So you got your server up, the SSH secure, the firewall on, and some monitoring filters for certain services in case someone tries to gain access through one of them.

What else can be done to secure your system?

This is a continuation from 4 previous parts:

Security Tangent - Disable Unused Services

One thing is to reduce the amount of running services to only be those services you need running.

sudo netstat -tulpn

Now, if you have a minimal installation with only the base server components, and you followed my previous "Securing Your Linux Server" guides, you will not have any more to do here, as you will see something like:

If you see that, then everything is good. Nothing to be done.

If you have more programs, you will need to look into securing them individually. But all unused services should be disabled.

To uninstall a service that is no longer needed in Ubuntu, do:

sudo apt-get purge package_name

Then do sudo netstat -tulpn again. The service name you removed, is no longer there.

Please go do more research into removing specific applications if you are unsure about them.

Intrusion Detection

With a server setup, there are still ways for intruders to get in, despite the feature already installed. Vulnerabilities exist and unauthorized access can allow someone to make changes to your system or take it down. They could even hijack your system, and use it to launch attacks on others, such as DDoS attacks.

Linux can secure your system in away that ensures you can detect and track all changes. One popular tool is AIDE, Advanced Intrusion Detection Environment. This is what we are going to install for some base IDS security. There are other more complex and comprehensive tools, like Tripwire and OSSEC.

* Note: use root login to install. I tried with sudo and don't have access to certain areas to create the DB. Don't waste your time, use root from the start.

1. Installation

apt-get install -y aide

2. Verify

aide -v

3. Create the AIDE database

Newer versions of Ubuntu after 14.04 use a newer manager for AIDE, called aide-common.

If you're not sure, try the old way:

aide --init

and if you get the error:

Couldn't open file /var/lib/aide/please-dont-call-aide-without-parameters/aide.db.new for writing

then that means you are on the newer version.

Configuration files are in different places, and we need to use the executables from "aide-common" instead:

aideinit
aide.wrapper
update-aide.conf
aide-attributes

AIDE files are located at:

/etc/default/aide - The AIDE general configuration file.
/etc/aide/aide.conf - The AIDE rules configuration file.
/etc/cron.daily/aide - Daily AIDE cron scripts.

Init Command

Instead of aide --init, use:

aideinit

This takes some time to run.

If you get asked:

Overwrite existing /var/lib/aide/aide.db.new [Yn]?

Type y and hit enter.

Error?

However, if inputing the aideinit cmd gives you an error about not being installed or not a recognized command, then you need to install aide-common:

apt-get install -y aide-common

Now try aideinit and it will work. But again, it takes time.

Result

When it completes, you should see:

Start timestamp: 2016-12-13 12:38:21 -0500
Verbose level: 6

Number of entries:      208403

---------------------------------------------------
The attributes of the (uncompressed) database(s):
---------------------------------------------------

/var/lib/aide/aide.db.new
  RMD160   : uyyN564Fdf446HHFHfh24Q/z6/4=
  TIGER    : 8G/Frmndfg76jhyN564Fdf4kRNQyqbB
  SHA256   : fgf564Fdf4fg5gfg4dfeDDGDG3sffGH5+D7q
             NVoi6RhnMUU=
  SHA512   : aRnSogD8PFdf4fg5Nysgs9oVf4yeuPaAMxHL
             Ce4lYsWuIQcBHHFHfh24Q/230uN3Vet+9l8
             yN564Fdf4IdriQogA==
  CRC32    : 564Fdf==
  HAVAL    : LRdNysgs9oVf48PFdf4fg57/RJDFHRe
             9TZxfWzXpwo=
  GOST     : zF8JRIaCyQ577UcUzF8JRIyVVIz
             JMx64Fdf4f=


End timestamp: 2016-12-13 12:44:52 -0500 (run time: 6m 31s)
Overwrite /var/lib/aide/aide.db [yN]?

The last line may not be there for you. If it is, type y and hit enter.

Jut check to make sure the db was created:

cd /var/lib/aide
ls -lt

and you should see:

aide.conf.autogenerated aide.db aide.db.new

4. Check aide

This is how to check the DB with the new "aide-common" package and command:

aide.wrapper --check

You can also do longer form as aide --config /var/lib/aide/aide.conf.autogenerated --check, or sudo /usr/bin/aide.wrapper -c /etc/aide/aide.conf --check, in case you have issues at some point these are alternatives.

It should give you the similar output from earlier with aideinit.

5. Create test file for AIDE to pick up

Just to make sure AIDE is working, create a dummy file:

touch /tmp/mytestfile.txt

Then check for changes:

aide.wrapper --check

You could see a lot of stuff after it finishes.

If you want to better manage what is displayed, instead of a limited scroll window, use aide.wrapper --check > /root/aidecheck.txt. Then open it with nano /root/aidecheck.txt.

If you are satisfied witht he results, then you can update the DB and forget about these changes:

update-aide.conf

6. Automate a schedule with a cron job

Create a file anywhere, for example:

nano /root/aide.sh

Add the following, and change the email at least, and the "tmp" file locations and names if you wish:

#! /bin/sh
MYDATE=`date +%Y-%m-%d`
MYFILENAME="Aide-"$MYDATE.txt
/bin/echo "Aide check !! `date`" > /tmp/$MYFILENAME
/usr/bin/aide.wrapper --check > /tmp/myAide.txt
/bin/cat /tmp/myAide.txt|/bin/grep -v failed >> /tmp/$MYFILENAME
/bin/echo "**************************************" >> /tmp/$MYFILENAME
/usr/bin/tail -20 /tmp/myAide.txt >> /tmp/$MYFILENAME
/bin/echo "****************DONE******************" >> /tmp/$MYFILENAME
/usr/bin/mail -s"$MYFILENAME `date`" email@domain.com < /tmp/$MYFILENAME

Make it executable:

chmod +x /root/aide.sh

Edit crontab:

crontab -e

Add at the bottom:

00 01 * * * /root/aide.sh

Then ctrl+x and save.

This shell script will run each day of the week (*), every month (*), each day of the month (*), at 1am. You can change any of the parameters for the interval. Please consult additional help to know how to modify each parameter.