Securing Your Linux System with SELinux: A Step-by-Step Installation Guide

Security-Enhanced Linux (SELinux) is a powerful security mechanism built into the Linux kernel. It provides an additional layer of protection beyond standard user permissions, helping to prevent unauthorized access and malicious activity. If you're serious about Linux security, understanding and using SELinux is a must.

In this guide, we'll walk you through the process of installing and configuring SELinux on your system.

Step 1: Install the SELinux Packages

Open your terminal and run the following command as the root user:

yum install -y selinux-policy-targeted selinux-policy libselinux libselinux-python libselinux-utils policycoreutils policycoreutils-python setroubleshoot setroubleshoot-server setroubleshoot-plugins

Verify that the packages are installed correctly:

rpm -qa | grep selinux
rpm -q policycoreutils
rpm -qa | grep setroubleshoot

Step 2: Prepare for Labeling

Before enabling SELinux, you need to label every file on your system with an SELinux context. To ensure a smooth boot, set SELinux to permissive mode in the /etc/selinux/config file:

SELINUX=permissive SELINUXTYPE=targeted

Step 3: Reboot and Label

Reboot your system. During the boot process, watch for a message indicating that files are being labeled with an SELinux context:

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

Step 4: Check for Denials (Permissive Mode)

While in permissive mode, SELinux doesn't enforce policies but logs any actions that would be denied in enforcing mode. Run the following command to check the logs:

grep "SELinux is preventing" /var/log/messages

If you see no output, it means there were no denied actions.

Step 5: Enable Enforcing Mode

If everything looks good, switch SELinux to enforcing mode in /etc/selinux/config:

SELINUX=enforcing SELINUXTYPE=targeted

Reboot again.

Step 6: Verify SELinux Status

After the reboot, verify that SELinux is running in enforcing mode:

getenforce

You should see the output "Enforcing."

Step 7: Check User Mappings

Finally, run this command to view the mapping between SELinux and Linux users:

semanage login -l

If the mappings aren't correct, follow the instructions in the content you provided to fix them.

The output should look like this:

Login Name      SELinux User         MLS/MCS Range

__default__     unconfined_u         s0-s0:c0.c1023
root            unconfined_u         s0-s0:c0.c1023
system_u        system_u            s0-s0:c0.c1023

Fixing Incorrect User Mappings:

If your output doesn't match the above, run the following commands as the root user. These commands ensure the correct mapping between Linux user accounts and their SELinux roles. If you see warnings about "SELinux-user username is already defined," you can safely ignore them.

semanage user -a -S targeted -P user -R "unconfined_r system_r" -r s0-s0:c0.c1023 unconfined_u
semanage login -m -S targeted -s "unconfined_u" -r s0-s0:c0.c1023 __default__
semanage login -m -S targeted -s "unconfined_u" -r s0-s0:c0.c1023 root
semanage user -a -S targeted -P user -R guest_r guest_u
semanage user -a -S targeted -P user -R xguest_r xguest_u
 

Important Considerations:

• Permissive Mode vs. Enforcing Mode: Start with permissive mode to identify potential issues before switching to enforcing mode, where SELinux actively blocks unauthorized actions.
• Troubleshooting: SELinux denials can be cryptic. To resolve issues, familiarize yourself with SELinux logs and troubleshooting tools like troubleshoot.
• Customization: SELinux policies are highly customizable. Learn how to create custom policies to tailor SELinux to your specific environment.

By following these steps, you can effectively leverage SELinux to enhance the security of your Linux system.

How to Create Cross-Account Alias Records in AWS Route 53 for an ELB

Managing DNS records across multiple AWS accounts can be challenging, especially when dealing with resources like Elastic Load Balancers (ELBs). If you have a domain hosted in one AWS account and an ELB in another, you might wonder how to create an alias record that links the two. Fortunately, AWS Route 53 supports cross-account alias records, making this process straightforward. Here’s how you can set it up.

Scenario

Account A: Contains the Route 53 hosted zone for your domain.
Account B: Contains the ELB.

Step-by-Step Guide

Step 1: Obtain the ELB DNS NameLog in to AWS Account B.

1. Log in to AWS Account B.
2. Navigate to the EC2 Console: Go to the EC2 dashboard.
3. In the navigation pane, select Load Balancers.
4. Copy the DNS Name of the ELB:Select your target ELB.
5. Note down its DNS name (e.g., my-elb-1234567890.us-west-2.elb.amazonaws.com).

Step 2: Create Alias Record in Route 53

• Log in to AWS Account A.
• Open the Route 53 Console: Go to the Route 53 dashboard.
• Navigate to Hosted Zones and select the hosted zone for your domain.
• Create a New Record:Click on Create Record.
• Choose Simple Routing.
• Configure the Alias Record:Record Name: Leave this blank if you are configuring the zone apex (e.g., example.com), or enter the desired subdomain (e.g., www).
• Record Type: Choose A - IPv4 address.
• Alias: Select Yes.
• Alias Target: Paste the ELB DNS name copied from Account B.
• AWS will automatically resolve the Alias Hosted Zone ID associated with the ELB DNS name.
• Save the Record:Click Create records to save your changes.

Step 3: Verify the Configuration

Check the DNS Record:Use a DNS query tool like dig or nslookup to verify that the domain points to the ELB

dig example.com

The response should include the ELB DNS name.

Updated AWS Documentation

AWS has updated its documentation to clarify the process of creating cross-account alias records. You can refer to the AWS Route 53 Developer Guide for detailed information.
Conclusion

By following these steps, you can successfully create an alias record in AWS Route 53 that points to an ELB in another AWS account. This method ensures seamless integration of your domain with resources across multiple AWS accounts, enhancing your infrastructure’s flexibility and security.

Creating a New ReiserFS Partition for /var on HDD Using GParted: A Step-by-Step Guide

I will walk you through the process of creating a new ReiserFS partition for your /var directory on your hard drive using GParted, and configuring your system to use it. This can help in managing disk space more efficiently and improving system performance.

Step 1: Create a New ReiserFS Partition

Open GParted:Boot into a live session of your preferred Linux distribution and open GParted.
Identify the hard drive where you want to create the new partition (e.g., /dev/sda).


Create the Partition:Select the unallocated space or the partition you want to resize.
Create a new partition and choose "ReiserFS" as the file system.
Label the new partition as "var".

Step 2: Reboot into Emergency Mode

Reboot your system into emergency mode:This can be done by adding systemd.unit=emergency.target to the kernel parameters in your bootloader.


Remount Root as Read-Write:Once in emergency mode, remount the root filesystem as read-write

mount -o remount,rw /

Step 3: Mount the New PartitionMount the new partition to a temporary location

mount /dev/sda8 /mnt/new_var

Step 4: Copy the Existing /var Contents

Copy the contents of /var to the new partition

cd /var cp -Rax * /mnt/new_var/

Move back to the root directory

cd /

Rename the old /var directory

mv var var.old

Unmount the new partition from the temporary location

umount /mnt/new_var

Step 5: Mount the New Partition as /var

Create a new empty /var directory

mkdir /var

Mount the new partition to /var

mount /dev/sda8 /var

Step 6: Update /etc/fstabAdd the new partition to /etc/fstab for automatic mounting on boot:Open /etc/fstab in your preferred text editor

nano /etc/fstab

Add the following line

/dev/sda8 /var reiserfs defaults 0 2

Conclusion

By following these steps, you have successfully created a new ReiserFS partition for your /var directory and configured your system to use it. This process can help improve system performance and manage disk space more efficiently. If you encounter any issues, you can always revert to the old /var by mounting it back from the renamed var.old directory.

Remember to double-check your backups and ensure all critical data is secured before making such changes to your filesystem. Happy partitioning!