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 and IPv6 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.

In the Zyxel Device, use the VPN Connection tab to set up Phase 2 and the VPN Gateway tab to set up Phase 1.

Some differences between IKEv1 and IKEv2 include:

SSL VPN uses remote users’ web browsers to provide the easiest-to-use of the Zyxel Device’s VPN solutions. A user just browses to the Zyxel Device’s web address and enters his user name and password to securely connect to the Zyxel Device’s network. Remote users do not need to configure security settings.

L2TP VPN uses the L2TP and IPSec client software included in remote users’ Android, iOS, or Windows operating systems for secure connections to the network behind the Zyxel Device. The remote users do not need their own IPSec gateways or third-party VPN client software. For example, configure sales representatives’ laptops, tablets, or smartphones to securely connect to the Zyxel Device’s network.

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.

This section briefly explains the relationship between VPN tunnels and other features. It also gives some basic suggestions for troubleshooting.

You should set up the following features before you set up the VPN tunnel.

The VPN Connection screen lists the VPN connection policies and their 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 VPN Connection 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 Configuration > VPN Connection screen , and click either the Add icon or an Edit icon.

The VPN Gateway summary screen displays the IPSec VPN gateway policies in the Zyxel Device, as well as the Zyxel Device’s address, remote IPSec router’s address, and associated VPN connections for each one. In addition, it also lets you activate and deactivate each VPN gateway.

The VPN Gateway Add/Edit screen allows you to create a new VPN gateway policy or edit an existing one.

A VPN concentrator combines several IPSec VPN connections into one secure network.

In a fully-meshed VPN topology, there is a VPN connection between every pair of routers. In a hub-and-spoke VPN topology, there is a VPN connection between each spoke router and the hub router, which uses the VPN concentrator. The VPN concentrator routes VPN traffic between the spoke routers and itself.

A VPN concentrator reduces the number of VPN connections that you have to set up and maintain on the network. You might also be able to consolidate the policy routes in each spoke router, depending on the IP addresses and subnets of each spoke.

However a VPN concentrator is not for every situation. The hub router is a single failure point, so a VPN concentrator is not as appropriate if the connection between spoke routers cannot be down occasionally (maintenance, for example). There is also more burden on the hub router. It receives VPN traffic from one spoke, decrypts it, inspects it to find out to which spoke to route it, encrypts it, and sends it to the appropriate spoke. Therefore, a VPN concentrator is more suitable when there is a minimum amount of traffic between spoke routers.

Consider the following when using the VPN concentrator.

The VPN Concentrator summary screen displays the VPN concentrators in the Zyxel Device.

Use the VPN Concentrator Add/Edit screen to create or edit a VPN concentrator.

Use the Configuration > VPN > IPSec VPN > Configuration Provisioning screen to configure who can retrieve VPN rule settings from the Zyxel Device using the Zyxel Device IPSec VPN Client. In the Zyxel Device IPSec VPN Client, you just need to enter the IP address of the to get all the VPN rule settings automatically. You do not need to manually configure all rule settings in the Zyxel Device IPSec VPN client.

VPN rules for the Zyxel Device IPSec VPN Client have certain restrictions. They must not contain the following settings:

The following VPN Gateway rules configured on the Zyxel Device cannot be provisioned to the IPSec VPN Client:

In the Zyxel Device Quick Setup wizard, you can use the VPN Settings for Configuration Provisioning wizard to create a VPN rule that will not violate these restrictions.

 

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--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 and content. The ID type can be domain name, IP address, or email address, and the content is a (properly-formatted) 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 and content refers to the ID type and content that applies to the router itself, and peer ID type and content refers to the ID type and 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 set the peer ID type to Any. 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--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.

X-Auth / Extended authentication is often used when multiple IPSec routers use the same VPN tunnel to connect to a single IPSec router. For example, this might be used with telecommuters.

In extended authentication, one of the routers (the Zyxel Device or the remote IPSec router) provides a user name and password to the other router, which uses a local user database and/or an external server to verify the user name and password. If the user name or password is wrong, the routers do not establish an IKE SA.

You can set up the Zyxel Device to provide a user name and password to the remote IPSec router, or you can set up the Zyxel Device to check a user name and password that is provided by the remote IPSec router.

If you use extended authentication, it takes four more steps to establish an IKE SA. These steps occur at the end, regardless of the negotiation mode (steps 7-10 in main mode, steps 4-7 in aggressive mode).

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.

The Zyxel Device can translate the following types of network addresses in IPSec SA.

This translation lets the Zyxel Device route packets from computers that are not part of the specified local network (local policy) through the IPSec SA.

To set up this NAT, you have to specify the following information:

You can set up this translation if you want to change the source address of computers in the remote network. To set up this NAT, you have to specify the following information:

You can set up this translation if you want the Zyxel Device to forward some packets from the remote network to a specific computer in the local network.

You have to specify one or more rules when you set up this kind of NAT. The Zyxel Device checks these rules similar to the way it checks rules for a security policy. The first part of these rules define the conditions in which the rule apply.

The second part of these rules controls the translation when the condition is satisfied.

The original port range and the mapped port range must be the same size.