Troubleshooting With Syslog

Troubleshooting With Syslog

There are hundreds of Linux applications on the market, each with their own configuration files and help pages. This variety makes Linux vibrant, but it also makes Linux system administration daunting. Fortunately, in most cases, Linux applications use the Syslog utility to export all their errors and status messages to files located in the /var/log directory.

This can be invaluable in correlating the timing and causes of related events on your system. It is also important to know that applications frequently don’t display errors on the screen, but will usually log them somewhere. Knowing the precise message that accompanies an error can be vital in researching malfunctions in product manuals, online documentation, and Web searches.

syslog, and the logrotate utility that cleans up log files, are both relatively easy to configure but they frequently don’t get their fair share of coverage in most texts.


syslog is a utility for tracking and logging all manner of system messages from the merely informational to the extremely critical. Each system message sent to the syslog server has two descriptive labels associated with it that makes the message easier to handle.

  • The first describes the function (facility) of the application that generated it. For example, applications such as mail and cron generate messages with easily identifiable facilities named mail and cron.
  • The second describes the degree of severity of the message. There are eight in all and they are listed in Table 5-1:

You can configure syslog’s /etc/syslog.conf configuration file to place messages of differing severities and facilities in different files. This procedure will be covered next.

Table 5-1 Syslog Facilities

Severity LevelKeywordDescription
0emergenciesSystem unusable
1alertsImmediate action required
2criticalCritical condition
3errorsError conditions
4warningsWarning conditions
5notificationsNormal but significant conditions
6informationalInformational messages
7debuggingDebugging messages

The /etc/syslog.conf file

The files to which syslog writes each type of message received is set in the /etc/syslog.conf configuration file. This file consists of two columns. The first lists the facilities and severities of messages to expect and the second lists the files to which they should be logged.

By default, RedHat/Fedora’s /etc/syslog.conf file is configured to put most of the messages in the file /var/log/messages. Here is a sample:

.info;mail.none;authpriv.none;cron.none           /var/log/messages

In this case, all messages of severity “info” and above are logged, but none from the mail, cron or authentication facilities/subsystems. You can make this logging even more sensitive by replacing the line above with one that captures all messages from debug severity and above in the /var/log/messages file. This may be more suitable for troubleshooting.

*.debug                                           /var/log/messages

Certain applications will additionally log to their own application specific log files and directories independent of the syslog.conf file. Here are some common examples:

  • Files:
/var/log/maillog             : Mail
/var/log/httpd/access_log    : Apache web server page access logs
  • Directories:
/var/log/samba                      : Samba messages
/var/log/mrtg                       : MRTG messages
/var/log/httpd                      : Apache webserver messages

Note: In some older versions of Linuxthe /etc/syslog.conf file was very sensitive to spaces and would recognize only tabs. The use of spaces in the file would cause unpredictable results. Check the formatting of your /etc/syslog.conffile to be safe.

Activating Changes to the syslog Configuration File

Changes to /etc/syslog.conf will not take effect until you restart syslog. Issue this command to do so:

[root@bigboy tmp]# service syslog restart

How to View New Log Entries as They Happen

If you want to get new log entries to scroll on the screen as they occur, then you can use this command:

[root@bigboy tmp]# tail -f /var/log/messages

Similar commands can be applied to all log files. This is probably one of the best troubleshooting tools available in Linux. Another good command to use apart from tail is grep. grep will help you search for all occurrences of a string in a log file; you can pipe it through the more command so that you only get one screen at a time. Here is an example:

[root@bigboy tmp]# grep string /var/log/messages | more

You can also just use the plain old more command to see one screen at a time of the entire log file without filtering with grep. Here is an example:

[root@bigboy tmp]# more /var/log/messages

Logging syslog Messages to a Remote Linux Server

Logging your system messages to a remote server is a good security practice. With all servers logging to a central syslog server, it becomes easier to correlate events across your company. It also makes covering up mistakes or malicious activities harder because the purposeful deletion of log files on a server cannot simultaneously occur on your logging server, especially if you restrict the user access to the logging server.

Configuring the Linux Syslog Server

By default syslog doesn’t expect to receive messages from remote clients. Here’s how to configure your Linux server to start listening for these messages.

As we saw previously, syslog checks its /etc/syslog.conf file to determine the expected names and locations of the log files it should create. It also checks the file /etc/sysconfig/syslog to determine the various modes in which it should operate. Syslog will not listen for remote messages unless the SYSLOGD_OPTIONS variable in this file has a -r included in it as shown below.

# Options to syslogd
# -m 0 disables 'MARK' messages.

# -r enables logging from remote machines

# -x disables DNS lookups on messages received with -r

# See syslogd(8) for more details


# Options to klogd

# -2 prints all kernel oops messages twice; once for klogd to decode, and

#    once for processing with 'ksymoops'

# -x disables all klogd processing of oops messages entirely

# See klogd(8) for more details


You will have to restart syslog on the server for the changes to take effect. The server will now start to listen on UDP port 514, which you can verify using either one of the following netstat command variations.

[root@bigboy tmp]# netstat -a | grep syslog
udp        0      0 *:syslog                *:*

[root@bigboy tmp]# netstat -an | grep 514

udp        0      0   *

[root@bigboy tmp]#

Configuring the Linux Client

The syslog server is now expecting to receive syslog messages. You have to configure your remote Linux client to send messages to it. This is done by editing the /etc/hosts file on the Linux client named smallfry. Here are the steps:

  • Determine the IP address and fully qualified hostname of your remote logging host.
  • Add an entry in the /etc/hosts file in the format:
IP-address    fully-qualified-domain-name    hostname    "loghost"

Example:    bigboy     loghost

Now your /etc/hosts file has a nickname of “loghost” for server bigboy.

  • The next thing you need to do is edit your /etc/syslog.conf file to make the syslog messages get sent to your new loghost nickname.
*.debug                                       @loghost

*.debug                                       /var/log/messages

You have now configured all debug messages and higher to be logged to both server bigboy (“loghost”) and the local file /var/log/messages. Remember to restart syslog to get the remote logging started.

You can now test to make sure that the syslog server is receiving the messages with a simple test such as restarting the lpd printer daemon and making sure the remote server sees the messages.

  • Linux Client
[root@smallfry tmp]# service lpd restart

Stopping lpd: [  OK  ]

Starting lpd: [  OK  ]

[root@smallfry tmp]#
  • Linux Server
[root@bigboy tmp]# tail /var/log/messages
Apr 11 22:09:35 smallfry lpd: lpd shutdown succeeded
Apr 11 22:09:39 smallfry lpd: lpd startup succeeded
[root@bigboy tmp]#

Syslog Configuration and Cisco Network Devices

syslog reserves facilities “local0” through “local7” for log messages received from remote servers and network devices. Routers, switches, firewalls and load balancers each logging with a different facility can each have their own log files for easy troubleshooting. Appendix 4 has examples of how to configure syslog to do this with Cisco devices using separate log files for the routers, switches, PIX firewalls, CSS load balancers and LocalDirectors.

syslog and Firewalls

syslog listens by default on UDP port 514. If you are logging to a remote syslog server via a firewall, you’ll have to allow traffic on this port to pass through the security device. syslog messages usually have UDP port 514 for both their source and destination ports.


The Linux utility logrotate renames and reuses system error log files on a periodic basis so that they don’t occupy excessive disk space.

The /etc/logrotate.conf File

This is logrotate’s general configuration file in which you can specify the frequency with which the files are reused.

  • You can specify either a weekly or daily rotation parameter. In the case below the weekly option is commented out with a #, allowing for daily updates.
  • The rotate parameter specifies the number of copies of log files logrotate will maintain. In the case below the 4 copy option is commented out with a #, while allowing 7 copies.
  • The create parameter creates a new log file after each rotation

Therefore, our sample configuration file will create daily archives of allthe logfiles and store them for seven days. The files will have the following names with, logfile being current active version:









Sample Contents of /etc/logrotate.conf

# rotate log files weekly

# rotate log files daily

# keep 4 weeks worth of backlogs
#rotate 4

# keep 7 days worth of backlogs
rotate 7

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

The /etc/logrotate.d Directory

Most Linux applications that use syslog will put an additional configuration file in this directory to specify the names of the log files to be rotated. It is a good practice to verify that all new applications that you want to use the syslog log have configuration files in this directory. Here are some sample files that define the specific files to be rotated for each application.

The /etc/logrotate.d/syslog File (for General System Logging)

/var/log/messages /var/log/secure /var/log/maillog /var/log/spooler /var/log/boot.log /var/log/cron {
    /bin/kill -HUP `cat /var/run/ 2> /dev/null` 2> /dev/null || true


The /etc/logrotate.d/apache File (for Apache)

/var/log/httpd/access_log /var/log/httpd/agent_log /var/log/httpd/error_log /var/log/httpd/referer_log {

    /bin/kill -HUP `cat /var/run/ 2>/dev/null` 2> /dev/null || true

The /etc/logrotate.d/samba File (for SAMBA)

/var/log/samba/*.log {

    /bin/kill -HUP `cat /var/lock/samba/*.pid 2> /dev/null` 2> /dev/null || true

Activating logrotate

The above logrotate settings in the previous section will not take effect until you issue the following command:

[root@bigboy tmp]# logrotate -f

If you want logrotate to reload only a specific configuration file, and not all of them, then issue the logrotate command with just that filename as the argument like this:

[root@bigboy tmp]# logrotate -f /etc/logrotate.d/syslog

Compressing Your Log Files

On busy Web sites the size of your log files can become quite large. Compression can be activated by editing the logrotate.conf file and adding the compress option.


# File: /etc/logrotate.conf


# Activate log compression


The log files will then start to become archived with the gzip utility, each file having a .gz extension.

[root@bigboy tmp]# ls /var/log/messages*

/var/log/messages      /var/log/messages.1.gz /var/log/messages.2.gz

/var/log/messages.3.gz /var/log/messages.4.gz /var/log/messages.5.gz

/var/log/messages.6.gz /var/log/messages.7.gz

[root@bigboy tmp]#

Viewing the contents of the files still remains easy because the zcat command can quickly output their contents to the screen. Use the command with the compressed file’s name as the argument as seen below.

[root@bigboy tmp]# zcat /var/log/messages.1.gz



Nov 15 04:08:02 bigboy httpd: httpd shutdown succeeded

Nov 15 04:08:04 bigboy httpd: httpd startup succeeded

Nov 15 04:08:05 bigboy sendmail[6003]: iACFMLHZ023165: to=<>, delay=2+20:45:44, xdelay=00:00:02, mailer=esmtp, pri=6388168, [], dsn=4.0.0, stat=Deferred: Connection refused by

[root@bigboy tmp]#


In the next chapter we cover the installation of Linux applications, and the use of syslog will become increasingly important especially in the troubleshooting of Linux-based firewalls which can be configured to ignore and then log all undesirable packets; the Apache Web server which logs all application programming errors generated by some of the popular scripting languages such as PERL and PHP; and finally, Linux mail whose configuration files are probably the most frequently edited system documents of all and which correspondingly suffer from the most mistakes.

This syslog chapter should make you more confident to learn more about these applications via experimentation because you’ll at least know where to look at the first sign of trouble.

Stefan Pejcic
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