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Managing Log files on a Linux System

Linux"s Log Files

All Linux systems generate systems logs that can be inspected to find information about your running system. These log files can contain a wealth of information from simple information messages to critical system issues. Most of the logging files that are created are in plain text. This means that it is very easy to view these using standard commands such as "more", "less", "cat", "head", "tail", "pg" or by using your preferred text editor such as "vi or vim".

By convention, most of the log files that are created are found under the directory "/var/log/". This is a standard area where system messages and logged/recorded. Depending on which Linux distribution you are using you will probably have a "message" file or a "syslog" file that contains recent activity. Logfiles are generally created by either a "syslogd" or "rsyslogd" logging demon. These demons are highly configurable and can filter messages into specified files. As well as handling local events, it is possible to log messages to remote servers dedicated to receiving these type of messages. It is quite common within larger organisations to have a dedicated syslog server. Some basic configuration options will be covered later. It is also common practice to have some form of log rotation process.

Below is a list of some of the more common log files that you will find. Some of these are distribution specific:

Although the above is not an exhaustive view of all the files that can be found within the "/var/log" area. It does give you a rough idea of what is logged. It is important to remember that many third part products (software) will also write to this area. Often a sub-directory is created with various log information held within. Samba is a good example of this.

As mentioned earlier it is the "syslogd" or "rsyslogd" daemon that handles the majority of logging on your systems.

rsyslogd - logging utility

Rsyslogd is a reliable extended version of the syslogd service. Linux uses rsyslogd as its mechanism to record log files either in a central area or split into separate directories for clarity. It is also possible to send information to a dedicated logging server. Multiple processes may write to the same area without causing file locking. Simple commands can be used directly from scripts to write to this area.

Configuration Files

How rsyslogd behaves on your system is down to its configuration. This file is generally located in "/etc/rsyslog.conf". This file contains text which describes what should happen to messages when they are logged. It is here that you can specify specific directories for specific message types. Default logging rules are generally located under "/etc/rsyslog.d/"

Example of rsyslogd.conf under Ubuntu

/etc/rsyslog.conf

Hashes "#" are used to denote a comment or for commenting out a function that is not required.
Notice the last line $IncludeConfig /etc/rsyslog.d/*.conf . This is where we can specify custom rules/mappings.

The default logging area is called "syslog", see below exert

What are Facilities and Levels?

Whenever the rsyslogd daemon receives a logging message, it acts based on the message type (Facility) and a Level (Priority). These mappings can be seen in your "/etc/syslog.conf" file or your included "/etc/syslog.d/*.conf" files.

Each entry within the configuration file can specify one or more facility/level selectors followed by an action. A selector consists of a facility or facilities followed by a single action. Action is normally the name of the directory and file that is to receive the messages into.

facility.level action

Example: mail.* -/var/log/mail - Here, "mail" is the facility, level is set to "*" and action is "/var/log/mail"

Facility

A facility represents the creator of the message, these normally consist of:

auth, authpriv, cron, daemon, kern, lpr, mail, mark, news, syslog, user, local0 - local7, "* " signifies any facility

These facilities give us the ability to control where messages from certain sources are sent to. The facilities local0 - local7 are for use by your own scripts.

Level (Priority)

The level specifies the severity threshold. These can be: (lowest priority first)

debug, info, notice, warning, err, crit, alert, emerg .

On older systems you may see "warn, error or panic". A level of none will disable the associated facility. These priorities control the amount of detail that is sent to each logfile. A single period "." separates the facility from the level. Together these are known as the message selector . An asterisk "* " may be used to specify all facilities or levels. Similar to facilities, wildcards "*" can be used along with "none". Only one level or wildcard may be specified with each selector. The following modifiers may be used "= " and "! "

If you specify only one level within a selector without any modifiers, you are actually specifying that level plus all other priorities. For example the selector user.notice is actually saying all user related messages having a level of notice or higher will be sent to the specified action area. If you require only a level of "notice", then you will have to use the "= " modifier:

user.=notice - Now means any user related messages with a level priority of "notice" only will be sent to the relevant logging area.

If you use the "! " modifier, this will negate your level priority. So if we specified user.!notice is the equivalent of all user related messages with a level priority of "notice" or higher. You can also specify user.!=notice which specify all user related messages except for ones with the level priority of "notice".

Actions

The action section is the destination for the messages. The action can be a filename such as "/var/log/syslog" or a hostname or IP address prefixed with the "@" sign. The latter option is popular in large organisations and enterprises. Quite often security related messages may be sent to a central logging server for further scrutiny.

rsyslog.conf structures

As rsyslogd is an enhanced version of the syslogd it can handle the older legacy style constructs known as sysklogd . It also handles legacy versions of rsyslog. However, the true power of rsyslog comes into play when you use what is known as "RainerScript ". This is the new style format for rsyslog which can handle complex cases with ease. In the example below you can see old format entries along with newer entries that use "if - then" constructs for a more precise handling.

Example section of "/etc/rsyslog.conf taken from openSUSE

Message Testing with the logger command

logger is a shell command interface into the syslog module. Logger allows you to make entries directly into the system log. This is very useful when incorporated into a script or when you want to test your message selector and mappings.

In its simplest form we can issue logger "I am a test" . This message would then go to our default area (probably /var/log/syslog or /var/log/messages) depending on how you have configured your rules. You can also specify a priority using the "-p or --priority" option. Examples of logger inaction:

Basic Logger Usage

Getting help with Rsyslog

The above is intended as an overview to the processes that takes place for logging of messages to occur. For further information you can issue "man rsyslogd" from your console for an overview of the many options. For further reading you can head to the main "rsyslog" website: www.rsyslog.com

dmesg

"dmesg" is a special command that stands for display message. dmesg will display the message buffer of the kernel. dmesg is very useful if you want to view the messages that flew past your screen quickly during the boot process. Another useful trick is to redirect the output from the dmesg command to a temporary file: dmesg > /tmp/temp.txt .

dmesg is also useful if you are having issues with an I/O device or a "USB" device. dmesg can be used in combination with the grep command to find exactly what you are looking for quickly: dmesg | grep -i usb

dmesg basic Overview