Home Sweet Home

Not all distributions tighten up your home directories by default. Assume my user name is lionel, and I don't want Luis or Neymar to see the files in my home directory. It's easy to keep other users out of my home directory by running the following command as root:

# chmod -R 750 /home/lionel

If you're keen to stop all home directories being accessed by others, simply replace lionel with an asterisk.

Hey, Mr. Postman

The next step is to set up the logwatch tool [1] and configure it to send you a daily digest of any important events found in your system logs. Bear in mind that, when you're finished with these simple steps, you'll also have a fully functional SMTP server. Clearly, I'm zooming through these commands for brevity; I encourage you to take a closer look if you're not sure what you're doing.

Straight out of the box, the Postfix mail server will be fully functioning. Choose Internet Site and add your full hostname (including FQDN) during the configuration process. With a quick shuffle, you can then change the old-school, Sendmail-style /etc/aliases file so that your personal address receives your root user email.

Install Postfix as follows:

# apt-get install postfix

Edit the aliases file and add your external address so you receive the root user's mail:

# pico -w /etc/aliases

Make sure the line starting root: looks similar to the following:

root: chris@binnie.tld

Update the aliases DB file by executing the Postfix newaliases command:

# newaliases

Finally, install the Logwatch package; even as a fresh install it's fabulously functional:

# apt-get install logwatch

The easiest way to execute Logwatch is to set up a cron job that runs at the default cron.daily predefined time, which is 06:25hr on my Jessie 8.1 build. Adjust the time in the /etc/crontab file or move the /etc/cron.daily/00logwatch file somewhere else and execute it specifically at another time from within the /etc/crontab file.

Next up run Logwatch and then check your inbox:

# /etc/cron.daily/00logwatch

The results are mind-blowingly useful. For more on monitoring your logs with logwatch, see the article at the ADMIN Magazine website [2].

Logwatch provides invaluable information about your system (including disk space usage, logins over SSH, new users/groups, service restarts, and unusual log entries). Additionally, you receive a list of the packages that you've installed following the completion of your minimal install.

The packages are also conveniently separated into Installed, Upgraded, and Removed sections. I find these lists are an excellent point for future reference, archived in my email with the subject line "Logwatch for <hostname> (Linux)."

If you ever encounter discrepancies between two servers due to package version or functionality, it is a two-second lookup to scan over how they were built. If you don't want root to directly receive mail via your aliases file, you can add MailTo = followed by an email address in the config file /etc/logwatch/conf/logwatch.conf. You might need to create this file if it doesn't already exist.

I would be remiss not to mention the task of securing of your shiny new SMTP server further. Even if relaying is switched off by default, you still need to take some steps to secure your mail server. The Ask Ubuntu site has a useful post on securing a Postfix mail server [3].

Passwords

If I am expecting a few different (non-sys admin users) to be logging into this box, I should also consider password security. If you have ever stumbled across a list of common passwords, you'll know why admins are very concerned about letting everyday users have access to server systems [4].

The quality of the typical user password is not good news if you haven't locked down access to your server by IP address or some other alternative method.

Without these precautions you are definitely vulnerable to automated dictionary attacks. Thankfully, you can strengthen your password rules on Linux with a sophisticated package called libpam-cracklib.

The following configuration works on my Debian "Wheezy" system. Install libpam-cracklib with:

# apt-get install libpam-cracklib

Then, edit the following file:

# pico -w /etc/pam.d/common-password

Adjust the following command with the necessary settings (comment out the old config line rather than deleting it so you can revert if necessary):

password requisite pam_cracklib.so retry=2 minlen=10 difok=6 dcredit=2 ocredit=2

This hardened configuration means a user is only permitted two password error retries and demands at least 10 characters for a password.

My favorite setting (to annoy colleagues) is difok=6, which means only six characters of the last password can be reused and the last two entries must be a minimum of two numbers and two symbols (or "special characters") present within the password. To activate these settings, on Wheezy at least, you can run the following command:

# pam-auth-update

Try a few tests – carefully, so you can still log back in. For troubleshooting, you might want to check whether UsePAM yes is present in the file /etc/ssh/sshd_config file. Restart the OpenSSH server with systemctl restart ssh if you change the configuration.

Table 1 shows some of the excellent options for enforcing stricter passwords from the pam_cracklib manual, which covers libpam-cracklib. Experiment with these settings to your heart's content.

If you want to flex your Cracklib muscles, you could also go one step further and introduce your own dictionaries for Cracklib to check against. Look online for an old but nicely-written piece on using Cracklib with Django [5].