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Linux HOWTO

Configuring and Installing NRPE and NSCA into Nagios Core 4 on CentOS 6

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Introduction

About a month ago I wrote (and updated) an article on how to install Nagios Core 4 onto your system. I’m a bit of a perfectionist, so I’ve rebuilt the packages a little to accommodate my needs. Now I thought it might be a good idea to introduce some of the powerful extensions you can get for Nagios.

For an updated solution, you may wish to check out the following:

  • NRDP for Nagios Core on CentOS 7.x: This blog explains how awesome NRDP really is and why it might become a vital asset to your own environment. This tool can be used to replace NSCA’s functionality. The blog also provides the first set of working RPMs (with SELinux support of course) of it’s kind to support it.
  • NRPE for Nagios Core on CentOS 7.x: This blog explains how to set up NRPE (v3.x) for your Nagios environment. At the time this blog was written, there was no packaging of it’s kind for this version.

RPM Solution

RPMs provide a version control and an automated set of scripts to configure the system how I want it. The beauty of them is that if you disagree with something the tool you’re packaging does, you can feed RPMs patch files to accommodate it without obstructing the original authors intention.

Now I won’t lie and claim I wrote these SPEC files from scratch because I certainly didn’t. I took the stock ones that ship with these products (NRPE and NSCA) and modified them to accommodate and satisfy my compulsive needs. 🙂

My needs required a bit more automation in the setup as well as including:

  • A previous Nagios requirement I had was a /etc/nagios/conf.d directory to behave similar to how Apache works. I wanted to be able to drop configuration files into here and just have it work without re-adjusting configuration files. In retrospect of this, these plugins are a perfect example of what can use this folder and work right out of the box.
  • These new Nagios plugins should adapt to the new nagiocmd permissions. The nagioscmd group permission was a Nagios requirement I had made in my previous blog specifically for the plugin access.
  • NSCA should prepare some default configuration to make it easier on an administrator.
  • NSCA servers that don’t respond within a certain time should advance to a critical state. This should be part of the default (optional) configuration one can use.
  • Both NRPE and NSCA should plug themselves into Nagios silently without human intervention being required.
  • Both NRPE and NSCA should log independently to their own controlled log file that is automatically rotated by the system when required.

Nagios Enhancement Focus

The key things I want to share with you guys that you may or may not find useful for your own environment are the following:

  • Nagios Remote Plugin Executor (NRPE): NRPE (officially accessed here) provides a way to execute all of the Nagios monitoring tools on a remote server. These actions are all preformed through a secure (private) connection to the remote server and then reported back to Nagios. NRPE can allow you to monitor servers that are spread over a WAN (even the internet) from one central monitoring server. This is truly the most fantastic extension of Nagios in my opinion.
    NRPE High Level Overview
    NRPE High Level Overview
  • Nagios Service Check Acceptor (NSCA): NSCA (officially accessed here) provides a way for external applications to report their status directly to the Nagios Server on their own. This solution still allows the remote monitoring of a system by taking the responsibility off of the status checks off of Nagios. However the fantastic features of Nagios are still applicable: You are still centrally monitoring your application and Nagios will immediately take action in notifying you if your application stops responding or reports a bad status. This solution is really useful when working with closed systems (where opening ports to other systems is not an option).
    NSCA High Level Overview
    NSCA High Level Overview

Just give me your packaged RPMS

Here they are:

How do I make these packages work for me?

In all cases, the RPMs take care of just about everything for you, so there isn’t really much to do at this point. Some considerations however are as follows:

  • NRPE
    NRPE - Nagios Remote Plugin Executor
    NRPE – Nagios Remote Plugin Executor

    In an NRPE setup, Nagios is always the client and all of the magic happens when it uses the check_nrpe plugin. Most of NRPE’s configuration resides at the remote server that Nagios will monitor. In a nutshell, NRPE will provide the gateway to check a remote system’s status but in a much more secure and restrictive manor than the check_ssh which already comes with the nagios-plugins package. The check_ssh requires you to create a remote user account it can connect with for remote checks. This can leave your system vulnerable to an attack since you can do a lot more damage with a compromised SSH account. However check_nrpe uses the NRPE protocol and can only return what you let it; therefore making it a MUCH safer choice then check_ssh!

    You’ll want to install nagios-plugins-nrpe on the same server your hosting Nagios on:

    # Download NRPE
wget --output-document=nagios-plugins-nrpe-2.15-1.el6.x86_64.rpm http://repo.nuxref.com/centos/6/en/x86_64/custom/nagios-plugins-nrpe-2.15-4.el6.nuxref.x86_64.rpm

# Now install it
yum -y localinstall nagios-plugins-nrpe-2.15-1.el6.x86_64.rpm

    Again I must stress, the above setup will work right away presuming you chose to use my custom build of Nagios introduced in my blog that went with it.

    Just to show you how everything works, we’ll make the Nagios Server the NRPE Server as well. In real world scenario, this would not be the case at all! But feel free to treat the setup example below on a remote system as well because it’s configuration will be identical! 🙂

    # Install our NRPE Server
wget --output-document=nrpe-2.15-1.el6.x86_64.rpm http://repo.nuxref.com/centos/6/en/x86_64/custom/nrpe-2.15-4.el6.nuxref.x86_64.rpm

# Install some Nagios Plugins we can configure NRPE to use
wget --output-document=nagios-plugins-1.5-1.x86_64.rpm http://repo.nuxref.com/centos/6/en/x86_64/custom/nagios-plugins-1.5-5.el6.nuxref.x86_64.rpm

# Now Install it
yum -y localinstall nrpe-2.15-1.el6.x86_64.rpm 
   nagios-plugins-1.5-1.x86_64.rpm
# This tool requires xinetd to be running; start it if it isn't
# already running
service xinetd status || service xinetd start

# Make sure our system will always start xinetd
# even if it's rebooted
chkconfig --level 345 xinetd on

    Now we can test our server by creating a test configuration:

    # Create a NRPE Configuration our server can accept
cat << _EOF > /etc/nrpe.d/check_mail.cfg
command[check_mailq]=/usr/lib64/nagios/plugins/check_mailq -c 100 -w 50
_EOF

# Create a temporary test configuration to work with:
cat << _EOF > /etc/nagios/conf.d/nrpe_test.cfg
define service{
   use                 local-service
   service_description Check Users
   host_name           localhost
   # check_users is already defined for us in /etc/nagios/nrpe.cfg
	check_command		  check_nrpe!check_users
}

# Test our new custom one we just created above
define service{
   use                 local-service
   service_description Check Mail Queue
   host_name           localhost
   # Use the new check_mailq we defined above in /etc/nrpe.d/check_mail.cfg
	check_command		  check_nrpe!check_mailq
}
_EOF

# Reload Nagios so it sees our new configuration defined in
# /etc/nagios/conf.d/*
service nagios reload

# Reload xinetd so nrpe sees our new configuration defined in
# /etc/nrpe.d/*
service xinetd reload

    We can even test our connection manually by calling the command:

    # This is what the output will look like if everything is okay:
/usr/lib64/nagios/plugins/check_nrpe -H localhost -c check_mailq
OK: mailq is empty|unsent=0;50;100;0

    Another scenario you might see (when setting on up on your remote server) is:

    /usr/lib64/nagios/plugins/check_nrpe -H localhost -c check_mailq
CHECK_NRPE: Error - Could not complete SSL handshake.

    Uh oh, Could not complete SSL handshake.! What does that mean?
    This is the most common error people see with the NRPE plugin. If you Google it, you’ll get an over-whelming amount of hits suggesting how you can resolve the problem. I found this link useful.
    That all said, I can probably tell you right off the bat why it isn’t working for you. Assuming you’re using the packaging I provided then it’s most likely because your NRPE Server is denying the requests your Nagios Server is making to it.

    To fix this, access your NRPE Server and open up /etc/xinetd/nrpe in an editor of your choice. You need to allow your Nagios Server access by adding it’s IP address to the only_from entry. Or you can just type the following:

    # Set your Nagios Server IP here:
NAGIOS_SERVER=192.168.192.168

# If you want to keep your previous entries and append the server
# you can do the following (spaces delimit the servers):
sed -i -e "s|^(.*only_from[^=]+=)[ t]*(.*)|1 2 $NAGIOS_SERVER|g" 
   /etc/xinetd.d/nrpe

# The below command is fine too to just replace what is there
# with the server of your choice (you can use either example
sed -i -e "s|^(.*only_from[^=]+=).*|1 $NAGIOS_SERVER|g" 
   /etc/xinetd.d/nrpe

# When your done, restart xinetd to update it's configuration
service xinetd reload

    Those who didn’t receive the error I showed above, it’s only because your using your Nagios Server as your NRPE Server too (which the xinetd tool is pre-configured to accept by default). So please pay attention to this when you start installing the NRPE server remotely.

    You will want to install nagios-plugins-nrpe on to your NRPE Server as well granting you access to all the same great monitoring tools that have already been proven to work and integrate perfectly with Nagios. This will save you a great deal of effort when setting up the NRPE status checks.

    As a final note, you may want to make sure port 5666 is open on your NRPE Server’s firewall otherwise the Nagios Server will not be able to preform remote checks.

    ## Open NRPE Port (as root)
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 5666 -j ACCEPT

# consider adding this change to your iptables configuration
# as well so when you reboot your system the port is
# automatically open for you. See: /etc/sysconfig/iptables
# You'll need to add a similar line as above (without the
# iptables reference)
# -A INPUT -m state --state NEW -m tcp -p tcp --dport 5666 -j ACCEPT
  • NSCA
    NSCA - Nagios Service Check Acceptor
    NSCA – Nagios Service Check Acceptor

    Remember, NSCA is used for systems that connect to you remotely (instead of you connecting to them (what NRPE does). This is a perfect choice plugin for systems you do not want to open ports up to unnecessarily on your remote system. That said, it means you need to open up ports on your Monitoring (Nagios) server instead.

    You’ll want to install nsca on the same server your hosting Nagios on:

    # Download NSCA
wget --output-document=nsca-2.7.2-9.el6.x86_64.rpm http://repo.nuxref.com/centos/6/en/x86_64/custom/nsca-2.7.2-10.el6.nuxref.x86_64.rpm

# Now install it
yum -y localinstall nsca-2.7.2-9.el6.x86_64.rpm

# This tool requires xinetd to be running; start it if it isn't
# already running
service xinetd status || service xinetd start

# Make sure our system will always start xinetd
# even if it's rebooted
chkconfig --level 345 xinetd on

# SELinux Users may wish to turn this flag on if they intend to allow it
# to call content as root (using sudo) which it must do for some status checks.
setsebool -P nagios_run_sudo on

    The best way to test if everything is working okay is by also installing the nsca-client on the same machine we just installed NSCA on (above). Then we can simply create a test passive service to test everything with. The below setup will work presuming you chose to use my custom build of Nagios introduced in my blog that went with it.

    # First install our NSCA client on the same machine we just installed NSCA
# on above.
wget http://repo.nuxref.com/centos/6/en/x86_64/custom/nsca-client-2.7.2-10.el6.nuxref.x86_64.rpm

# Now install it
yum -y localinstall nsca-client-2.7.2-9.el6.x86_64.rpm

# Create a temporary test configuration to work with:
cat << _EOF > /etc/nagios/conf.d/nsca_test.cfg
# Define a test service. Note that the service 'passive_service'
# is already predefined in /etc/nagios/conf.d/nsca.cfg which was
# placed when you installed my nsca rpm
define service{
   use                 passive_service
   service_description TestMessage
   host_name           localhost
}
_EOF

# Now reload Nagios to it reads in our new configuration
# Note: This will only work if you are using my Nagios build
service nagios reload

    Now that we have a test service set up, we can send it different nagios status through the send_nsca binary that was made available to us after installing nsca-client.

    # Send a Critical notice to Nagios using our test service
# and send_nsca. By default send_nsca uses the '<tab>' as a
# delimiter, but that is hard to show in a blog (it can get mixed up
# with the space.  So in the examples below i add a -d switch
# to adjust what the delimiter in the message.
# The syntax is simple:
#    hostname,nagios_service,status_code,status_msg
#
# The test service we defined above identifies both the
# 'host_name' and 'service_description' define our first 2
# delimited columns below. The status_code is as simple as:
#       0 : Okay
#       1 : Warning
#       2 : Critical
# The final delimited entry is just the human readable text
# we want to pass a long with the status.
#
# Here we'll send our critical message:
cat << _EOF | /usr/sbin/send_nsca -H 127.0.0.1 -d ','
localhost,TestMessage,2,This is a Test Error
_EOF

# Open your Nagios screen (http://localhost/nagios) at this point and watch the
# status change (it can take up to 4 or 5 seconds or so to register
# the command above).

# Cool?  Here is a warning message:
cat << _EOF | /usr/sbin/send_nsca -H 127.0.0.1 -d ',' -c /etc/nagios/send_nsca.cfg
localhost,TestMessage,1,This is a Test Warning
_EOF

# Check your warning on Nagios, when your happy, here is your
# OKAY message:
cat << _EOF | /usr/sbin/send_nsca -H 127.0.0.1 -d ',' -c /etc/nagios/send_nsca.cfg
localhost,TestMessage,0,Life is good!
_EOF

    Since NSCA requires you to listen to a public port, you’ll need to know this last bit of information to complete your NSCA configuration. Up until now the package i provide only open full access to localhost for security reasons. But you’ll need to take the next step and allow your remote systems to talk to you.

    NSCA uses port 5667, so you’ll want to make sure your firewall has this port open using the following command:

    ## Open NSCA Port (as root)
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 5667 -j ACCEPT

# consider adding this change to your iptables configuration
# as well so when you reboot your system the port is
# automatically open for you. See: /etc/sysconfig/iptables
# You'll need to add a similar line as above (without the
# iptables reference)
# -A INPUT -m state --state NEW -m tcp -p tcp --dport 5667 -j ACCEPT

    Another security in place with the NSCA configuration you installed out of
    the box is that it is being managed by xinetd. The configuration can
    be found here: /etc/xinetd.d/nsca. The security restriction in place that you’ll want to pay close attention to is line 16 which reads:

    only_from = 127.0.0.1 ::1

    If you remove this line, you’ll allow any system to connect to yours; this is a bit unsafe but an option. Personally, I recommend that you individually add each remote system you want to monitor to this line. Use a space to separate more the one system.

    You can consider adding more security by setting up a NSCA paraphrase which will reside in /etc/nagios/nsca.cfg to which you can place the same paraphrase in all of the nsca-clients you set up by updating /etc/nagios/send_nsca.cfg.

    Consider our example above; I can do the following to add a paraphrase:

    # Configure Client
sed -i -e 's/^#*password=/password=ABCDEFGHIJKLMNOPQRSTUVWXYZ/g' 
   /etc/nagios/send_nsca.cfg
# Configure Server
sed -i -e 's/^#*password=/password=ABCDEFGHIJKLMNOPQRSTUVWXYZ/g' 
   /etc/nagios/nsca.cfg
# Reload xinetd so it rereads /etc/nagios/nsca.cfg
service xinetd reload

I don’t trust you, I want to repackage this myself!

As always, I will always provide you a way to build the source code from scratch if you don’t want to use what I’ve already prepared. I use mock for everything I build so I don’t need to haul in development packages into my native environment. You’ll need to make sure mock is setup and configured properly first for yourself:

# Install 'mock' into your environment if you don't have it already.
# This step will require you to be the superuser (root) in your native
# environment.
yum install -y mock

# Grant your normal every day user account access to the mock group
# This step will also require you to be the root user.
usermod -a -G mock YourNonRootUsername

At this point it’s safe to change from the ‘root‘ user back to the user account you granted the mock group privileges to in the step above. We won’t need the root user again until the end of this tutorial when we install our built RPM.

Just to give you a quick summary of what I did, here are the new spec files and patch files I created:

  • NSCA RPM SPEC File: Here is the enhanced spec file I used (enhancing the one already provided in the EPEL release found on pkgs.org). At the time I wrote this blog, the newest version of NSCA was v2.7.2-8. This is why I repackaged it as v2.7.2-9 to include my enhancements. I created 2 patches along with the spec file enhancements.
    nrpe.conf.d.patch was created to provide a working NRPE configuration right out of the box (as soon as it was installed) and nrpe.xinetd.logrotate.patch was created to pre-configure a working xinetd server configuration.
  • NRPE RPM SPEC File: Here is the enhanced spec file I used (enhancing the one already provided in the EPEL release found on pkgs.org). At the time I wrote this blog, the newest version of NRPE was v2.14-5. However v2.15 was available off of the Nagios website so this is why I repackaged it as v2.15-1 to include my enhancements.
    nsca.xinetd.logrotate.patch was the only patch I needed to create to prepare a NSCA xinetd server working out of the box.

Everything else packaged (patches and all) are the same ones carried forward from previous versions by their package managers.

Rebuild your external monitoring solutions:

Below shows the long way of rebuilding the RPMs from source. 

# Perhaps make a directory and work within it so it's easy to find
# everything later
mkdir nagiosbuild
cd nagiosbuild
###
# Now we want to download all the requirements we need to build
###
# Prepare our mock environment
###
# Initialize Mock Environment
mock -v -r epel-6-x86_64 --init

# NRPE (v2.15)
wget http://repo.nuxref.com/centos/6/en/source/custom/nrpe-2.15-4.el6.nuxref.src.rpm 
mock -v -r epel-6-x86_64 --copyin nrpe-2.15-1.el6.src.rpm /builddir/build

# NSCA (v2.7.2)
wget http://repo.nuxref.com/centos/6/en/source/custom/nsca-2.7.2-10.el6.nuxref.src.rpm 
mock -v -r epel-6-x86_64 --copyin nsca-2.7.2-9.el6.src.rpm /builddir/build

#######################
### THE SHORT WAY #####
#######################
# Now, the short way to rebuild everything is through these commands:
mock -v -r epel-6-x86_64 --resultdir=$(pwd)/results 
   --rebuild  nrpe-2.15-1.el6.src.rpm  nsca-2.7.2-9.el6.src.rpm

# You're done; You can find all of your rpms in a results directory
# in the same location you typed the above command in.  You can 
# alternatively rebuild everything the long way allowing you to
# inspect the content in more detail and even change it for your
# own liking

#######################
### THE LONG WAY  #####
#######################
# Install NRPE Dependencies
mock -v -r epel-6-x86_64 --install 
   autoconf automake libtool openssl-devel tcp_wrappers-devel

# Install NSCA Dependencies
mock -v -r epel-6-x86_64 --install 
   tcp_wrappers-devel libmcrypt-devel

###
# Build Stage
###
# Shell into our enviroment
mock -v -r epel-6-x86_64 --shell

# Change to our build directory
cd builddir/build

# Install our SRPMS (within our mock jail)
rpm -Uhi nsca-*.src.rpm nrpe-*.src.rpm

# Now we'll have placed all our content in the SPECS and SOURCES
# directory (within /builddir/build).  Have a look to verify
# content if you like

# Build our RPMS
rpmbuild -ba SPECS/*.spec

# we're now done with our mock environment for now; Press Ctrl-D to
# exit or simply type exit on the command line of our virtual
# environment
exit

###
# Save our content that we built in the mock environment
###

#NRPE
mock -v -r epel-6-x86_64 --copyout /builddir/build/SRPMS/nrpe-2.15-1.el6.src.rpm .
mock -v -r epel-6-x86_64 --copyout /builddir/build/RPMS/nrpe-2.15-1.el6.x86_64.rpm .
mock -v -r epel-6-x86_64 --copyout /builddir/build/RPMS/nagios-plugins-nrpe-2.15-1.el6.x86_64.rpm .
mock -v -r epel-6-x86_64 --copyout /builddir/build/RPMS/nrpe-debuginfo-2.15-1.el6.x86_64.rpm .

#NSCA
mock -v -r epel-6-x86_64 --copyout /builddir/build/SRPMS/nsca-2.7.2-9.el6.src.rpm .
mock -v -r epel-6-x86_64 --copyout /builddir/build/RPMS/nsca-2.7.2-9.el6.x86_64.rpm .
mock -v -r epel-6-x86_64 --copyout /builddir/build/RPMS/nsca-client-2.7.2-9.el6.x86_64.rpm .
mock -v -r epel-6-x86_64 --copyout /builddir/build/RPMS/nsca-debuginfo-2.7.2-9.el6.x86_64.rpm .

# *Note that all the commands that interact with mock I pass in 
# the -v which outputs a lot of verbose information. You don't
# have to supply it; but I like to see what is going on at times.

# **Note: You may receive this warning when calling the '--copyout'
# above:
# WARNING: unable to delete selinux filesystems 
#    (/tmp/mock-selinux-plugin.??????): #
#    [Errno 1] Operation not permitted: '/tmp/mock-selinux-plugin.??????'
#
# This is totally okay; and is safe to ignore, the action you called
# still worked perfectly; so don't panic!

So where do I go from here?
NRPE and NSCA are both fantastic solutions that can allow you to tackle any monitoring problem you ever had. In this blog here I focus specifically on Linux, but these tools are also available on Microsoft Windows as well. You can easily have 1 Nagios Server manage thousands of remote systems (of all operating system flavours). There are hundreds of fantastic tools to monitor all mainstream applications used today (Databases, Web Servers, etc). Even if your trying to support a custom application you wrote. If you can interface with your application using the command line interface, well then Nagios can monitor it for you. You only need to write a small script with this in mind:

  • Your script should always have an exit code of 0 (zero) if everything is okay, 1 (one) if you want to raise a warning, and 2 (two) if you want to raise a critical alarm.
  • No matter what the exit code is, you should also echo some kind of message that someone could easily interpret what is going on.

There is enough information in this blog to do the rest for you (as far as creating a Nagios configuration entry for it goes). If you followed the 2 rules above, then everything should ‘just work’. It’s truely that easy and powerful.

How do I decide if I need NSCA or NRPE?

NRPE & NSCA High Level Overview
NRPE & NSCA High Level Overview

NRPE makes it Nagios’s responsibility to check your application where as NSCA makes it your applications responsible to report its status. Both have their pros and cons. NSCA could be considered the most secure approach because at the end of the day the only port that requires opening is the one on the Nagios server. NSCA does not use a completely secure connection (but there is encryption none the less). NRPE is very secure and doesn’t require you to really do much since it just simply works with the nagios-plugins already available. It litterally just extends these existing Nagios local checks to remote ones. NSCA requires you to configure a cron, or adjust your applications in such a way that it frequently calls the send_nsca command. NSCA can be a bit more difficult to set up but creates some what of a heartbeat between you and the system monitoring it (which can be a good thing too). I pre-configured the NSCA server with a small tweak that will automatically set your application to a critical state if a send_nsca call is missed for an extended period of time.

Always consider that the point of this blog was to state that you can use both at the same time giving you total flexibility over all of your systems that require monitoring.

Credit

All of the custom packaging in this blog was done by me personally. I took the open source available to me and rebuilt it to make it an easier solution and decided to share it. If you like what you see and wish to copy and paste this HOWTO, please reference back to this blog post at the very least. It’s really all I ask.

Sources

I referenced the following resources to make this blog possible:

  • The blog I wrote earlier that is recommended you read before this one:Configuring and Installing Nagios Core 4 on CentOS 6
  • Official NRPE download link; I used all of the official documentation to make the NRPE references on this blog possible.
  • A document identifying the common errors you might see and their resolution here.
  • Official NSCA download link; I used all of the official documentation to make the NSCA references on this blog possible.
  • The NRPE and NSCA images I’m reposting on this blog were taking straight from their official sites mentioned above.
  • Linux Packages Search (pkgs.org) was where I obtained the source RPMs as well as their old SPEC files. These would be a starting point before I’d expand them.
  • A bit outdated, but a great (and simple) representation of how NSCA works with Nagios can be seen here.
Linux HOWTO

Creating a Swap File for Your VPS

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Introduction

RAM is expensive when compared to physical hard disk (storage) in today’s market. Combine this knowledge with Virtual Private Server (VPS) solutions offered by vendors such as Amazon EC2 who will literately give us our own Linux box in The Cloud. They’ll also grant us full root level privileges to do whatever we want in these new sandboxes of ours. These vendors usually provide their product to us with a ton of disk space and very little RAM. That said, the more cash we’re willing to shell out of our wallet, the more ram and disk space we’ll receive in return (just like everything else in our consumer driven world).

If you chose a VPS as your preliminary (or final) hosting solution, then you (like everybody else) are on a budget. There is a good chance you’re working with a very minimal amount of RAM (probably 512MB to 4GB) regardless of how much cash you forked out of your pocket. Now you could argue that this is all you need, but this is where I say: you still need swap space no matter what.

This is truly worth blogging about because if you install just about any Linux distribution, it will (usually) set up a swap partition for you as part of it’s default out of the box settings. But most VPS solutions out there provide you with a pre-configured distribution of Linux that is not equipped with any swap space whatsoever. I wrote this blog because popular VPS vendors such as Linode and Digital Ocean do not provide swap space with their virtual machine (or at least at the time of writing this).

You were saying…

Did you ever think that maybe they don’t provide it because you don’t need it?

No, that’s simply not the case. The fact of the matter is; they offer packages on their website such as 1GB of RAM and 20GB of storage space and that is exactly what they give you. Could you just imagine how frustrated you’d be as a customer if you paid for that deal and found out you actually only had 512MB (0.5GB) of physical RAM while the remaining 512MB was just your swap partition? That would be just false advertising in my opinion. The issue could get even more complicated then that… would that mean they’d be providing you with 20GB of storage space minus (-) 512MB because it’s reserved for this swap partition? Thankfully at the end of the day we get what we pay for. In this example we’d have purchased a Linux distribution with 1 root partition (/) that is exactly 20GB in size along with 1GB of physical RAM. We could type the command free -m and see this 1GB of physical RAM for ourselves.

Why do I need swap space?

  • You grant your system more (virtual) memory (RAM) to work with. This is especially useful in dire situations when your memory may be almost exhausted. Remember, if you run out of RAM, the services running on your server WILL start to fail on you.
  • It can save you money! Why opt for a more expensive VPS solution that comes with more RAM when your application you choose to host (A mail server for example) may only utilize all of the RAM during it’s start up. Once the application has finished starting up, it may only use 2GB of ram thereafter. Was it really worth spending $100+ more a year just to handle your applications startup? A swap disk can alleviate this for you allowing you to choose a cheaper server.
  • You will free up physical RAM while swap space is present.

    Consider this: you’re running a program that has 200 features in it, but you’re only using 10 of them. Without swap space, all 200 features may (and most probably will) be loaded into physical (expensive) memory; even the features you’re not using! But with a swap file/partition, the system will detect the sections of the code not being referenced/used (in RAM) and move it into swap space. This process frees up physical memory for other services you want to run. The program won’t care you’re doing this; as far as it’s concerned, the resources are still available to it (which they are).

    In other words: The operating system is smart enough to note the frequency of code being executed. These pieces of accessed code will always remain in memory while less frequently accessed code will be swapped to the (swap) disk until it is needed again. The process is called paging when the operating system goes back into the swap space to retrieve this data (returning it to RAM for the program that requested it). We have some control over the swapping process (to limit paging) which I talk about later (swappiness).

Swap files are slow and can cause extensive i/o:

Swapping content from RAM back to a file storage (and back) is slow; there is no question about this. I also will not argue with you that this process does generate i/o. You obviously want to minimize the swapping of the content that you’re quite sure will be used again. But you still never want to eliminate it completely as it plays a huge role in optimizing your system.

In fact, the cons of swap space only outweigh it’s benefits if you abuse it by treating it as if it were additional physical RAM.

Hence you DO NOT create virtual swap space because you are out of memory! You create a swap space to optimize system resources. You create swap space to provide a safety net for your applications during peak operation times. You create swap space to increase system performance by freeing up sections of memory that are rarely being used.

If you only paid for 0.5GB (512MB) of RAM from your VPS provider, then do not consider yourself as having 2.5GB of RAM after you create a 2GB swap file. Sure, let your applications think this, but you yourself don’t want to exceed this 0.5GB memory boundaries any more then you have to. Obviously you’re creating more (virtual) RAM so that you can venture into the higher ranges in dire situations, but thats it. Otherwise you’re not using swap space effectively.

A Final Word of Caution…

With some vendors such as Amazon EC, you’ll pay for the disk i/o you generate. So improper use of swap space would work against you (costing you). Meanwhile companies like Crissic Solutions go out of their way to micro-manage the virtual machines they host. They specifically look for people violating disk i/o unnecessarily and are quick to launch their own internal investigations to see if you’re abusing their privileges. It’s this tight environment they enforce on everyone that allows them to promise their clients a very responsive experience.

Word of Caution
Just a Final Word of Caution
If you decide you need swap space (as I did) along with your services (and you should), then do not treat your results as free RAM.

The decision you make in choosing how much extra RAM you will give yourself with a swap file will not decide if you intend to abuse your hosting privileges. However the amount of this new free memory you intend to exhaust at once could… so just be considerate! Most vendors will generally just trust that you are not doing anything malicious; in fact you acknowledged this as part of the Terms of Service when you first signed up. So with that said, please don’t abuse the information in this blog or your VPS provider WILL catch you if their system disk i/o spikes for long durations of time. Trust me when I say: “it won’t take rocket science to track the source to be your VPS if you abuse their services.”

Its also worth noting that if you already have a swap partition on your system, then it’s really not worth adding another. You probably want to consider adding or paying for more RAM if your system is still struggling.

How much swap space do I need?

This really is the magic question, there are lots of people who have tons of different considerations for this. Personally, it honestly depends on what your going to run. Some applications utilize an insane amount of ram up front, but once they’re past their start up phase, they only use a fraction of RAM. Meanwhile, other systems are just generally memory hogs.

At the end of the day, the size really isn’t important. In fact it’s probably safer to have to much then too little. I mean if you had 16GB of physical RAM in your VPS, I’d (arbitrarily) say create a 10GB swap partition. This allows you to run a TON of applications and all of them have plenty of space to dump their unused code to. It also leaves you with a bit of a buffer (of extra RAM) in critical situations. If you have 512MB to 4GB of physical RAM, then I’d say create a partition that is about 2GB in size. Disk space is cheap and 2GB really isn’t that much.

Again, the decision is really up to you… more space is better then less (but too much just becomes wasted disk space). There is also no reason for your swap space to ever be larger then your actual alloted physical RAM. You’ll want to watch and make sure that too much paging is taking place at the end of the day regardless of what size you pick. Excessive paging means you’ve exhausted your physical RAM and you’re relying on the swap space you created to fill the void. You need to consider paying for more RAM at this point or not running so many applications on the Virtual Machine your VPS provided you.

Commands

Here is how you can create your own swap space (as a swap file) on an already running system.

# Creating a directory to host our swap file
mkdir -p /var/swap

# Agree on a size... in my case I want a 2GB buffer.
# Now ideally this would be the easy to read command to run
# as the following:
#   dd if=/dev/zero of=/var/swap/swap.fs bs=2G count=512
#
# But.. no... it's not that simple, the above command would
# need 2GB of ram which you may or may not have (yet).... so we
# need to work with a smaller block size and a higher count to
# achieve the same results.
#
# The below command is more i/o intensive but less memory
# intensive which is the situation for must of us reading this today.
#
# 1024MB = 1GB
# (1024 * 1024MB * 2) = 2097152 block size
#
dd if=/dev/zero of=/var/swap/swap.fs bs=1024 count=2097152

# Format swap file (allowing it to be a recognizable swap space
# partition)
mkswap -L swapfs /var/swap/swap.fs

# You may see a warning about including -f; don't bother... you're
# done after the above command, Now you want to protect
# the new file you created
chmod 600 /var/swap/swap.fs
chmod 100 /var/swap

# Update fstab file:
echo '/var/swap/swap.fs swap swap defaults 0 0'  >> /etc/fstab

# Enable the swap partition
swapon /var/swap/swap.fs

You’re done… You can use the following command to test to see if your partition got activated or not:

# Test to see swap correctly activated itself:
free -m

Swappiness

Yes, swappiness is a word… at least it is in respect to disk swapping/paging it is. Your system is most likely set to the same level of swappiness as the rest of us have (60). Here is how you can check what yours is set to:

[root@node01 ~]# cat /proc/sys/vm/swappiness
60

Swappiness is a number from 0 to 100 where the higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Generally 60 is pretty safe to use, but if this isn’t your machine that is hosting it. You may want to tweak this value a bit. Bring it down a bit so you only swap in situations where you absolutely have to. This will minimizing the i/o impact on the server provided to you at such a great cost.

This can be done using the following command:

# Note this is temporary and will be lost on a reboot
echo 40 > /proc/sys/vm/swappiness

Or if your happy with your change, you can permanently save this change by updating your /etc/sysctl.conf file:

# first make sure an existing entry doesn't already exist
sed -i -e '/[ t]*vm.swappiness/d' /etc/sysctl.conf

# Now create it with our entry
echo "vm.swappiness=40" >> /etc/sysctl.conf

# Now future reboots will hold this setting for you

It’s not really necessary to play with the default swappiness value unless this isn’t your server you just created a swap file on. Then it becomes a nice way to minimize unnecessary i/o when using a VPS as your solution.

Credit

If you like what you see and wish to copy and paste this HOWTO, please reference back to this blog post at the very least. It’s really all I ask.

Sources

Swap file generation is really well documented everywhere and certainly isn’t a new concept. My goal was just to bring it to your attention that most VPS solutions do not provide you with any swap space at all initially and you should consider adding it right away. Here are some helpful links on swapping:

Virtual Private Server (VPS)

I made reference to a few of them in this blog; I don’t want to promote one over the other either. It’s still worth sharing the different VPS solutions out there since they are a cost-effective way to host content for your personal use or business.

Linux HOWTO

Secure and Protect Your CentOS 6 System

1 Comment

Introduction

Security is important these days but sadly there are a scary amount of blogs and comments from people out there who solve their problems by turning off their firewall and/or disabling SELinux. These steps may work around a problem, but it will make your system much more vulnerable to cyber attacks. Learning alternative ways of solving your applications woes without disabling key safety components on your system is a better approach to keeping it secure. The goal of this blog is to offer a quick primer on some key security components that will protect you an your data!

The topics covered in this blog to keep your systems secure are:

  • Fail2Ban
    Intrusive detection and prevention
  • Firewall
    The front line to the internet (where all the bad stuff happens)
  • Restricting SSH Access
    A couple tweaks to help bulletproof yourself from possible intrusion.
  • System Auditing
    Find out what got accessed and by who and what major system calls were made.
  • System Monitoring
    Monitor your system for key things that could be symptoms of a hacked or compromised system.
  • Enable SELinux
    This is literally your last line of defense, and its a lot better then people make it out to be.
  • Disk Quotas
    Prevent a user from violating disk space on a production system.

Fail2Ban

For those of you who run a system on the front end of the internet have to deal with firewalls and security. Fail2Ban is a python based tool that wraps itself around iptables. It’s can cleverly watch system log files and detect abuse; it then proceeds to temporary (or permanently) block/ban the culprit by using their IP address in conjunction with your firewall.

At the time I blogged this, Fail2Ban was in the pre-release stages of v0.8.11. For this reason I’ll focus on v0.8.10.

Get the Software
Now the EPEL Repositories already provide us with software to do the installation here (and source rpm here).

Alternatively, I rebuilt the source myself and am hosting it here (in case the version changes significantly enough that this tutorial no longer works). My hosted version of version of Fail2Ban rpm can be retrieved here (and source rpm here).

Setting Up Fail2Ban

# Configure epel (if not already)
rpm -Uhi http://fedora.mirror.nexicom.net/epel/6/i386/epel-release-6-8.noarch.rpm

# Install the goods
yum -y install fail2ban

# Optionally install 'jwhois'. This tool gives you great detail on people
# accessing your system by performing a whois lookup on them and including
# the results of this in the automated email sent. Honestly it's worth it.
yum -y install jwhois

Out of the box the settings will work with SSHD only and for now that is all I need. However I’ve made it a bit more restrictive to satisfy my needs:

# Create a backup of our original configuration files before we apply
# any changes to them:
[ ! -f /etc/fail2ban/fail2ban.conf.orig ] && \
   cp -af /etc/fail2ban/fail2ban.conf /etc/fail2ban/fail2ban.conf.orig
[ ! -f /etc/fail2ban/jail.conf.orig ] && \
   cp -af /etc/fail2ban/jail.conf /etc/fail2ban/jail.conf.orig

# var/log/messages is busy enough; I prefer to use
# /var/log/fail2ban.log personally (it's already pre-configured
# with a logrotate.d entry to accommodate this)
sed -i -e 's|^logtarget[ t]=.*|logtarget = /var/log/fail2ban.log|g' \
	/etc/fail2ban/fail2ban.conf

# The below ;'sed' commands only change the first occurrence in the file
# # which is where the [default] is identified
# Default Ban for 1 day (86400 seconds = 1 day)
sed -i -e '0,/^bantime[ t]*=.*/s//bantime = 86400/' \
	/etc/fail2ban/jail.conf

# Default Ban if more then 3 unsuccessful attempts are made within 30 min
# (1800 seconds = 30 min).
sed -i -e '0,/^maxretry[ t]*=.*/s//maxretry = 3/' \
	/etc/fail2ban/jail.conf
sed -i -e '0,/^findtime[ t]*=.*/s//findtime = 1800/' \
	/etc/fail2ban/jail.conf

# These IPs don't conform to our very restrictive checks and will
# bypass the Fail2Ban security. For my own personal privacy, I've
# adjusted them from my own personal IPs. You might want to change
# this to reflect what you want (use spaces to delimit them). You
# can use masks too such as 10.128.3.0/16 (adding the slash (/)).
# but remember you need to escape () the slash in the below
# sed statement.  For example, the below will add the following:
#     127.0.0.1, 1.2.3.4, and 7.8.9.0/24
#
# Note: At a minimum, make sure to include 127.0.0.1
sed -i -e '0,/^ignoreip[ t]*=.*/s//ignoreip = 127.0.0.1 1.2.3.4 7.8.9.0\/24/' \
	/etc/fail2ban/jail.conf

# Configure the system to start fail2ban after every reboot
chkconfig --levels 345 fail2ban on

# Start it up now for the first time (use 'restart' instead
# of 'start)' if it was already running:
service fail2ban start

There are lots of configurations already available that ship with this tool (but disabled by default). Have a look at /etc/fail2ban/jails.conf, perhaps there are others you might be interested in. If you’re uncertain what some of them are, or what they do; just have a look at the fail2ban manual.

Firewall / iptables

This literally your front line runner to all the security between you and the violent internet. Your firewall is your only shield and one of the last line of defense you have in some scenarios. SELinux would be the last line of defense you have which I talk about later. If troubleshooting a product has boiled down to stopping your firewall, then you’ve done something horribly wrong. There are other ways to debug firewall issues and stopping it shouldn’t be one of them.

Try running the following command just to see what ports your system is already listening on:

# The below lists all listening connections that could be
# being access remotely if your firewall is disabled
#
netstat -pnat | egrep LISTEN | 
   tr -s ' ' | cut -f4,7 -d' ' | sed '/^0.0.0/d'

It’s the above list you’re trying to protect! It’s not uncommon for an application to communicate to another through ip (such as a database does); but these ports do not need to be open to the entire internet!

Setting Up a Simple Firewall
Here is a very simple firewall you can use to get you started:

# Set this to your internet interface (it might be ppp0 too)
# run ifconfig to see what interfaces you have
PUBLIC=eth0
cat << _EOF > /etc/sysconfig/iptables
#
# A Simple firewall that allows access to SSH and Web Based inbound
# connections but will allow you to access everything outside
#
*filter
#---------------------------------------------------------------
# Drop Everything by default
#---------------------------------------------------------------
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]

#---------------------------------------------------------------
# Internal Traffic is Okay
#---------------------------------------------------------------
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT

#---------------------------------------------------------------
# Always accept already established connections
#---------------------------------------------------------------
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

#---------------------------------------------------------------
# Deny traffic from internet that spoofs addresses used internally
#---------------------------------------------------------------
-A INPUT -i $PUBLIC -s 192.168.0.0/24 -j DROP
-A INPUT -i $PUBLIC -s 127.0.0.0/8 -j DROP
-A INPUT         -d 10.0.0.0/8 -j DROP
-A INPUT         -d 169.254.0.0/16 -j DROP

#---------------------------------------------------------------
#  All Outbound Traffic Accepted (for now)
#---------------------------------------------------------------
-A OUTPUT -o $PUBLIC -j ACCEPT

#---------------------------------------------------------------
# SSH Allowed
#---------------------------------------------------------------
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

#---------------------------------------------------------------
# Web Traffic Allowed
#---------------------------------------------------------------
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT

#---------------------------------------------------------------
# The default is to drop everything else
# but for read-ability and peace of mind
# we force it again anyway
#---------------------------------------------------------------
-A INPUT -i $PUBLIC -j DROP
-A FORWARD -i $PUBLIC -j DROP

# End
COMMIT
_EOF

# Now restart iptables for the new rules to take affect
service iptables restart

# If you're running fail2ban then you'll need to restart it too
# since the extra chains it creates into iptables would have just
# got wiped with the last command.  Restarting it will rebuild
# everything the way it should be
service fail2ban restart

# Ensure this file is not accessible by anyone
chmod 600 /etc/sysconfig/iptables

When or if you add a new program into your environment, it should just work… if you need to share or host it’s services to others, find out what ports it uses and ‘ONLY’ open them. Consider the security risks that become available to you once these ports are open to the world as well. Hence you are putting all your trust in the application listening on every port you open. Also note that every port your open is a potential point of entry a hacker can use into your system.

Restrict SSH Access

SSH is a great way for you to connect remotely to your server and see how things are going, make changes etc. But consider other people (whom you don’t know) might be trying to access it also (usually for malicious reasons). If you’ve set up fail2ban already, then you’re already in really good shape. But consider restricting the the SSH Daemon even more for precautionary reasons. Here is what I’m suggesting:

  • Disable remote SSH access for the root user login: 
    # Before you do this, be sure you have an non-root account you
# can still connect to the system as that you will use instead of
# root
useradd nuxref
# I always add my users to the users group, you don't have to do
# this:
usermod -G users nuxref
# Set a password for the user you created
passwd nuxref

# In the above example I can use 'nuxref' as an entrance into the
# system to which I can switch to the root after I establish a
# my connection.
# 90% of the constant connections your server will face when
# directly connected to the internet will be as the 'root' user.
# So why even bother enabling that account? Fail2Ban will end up
# blocking them shortly anyway, but why even give them 3 lucky
# guesses? It's really not worth it. It's much safer to use another
# account and switch to root if needed later.

# This also means that if for some miraculous reason someone
# guesses your non-root account to gain access to the system, the
# damage they can do will be as minimal as the access you've given
# that account.

# Now Deny Root Login Attempts
sed -i -e 's/^[# t]*PermitRootLogin .*/PermitRootLogin no/g' /etc/ssh/sshd_config

# To prevent the system from allowing additional users you add to
# your system from accessing it remotely you'll want to consider
# doing the following:
# First get rid of an existing entry
sed -i -e '/^[# t]*AllowUsers .*/d' /etc/ssh/sshd_config
# Now add our user restrictions (if you have more then one
# user you want to add, separate them using spaces
echo "AllowUsers nuxref" >> /etc/ssh/sshd_config
  • Consider using a banner message as a warning to let people know they’ve inadvertently accessed a system they shouldn’t. Sure a hacker will ignore this message, but for the poor fellow who really did just mistype an ip or host; this will give your server some character and notify them that you are monitoring them. Nothing but a scare tactic; but it’s still worth doing. 
    # A simple scare banner
cat << _EOF > /etc/banner
* - - - - - - - W A R N I N G - - - - - - - - - - W A R N I N G - - - - - - - *
*                                                                             *
* The use of this system is restricted to authorized users. All information   *
* and communications on this system are subject to review, monitoring and     *
* recording at any time, without notice or permission.                        *
*                                                                             *
* Unauthorized access or use shall be subject to prosecution.                 *
*                                                                             *
* - - - - - - - W A R N I N G - - - - - - - - - - W A R N I N G - - - - - - - *
_EOF

# Make sure it's not accessible by others
chmod 640 /etc/banner

# SELinux Handling
restorecon /etc/banner

# Now tell the SSH Daemon to reference it:
sed -i -e 's|^[# t]*Banner .*|Banner /etc/banner|g' /etc/ssh/sshd_config
  • Disable X11 forwarding and Tcp Forwarding; These are what hackers will want to utilize if they ever successfully gain access to your system: 
    # X11 grants someone the ability to launch X applications locally
# at their machine that are associated with your server.  In some
# cases this is okay. But if you're just hosting web servers and
# databases; you shouldn't offer free candy to a potential unknown
# users who may have just connected to your production system.
sed -i -e 's|^X11Forwarding .*|X11Forwarding no|g' /etc/ssh/sshd_config
# Yet another service that just shouldn't be open no a production
# system. Someone is trying to access something they couldn't
# otherwise have done it if they're invoking this. Not saying this
# feature isn't powerful, I'm just saying disable it until you
# find a reason not to.
sed -i -e 's|^[#]*AllowTcpForwarding .*|AllowTcpForwarding no|g' /etc/ssh/sshd_config
  • Disable UseDNS reference. This is optional; I just do this because it’s faster. Also IPs are easier to scan later. The DNS lookup can slow things down sometimes which can be annoying. This isn’t a security thing at all; it just makes things faster. 
    # Disable UseDNS
sed -i -e 's|^[# t]*UseDNS .*|UseDNS no|g' /etc/ssh/sshd_config

When you’re comfortable with all your changes, you can restart the SSH Daemon to take them on. If you were logged in remotely already, don’t worry, you won’t lose your connection when you do this.

# Restart SSHD so it will reread it's configuration 
service sshd restart

Enable Auditing

If you have to resort to auditing, then your system may have already been compromised, but this will play a key role in figuring out what happened. Hopefully you’ll never have to rely on this step, but being cocky and going without it might become problematic in the future. This tool will help make your system better in the long run.

But auditing gives you something more as well; once it’s enabled you can set up the monitoring of it’s log file (/var/log/audit/audit.log) for suspicious activity. You can even go as far to write your own plugin for Fail2Ban to study the audit.log and react if certain suspicious system transaction takes place by a non-root user.

Auditing should be a considered mandatory service you implement on all of your servers.

Setting Up Some Simple Working Audit Rules
Here is a quick audit file you can use (and tailor to your liking) just to get you started; even if you added nothing else to this file, it’s configuration as is may save you one day:

# Install Auditing if it isn't already (most systems install this out of
# the box)
yum -y install audit
# Ensure your system will always run it
chkconfig --levels 345 auditd on
# Start it if it isn't already started:
service auditd status || service auditd start
# Install Audit Rules
cat << _EOF > /etc/audit/audit.rules
# First rule - delete all
-D

# increase the buffers to survive stress events. make this bigger for
# busy systems.
-b 1024

# monitor unlink() and rmdir() system calls.
-a exit,always -F arch=x86_64 -S unlink -S rmdir
# settimeofday so we know no one is adjusting the system times
-a exit,always -F arch=x86_64 -S settimeofday
# setrlimit.* so we know when kernel resources are being adjusted
-a exit,always -F arch=x86_64 -S setrlimit
# Filesystem Mounting (umount = 32bit, umount2 = 64bit)
-a exit,always -F arch=x86_64 -S mount -S umount2

#Ensure that failed use of the following system calls is audited
-a exit,always -F arch=x86_64 -S quotactl -S kill -S chroot -F success=0 -F auid=-1 -F auid=0

# some file and directory watches
-w /var/log/audit/ 
-w /etc/audit/auditd.conf -p rxwa
-w /etc/audit/audit.rules -p rxwa

# monitor write-access and change in file properties (read/write/execute)
# of the following files.
-w /etc/group -p wa
-w /etc/passwd -p wa
-w /etc/shadow -p wa
-w /etc/sudoers -p wa

# monitor write-access to the following directories
-w /etc/fail2ban -p wa
-w /etc/httpd -p wa
-w /etc/cron.d -p wa
-w /var/www -p wa

# lock the audit configuration to prevent any modification of this file.
#-e 2
_EOF

# Restart Audit To take on new configuration
service auditd restart

Now you can check for changes as root using commands like:

# Search for anyone touching the /etc/passwd file
ausearch -f /etc/passwd
# Search for anyone accessing the /etc/fail2ban/fail2ban.conf
ausearch -f /etc/fail2ban/fail2ban.conf

Don’t forget to uncomment the very last line of the /etc/audit/audit.rules if you’re using sample configuration file I have above as your template. The -e 2 will prevent someone from turning the auditing off before they wreck havoc on your system. the -e 2 will make it so a reboot is required for the rules to change. This IS what you want; trust me! Hackers aren’t stupid; disabling auditing is the first thing they’ll attempt before they begin creating their chaos.

Monitoring Strategies

I already wrote a tutorial on using Nagios here. Consider using this; it can even be configured to monitor the audit logs and set alarms off when something unusual is matched.

But consider monitoring things such as the following as well:

  • The System Load
  • Remaining Disk Space
  • Bandwidth Utilization
  • Number of Users Remotely Logged in

All of the suggestions above can help you quickly identify unusual behavior and allow you to take action immediately (instead of just the next time you happen to be on the system). Obviously you’ll know your system better than anyone else, so if you expect the system load to go high at night for certain tasks, you can consider this in your monitoring as well.

It’s the times that nothing should be out of the ordinary that an alarm could help you resolve a problem just minutes (if not seconds) after it occurs. Monitoring also works in your favour for checking other system commands such as web page availability (if you’re hosting one) etc. Find out something is wrong before your very own customers do is the key here.

In fact, there really isn’t a good reason you should ever consider snuffing monitoring from your security TODO list.

Enable SELinux

There are a lot of people who seem to be really against using SELinux when in fact it is totally amazing and really easy to work with once you understand it. The biggest problem most people have is they don’t ever take the time to learn it. Hell, even I’ll admit it’s a frustrating learning curve in it’s unknown state.

But honestly: it effectively allows you to revoke sections of your file system as well as major system calls based on the executable code (not just the user/group). Why is this so important? Well take Apache for example. If someone were to successfully compromise it from the outside (using a buffer overflow), they will have gained full user access granted to the apache user and can literally browse your entire system. Their goal would be to potentially look for other exploits that they can use to gain higher privileges (such as root access). Heck even as the Apache user, you can run quite a lot of programs including ones that access the internet.

With respect to the Apache example just explained, if the administrator had SELinux running (in Enforcing Mode) the hacker is restricted to only reading and accessing the permissions assigned to the /usr/sbin/httpd binary. At most they’d be able to view your Apache configuration file and read html files… That’s about it.

SELinux is an amazing tool for locking down your system. In fact, SELinux is your very last line of defense. If this isn’t running and an application gets compromised, you’re going to have to rely completely on your audit logs to find what damage was done to your system and attempt to repair it (hopefully from backups).

I won’t lie though, the big problem with SELinux is that the documentation is poor. Not to mention that no one really promotes it’s fantastic functionality. Instead everyone just disables it and moves on.

One of the simplest things you can do upfront when trying to work with SELinux is just keep it in enforcing mode. RHEL/CentOS packages have already looked after most of the settings for you. In the rare case something doesn’t work for you; instead of panicking and disabling SELinux (which all websites tell you to do). Set it to ‘Permissive’ mode instead. This is effectively the same as disabling it except it still records violations in /var/log/audit/audit.log what it would have otherwise denied.

# Set SELinux into Permissive mode
setenforce 0

At this point your application (that may have not worked prior to this call) may suddenly work again. It is at this point you you can begin seeing what SELinux was denying by referencing these very audit logs. These logs play a key in making everything work for you again.

Making a New Product SELinux Compatible
Take Nagios for example and the blog I did for it. This is really easy to protect and still have SELinux running in Enforcing mode. Assuming you set SELinux in Permissive mode already (explained above), you can now generate a fast set of rules to allow the product to work with it enabled. Here is how you can do it:

# First make sure you have the right package installed to work
# with:
yum install -y policycoreutils

# For alarms to be generated, you'll want to run Nagios with
# SELinux in Permissive mode for a while (preferably a day would
# be great to get everything) 

# Filter out only nagios alarms that were generated by SELinux
cat /var/log/audit/audit.log | grep denied | 
	grep nagios > nagios.audit.log

# Now tailor the file if necessary (strip out lines you don't
# want to grant access to). Use a simple editor (like vi) to do
# this. It's really not that hard to read, and the output will
# show you precisely what nagios tried to access.  In some
# cases, you may never want nagios to access these things. so
# remove these entries from this list. Only keep the denied
# messages you want to reverse (and allow)

# Create an installable Module (prefix with 'my' to avoid
# conflicts with other package management that it might be using)
audit2allow -v --input nagios.audit.log -M mynagios

## Install the new module you just created
semodule -i mynagios.pp

That’s it; now you can set SELinux back to enforcing mode for the added security it offers you:

# Set SELinux back to Enforcing mode
setenforce 1

Was that really so hard? Every now and then grep through your audit logs (like you already did above) and scan for the keyword ‘nagios’. It’s possible it may still try to do things to which it’s being denied access to. You may even need to update your mynagios.pp SELinux module with extra entries found using audit2allow again. Consider also that it may be doing things you’re quite happy to leave the way it is. You don’t need to grant it access to absolutely everything, just want is necessary for it to operate.

Remember; if your project is compromised it will try to do all sorts of things it shouldn’t do and SELinux will be the barrier that will save your system.

Enable User Disk Quotas

Even if you’re running SELinux and have locked your system with auditing. A hacker can still try to bring your system down completely by filling it’s disk space until it’s full. This can cause other applications to catastrophically fail or stop behaving as they should which will is not cool at all.

By enabling user disk quota’s you can restrict the amount disk space imposed by a user (and even group level). Above, in the "Restricting SSH Access" points, I created a user called nuxref who i’d use as the entry point into my system. If you followed that piece of advice and significantly restricted remote access to your system you’re already halfway there. But… in the extremely unlikely circumstances that remote entry account gets compromised, you will want to be sure you keep the hackers options to a minimum.

Since I’ll only use that account to access the system and either check a few things or switch to the root user to make a change, I want to eliminate it’s ability to do much anything else. Most hackers start transferring all their hacking tools onto the system the second they gain access. It’s through these tools they can exploit more content. Other hackers might just want to completely fill your disk space causing your production data to crash.

An Example When You Might Want To Consider Disk Quotas:
Some cloud hosting services that provide you a bare bone virtual machine do not partition/carve out it’s disks in ways you’d have otherwise done differently. They do this primarily because they usually offer such small amounts of disk space (like 20-40GB) and it would be unfair of them to carve it in a way you might complain about. It’s better for them to just give your the entire space as one partition. But hey! When you pay for a very cheap hosting service; you take what you get. 🙂

Here is an example of what one of the virtual hosting providers I use did to my system when they gave it to me:

[root@node01 ~]# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/vda              20642428   1437052  18156800   8% /
none                    251228         0    251228   0% /dev/shm

In this example, /home (where users log into) and /tmp are not carved into their own partition. These directories are critical because all users usually have full read/write permissions here. If this were your production environment (and it is for many), a user could completely fill these unprotected directories until the file system was full. So in this example, Disk Quotas would be a very good idea!

Setting Disk Quotas Up
I will set up a assuming the file system I have to work with is identical to that displayed above (where I only have a ‘/’ partition) and a single user account that is accessible by SSHD. I’ll keep using the nuxref account to make things consistent.

Up until now I also gave you ‘one-liners’ to automate everything by simply copying and pasting from your browser to your command line. Well this one here will differ across systems, so it’s not really fair for me to give you this.

  1. First make sure the correct software is installed onto your system: 
    yum -y install quota
  2. Identify what partition needs to be updated. You need to open up your /etc/fstab file for this. I’m personally a vi fan, but use whatever editor your comforatble with. 
    # Here is a before snapshot of the /etc/fstab file before I edited it:
[root@node01 ~]# cat /etc/fstab             
LABEL=DOROOT       /               ext4    errors=remount-ro 0       1
none             /dev/shm      tmpfs   defaults                    0 0
  3. I want to adjust the ‘/’ partition (column 4); I need to update the line that already reads errors=remount-ro with errors=remount-ro,usrquota,grpquota. The fstab file will look like this for me when i’m finished: 
    # Here is an after snapshot of the /etc/fstab file after I edited it:
[root@node01 ~]# cat /etc/fstab 
LABEL=DOROOT       /               ext4    errors=remount-ro,usrquota,grpquota 0       1
none             /dev/shm      tmpfs   defaults                    0 0
  4. Now remount the ‘/’ filesystem (or whatever mount you chose to update in your /etc/fstab file) by typing the following: 
    mount -o remount /
  5. Now allow your system to scan your filesystem for files so it can begin tracking them: 
    quotacheck -avugm

    The step above could take hours depending on how busy your system is or the number of files that reside on the partition your scanning.

    You may see some output like this (don’t worry; nothing bad has happened) 🙂

    quotacheck: Scanning /dev/vda [/] done
quotacheck: Cannot stat old user quota file: No such file or directory
quotacheck: Cannot stat old group quota file: No such file or directory
quotacheck: Cannot stat old user quota file: No such file or directory
quotacheck: Cannot stat old group quota file: No such file or directory
quotacheck: Checked 4404 directories and 30017 files
quotacheck: Old file not found.
quotacheck: Old file not found.

    You may also see a warning like this:

    quotacheck: Your kernel probably supports journaled quota but you are not using it. Consider switching to journaled quota to avoid running quotacheck after an unclean shutdown.

    Journaling is a way of managing the transactions that are written and removed from your hard disk. It makes for a faster and more promising data recovery should your server ever suffers a hard crash (power outage or kernel crash). Journals sure do have their merits, but there are times when you don’t want them either. One reason is because your server is using a Solid State disk. These drives are SO much faster then traditional (mechanical) hardrives, but the trade off is they don’t like unnecessary writes. Disabling journaling is a way of prologging the life of these kind of drives.

    In a nutshell… if you’re getting this warning about journaled quota being disabled, it’s not a bad thing. In the example I’m using, the ‘/’ (ext4) partition is in fact residing on a Solid State hardrive, so I’m already fully aware of my scenario and know that the filesystem was formatted with out journaling enabled. So if it’s disabled already on your system too, think twice before enabling it; it is probably disabled for a reason and is completely safe to leave off (despite this warning).

  6. Now we can activate the quota system:

    quotaon -av

    You should see the following output (/dev/vda will be replaced with whatever device your currently watching quotas with):

    /dev/vda [/]: group quotas turned on
/dev/vda [/]: user quotas turned on
  7. Now you can look at the restrictions put in place:

    [root@node01 ~]# repquota -a
*** Report for user quotas on device /dev/vda
Block grace time: 7days; Inode grace time: 7days
                        Block limits                File limits
User            used    soft    hard  grace    used  soft  hard  grace
----------------------------------------------------------------------
root      -- 1212596       0       0          32284     0     0       
postfix   --      60       0       0             38     0     0       
nuxref    --      60       0       0             10     0     0 
apache    --      12       0       0              3     0     0       
postgres  --      16       0       0              4     0     0

    The quota are identified under the soft and hard columns which are all zero’s (0). This means there are no restrictions in place at all.

  8. We want to impost a restriction on the nuxref user in my case. I want to restrict this user to 10MB as there is no excuse why I should need more then that if I’m only using the account to check on things or switch to root. This is done using the command edquota username which will pop up an editor allow you to change these fields.

    # Edit the nuxref user's quota
edquota nuxref
# Adjusted both the soft and hard limits to 8 and 10 respectively.

[root@node01 ~]# repquota -a    
*** Report for user quotas on device /dev/vda
Block grace time: 7days; Inode grace time: 7days
                        Block limits                File limits
User            used    soft    hard  grace    used  soft  hard  grace
----------------------------------------------------------------------
root      -- 1212596       0       0          32284     0     0       
postfix   --      60       0       0             38     0     0       
nuxref    --      60    8000   10000             10     0     0 
apache    --      12       0       0              3     0     0       
postgres  --      16       0       0              4     0     0

    As you can see, the nuxref user is now restricted to 10 MB. If soft limit (set to 8MB in this example) is reached, an email will be sent to the user and for a ‘grace’ period (default is 6 days), the user will still be able to write more content to this directory. But after the grace period is reached the ‘soft’ limit is identical to the ‘hard’ limit and restricts further writing (until you clean up some space).

    It’s worth noting too that we could have acomplished the exact same effect (without using an editor) as above with the following one liner:

    # If you script the following; make sure you specify the correct device!
# /dev/vda is used below only because that follows inline with all my
# examples above.
setquota -u nuxref  8000 10000 0 0 /dev/vda

Disk Quotas Key Point If You Use Them:
I want to be clear about this: If you choose to use Disk Quotas, you only really need to lock down the accounts that ‘could’ be compromised. In my examples in this blog, I only have one account (nuxref). It’s not necessary to lock down system accounts like apache, postgres, mysql, etc. NEVER lock down the root user!. Most of you may not even need to do this step; It truly is an extreme (and in most cases unnecessary) precautionary step that only ‘some’ should take.

Heartbleed Security Exploit

This is probably one of the biggest security exploits that has surfaced in the past few years that can compromise many systems (and potentially has been for years). This includes systems such as CentOS 6.5 (or less) allowing attackers to access random (64K) chunks of your RAM (memory) at a time (which is almost never encrypted). This can allow them to extract a lot of sensitive information that they shouldn’t have otherwise been granted access to. If you’ve installed a newer version of CentOS (or Red Hat) than v6.5 you are most likely okay. But with that all said: Make sure your version of OpenSSL is at least 1.0.1e-16!

You can check your version by typing:

# Check the details of your openssl:
rpm -qi openssl
# The output should read this version (at least):
#
Name        : openssl                      Relocations: (not relocatable)
Version     : 1.0.1e                            Vendor: CentOS
Release     : 16.el6_5.7                    Build Date: Mon 07 Apr 2014 10:43:19 PM EDT
# ....

If your version of openssl is not satisfactory, then you should update it immediately to keep your system secure! Download it from the CentOS repositories directly, or use yum:

# Get the latest version of openssl
yum update -y openssl

The website heartbleed.com does a fantastic job explaining the details on this exploit if you are interested. The Common Vulnerabilities and Exposures is defined here: CVE-2014-0160.

Credit

This blog took me a very long time to put together and test! If you like what you see and wish to copy and paste this HOWTO, please reference back to this blog post at the very least. It’s really all I ask.

Sources

A lot of what I wrote here was accumulated knowledge over time. I can’t recall where all my sources came from and for that I’m sorry. This blog is more of a brain dump along content I keep in a personal Wiki containing information I’ve been meaning to share with everyone.

Here are some great links that may help you understand some topics discussed here a bit better though: