A virtual private network (VPN) provides secure communications between sites without the expense of leased site-to-site lines. A secure VPN is a combination of tunneling, encryption, authentication, access control and auditing. It is used to transport traffic over the Internet or any insecure network that uses TCP/IP for communication.

Internet Protocol Security (IPSec) VPN connects IPSec routers or remote users using IPSec client software. This standards-based VPN offers flexible solutions for secure data communications across a public network. IPSec is built around a number of standardized cryptographic techniques to provide confidentiality, data integrity and authentication at the IP layer. The Zyxel Device can also combine multiple IPSec VPN connections into one secure network.

The Zyxel Device supports IKEv1 and IKEv2 for IPv4 traffic. IKE (Internet Key Exchange) is a protocol used in setting up security associations that allows two parties to send data securely.

IKE uses certificates or pre-shared keys for authentication and a Diffie–Hellman key exchange to set up a shared session secret from which encryption keys are derived. A security policy for each peer must be manually created.

IPSec VPN consists of two phases: Phase 1 and Phase 2. Phase 1's purpose is to establish a secure authenticated communication channel by using the Diffie–Hellman key exchange algorithm to generate a shared secret key to encrypt IKE communications. This negotiation results in one single bi-directional ISAKMP Security Association (SA). The authentication can be performed using either pre-shared key (shared secret), signatures, or public key encryption. Phase 1 operates in either Main Mode or Aggressive Mode. Main Mode protects the identity of the peers, but Aggressive Mode does not.

During Phase 2, the remote IPSec routers use the secure channel established in Phase 1 to negotiate Security Associations for IPSec. The negotiation results in a minimum of two unidirectional security associations (one inbound and one outbound). Phase 2 uses Quick Mode (only). Quick mode occurs after IKE has established the secure tunnel in Phase 1. It negotiates a shared IPSec policy, derives shared secret keys used for the IPSec security algorithms, and establishes IPSec SAs. Quick mode is also used to renegotiate a new IPSec SA when the IPSec SA lifetime expires.

Some differences between IKEv1 and IKEv2 include:

Here is some more detailed IPSec VPN background information.

The IKE SA provides a secure connection between the Zyxel Device and remote IPSec router.

It takes several steps to establish an IKE SA. The negotiation mode determines how many. There are two negotiation modes for IKEv1--main mode and aggressive mode. Main mode provides better security, while aggressive mode is faster.

The Zyxel Device supports IKEv1 and IKEv2. See Virtual Private Networks (VPN) Overview for more information.

To set up an IKE SA, you have to specify the IP addresses of the Zyxel Device and remote IPSec router. You can usually enter a static IP address or a domain name for either or both IP addresses. Sometimes, your Zyxel Device might offer another alternative, such as using the IP address of a port or interface, as well.

The IKE SA proposal is used to identify the encryption algorithm, authentication algorithm, and Diffie-Hellman (DH) key group that the Zyxel Device and remote IPSec router use in the IKE SA. In main mode, this is done in steps 1 and 2.

The Zyxel Device sends one or more proposals to the remote IPSec router. (In some devices, you can only set up one proposal.) Each proposal consists of an encryption algorithm, authentication algorithm, and DH key group that the Zyxel Device wants to use in the IKE SA. The remote IPSec router selects an acceptable proposal and sends the accepted proposal back to the Zyxel Device. If the remote IPSec router rejects all of the proposals, the Zyxel Device and remote IPSec router cannot establish an IKE SA.

In most Zyxel Devices, you can select one of the following encryption algorithms for each proposal. The algorithms are listed in order from weakest to strongest.

Some Zyxel Devices also offer stronger forms of AES that apply 192-bit or 256-bit keys to 128-bit blocks of data.

In most Zyxel Devices, you can select one of the following authentication algorithms for each proposal. The algorithms are listed in order from weakest to strongest.

The Zyxel Device and the remote IPSec router use DH public-key cryptography to establish a shared secret. The shared secret is then used to generate encryption keys for the IKE SA and IPSec SA. In main mode, this is done in steps 3 and 4.

DH public-key cryptography is based on DH key groups. Each key group is a fixed number of bits long. The longer the key, the more secure the encryption, but also the longer it takes to encrypt and decrypt information. For example, DH2 keys (1024 bits) are more secure than DH1 keys (768 bits), but DH2 keys take longer to encrypt and decrypt.

Before the Zyxel Device and remote IPSec router establish an IKE SA, they have to verify each other’s identity. This process is based on pre-shared keys and router identities.

In main mode, the Zyxel Device and remote IPSec router authenticate each other in steps 5 and 6. The identities are also encrypted using the encryption algorithm and encryption key the Zyxel Device and remote IPSec router selected in previous steps.

You have to create (and distribute) a pre-shared key. The Zyxel Device and remote IPSec router use it in the authentication process, though it is not actually transmitted or exchanged.

Router identity consists of ID type. The ID type can be domain name, IP address, or email address. The content is only used for identification. Any domain name or email address that you enter does not have to actually exist. Similarly, any domain name or IP address that you enter does not have to correspond to the Zyxel Device’s or remote IPSec router’s properties.

The Zyxel Device and the remote IPSec router have their own identities, so both of them must store two sets of information, one for themselves and one for the other router. Local ID type refers to the content that applies to the router itself, and remote ID type refers to the content that applies to the other router.

For example, in the next table, the Zyxel Device and the remote IPSec router authenticate each other successfully. In contrast, in the following table, the Zyxel Device and the remote IPSec router cannot authenticate each other and, therefore, cannot establish an IKE SA.

It is also possible to configure the Zyxel Device to ignore the identity of the remote IPSec router. In this case, you usually leave the remote ID type field empty. This is less secure, so you should only use this if your Zyxel Device provides another way to check the identity of the remote IPSec router (for example, extended authentication) or if you are troubleshooting a VPN tunnel.

This section provides more information about IKE SA.

There are two negotiation modes for IKEv1--main mode and aggressive mode. Main mode provides better security, while aggressive mode is faster.

Main mode takes six steps to establish an IKE SA.

Steps 1 - 2: The Zyxel Device sends its proposals to the remote IPSec router. The remote IPSec router selects an acceptable proposal and sends it back to the Zyxel Device.

Steps 3 - 4: The Zyxel Device and the remote IPSec router exchange pre-shared keys for authentication and participate in a Diffie-Hellman key exchange, based on the accepted DH key group, to establish a shared secret.

Steps 5 - 6: Finally, the Zyxel Device and the remote IPSec router generate an encryption key (from the shared secret), encrypt their identities, and exchange their encrypted identity information for authentication.

In contrast, aggressive mode only takes three steps to establish an IKE SA. Aggressive mode does not provide as much security because the identity of the Zyxel Device and the identity of the remote IPSec router are not encrypted. It is usually used in remote-access situations, where the address of the initiator is not known by the responder and both parties want to use pre-shared keys for authentication. For example, the remote IPSec router may be a telecommuter who does not have a static IP address.

You have to do the following things to set up NAT traversal.

The extra header may be UDP port 500 or UDP port 4500, depending on the standard(s) the Zyxel Device and remote IPSec router support.

It is possible for the Zyxel Device and remote IPSec router to authenticate each other with certificates. In this case, you do not have to set up the pre-shared key, local identity, or remote identity because the certificates provide this information instead.

Once the Zyxel Device and remote IPSec router have established the IKE SA, they can securely negotiate an IPSec SA through which to send data between computers on the networks.

This section introduces the key components of an IPSec SA.

In an IPSec SA, the local network, the one(s) connected to the Zyxel Device, may be called the local policy. Similarly, the remote network, the one(s) connected to the remote IPSec router, may be called the remote policy.

The active protocol controls the format of each packet. It also specifies how much of each packet is protected by the encryption and authentication algorithms. IPSec VPN includes two active protocols, AH (Authentication Header, RFC 2402) and ESP (Encapsulating Security Payload, RFC 2406).

Usually, you should select ESP. AH does not support encryption, and ESP is more suitable with NAT.

There are two ways to encapsulate packets. Usually, you should use tunnel mode because it is more secure. Transport mode is only used when the IPSec SA is used for communication between the Zyxel Device and remote IPSec router (for example, for remote management), not between computers on the local and remote networks.

In tunnel mode, the Zyxel Device uses the active protocol to encapsulate the entire IP packet. As a result, there are two IP headers:

In transport mode, the encapsulation depends on the active protocol. With AH, the Zyxel Device includes part of the original IP header when it encapsulates the packet. With ESP, however, the Zyxel Device does not include the IP header when it encapsulates the packet, so it is not possible to verify the integrity of the source IP address.

An IPSec SA proposal is similar to an IKE SA proposal (see IKE SA Proposal), except that you also have the choice whether or not the Zyxel Device and remote IPSec router perform a new DH key exchange every time an IPSec SA is established. This is called Perfect Forward Secrecy (PFS).

If you enable PFS, the Zyxel Device and remote IPSec router perform a DH key exchange every time an IPSec SA is established, changing the root key from which encryption keys are generated. As a result, if one encryption key is compromised, other encryption keys remain secure.

If you do not enable PFS, the Zyxel Device and remote IPSec router use the same root key that was generated when the IKE SA was established to generate encryption keys.

The DH key exchange is time-consuming and may be unnecessary for data that does not require such security.

PFS is ignored in initial IKEv2 authentication but is used when re-authenticating.

This section provides more information about IPSec SA in your Zyxel Device.

For authentication, the Zyxel Device and remote IPSec router use the SPI, instead of pre-shared keys, ID type and content. The SPI is an identification number.

An IPSec VPN tunnel is usually established in two phases. Each phase establishes a security association (SA), a contract indicating what security parameters the Zyxel Device and the remote IPSec router will use. The first phase establishes an Internet Key Exchange (IKE) SA between the Zyxel Device and remote IPSec router. The second phase uses the IKE SA to securely establish an IPSec SA through which the Zyxel Device and remote IPSec router can send data between computers on the local network and remote network.

The Zyxel Device’s application scenarios make it easier to configure your VPN connection settings.

The Site to Site VPN screen lists the VPN connection associated VPN gateway(s), and various settings. In addition, it also lets you activate or deactivate and connect or disconnect each VPN connection (each IPSec SA). Click a column’s heading cell to sort the table entries by that column’s criteria. Click the heading cell again to reverse the sort order.

The Site to Site VPN Add/Edit Gateway screen allows you to create a new VPN connection policy or edit an existing one. To access this screen, go to the VPN > Site to Site VPN screen, and click either the Add icon or an Edit icon. Select Site-to-Site in VPN > Site to Site VPN > Add/Edit > Scenario > Type to create a VPN rule using the wizard.

Use this screen to configure the VPN connection name and select the scenario that best describes your intended VPN connection.

Each field is described in the following table.

Use this screen to configure the Zyxel Device interface and remote IPSec device settings.

Each field is described in the following table.

Use this screen to configure the authentication type and settings.

Each field is described in the following table.

Use this screen to configure the IP addresses of the computer on your network and the computer behind the remote IPSec device.

Each field is described in the following table.

Use this screen to view a summary of the VPN tunnel configurations. You can click Edit to change the VPN tunnel configuration settings.

The Site to Site VPN Add/Edit Gateway screen allows you to create a new VPN connection policy or edit an existing one. To access this screen, go to the VPN > Site to Site VPN screen, and click either the Add icon or an Edit icon. Select Custom in VPN > Site to Site VPN> Add/Edit > Scenario > Type to create a customized VPN rule with advanced settings.

See Virtual Private Networks (VPN) Overview for more information on phase 1 and phase 2 settings; see IPSec VPN Background Information for more information on IKE SA proposals.

Each field is described in the following table.

The following are application scenarios for SNAT and 1-1 NAT.

Here is an example of SNAT VPN scenario. Use this when there are no overlapping local and remote VPN IP addresses. Map the source IP address of the sender to an IP address in the Local IP address range (in the Mapped IP field) for the VPN rule. The headquarters (HQ) and branch sites A and B need to access the remote data center (D). The source IP addresses of sites A and B are not in the range of the local policy's IP address (192.168.168.0/24) for Phase 2. NAT rules need to be configured to translate the source IP addresses of sites A and B to an IP address in the 192.168.168.0/24 range before entering the IPsec tunnel.

The administrator need to set up VPN policy on both sites.

Here is an example of a 1:1 NAT VPN scenario.Use this to avoid overlapping local and remote VPN IP addresses. IPSec router A and IPSec router B need to access each other, but they have overlapping subnets. To avoid conflicts, both IPSec routers need to create identical 1:1 NAT rules that map their local subnet to a non-overlapping subnet.

In the following example, IPSec router A is sending traffic to router B. Before data entering the VPN tunnel, the source IP address (set in Origin IP) from router A is translated to a mapped IP address (set in Mapped IP). After data exiting the VPN tunnel, router B translates the destination IP address (set in Mapped IP) back to the Origin IP.

The administrator need to set up VPN policy on both sites.

Configure the settings in this screen to create a new or edit an existing remote access VPN rule to securely access the Zyxel Device local networks from anywhere. See Virtual Private Networks (VPN) Overview for more information on phase 1 and phase 2 settings; see IPSec VPN Background Information for more information on IKE SA proposals.

SecuExtender is a Zyxel subscription-based VPN client. A remote access VPN client must have SecuExtender VPN client installed on his device and uses a supported computer operating system.

Make sure the settings configured on the IPSec VPN client matches the settings you configured on the Zyxel Device.

The following table describes the labels in this screen.

See the following table for VPN types and remote software options.

Select SSL or IKEv2 to configure the Zyxel Device in the office. See SSL / IKEv2 VPN Comparison for the comparison between two VPN types.

Follow the table below to configure the VPN > SSL VPN screen.

Follow the table below to configure the VPN > IPSec VPN > Remote Access VPN screen.

The administrator has now finished setting up the VPN configuration on the Zyxel Device. Now, the home user needs to set up a VPN client software on their computer or mobile device to connect to the office network. See SSL / IKEv2 VPN Comparison for VPN software options for home user and more details.

SecuExtender is a Zyxel subscription-based VPN client.

Home users using SecuExtender need the following:

Follow these steps to establish a VPN connection to the office’s network through SecuExtender:

Follow these steps to establish a VPN connection to the office’s network through OpenVPN Connect:

Use the built-in VPN client in Windows, macOS, iOS, or Android (strongSwan).

Home users using the IPSec VPN client on their computers need the following:

Follow these steps to establish a VPN connection to the office’s network through the IPSec VPN client on a computer with a Windows operating system:

To test if the home user’s computer can successfully connect to the office’s network, they should open the Command Prompt and ping the IP address of a device in the LAN.