pfSense OpenVPN Hardware Crypto: How AES-NI and Intel RDRAND Improve Performance
KEY TAKEAWAYS
- OpenVPN secures data with AES encryption, but software-based encryption alone can overwhelm CPUs, especially in high-traffic networks
- By offloading AES operations to dedicated CPU instructions, AES-NI significantly boosts encryption speed.
- RDRAND supplies cryptographic-quality; it doesn’t speed up bulk encryption, but generates secure random numbers faster, useful for key generation and session establishment.
- AES-NI handles the heavy lifting of packet encryption and decryption, while RDRAND ensures key management and entropy needs don’t slow things down.
- Modern CPU support is crucial: Most Intel Xeon, Core i3/i5/i7, and AMD Ryzen processors from recent years support AES-NI and RDRAND, but older CPUs do not.
VPNs (Virtual Private Networks) have become crucial for ensuring privacy and secure communications. OpenVPN is one of the most popular open-source VPN solutions, widely used in firewall platforms like pfSense due to its robust security, flexibility, and open architecture.
Yet, VPN encryption can be CPU-intensive, sometimes limiting throughput and increasing latency. Fortunately, modern CPUs and pfSense offer hardware cryptographic acceleration features like AES-NI and Intel RDRAND to alleviate these performance bottlenecks.
This article explores how AES-NI and Intel RDRAND improve OpenVPN performance on pfSense systems and why enabling these features can significantly enhance VPN throughput and reduce CPU load.
Understanding OpenVPN Encryption and Performance Challenges
OpenVPN secures network traffic by encrypting data packets using cryptographic algorithms such as AES (Advanced Encryption Standard). The encryption process, while essential for privacy, requires considerable computational resources. When a pfSense firewall is running OpenVPN, the CPU must perform encryption and decryption for each packet, which can become a performance bottleneck, especially on busy networks handling gigabit traffic or higher.
By default, OpenVPN uses software-based encryption routines, which rely on the CPU to execute cryptographic algorithms through software libraries like OpenSSL. Although well optimized, software encryption cannot fully utilize specialized CPU instructions designed for encryption acceleration, limiting maximum VPN throughput.
Hardware acceleration technologies embedded in CPUs help address these limits by offloading encryption tasks from general-purpose CPU instructions to specialized hardware instructions or modules, thereby increasing the speed of cryptographic operations and lowering CPU usage.
AES-NI: Hardware Acceleration for AES Encryption
AES-NI (Advanced Encryption Standard New Instructions) is an extension set of instructions provided by Intel and AMD processors designed to accelerate AES encryption and decryption. AES-NI instructions are implemented directly in the CPU silicon, enabling efficient execution of AES cipher routines that are typically used by VPNs like OpenVPN for data confidentiality.
The benefits of AES-NI for OpenVPN in pfSense include:
- Increased VPN Tunnel Throughput: AES-NI allows OpenVPN to encrypt and decrypt data faster, which translates into higher VPN throughput. Reports suggest throughput improvements of 30% to 50% or more when AES-NI is active compared to purely software-based AES encryption.
- Reduced CPU Load: By offloading AES operations to hardware instructions, AES-NI reduces the processing burden on the CPU. This leads to lower CPU utilization, reducing the chance of the firewall becoming a performance bottleneck and freeing processing power for other tasks.
- Compatibility with AES-GCM: OpenVPN versions integrated into pfSense fully support AES-GCM (Galois/Counter Mode) encryption utilizing AES-NI, which is an authenticated encryption mode preferred for its performance and security properties.
To leverage AES-NI with OpenVPN on pfSense, the underlying CPU must support AES-NI instructions (common on most Intel and AMD processors produced in the past several years).
Additionally, pfSense 2.4.3 and later versions have integrated OpenVPN versions that automatically utilize AES-NI if available, without necessarily selecting additional options. However, selecting appropriate cryptographic settings, such as AES-GCM encryption, enhances compatibility and performance gains.
Intel RDRAND: Cryptographically Secure Random Number Generation
Intel RDRAND is another hardware instruction available in Intel processors that generates high-quality random numbers, which are essential for cryptographic operations like key generation and session establishment. While not directly accelerating the bulk encryption process like AES-NI, Intel RDRAND contributes to the cryptographic processes by providing efficient, hardware-generated randomness.
In pfSense OpenVPN configurations, there is often an option labeled “Hardware Crypto” with a choice like “Intel RDRAND engine – RAND.” This instructs the system to use the Intel RDRAND engine for generating random numbers during cryptographic operations.
The advantages of Intel RDRAND in OpenVPN operations are:
- Faster and More Secure Key Generation: High-quality hardware-based randomness improves the speed and security of key generation, which is critical for establishing secure VPN tunnels.
- Lower CPU Usage for Random Number Generation: Using hardware to generate random data reduces the CPU time otherwise spent generating pseudo-random numbers in software.
- Overall Cryptographic Efficiency: While RDRAND does not speed encryption/decryption directly, faster cryptographic operations related to session setup can improve VPN responsiveness and throughput under load.
Practical pfSense Considerations: Enabling Hardware Crypto
In pfSense, enabling hardware cryptographic acceleration involves a few steps and considerations:
- Verify CPU Capabilities: Confirm that the pfSense firewall’s CPU supports AES-NI (and Intel RDRAND if desired). Most Xeon, Core i3/i5/i7, and AMD Ryzen processors from recent years support AES-NI.
- Enable AES-NI in BIOS/UEFI: Sometimes AES-NI support is disabled by default in the system BIOS/UEFI and must be explicitly enabled.
- Check pfSense System Information: The System Information panel in pfSense should indicate whether AES-NI CPU Crypto is “Yes (active).”
- Configure OpenVPN Server Settings: Within the OpenVPN server configuration, select AES-based encryption algorithms (preferably AES-GCM). The “Hardware Crypto” dropdown often offers options like “No Hardware Crypto Acceleration” or “Intel RDRAND engine – RAND.” Typically, OpenVPN uses AES-NI automatically if available, so this setting may be optional for AES acceleration but can be set for RDRAND usage.
- Testing Performance: Testing VPN throughput and CPU load before and after enabling hardware acceleration helps validate the performance gains. Throughput should increase, and CPU usage during VPN traffic should decrease.
Potential Pitfalls and Myths
Below are the potential myths and pitfalls of OpenVPN Hardware Crypto:
- Selecting “No Hardware Crypto Acceleration” in pfSense OpenVPN often still results in AES-NI being utilized automatically by OpenSSL libraries if the CPU supports it, which can lead to confusion about why disabling hardware crypto on the VPN server page does not seem to impact performance.
- The “Hardware Crypto” option may not accelerate bulk encryption, but it can influence the use of hardware random number generation (Intel RDRAND), so enabling it has benefits for cryptographic operations beyond just encryption speed.
- Not all older CPUs support AES-NI or Intel RDRAND, limiting hardware acceleration benefits on those systems.
- VPN performance can still be limited by other factors such as CPU clock speed, single-thread performance, network interface cards, and OpenVPN configuration parameters.
Performance Gains in Real-World Usage
Users deploying OpenVPN on AES-NI-capable pfSense firewalls often report significant performance improvements, such as VPN speeds increasing from tens of Mbps to hundreds of Mbps and reduced CPU usage under load.
For example, a pfSense box with an Intel Xeon D-1537 SoC employing AES-NI can handle OpenVPN VPN throughput in the hundreds of Mbps range with low CPU utilization, provided the clients also support AES-NI.
Similarly, enabling Intel RDRAND for hardware random number generation contributes to more efficient key management during session negotiation phases, helping maintain responsive VPN connections even under many simultaneous sessions.
Maximizing pfSense VPN Performance with Hardware Crypto
AES-NI is the single most impactful CPU feature for accelerating OpenVPN encryption on pfSense when encryption is the CPU bottleneck, allowing higher throughput and lower CPU usage for the same VPN load.
Intel RDRAND, by contrast, is an entropy convenience and safety feature that reduces blocking for RNG-heavy tasks (key generation, seeding), but it rarely changes steady-state tunnel throughput.
Together, they improve the reliability and scalability of VPN gateways when used appropriately: AES-NI to speed bulk encryption; RDRAND to keep cryptographic operations well-seeded and non-blocking. As always, measure before and after, choose AEAD ciphers, and match hardware selection to expected VPN throughput for the best results.
FAQ
What is AES-NI, and why does it matter for OpenVPN?
AES-NI (Advanced Encryption Standard New Instructions) is a CPU instruction set that accelerates AES encryption and decryption. In pfSense OpenVPN, it increases VPN throughput and reduces CPU load.
Does pfSense automatically use AES-NI if my CPU supports it?
Yes. Modern pfSense versions (2.4.3 and above) and OpenVPN automatically leverage AES-NI if the CPU supports it. Even if “No Hardware Crypto” is selected in OpenVPN settings, AES-NI may still be active through OpenSSL.
What is Intel RDRAND, and how does it improve OpenVPN performance?
RDRAND is a CPU instruction that generates cryptographically secure random numbers. It improves key generation speed and cryptographic efficiency during VPN session establishment, though it does not accelerate bulk encryption.
Will enabling RDRAND boost VPN throughput like AES-NI?
No. RDRAND mainly helps with randomness for secure keys and session negotiation. AES-NI is the feature that boosts actual VPN data throughput.
How can I check if AES-NI is active on my pfSense firewall?
Go to System Information in the pfSense dashboard. If the CPU supports AES-NI, it will display “AES-NI CPU Crypto: Yes (active).” You can also verify via BIOS/UEFI if needed.
What encryption algorithms work best with AES-NI?
AES-GCM (Galois/Counter Mode) is recommended because it’s efficient, secure, and optimized for AES-NI. AES-128-GCM is often the best balance between speed and strong security.
Can all CPUs benefit from AES-NI and RDRAND?
No. Only Intel and AMD CPUs from the last several years support AES-NI and RDRAND. Older processors may lack these features, limiting hardware acceleration benefits.