Zyxel ZyWALL-USG50 Router Manual PDF (Setup & Configuration Guide)

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Chapter 23 IPSec VPN

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keys for the IKE SA and IPSec SA. In main mode, this is done in steps 3 and 4, as

illustrated next.

Figure 235

IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange

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.

Authentication

Before the ZyWALL 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 ZyWALL and remote IPSec router authenticate each other in

steps 5 and 6, as illustrated below. The identities are also encrypted using the

encryption algorithm and encryption key the ZyWALL and remote IPSec router

selected in previous steps.

Figure 236

IKE SA: Main Negotiation Mode, Steps 5 - 6: Authentication (continued)

You have to create (and distribute) a pre-shared key. The ZyWALL and remote

IPSec router use it in the authentication process, though it is not actually

transmitted or exchanged.

Note: The ZyWALL and the remote IPSec router must use the same pre-shared key.

Diffie-Hellman key exchange

3

4

X

Y

Step 5:

pre-shared key

ZyWALL identity, consisting of

- ID type

- content

Step 6:

pre-shared key

Remote IPSec router identity, consisting of

- ID type

- content

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Router identity consists of ID type and content. The ID type can be domain name,

IP address, or e-mail address, and the content is a (properly-formatted) domain

name, IP address, or e-mail address. The content is only used for identification.

Any domain name or e-mail 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 ZyWALL’s or remote IPSec router’s properties.

The ZyWALL 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.

Note: The ZyWALL’s local and peer ID type and content must match the remote

IPSec router’s peer and local ID type and content, respectively.

For example, in

Table 117 on page 402

, the ZyWALL and the remote IPSec router

authenticate each other successfully. In contrast, in

Table 118 on page 402

, the

ZyWALL and the remote IPSec router cannot authenticate each other and,

therefore, cannot establish an IKE SA.

It is also possible to configure the ZyWALL 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 ZyWALL provides another way to check

Table 117

VPN Example: Matching ID Type and Content

ZYWALL

REMOTE IPSEC ROUTER

Local ID type: E-mail

Local ID type: IP

Local ID content: [email protected]

Local ID content: 1.1.1.2

Peer ID type: IP

Peer ID type: E-mail

Peer ID content: 1.1.1.2

Peer ID content: [email protected]

Table 118

VPN Example: Mismatching ID Type and Content

ZYWALL

REMOTE IPSEC ROUTER

Local ID type: E-mail

Local ID type: IP

Local ID content: [email protected]

Local ID content: 1.1.1.2

Peer ID type: IP

Peer ID type: E-mail

Peer ID content: 1.1.1.20

Peer ID content: [email protected]

5

6

X

Y

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the identity of the remote IPSec router (for example, extended authentication) or

if you are troubleshooting a VPN tunnel.

Additional Topics for IKE SA

This section provides more information about IKE SA.

Negotiation Mode

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 ZyWALL sends its proposals to the remote IPSec router. The

remote IPSec router selects an acceptable proposal and sends it back to the

ZyWALL.

Steps 3 - 4: The ZyWALL 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 ZyWALL 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

ZyWALL 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.

VPN, NAT, and NAT Traversal

In the following example, there is another router (

A

) between router

X

and router

Y

.

Figure 237

VPN/NAT Example

A

X

Y

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If router

A

does NAT, it might change the IP addresses, port numbers, or both. If

router

X

and router

Y

try to establish a VPN tunnel, the authentication fails

because it depends on this information. The routers cannot establish a VPN tunnel.

Most routers like router

A

now have an IPSec pass-thru feature. This feature helps

router

A

recognize VPN packets and route them appropriately. If router

A

has this

feature, router

X

and router

Y

can establish a VPN tunnel as long as the active

protocol is ESP. (See

Active Protocol on page 405

for more information about

active protocols.)

If router A does not have an IPSec pass-thru or if the active protocol is AH, you

can solve this problem by enabling NAT traversal. In NAT traversal, router

X

and

router

Y

add an extra header to the IKE SA and IPSec SA packets. If you configure

router

A

to forward these packets unchanged, router

X

and router

Y

can establish

a VPN tunnel.

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

•

Enable NAT traversal on the ZyWALL and remote IPSec router.

•

Configure the NAT router to forward packets with the extra header unchanged.

(See the field description for detailed information about the extra header.)

The extra header may be UDP port 500 or UDP port 4500, depending on the

standard(s) the ZyWALL and remote IPSec router support.

Extended Authentication

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 ZyWALL 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 ZyWALL to provide a user name and password to the remote

IPSec router, or you can set up the ZyWALL 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).

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Certificates

It is possible for the ZyWALL 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.

•

Instead of using the pre-shared key, the ZyWALL and remote IPSec router check

the signatures on each other’s certificates. Unlike pre-shared keys, the

signatures do not have to match.

•

The local and peer ID type and content come from the certificates.

Note: You must set up the certificates for the ZyWALL and remote IPSec router first.

IPSec SA Overview

Once the ZyWALL 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.

Note: The IPSec SA stays connected even if the underlying IKE SA is not available

anymore.

This section introduces the key components of an IPSec SA.

Local Network and Remote Network

In an IPSec SA, the local network, the one(s) connected to the ZyWALL, 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.

Active Protocol

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).

Note: The ZyWALL and remote IPSec router must use the same active protocol.

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

suitable with NAT.

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