The Jedi Master’s Guide to RHEL Security: May the Logs Be With You

Once you’ve locked down the basics of RHEL security, it’s time to move into more advanced territory. These next-level techniques help safeguard your systems against sophisticated threats and improve your overall security posture.

1. Implement Advanced SELinux Policies

SELinux isn’t just “on or off.” You can customize it with advanced policy modules to control how applications and services interact.

• Create Custom SELinux Policies:
  Use audit2allow to generate specific rules for services triggering denied actions.

  sudo audit2allow -a -M custom_policy
  sudo semodule -i custom_policy.pp
  

• Use Targeted Policy Modules:
  Enable and configure targeted SELinux policies for specific services like Apache, MySQL, or Postfix.

This fine-tuning helps ensure services run with the least privilege possible.

2. Leverage Advanced auditd Rules

Basic logging is helpful, but you can take things further by auditing critical files and user behavior.

• Track Privilege Escalation Attempts:

  -a always,exit -F arch=b64 -S execve -C uid!=euid -k priv_escalation
  

• Monitor Access to Sensitive Files:

  -w /etc/shadow -p wa -k shadow_file
  

• Set Immutable Rules:
  Once you’ve configured your audit rules, make them immutable (cannot be changed without a reboot):

  -e 2
  

This ensures your auditing policies can’t be tampered with during runtime.

3. Enforce Secure Boot and Kernel Module Signing

• Enable Secure Boot in BIOS/UEFI:
  Secure Boot verifies signed bootloaders and kernels before loading.

• Sign Kernel Modules Manually:
  If you develop custom kernel modules, sign them using your own keys:

  openssl req -new -x509 -newkey rsa:2048 -keyout MOK.priv -outform DER -out MOK.der -nodes -days 365
  sudo mokutil --import MOK.der
  

• Require Signed Modules Only:
  Modify kernel boot parameters to enforce loading only signed modules:

  sudo grubby --args="module.sig_enforce=1" --update-kernel=ALL
  

This protects against unauthorized or malicious kernel modules.

4. Apply Network Namespace Isolation

For high-security environments, isolating services into network namespaces helps compartmentalize traffic.

• Create Isolated Namespaces:

  sudo ip netns add ns1
  sudo ip link add veth0 type veth peer name veth1
  sudo ip link set veth1 netns ns1
  

• Configure Dedicated Interfaces:
  Assign IPs and routes unique to each namespace. This isolates traffic and helps prevent lateral movement during a breach.

Namespaces are like creating mini-networks inside your server.

5. Harden Compilers and Build Chains

Limiting who can compile or execute code on production servers is another layer of defense.

• Restrict Compiler Usage:
  Remove compilers from production servers or restrict them to trusted users:

  sudo chmod 700 /usr/bin/gcc
  

• Enforce Compiler Security Flags:
  Standardize the use of secure build flags for your applications:

  export CFLAGS="-fstack-protector-strong -D_FORTIFY_SOURCE=2 -O2"
  export LDFLAGS="-Wl,-z,relro,-z,now"
  

These flags harden applications against memory-based attacks.

6. Configure Advanced firewalld Rules

Move beyond basic port blocking and implement granular, context-aware firewall rules.

• Use Rich Rules:
  Set up specific rules for logging or restricting by IP subnet:

  sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/24" service name="ssh" accept'
  

• Implement Port Knocking:
  This technique requires users to “knock” on ports in a specific sequence to open critical services like SSH.

Port knocking reduces exposure of open ports to external scans.

7. Secure NTP and Time Synchronization

A compromised time source can disrupt logs and security systems.

• Configure NTP with Authentication:
  Use authenticated time sources with tools like chrony:

  sudo yum install chrony
  sudo vi /etc/chrony.conf
  

• Restrict NTP Traffic:
  Limit which hosts can interact with your NTP service to trusted sources only.

Consistent and secure time sync is critical for audits and incident forensics.

8. Implement AppArmor or Additional MAC Frameworks

While SELinux is powerful, some environments also leverage AppArmor for specific workloads.

• Install AppArmor:

  sudo yum install apparmor
  

• Apply Profiles:
  Configure profiles to confine applications like databases or web servers within specific security policies.

Using multiple MAC systems can create layered defense models.

9. Deploy Host Intrusion Detection (HIDS)

• Install OSSEC or AIDE:
  These tools detect unauthorized changes to files, configurations, and binaries.

  sudo yum install aide
  sudo aide --init
  

• Automate Checks:
  Schedule daily integrity scans and receive alerts when critical files are modified.

HIDS adds a crucial layer of monitoring for persistent threats.

10. Automate Compliance with SCAP and OpenSCAP

Ensure your RHEL system meets industry standards like CIS, DISA STIG, or PCI DSS.

• Install OpenSCAP Toolkit:

  sudo yum install scap-security-guide openscap-scanner
  

• Run Compliance Scans:

  sudo oscap xccdf eval --profile xccdf_org.ssgproject.content_profile_stig --results results.xml /usr/share/xml/scap/ssg/content/ssg-rhel8-ds.xml
  

• Remediate Automatically:
  OpenSCAP provides suggestions and scripts to close security gaps.

Automating compliance audits reduces manual effort and helps avoid regulatory fines.

Wrapping Up

Advanced security hardening on RHEL goes far beyond basic firewall rules and password policies. From namespace isolation and module signing to compliance automation and intrusion detection, these techniques help you protect your systems against sophisticated threats.

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