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VPN Scaling

The advice on this page is intended to help firewall administrators handle increased VPN volume, both in terms of throughput and number of connected users.

Warning

The advice on this page is relayed from experience and from community members. The advice on this page may not apply to all environments or use cases, and has not been definitively proven to help, but is offered in case others find it useful.

See also

This document won’t cover all factors of choosing between VPN types or how to setup VPNs, that information can be found elsewhere in this documentation and in pfSense Hangouts on Youtube.

General Advice

No pfSense Limits

pfSense does not place any artificial limits on VPN connections. Any limitations encountered are due to settings, the hardware/environment, or the underlying technology.

IPsec is Faster

IPsec is faster than OpenVPN, so if both client and server support IPsec, use IPsec.

Use External Authentication

For user-based authentication, the most efficient method of user management for large numbers of accounts is an external authentication source, such as a RADIUS server, LDAP server, Active Directory (Via LDAP or RADIUS/NPS), etc.

Check Logs

If additional users are unable to connect, look in the logs on both the client and server side for specific error messages before seeking support.

Use Hardware Acceleration

Using a cryptographic accelerator such as a CPU with AES-NI will help greatly with throughput and crypto-related tasks. Enable the AES-NI and BSD cryptodev modules under System > Advanced on the Miscellaneous tab.

Use AES-GCM

Using efficient encryption like AEAD ciphers, which combine encryption and authentication, will increase security and performance. Both IPsec and OpenVPN can use AES-GCM, which is an AEAD cipher. Client support may vary by platform.

Use Accelerated Ciphers

As above, using AES-NI and AES-GCM is the best possible combination at this time. That said, certain hardware may accelerate other ciphers so that alternate choices are faster or more efficient. For hardware sold by Netgate, see the Netgate Appliances page for performance data and recommendations.

Disable Performance-Limiting Mitigation Settings

While we do not have any data on if or by how much they may impact VPNs, CPU vulnerability mitigation methods such as Kernel PTI and MDS mode can potentially degrade total performance. The potential for exploitation is minimal since arbitrary code cannot be run on the firewall except by users which already have the equivalent of administrator-level access. To ensure this risk stays low, only allow trusted administrators to access the firewall GUI and shell (SSH or console). The settings to enable/disable these features are under System > Advanced on the Miscellaneous tab.

Check Tunnel Network/Virtual Address Pool Sizes

Both IPsec and OpenVPN can assign addresses to clients out of a pool for remote access/mobile VPNs. The sizing of this pool limits how many clients can connect. For example, the maximum number of users in a /24 pool is 252, but other settings may reduce that value. See the sections below for more specific advice.

Use “Secure Enough” Settings

While we do not recommend deliberately using weak configurations, in some cases trade-offs are made for security between two secure ciphers or settings where one may offer even better security, but the lower of the two is still secure. In these cases, using the “Secure Enough” option can provide efficiency vs increased security. So long as the decision is informed, there may be some performance gained without compromising security in an unacceptable way. For example, with AES-GCM a key length of 128 bits is still considered secure. A 256 bit key is more secure, but the 256 bit key could put more of a burden on the hardware.

Consider Split Tunneling

Configurations which send all client data over the VPN, including Internet-bound traffic, will consume more resources than those which only send traffic for specific subnets. There are plenty of valid reasons to use either kind of configuration, however, when resources are stretched thin, easing the traffic burden on the VPN may justify switching to split tunneling rather than tunneling everything. Depending on the type of VPN and client, this may require adjustments on the server, the client, or both. See the sections below for specific recommendations.

Use Multiple Firewalls

In some instances, the burden may be too great for any single pfSense firewall to handle. In cases like this, multiple firewalls can be used to handle the required number of clients or throughput, at a cost of greatly increased complexity. There is no way to automatically balance between nodes in this manner, but such a configuration could be manually managed. This would also likely require the capability to have multiple external addresses on the WAN so each firewall can work in parallel, and also increases the complexity of routing on the internal side.

Use TNSR

TNSR is capable of vastly increased total IPsec throughput compared to pfSense. If pfSense is unable to reach the throughput needs for a given use case, see the TNSR product page for more information.

Scaling IPsec

IPsec is well-suited to high throughput by default, especially given the advice above, but there are additional IPsec-specific tweaks which may help.

Note

See the TNSR product page for information about using TNSR for even larger total site-to-site throughput needs.

Optimal Encryption Settings

  • Use AES-NI capable hardware.

  • In Phase 1 (IKE) settings, use:

    • AES128-GCM with 128 bit key length for the Algorithm

    • AES-XCBC for the hash, which in this case is effectively a Pseudo-Random Function (PRF).

  • In Phase 2 (Child SA) settings, use:

    • AES128-GCM with 128 bit key length for the Algorithm

    • Do not select any Hash Algorithms. A hash algorithm is unnecessary for AES-GCM as it already includes authentication.

Enable Multiple Phase 1 and Phase 2 Proposals

Multiple Phase 1 and Phase 2 encryption proposals may be configured in the GUI. Enabling multiple combinations of settings will allow peers to choose the most optimal settings which both sides support.

Enable Asynchronous Cryptography

IPsec cryptography jobs can be dispatched multi-threaded to run in parallel and increase performance. However, not all platforms and configurations fully support this function. To enable this capability, check Asynchronous Cryptography under VPN > IPsec on the Advanced tab.

Warning

Be on the lookout for IPsec traffic drops/failures to pass with this setting enabled. See https://redmine.pfsense.org/issues/8964 for more information.

Split Tunneling

As mentioned above, split tunneling would only send traffic for specific subnets across the VPN rather than sending all traffic. On IPsec, this can be done in some cases by listing the specific networks in Phase 2 entries for the Mobile IPsec P1 rather than 0.0.0.0/0. On the mobile clients tab, set Provide a list of accessible networks to clients. Even with that set, certain cases such as Windows 10 may require additional changes to direct clients to send only specific traffic over the tunnel.

Scaling OpenVPN

Use IPsec Instead

As mentioned previously, where possible, use IPsec instead. IPsec is much more efficiently integrated into the operating system, and is capable of much greater throughput than OpenVPN.

Use UDP

UDP has less overhead for tunneled data, and if a client has to retransmit, it won’t compound the problem by retransmitting both inside and outside the tunnel. Unless there are extenuating circumstances which require TCP, use UDP.

Use TLS for Authentication Only

OpenVPN can use TLS for both authentication and for encryption of the control channel. Performing control channel encryption adds more overhead, which can add up with many clients. If control channel encryption is not required, consider using TLS for only authentication instead. No matter which option is chosen, traffic carried by OpenVPN is encrypted.

Encryption Algorithm

Use a CPU with AES-NI when possible, and use AES-GCM for the Encryption Algorithm when possible. Note that for AEAD ciphers such as AES-GCM, OpenVPN ignores the setting for Auth Digest Algorithm.

Note

AES-GCM can only be used in SSL/TLS mode, not Shared Key mode.

Use Negotiable Crypto Parameters

NCP can be used to set preferences so that more efficient ciphers can be preferred by clients where possible, but others can be used when necessary. Set high-priority selections such as AES-128-GCM first, followed by others like AES-128-CBC.

Split Tunneling

As mentioned in the general section above, split tunneling only sends traffic for specific subnets across the VPN rather than sending all traffic. With OpenVPN, this can be done by Unchecking the Redirect IPv4/IPv6 Gateway option(s) and configuring IPv4/IPv6 Local Network(s) entries instead. Clients may still override this behavior remotely, however, so check the client configurations as well.

Concurrent Connections

pfSense does not impose any connection limits by default, but an administrator may have chosen to configure a limit on the number of connections via the Concurrent Connections setting on servers. Ensure this is either unset or set high enough to accommodate the required number of users.

Disable Compression

Though using compression is tempting to squeeze extra throughput out of slower links, it is both inefficient and insecure. Most data sent across VPNs in modern environments is already encrypted or otherwise uncompressible, which wastes CPU when attempting to compress. Additionally, vulnerabilities such as VORACLE can allow attackers to glean information about encrypted data when it has been compressed. Disabling encryption will mitigate that attack and also reduce CPU overhead. On the server, set Compression to Disable Compression.

Duplicate Connections

Normally, if an OpenVPN client connects using the same username or certificate CN, the older connection is broken in favor of the new connection. This is more secure, but does not allow any given user to connect multiple times. Circumstances may necessitate supporting this, and in some environments it’s not possible to give every device a unique username and/or certificate. Check Duplicate Connection in the OpenVPN server settings to allow multiple connections from the same user.

Topology

On recent versions of pfSense, OpenVPN defaults to subnet topology which uses addresses more efficiently, but if the VPN was configured initially on older versions, or if an older guide was followed, it may still be using net30 topology. Using a common example tunnel network of 10.0.8.0/24, with subnet topology, the VPN can have a maximum of 252 users but with net30, it can only have 63. This is because in net30 mode, each user receives a /30 subnet which utilizes four IP addresses for each user. In subnet mode, the server uses a single address and the client uses a single address, which is much more efficient.

Use UDP Fast I/O

This option is experimental but for those who have used it, it can result in much higher throughput. Not all platforms support it, however.

Increase Send/Receive Buffer

The default buffer size is safe, but not optimal. Increasing the buffer size to 512KiB on both sides can result in greater throughput. Results will vary by platform, internet link speed, and other factors. May require experimenting with multiple values to find the most efficient setting for a given environment.

Use Multiple Servers

OpenVPN is not multi-threaded so any single instance of OpenVPN is limited to using a single CPU. If a router has fast cores and not too many users, that may be OK, but it does not scale well. A workaround for this is to split users onto multiple servers. There are various means to reach this goal, including (but not limited to):

  • Multiple servers on different WANs or ports, each with unique tunnel networks but otherwise identical settings (Same CA structure, encryption, etc).

    • Administrators could choose to manually configure pools of clients to connect to specific servers, but that does not scale well.

    • Clients may connect to any server configured in this manner so long as their settings line up properly.

    • Multiple servers can be listed in a single client configuration with additional remote statements.

    • Add remote-random to the client configuration so that clients will pick a random server when starting, which avoids overloading whichever server is listed first.

    • Servers could be run on multiple WANs to overcome single-circuit throughput limits.

  • Multiple servers with completely unique settings (Different CA structure, different clients, etc)

    • More secure but more difficult to manage.

    • Clients must use different configurations to reach each server, no automated/built-in way to pick between them unless a specific client supports that function.

    • Good for isolating separate security levels (e.g. remote workers, remote administrators, vendors).

Process Efficiency

As a counterpoint to the above, each server will incur additional memory and other overhead to manage the process. When dealing with site-to-site VPNs, it is more efficient from a memory standpoint to use a single server with multiple clients (Peer to Peer SSL/TLS) vs servers for every node (Peer to Peer Shared Key, or SSL/TLS with a /30 tunnel network). If memory is a limiting factor, use fewer servers. If CPU overhead is the limiting factor, use separate servers.