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If you manage any Linux machines, it is essential that you know where the log files are located, and what is contained in them. Such files are usually in /var/log. Logging is controlled by the associated .conf file.

Some log files are distribution specific and this directory can also contain applications such as samba, apache, lighttpd, mail etc.

From a security perspective, here are 5 groups of files which are essential. Many other files are generated and will be important for system administration and troubleshooting.

1. The main log file
a) /var/log/messages – Contains global system messages, including the messages that are logged during system startup. There are several things that are logged in /var/log/messages including mail, cron, daemon, kern, auth, etc.

2. Access and authentication
a) /var/log/auth.log – Contains system authorization information, including user logins and authentication machinsm that were used.
b) /var/log/lastlog – Displays the recent login information for all the users. This is not an ascii file. You should use lastlog command to view the content of this file.
c) /var/log/btmp – This file contains information about failed login attemps. Use the last command to view the btmp file. For example, “last -f /var/log/btmp | more”
d) /var/log/wtmp or /var/log/utmp – Contains login records. Using wtmp you can find out who is logged into the system. who command uses this file to display the information.
e) /var/log/faillog – Contains user failed login attemps. Use faillog command to display the content of this file.
f) /var/log/secure – Contains information related to authentication and authorization privileges. For example, sshd logs all the messages here, including unsuccessful login.

3. Package install/uninstall
a) /var/log/dpkg.log – Contains information that are logged when a package is installed or removed using dpkg command
b) /var/log/yum.log – Contains information that are logged when a package is installed using yum

4. System
a) /var/log/daemon.log – Contains information logged by the various background daemons that runs on the system
b) /var/log/cups – All printer and printing related log messages
c) /var/log/cron – Whenever cron daemon (or anacron) starts a cron job, it logs the information about the cron job in this file

5. Applications
b) /var/log/maillog /var/log/mail.log – Contains the log information from the mail server that is running on the system. For example, sendmail logs information about all the sent items to this file
b) /var/log/Xorg.x.log – Log messages from the XWindows system

Happy Logging!