Level: Introductory Chris Walden (dwinfo@us.ibm.com), e-business Architect, IBM
11 Nov 2003 IBM e-business architect Chris Walden is your guide through a nine-part developerWorks series on moving your operational skills from a Windows to a Linux environment. He covers everything from logging to networking, and from the command-line to help systems -- even compiling packages from available source code. In this part, we track, manipulate, and rotate logs for security and informational purposes.
One of the keys to success in managing any system is to know what is
happening on the system. Linux offers exceptional logging, and the detail
in the logs is configurable.
Linux logs are in plain text, so you can search and read them without
having to use special tools. You can also write scripts that scan through
logs and perform automatic functions based on the contents.
Linux logs are contained in the /var/log directory. There are several log
files that are maintained by the system, but other services and programs
may put their log files here too. Most logs are only readable by root, but
that can be changed by simply changing the access rights to the file.
/var/log/messages
The messages log is the core system log file. It contains the boot
messages when the system came up as well as other status messages as the
system runs. Errors with IO, networking, and other general system errors
are reported in this file. Other information, such as when someone becomes root,
is listed here as well. If services are running, such as DHCP servers, you
can watch the action in the messages file. /var/log/messages is generally
your first place to look when you are troubleshooting.
/var/log/XFree86.0.log
This log shows the results of the last execution of the Xfree86 Xwindows server.
If you are having problems getting the graphical mode to come up, this
file will usually provide answers as to what is failing.
Other logs
There will be other log files in the /var/log directory depending on your
distribution of Linux and the services and applications that you are
running. For example, there may be logs associated with running a mail
server, resource sharing, automatic tasks, and others.
Ready? Rotate!
You will see some files in the /var/log directory that end with a number.
These are rotated archives. Log files can get rather large and cumbersome.
Linux provides a command to rotate these logs so that you don't have
current log information mixed with older irrelevant data. Generally
logrotate runs automatically on a timed basis, but it can also be run
manually. When executed, logrotate will take the current version of the
log files and add a ".1" to the end of the filename. Then any other
previously rotated files are sequenced with ".2," ".3," etc. The larger
the number after a filename, the older the log is.
You can configure the automatic behavior for logrotate by editing the
/etc/logrotate.conf file. Learn the full details about logrotate with
man logrotate.
Log tools
Any text tool can be used to work with log files. Here are some tools
that are particularly helpful.
dmesg
To get a quick view of the boot log for the last system boot, use the
command dmesg. It generally puts out a lot of
text, so you will generally want to pipe it through a viewer.
dmesg | more
The command above will show the boot messages one screen page at a time.
tail
Sometimes you want to keep an eye on a log file as activity is occurring.
Tail is designed to show the last few lines of a text file. By adding the
-f switch, tail will continue to show
new output as it occurs.
tail -f /var/log/messages
The command above will show the last ten lines of /var/log/messages, then
continue to monitor the file and output any new activity. To stop the
tail -f command, use Ctrl + C to break the
processing.
more
More works the same as the DOS version. You can point it to a file, or
pipe output through it to see the information one screen page at a time. For example, to show the contents of the Xfree86 startup log file one screen page at a time:
more /var/log/XFree86.0.log
Use "q" or [Ctrl]-C to stop looking at a file.
less
Less is another text viewer, but it allows you to scroll through a file
and search for information.
less /var/log/messages
The command above will display the contents of the /var/log/messages
file. Use "q" to quit viewing the file. Use "h" to
get help on using less.
logger
You may want to put your own messages into the log file. You could just
append the log message to the correct text file, but you would have to
duplicate the log information style. Also, you would have to change your
code if the logging system had been customized. The logger command lets you send your own messages to the
logging facility. Use it in scripts to provide messages about execution
and errors.
Customized logging
There are two services, or daemons, that control logging, klogd and syslogd. klogd only deals with kernel messages. syslogd deals with other system messages, such as
applications. You can configure the behavior of both by editing the files
/etc/syslog.conf and /etc/sysconfig/syslog. Full custom logging is beyond
the scope of this article, but full details can be found in the Resources listed at the end of this article. You can
also learn much by looking at the man page for /etc/sylogd.conf.
Essentially, each message generated by software provides some information
to identify where the message came from and what message it is. The
/etc/syslog.conf file allows you to specify what you want done with that
kind of message. You can dump it to the messages file. You can dump it to
a custom file. You can have it sent to a remote host where that host will
process it according to its own syslogd configuration. Remote logging is
an excellent security feature. By placing your logs on a remote system,
you can prevent a security breach from easily covering its tracks by
altering the log files.
Here is an example of customized logging taken from the man /etc/syslog.conf page:
Customized logging
# Kernel messages are first, stored in the kernel
# file, critical messages and higher ones also go
# to another host and to the console
#
kern.* /var/adm/kernel
kern.crit @finlandia
kern.crit /dev/console
kern.info;kern.!err /var/adm/kernel-info
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The first rule directs any message that has the kernel facility to the
file /var/adm/kernel.
The second statement directs all kernel messages of the priority crit and
higher to the remote host finlandia. This is useful, because if the host
crashes and the disks get irreparable errors, you might not be able to
read the stored messages. If they're on a remote host, too, you still can
try to find out the reason for the crash.
The third rule directs these messages to the actual console, so the
person who works on the machine will get them, too.
The fourth line tells the syslogd to save all kernel messages that come
with priorities from info up to warning in the file /var/adm/kernel- info.
Everything from err and higher is excluded.
The ability to customize logging like this provides a great deal of
flexibility and control over the Linux environment.
Log configuration in Webmin
Webmin has a module for working with log files.
Figure 1. Webmin system log view
All configured log files are shown. Click on a log file to edit its
configuration.
Figure 2. Webmin log edit screen
Or you can click the View to see the contents of a log
file.
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Viewing log files from the console
Since log files in Linux are written in plain text, they do not require a
special tool to interpret them. Any text file viewer can show a Linux log
file. A browser, such as Mozilla, can display a log file, and provide search
capability. Linux also has console tools to view text files. more,
shows you a file one page at a time, just like the MS DOS version. The
less command will display the file in a read-only
viewer, which provides bi-directional scrolling and search
capabilities. Try it now by entering less
/var/log/messages at the command line.
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The Webmin module interacts with the /etc/syslog.conf file, so anything
you do in one is reflected in the other.
Logging in your life
Log files in Linux are critical to troubleshooting and maintaining your
system. Linux logging is done to text files, so no proprietary tools are
required to view the files. Text files are also easy to use with custom
scripts and programs.
Logs are rotated to keep them from getting too large and to separate the
current information from much older data. Log rotation is configurable.
Logging is highly configurable, and logs can even be stored on a separate
system for security or backup purposes. You can generate system log
messages out of your own scripts and programs that will be recognized and
processed by the syslogd daemon.
Resources Learn
Get products and technologies
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Order the SEK for Linux, a two-DVD set containing the latest IBM trial software for Linux from DB2®, Lotus®, Rational®, Tivoli®, and WebSphere®.
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With IBM trial software, available for download directly from developerWorks, build your next development project on Linux.
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About the author | | | Chris Walden is an e-business Architect for IBM Developer Relations Technical Consulting in Austin, Texas, providing education, enablement, and consulting to IBM Business Partners. He is the official Linux fanatic on his hallway and does his best to spread the good news to all who will hear it. In addition to his architect duties, he manages the area's all-Linux infrastructure servers, which include file, print, and other application services in a mixed-platform user environment. Chris has ten years of experience in the computer industry ranging from field support to Web application development and consulting. |
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