System logs deal primarily with the functioning of the Ubuntu system, not necessarily with additional applications added by users. Examples include authorization mechanisms, system daemons, system messages, and the all-encompassing system log itself, syslog.
The Authorization Log tracks usage of authorization systems, the mechanisms for authorizing users which prompt for user passwords, such as the Pluggable Authentication Module (PAM) system, the
sudo command, remote logins to
sshd and so on.
The daemon log contains information about running system and application daemons such as the Gnome Display Manager daemon
gdm, the Bluetooth HCI daemon
hcid, or the MySQL database daemon
The debug log provides detailed debug messages from the Ubuntu system and applications which log to
syslogd at the DEBUG level.
The kernel log provides a detailed log of messages from the Ubuntu Linux kernel. These messages may prove useful for trouble-shooting a new or custom-built kernel, for example.
Kernel Ring Buffer:
The kernel ring buffer is not really a log file per se, but rather an area in the running kernel you can query for kernel bootup messages via the
dmesg utility. To see the messages, use this:
dmesg | less
By default, the system initialization script
/etc/init.d/bootmisc.sh sends all bootup messages to the file
/var/log/dmesg as well.
The messages log contains informational messages from applications, and system facilities. This log is useful for examining message output from applications, and system facilities which log to the
sysklog daemon at the INFO level.
The system log typically contains the greatest deal of information by default about your Ubuntu system. It may contain information other logs do not. Consult the System Log when you can't locate the desired log information in another log.
Many applications also create logs in
/var/log. If you list the contents of your
/var/log subdirectory, you will see familiar names, such as
/var/log/apache2 representing the logs for the Apache 2 web server, or
/var/log/samba, which contains the logs for the Samba server.
Apache HTTP Server Logs:
The default installation for Apache2 on Ubuntu creates a log subdirectory. Within this subdirectory are two log files with two distinct purposes:
/var/log/apache2/access.log - records of every page served and every file loaded by the web server.
/var/log/apache2/error.log - records of all error conditions reported by the HTTP server
CUPS Print System Logs:
The Common Unix Printing System (CUPS) uses the default log file
/var/log/cups/error_log to store informational and error messages.
Rootkit Hunter Log:
The Rootkit Hunter utility (
rkhunter) checks your Ubuntu system for backdoors, sniffers and rootkits, which are all signs of compromise of your system.
Samba SMB Server Logs:
The Server Message Block Protocol (SMB) server, Samba is popularly used for sharing files between your Ubuntu computer and other computers which support the SMB protocol. Samba keeps three distinct types of logs in the subdirectory:
log.nmbd - messages related to Samba's NETBIOS over IP functionality (the network stuff)
log.smbd - messages related to Samba's SMB/CIFS functionality (the file and print sharing stuff)
log.[IP_ADDRESS] - messages related to requests for services from the IP address contained in the log file name.
X11 Server Log:
The default X11 Windowing Server in use with Ubuntu is the Xorg X11 server. This log is helpful for diagnosing issues with your X11 environment.
Some log files found in the
/var/log subdirectory are designed to be readable by applications, not necessarily by humans. Some examples follow.
Login Failures Log:
The login failures log located at
/var/log/faillog is actually designed to be parsed and displayed by the
Last Logins Log:
The last logins log at
/var/log/lastlog should not typically be parsed and examined by humans, but rather should be used in conjunction with the
Login Records Log:
/var/log/wtmp contains login records, but unlike
/var/log/wtmp is not used to show a list of recent logins, but is instead used by other utilities such as the
who command to present a listed of currently logged in users.