Skip to main content

SELinux: improve the security of your EC2 servers

SELinux provides tools to more finely control the activities allowed to users, processes, and daemons to limit the potential damage from vulnerabilities.

In the third and final part of our server security series, we will look at how we can enhance the security of Linux-based AWS EC2 instances with SELinux. We will learn how to set up SELinux on Amazon Linux, and we will walk through a simple example on Red Hat Enterprise Linux (RHEL).

In Linux, we can easily control access to an individual file or directory by modifying the standard file permissions. We can define if we want to allow read, or write, or even execute permissions to the file owner, to all members of a single group, or to everyone else. If the standard file permissions are insufficient, we can also define Access Control Lists (ACL) which allow us to set permissions on an even finer scale.

However, there is an obvious limitation to this type of access control: there is no good way to restrict or limit a process from accessing files and directories that it should not have access to in the first place. For example, an Apache web server should have access to /var/www/html/index.html, but not /etc/passwd.

This limitation can be addressed by using SELinux as an additional layer of access control. The main model used is called Type Enforcement where processes and file system objects are labelled based on their types. SELinux compartmentalizes processes by defining rules around the types in its policy to determine what the processes are allowed to access. SELinux policies deny everything by default unless it is explicitly allowed.

Activating SELinux

On Amazon Linux AMI release 2015.09, SELinux is disabled by default. I am not sure what the state of SELinux is on Amazon Linux but, in any case, you can enable it by performing the following steps.

$ sestatus
SELinux status:                 disabled

Install the selinux-policy, selinux-policy-targeted and policycoreutils-python packages, and ensure that SELinux is configured to be enforced.

# yum install selinux-policy selinux-policy-targeted policycoreutils-python
# egrep -v -e ^\# -e ^$ /etc/selinux/config

After that, edit the menu.lst file and append "selinux=1 security=selinux" at the end of the kernel command:

# cd /boot/grub
# cp -prv menu.lst menu.lst.default
‘menu.lst’ -> ‘menu.lst.default’
# vim menu.lst
# diff -u menu.lst.default menu.lst
--- menu.lst.default	2015-11-15 23:43:54.874843843 +0000
+++ menu.lst	2015-11-15 23:44:19.979125318 +0000
@@ -6,6 +6,6 @@
 title Amazon Linux 2015.09 (4.1.10-17.31.amzn1.x86_64)
 root (hd0,0)
-kernel /boot/vmlinuz-4.1.10-17.31.amzn1.x86_64 root=LABEL=/ console=ttyS0
+kernel /boot/vmlinuz-4.1.10-17.31.amzn1.x86_64 root=LABEL=/ console=ttyS0 selinux=1 security=selinux
 initrd /boot/initramfs-4.1.10-17.31.amzn1.x86_64.img

Finally, create an empty .autorelabel file in the root directory to label the entire system with the SELinux context after the instance is rebooted. It will take some time, especially since this is the first time SELinux is enforced on the instance. We need to perform this step to avoid potential issues arising from mislabelled files.

# touch /.autorelabel
# sync; init 6

In the system log, you will see the following message before it is rebooted again.

*** Warning -- SELinux targeted policy relabel is required.
*** Relabeling could take a very long time, depending on file
*** system size and speed of hard drives.

Once the system reboots by itself for the second time, you will find that SELinux is enforced.

$ getenforce

Now we’ll walk through a simple example of how SELinux enforces its policy. We will change the default port number of the OpenSSH service and see how SELinux reacts. We will use Red Hat Enterprise Linux 7.1 to modify the SELinux policy so that we can use the new port number – although we could perform (almost) the exact same steps on Amazon Linux.

Make sure you modify your EC2 instance’s Security Group to include this "Custom TCP Rule" before proceeding further.
On Red Hat Enterprise Linux 7.1, SELinux is enabled and enforced by default.

# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.1 (Maipo)
# sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Max kernel policy version:      28

We know that the default port number for the OpenSSH server is 22. We want to change it to port 31337. We can do so by modifying the /etc/ssh/sshd_config configuration file to include "Port 31337".

# cd /etc/ssh
# cp -prv sshd_config sshd_config.default
# vim sshd_config
# egrep -i ^.?port /etc/ssh/sshd_config
#Port 22
Port 31337

After modifying the sshd_config configuration file, we can restart the OpenSSH service.
But before we restart the service, let’s install the setroubleshoot-server RPM package for the sealert tool. I am unable to find this package on Amazon Linux, but it is available on Red Hat Enterprise Linux.

# yum -y install setroubleshoot-server

Everything that SELinux has denied will be logged in /var/log/audit/audit.log. In order to make sense of the logs, you can use the sealert to diagnose the denial messages in layman’s terms. It provides easy-to-understand explanations of the messages and suggests how we can go about addressing such denials next time.

Now we are ready to restart the OpenSSH service.

# systemctl restart sshd.service
# systemctl status sshd.service
sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
   Active: activating (auto-restart) (Result: exit-code) since Sun 2015-11-15 11:29:05 EST; 5s ago
  Process: 10871 ExecStart=/usr/sbin/sshd -D $OPTIONS (code=exited, status=255)
 Main PID: 10871 (code=exited, status=255)
Nov 15 11:29:05 ip-[redacted].ec2.internal systemd[1]: sshd.service: main process exited, code=.../a
Nov 15 11:29:05 ip-[redacted].ec2.internal systemd[1]: Unit sshd.service entered failed state.
Hint: Some lines were ellipsized, use -l to show in full.

You will find that the OpenSSH service will not start. This is because the SELinux policy insists that the OpenSSH service should only bind the default port 22. If it finds that it uses a different port, it will not allow it. You can look at the /var/log/audit/audit.log log file to find the offending SELinux denial message.


# grep 31337 /var/log/audit/audit.log  | tail -1
type=AVC msg=audit(1447605071.365:127): avc:  denied  { name_bind } for  pid=10893 comm="sshd" src=31337 scontext=system_u:system_r:sshd_t:s0-s0:c0.c1023 tcontext=system_u:object_r:unreserved_port_t:s0 tclass=tcp_socket


Now let’s see what the sealert tool says.


# sealert -a /var/log/audit/audit.log
 78% done'list' object has no attribute 'split'
100% done
found 1 alerts in /var/log/audit/audit.log
SELinux is preventing /usr/sbin/sshd from name_bind access on the tcp_socket port 31337.
*****  Plugin bind_ports (92.2 confidence) suggests   ************************
If you want to allow /usr/sbin/sshd to bind to network port 31337
Then you need to modify the port type.
# semanage port -a -t PORT_TYPE -p tcp 31337
    where PORT_TYPE is one of the following: ssh_port_t, vnc_port_t, xserver_port_t.
Additional Information:
Source Context                system_u:system_r:sshd_t:s0-s0:c0.c1023
Target Context                system_u:object_r:unreserved_port_t:s0
Target Objects                port 31337 [ tcp_socket ]
Source                        sshd
Source Path                   /usr/sbin/sshd
Port                          31337

Working with SELinux types

The sealert tool hinted that if we wanted to bind a different port, we would have to modify the SELinux port type to include the new port number.

# semanage port -l | grep ssh
ssh_port_t                     tcp      22
# semanage port -a -t ssh_port_t -p tcp 31337
# semanage port -l | grep ssh
ssh_port_t                     tcp      31337, 22


Once we have modified the SELinux SSH port type, let’s restart the OpenSSH service and see if it worked this time.


# systemctl restart sshd.service
# systemctl status sshd.service
sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
   Active: active (running) since Sun 2015-11-15 11:36:10 EST; 6s ago
 Main PID: 10965 (sshd)
   CGroup: /system.slice/sshd.service
           └─10965 /usr/sbin/sshd -D
Nov 15 11:36:10 ip-[redacted].ec2.internal systemd[1]: Starting OpenSSH server daemon...
Nov 15 11:36:10 ip-[redacted].ec2.internal systemd[1]: Started OpenSSH server daemon.
Nov 15 11:36:10 ip-[redacted].ec2.internal sshd[10965]: Server listening on port 31337.
Nov 15 11:36:10 ip-[redacted].ec2.internal sshd[10965]: Server listening on :: port 31337.
# netstat -napt | grep ssh | grep -i listen
tcp        0      0 *               LISTEN      10965/sshd
tcp6       0      0 :::31337                :::*                    LISTEN      10965/sshd


Awesome! It worked! I hope this article gives you the confidence to learn and explore using SELinux on your EC2 instance.

Written by

Eugene Teo

Eugene Teo is a director of security at a US-based technology company. He is interested in applying machine learning techniques to solve problems in the security domain.

Related Posts

Sanket Dangi
— February 11, 2019

WaitCondition Controls the Pace of AWS CloudFormation Templates

AWS's WaitCondition can be used with CloudFormation templates to ensure required resources are running.As you may already be aware, AWS CloudFormation is used for infrastructure automation by allowing you to write JSON templates to automatically install, configure, and bootstrap your ...

Read more
  • AWS
  • formation
Andrew Larkin
— January 24, 2019

The 9 AWS Certifications: Which is Right for You and Your Team?

As companies increasingly shift workloads to the public cloud, cloud computing has moved from a nice-to-have to a core competency in the enterprise. This shift requires a new set of skills to design, deploy, and manage applications in cloud computing.As the market leader and most ma...

Read more
  • AWS
  • AWS certifications
Andrew Larkin
— November 28, 2018

Two New EC2 Instance Types Announced at AWS re:Invent 2018 – Monday Night Live

The announcements at re:Invent just keep on coming! Let’s look at what benefits these two new EC2 instance types offer and how these two new instances could be of benefit to you. If you're not too familiar with Amazon EC2, you might want to familiarize yourself by creating your first Am...

Read more
  • AWS
  • EC2
  • re:Invent 2018
Guy Hummel
— November 21, 2018

Google Cloud Certification: Preparation and Prerequisites

Google Cloud Platform (GCP) has evolved from being a niche player to a serious competitor to Amazon Web Services and Microsoft Azure. In 2018, research firm Gartner placed Google in the Leaders quadrant in its Magic Quadrant for Cloud Infrastructure as a Service for the first time. In t...

Read more
  • AWS
  • Azure
  • Google Cloud
Khash Nakhostin
Khash Nakhostin
— November 13, 2018

Understanding AWS VPC Egress Filtering Methods

In order to understand AWS VPC egress filtering methods, you first need to understand that security on AWS is governed by a shared responsibility model where both vendor and subscriber have various operational responsibilities. AWS assumes responsibility for the underlying infrastructur...

Read more
  • Aviatrix
  • AWS
  • VPC
Jeremy Cook
— November 10, 2018

S3 FTP: Build a Reliable and Inexpensive FTP Server Using Amazon’s S3

Is it possible to create an S3 FTP file backup/transfer solution, minimizing associated file storage and capacity planning administration headache?FTP (File Transfer Protocol) is a fast and convenient way to transfer large files over the Internet. You might, at some point, have conf...

Read more
  • Amazon S3
  • AWS
Guy Hummel
— October 18, 2018

Microservices Architecture: Advantages and Drawbacks

Microservices are a way of breaking large software projects into loosely coupled modules, which communicate with each other through simple Application Programming Interfaces (APIs).Microservices have become increasingly popular over the past few years. The modular architectural style,...

Read more
  • AWS
  • Microservices
Stuart Scott
— October 2, 2018

What Are Best Practices for Tagging AWS Resources?

There are many use cases for tags, but what are the best practices for tagging AWS resources? In order for your organization to effectively manage resources (and your monthly AWS bill), you need to implement and adopt a thoughtful tagging strategy that makes sense for your business. The...

Read more
  • AWS
  • cost optimization
Stuart Scott
— September 26, 2018

How to Optimize Amazon S3 Performance

Amazon S3 is the most common storage options for many organizations, being object storage it is used for a wide variety of data types, from the smallest objects to huge datasets. All in all, Amazon S3 is a great service to store a wide scope of data types in a highly available and resil...

Read more
  • Amazon S3
  • AWS
Cloud Academy Team
— September 18, 2018

How to Optimize Cloud Costs with Spot Instances: New on Cloud Academy

One of the main promises of cloud computing is access to nearly endless capacity. However, it doesn’t come cheap. With the introduction of Spot Instances for Amazon Web Services’ Elastic Compute Cloud (AWS EC2) in 2009, spot instances have been a way for major cloud providers to sell sp...

Read more
  • AWS
  • Azure
  • Google Cloud
  • SpotInst
Guy Hummel and Jeremy Cook
— August 23, 2018

What are the Benefits of Machine Learning in the Cloud?

A Comparison of Machine Learning Services on AWS, Azure, and Google CloudArtificial intelligence and machine learning are steadily making their way into enterprise applications in areas such as customer support, fraud detection, and business intelligence. There is every reason to beli...

Read more
  • AWS
  • Azure
  • Google Cloud
  • Machine Learning
Stuart Scott
— August 17, 2018

How to Use AWS CLI

The AWS Command Line Interface (CLI) is for managing your AWS services from a terminal session on your own client, allowing you to control and configure multiple AWS services.So you’ve been using AWS for awhile and finally feel comfortable clicking your way through all the services....

Read more
  • AWS