by July 7, 2001 0 comments



Authored by Madhu M Kurup and Babu Kalakrishnan

The
first week of May 2000 saw the nasty “I Love You” virus destroy
data on PCs all over the world, causing damage worth billions of dollars.
And it isn’t the first time such an event has occurred (remember Melissa
just a few months back)? Significantly, not even one of the PCs running
Linux was affected. Does this mean that Linux is immune to viruses? Or is it
just that virus writers haven’t targeted Linux machines as yet?

Is Linux really immune?

A virus or
virus-like program can be written for any operating system. There have been
reports of at least one replicating virus–Bliss, May 1997–specifically
targeted at Linux executables. Destructive code can also be written very
easily using scripting languages like Perl available on most Linux
distributions. Even a bash script can be quite powerful.

The relative immunity of
Linux stems from the implementation of security at the operating system
level. If a user executes a piece of destructive code, the files that the
program can infect or destroy are restricted to those for which the user has
permissions to write to. All the binaries of the standard programs are
normally installed with write permissions only to the super-user (root) and
hence can’t be modified (unless you’re running the program as root).
Read the Linux Security HOWTO to understand why you shouldn’t be doing
that anyway.

Open Source is secure

Another
reason why Linux is relatively immune to virus attacks, is because most
software used in Linux is available either as Open Source or free software.
A major mode of propagation of viruses is through cracked or pirated
commercial software, where you can’t verify the authenticity of the copy
you obtained. As for Linux applications, most are freely downloadable from
the Net, and cryptographic signatures of packages are generally available at
the official Websites of the distributors. So, you can verify the
authenticity of the package, even if you obtained the software from a
different source.

In addition, the most vital
aspect of Linux-based software is that the source code is always available
for anyone to read, inspect, and verify. Backdoors, loopholes, and other
exploits quickly get detected, given that you can compile your own version
of a program. Sometimes, availability of the source is considered a
weakness, as access to the source code supposedly permits flaws in the
operating system to be found easily. However, you should realize that Linux
is a huge community effort, and that at any point, hundreds of people are
co-operating towards finding, solving, and securing loopholes in the
operating system.

To understand what logrotate can do….

To understand what logrotate
can do, first ask yourself what you want to do with your log files. The
table "Planning for a log processing and archiving policy" might
help you to start. The first row lists the processing and reporting to be
done, while the first column lists the files on which the processing is to
be done. Put down the different log files in column 1, tick out the log
processing of your choice, and you can come up with a policy for using
logrotate.

Let me briefly explain what
each column implies. A "yes" on column 2 indicates that you want
to retain the log file as a record, so it’s best kept compressed.
Similarly, a "yes" in column 3 indicates that you merely want to
scan the file, look for the unusual, and then discard it. You might want to
mail this file to yourself or to the relevant administrator. Column 4 says
that you want to discard the file straightaway. In the sysadmin world, this
obviously doesn’t qualify for best practice. Columns 5 and 6
mention the actions you want to perform before and after you do the log
processing. Column 7 is for an e-mail address to which errors during log
processing are to be reported, and column 8 indicates how often you want the
processing to be done. Note that you might want a time threshold with a
granularity of a day or choose to have a file size threshold to rotate the
logs. This table is not exhaustive or mandatory in nature–it’s is merely
an example of how you would go about the policy-making exercise. So, don’t
implement this, as is, as a policy. Evolve one to suit your needs.

If you’re ready with a
table such as the one above, you have a policy. You can now use logrotate to
implement this policy.

The policy is specified using
keywords, as well as with a script-like language comprising keywords
specific to logrotate. The script is intuitive and easy to understand. By
default, most logs are rotated four times, uncompressed, before they’re
removed from the system. This should explain the presence of files with the
extensions .1, .2, .3 and .4 in the /var/log directory. Take the file /var/log/messages
as an example. After a certain time period or after a certain file size is
reached (as specified in /etc/logrotate. conf), this file is renamed to
messages.1 and an empty file called messages is created to take in the new
log input. This is repeated until they’re rotated four times.

Let’s look at a portion of
the configuration from /etc/logrotate.conf from a standard install. The
first line mentions the name of the file for which the policy is laid out.
Notice the intuitive keywords–"monthly" indicates that the
rotation cycle is monthly, "create" specifies the permissions and
ownerships to be used when the old file is moved to another name and an
empty file is created. "Rotate 1" indicates that one rotated
logfile will be retained:

/var/log/wtmp
{ monthly

create 0664
root utmp

rotate 1

}

Here’s a portion of the
file /etc/logrotate.d/apache–the policy for processing apache log files.
The keyword missingok implies that if the log file isn’t found, continue
processing the rest. Notice the command in between the keywords postrotate
and endscript. This command is executed after log processing is done.
Surprisingly, you don’t find any other instructions such as the frequency
of rotation or the number of rotations, as in the previous case. When there’s
no explicit mention made, the definitions in the global configuration file
will apply.

/var/log/httpd/access_log
{

missingok

postrotate

/usr/bin/killall -HUP httpd 2> /dev/null || true

endscript

}

logrotate is typically run
once a day by the cron. If you are logged in as superuser, you would see an
entry similar to the one below in the crontab file:

0 0 * * * /usr/sbin/logrotate

The utility runs every
midnight. You can run it more often if you need to.

A good start towards
minimizing disk storage space would be to uncomment the compress option in
/etc/logrotate. conf, so that all the rotated log files are kept compressed.

Avinash
Shenoy
is a systems and network administrator at the NCBS, Bangalore,
and Gopi Garge is a technology
consultant with Exocore Consulting <www.exocore.com>

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