mateo
    January 18th, 2020

    Thank you! well illustrated.

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