syslog

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.

  Table 5.1. syslog Facilities

  Severity Level	Keyword	Description
  0	emergencies	System unusable
  1	alerts	Immediate action required
  2	critical	Critical condition
  3	errors	Error conditions
  4	warnings	Warning conditions
  5	notifications	Normal but significant conditions
  6	informational	Informational messages
  7	debugging	Debugging messages

  
  

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

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 severity of messages to expect, and the second lists the files to which they should be logged. By default, Red Hat/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
     /var/log/samba                : Samba messages
     /var/log/mrtg                 : MRTG messages
     /var/log/httpd                : Apache webserver messages

Note In some older versions of Linux the /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.conf file 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, 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 an -r included in it:

     # 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
     SYSLOGD_OPTIONS="-m 0 -r"
     # 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
     KLOGD_OPTIONS="-2"

You 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 0.0.0.0:514             0.0.0.0:*
     [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:

  IP-address    fully-qualified-domain-name    hostname    "loghost"

  192.168.1.100   bigboy.my-web-site.org    bigboy    loghost

*.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 balancerseach logging with a different facilitycan each have their own log files for easy troubleshooting. Appendix IV 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 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 UDP ports.