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documentation.suse.com / Dokumentacja systemu SUSE LINUX Enterprise Server / System Analysis and Tuning Guide / System monitoring / System log files
Applies to SUSE Linux Enterprise Server 15 SP4

3 System log files

System log file analysis is one of the most important tasks when analyzing the system. In fact, looking at the system log files should be the first thing to do when maintaining or troubleshooting a system. SUSE Linux Enterprise Server automatically logs almost everything that happens on the system in detail. Since the move to systemd, kernel messages and messages of system services registered with systemd are logged in systemd journal (see Chapter 21, journalctl: Query the systemd journal). Other log files (mainly those of system applications) are written in plain text and can be easily read using an editor or pager. It is also possible to parse them using scripts. This allows you to filter their content.

3.1 System log files in /var/log/

System log files are always located under the /var/log directory. The following list presents an overview of all system log files from SUSE Linux Enterprise Server present after a default installation. Depending on your installation scope, /var/log also contains log files from other services and applications not listed here. Some files and directories described below are placeholders and are only used, when the corresponding application is installed. Most log files are only visible for the user root.


AppArmor log files. For more information about AppArmor, see Part V, “Confining privileges with AppArmor.


Logs from the audit framework. See Part VII, “The Linux Audit Framework” for details.


Logs of the ConsoleKit daemon (daemon for tracking what users are logged in and how they interact with the computer).


Access and error logs of the Common Unix Printing System (cups).


Firewall logs.


Log files from the GNOME display manager.


Log files from the Kerberos network authentication system.


A database containing information on the last login of each user. Use the command lastlog to view. See man 8 lastlog for more information.


Log messages of some boot scripts, for example the log of the DHCP client.


Mail server (postfix, sendmail) logs.


This is the default place where all kernel and system log messages go and should be the first place (along with /var/log/warn) to look at in case of problems.


NetworkManager log files.


Log messages from a news server.


Logs from the Network Time Protocol daemon (chrony).


PackageKit (with libzypp back-end) log files.


Log files from Samba, the Windows SMB/CIFS file server.


Log of all system warnings and errors. This should be the first place (along with the output of the systemd journal) to look in case of problems.


Database of all login/logout activities, and remote connections. Use the command last to view. See man 1 last for more information.


X.Org start-up log file. Refer to these files in case you have problems starting X.Org.

The NUMBER in the file name is the display number. For example, the default Xorg.0.log is the log for display number 0, and Xorg.1.log is the log for display number 1. Copies from previous X.Org starts are named as Xorg.NUMBER.log.old.


The X.Org log files are available in the /var/log/ directory only if you start an X.Org session as root. If you start an X.Org session as any other user, you can locate the log files in the ~/.local/share/xorg/ directory.


All YaST log files.


libzypp log files. Refer to these files for the package installation history.


Logs from the command line installer zypper.

3.2 Viewing and parsing log files

To view log files, you can use any text editor. There is also a simple YaST module for viewing the system log available in the YaST control center under Miscellaneous › System Log.

For viewing log files in a text console, use the commands less or more. Use head and tail to view the beginning or end of a log file. To view entries appended to a log file in real-time use tail -f. For information about how to use these tools, see their man pages.

To search for strings or regular expressions in log files use grep. awk is useful for parsing and rewriting log files.

3.3 Managing log files with logrotate

Log files under /var/log grow on a daily basis and quickly become very large. logrotate is a tool that helps you manage log files and their growth. It allows automatic rotation, removal, compression, and mailing of log files. Log files can be handled periodically (daily, weekly, or monthly) or when exceeding a particular size.

logrotate is usually run daily by systemd, and thus usually modifies log files only once a day. However, exceptions occur when a log file is modified because of its size, if logrotate is run multiple times a day, or if --force is enabled. Use /var/lib/misc/logrotate.status to find out when a particular file was last rotated.

The main configuration file of logrotate is /etc/logrotate.conf. System packages and programs that produce log files (for example, apache2) put their own configuration files in the /etc/logrotate.d/ directory. The content of /etc/logrotate.d/ is included via /etc/logrotate.conf.

Example 3.1: Example for /etc/logrotate.conf

# see "man logrotate" for details
# rotate log files weekly

# keep 4 weeks worth of backlogs
rotate 4

# create new (empty) log files after rotating old ones

# use date as a suffix of the rotated file

# uncomment this if you want your log files compressed

# comment these to switch compression to use gzip or another
# compression scheme
compresscmd /usr/bin/bzip2
uncompresscmd /usr/bin/bunzip2

# RPM packages drop log rotation information into this directory
include /etc/logrotate.d
Important: Avoid permission conflicts

The create option pays heed to the modes and ownership of files specified in /etc/permissions*. If you modify these settings, make sure no conflicts arise.

3.4 Monitoring log files with logwatch

logwatch is a customizable, pluggable log-monitoring script. It parses system logs, extracts the important information and presents them in a human readable manner. To use logwatch, install the logwatch package.

logwatch can either be used at the command line to generate on-the-fly reports, or via cron to regularly create custom reports. Reports can either be printed on the screen, saved to a file, or be mailed to a specified address. The latter is especially useful when automatically generating reports via cron.

On the command line, you can tell logwatch for which service and time span to generate a report and how much detail should be included:

# Detailed report on all kernel messages from yesterday
logwatch --service kernel --detail High --range Yesterday --print

# Low detail report on all sshd events recorded (incl. archived logs)
logwatch --service sshd --detail Low --range All --archives --print

# Mail a report on all smartd messages from May 5th to May 7th to root@localhost
logwatch --service smartd --range 'between 5/5/2005 and 5/7/2005' \
--mailto root@localhost --print

The --range option has got a complex syntax—see logwatch --range help for details. A list of all services that can be queried is available with the following command:

> ls /usr/share/logwatch/default.conf/services/ | sed 's/\.conf//g'

logwatch can be customized to great detail. However, the default configuration should usually be sufficient. The default configuration files are located under /usr/share/logwatch/default.conf/. Never change them because they would get overwritten again with the next update. Rather place custom configuration in /etc/logwatch/conf/ (you may use the default configuration file as a template, though). A detailed HOWTO on customizing logwatch is available at /usr/share/doc/packages/logwatch/HOWTO-Customize-LogWatch. The following configuration files exist:


The main configuration file. The default version is extensively commented. Each configuration option can be overwritten on the command line.


Filter for all lines that should globally be ignored by logwatch.


The service directory holds configuration files for each service you can generate a report for.


Specifications on which log files should be parsed for each service.

3.5 Configuring mail forwarding for root

System daemons, cron jobs, systemd timers, and other applications can generate messages and send them to the root user of the system. By default, each user account owns a local mailbox and will be notified about new mail messages upon login.

These messages can contain security relevant reports and incidents that might require a quick response by the system administrator. To get notified about these messages in a timely fashion, it is strongly recommended to forward these mails to a dedicated remote e-mail account that is regularly checked.

Procedure 3.1: Configure mail forwarding for the root user

To forward mail for the root user, perform the following steps:

  1. Install the yast2-mail package:

    # zypper in yast2-mail
  2. Run the interactive YaST mail configuration:

    # yast mail
  3. Choose Permanent as Connection type and proceed with Next.

  4. Enter the address of the Outgoing mail server. If necessary, configure Authentication. It is strongly recommended to Enforce TLS encryption to prevent potentially sensitive system data from being sent unencrypted over the network. Proceed with Next.

  5. Enter the e-mail address to Forward root's mail to and Finish the configuration.

    Important: Do not accept remote SMTP connections

    Do not enable Accept remote SMTP connections, otherwise the local machine will act as a mail relay.

  6. Send a message to test whether mail forwarding works correctly:

    > mail root
    subject: test
  7. Use the mailq command to verify that the test message has been sent. Upon success, the queue should be empty. The message should be received by the dedicated mail address configured previously.

Depending on the number of managed machines and the number of persons who need to be informed about system events, different e-mail address models can be established:

  • Collect messages from different systems in an e-mail account that is only accessed by a single person.

  • Collect messages from different systems in a group e-mail account (aliases or mailing list) that can be accessed by all relevant persons.

  • Create separate e-mail accounts for each system.

It is crucial that administrators regularly check the related e-mail accounts. To facilitate this effort and identify important events, avoid sending unnecessary information. Configure applications to only send relevant information.

3.6 Forwarding log messages to a central syslog server

System log data can be forwarded from individual systems to a central syslog server on the network. This allows administrators to get an overview of events on all hosts, and prevents attackers that succeed in taking over a system from manipulating system logs to cover their tracks.

Setting up a central syslog server consists of two parts. First you configure the central log server, then the clients for remote logging.

3.6.1 Set up the central syslog server

Procedure 3.2: Configure the central rsyslog server

To set up a central syslog server, perform the following steps:

  1. Edit the configuration file /etc/rsyslog.d/remote.conf.

  2. Uncomment the following lines in the UDP Syslog Server or TCP Syslog Server section of the configuration file. Assign an IP address and port for rsyslogd.

    TCP example:

    $ModLoad imtcp.so
    $UDPServerAddress IP1
    $InputTCPServerRun PORT2

    UDP example:

    $ModLoad imudp.so
    $UDPServerAddress IP1
    $UDPServerRun PORT2


    IP address of the interface for rsyslogd to listen on. If no address is given, the daemon listens on all interfaces.


    Port for rsyslogd to listen on. Select a privileged port below 1024. The default is 514.

    Important: TCP versus UDP protocol

    Traditionally syslog uses the UDP protocol to transmit log messages over the network. This involves less overhead, but lacks reliability. Log messages can get lost under high load.

    The TCP protocol is more reliable and should be preferred over UDP.

    Note: UDPServerAddress with TCP

    The $UDPServerAddress configuration parameter in the TCP example is no error. Despite its name it is used for both TCP and UDP.

  3. Save the file.

  4. Restart the rsyslog service:

    > sudo systemctl restart rsyslog.service
  5. Open the respective port in the firewall. For firewalld with TCP on port 514 run:

    > sudo firewall-cmd --add-port 514/tcp --permanent
    > sudo firewall-cmd --reload

You have now configured the central syslog server. Next, configure clients for remote logging.

3.6.2 Set up the client machines

Procedure 3.3: Configure a rsyslog instance for remote logging

To configure a machine for remote logging to a central syslog server, perform the following steps:

  1. Edit the configuration file /etc/rsyslog.d/remote.conf.

  2. Uncomment the appropriate line (TCP or UDP) and replace remote-host with the address of the central syslog server set up in Section 3.6.1, “Set up the central syslog server”.

    TCP example:

    # Remote Logging using TCP for reliable delivery
    # remote host is: name/ip:port, e.g., port optional
    *.* @@remote-host

    UDP example:

    # Remote Logging using UDP
    # remote host is: name/ip:port, e.g., port optional
    *.* @remote-host
  3. Save the file.

  4. Restart the rsyslog service:

    > sudo systemctl restart rsyslog.service
  5. Verify the proper function of the syslog forwarding:

    > logger "hello world"

    The log message hello world should now appear on the central syslog server.

You have now configured a system for remote logging to your central syslog server. Repeat this procedure for all systems that should log remotely.

3.6.3 More information

This basic setup does not include encryption and is only suitable for trusted internal networks. TLS encryption is strongly recommended, but requires a certificate infrastructure.

In this configuration, all messages from remote hosts will be treated the same on the central syslog server. Consider filtering messages into separate files by remote host or classify them by message category.

For more information about encryption, filtering, and other advanced topics, consult the RSyslog documentation at https://www.rsyslog.com/doc/master/index.html#manual.

3.7 Using logger to make system log entries

logger is a tool for making entries in the system log. It provides a shell command interface to the rsyslogd system log module. For example, the following line outputs its message in /var/log/messages or directly in the journal (if no logging facility is running):

> logger -t Test "This message comes from $USER"

Depending on the current user and host name, the log contains a line similar to this:

Sep 28 13:09:31 venus Test: This message comes from tux