Zyxel NBG5615 Router Manual PDF (Setup & Configuration Guide)

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Chapter 18 IPSec VPN

NBG5715 User’s Guide

136

18.7.1

IPSec Architecture

The overall IPSec architecture is shown as follows.

Figure 83

IPSec Architecture

IPSec Algorithms

The

ESP

(Encapsulating Security Payload) Protocol (RFC 2406) and

AH

(Authentication Header)

protocol (RFC 2402) describe the packet formats and the default standards for packet structure

(including implementation algorithms).

The Encryption Algorithm describes the use of encryption techniques such as DES (Data Encryption

Standard) and Triple DES algorithms.

The Authentication Algorithms, HMAC-MD5 (RFC 2403) and HMAC-SHA-1 (RFC 2404, provide an

authentication mechanism for the

AH

and

ESP

protocols.

Key Management

Key management allows you to determine whether to use IKE (ISAKMP) or manual key

configuration in order to set up a VPN.

18.7.2

Encapsulation

The two modes of operation for IPSec VPNs are

Transport

mode and

Tunnel

mode. At the time of

writing, the NBG5715 supports

Tunnel

mode only.

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137

Figure 84

Transport and Tunnel Mode IPSec Encapsulation

Transport Mode

Transport

mode is used to protect upper layer protocols and only affects the data in the IP packet.

In

Transport

mode, the IP packet contains the security protocol (

AH

or

ESP

) located after the

original IP header and options, but before any upper layer protocols contained in the packet (such

as TCP and UDP).

With

ESP,

protection is applied only to the upper layer protocols contained in the packet. The IP

header information and options are not used in the authentication process. Therefore, the

originating IP address cannot be verified for integrity against the data.

With the use of

AH

as the security protocol, protection is extended forward into the IP header to

verify the integrity of the entire packet by use of portions of the original IP header in the hashing

process.

Tunnel Mode

Tunnel

mode encapsulates the entire IP packet to transmit it securely. A

Tunnel

mode is required

for gateway services to provide access to internal systems.

Tunnel

mode is fundamentally an IP

tunnel with authentication and encryption. This is the most common mode of operation.

Tunnel

mode is required for gateway to gateway and host to gateway communications.

Tunnel

mode

communications have two sets of IP headers:

•

Outside header

: The outside IP header contains the destination IP address of the VPN gateway.

•

Inside header

: The inside IP header contains the destination IP address of the final system

behind the VPN gateway. The security protocol appears after the outer IP header and before the

inside IP header.

18.7.3

IKE Phases

There are two phases to every IKE (Internet Key Exchange) negotiation – phase 1 (Authentication)

and phase 2 (Key Exchange). A phase 1 exchange establishes an IKE SA and the second one uses

that SA to negotiate SAs for IPSec.

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138

Figure 85

Two Phases to Set Up the IPSec SA

In phase 1 you must:

•

Choose a negotiation mode.

•

Authenticate the connection by entering a pre-shared key.

•

Choose an encryption algorithm.

•

Choose an authentication algorithm.

•

Choose a Diffie-Hellman public-key cryptography key group (

DH1

or

DH2

)

.

•

Set the IKE SA lifetime. This field allows you to determine how long an IKE SA should stay up

before it times out. An IKE SA times out when the IKE SA lifetime period expires. If an IKE SA

times out when an IPSec SA is already established, the IPSec SA stays connected.

In phase 2 you must:

•

Choose an encryption algorithm.

•

Choose an authentication algorithm

•

Choose a Diffie-Hellman public-key cryptography key group

.

•

Set the IPSec SA lifetime. This field allows you to determine how long the IPSec SA should stay

up before it times out. The NBG5715 automatically renegotiates the IPSec SA if there is traffic

when the IPSec SA lifetime period expires. If an IPSec SA times out, then the IPSec router must

renegotiate the SA the next time someone attempts to send traffic.

18.7.4

Negotiation Mode

The phase 1

Negotiation Mode

you select determines how the Security Association (SA) will be

established for each connection through IKE negotiations.

•

Main Mode

ensures the highest level of security when the communicating parties are

negotiating authentication (phase 1). It uses 6 messages in three round trips: SA negotiation,

Diffie-Hellman exchange and an exchange of nonces (a nonce is a random number). This mode

features identity protection (your identity is not revealed in the negotiation).

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139

•

Aggressive Mode

is quicker than

Main Mode

because it eliminates several steps when the

communicating parties are negotiating authentication (phase 1). However the trade-off is that

faster speed limits its negotiating power and it also does not provide identity protection. It is

useful in remote access situations where the address of the initiator is not know by the responder

and both parties want to use pre-shared key authentication.

18.7.5

IPSec and NAT

Read this section if you are running IPSec on a host computer behind the NBG5715.

NAT is incompatible with the

AH

protocol in both

Transport

and

Tunnel

mode. An IPSec VPN using

the

AH

protocol digitally signs the outbound packet, both data payload and headers, with a hash

value appended to the packet. When using

AH

protocol, packet contents (the data payload) are not

encrypted.

A NAT device in between the IPSec endpoints will rewrite either the source or destination address

with one of its own choosing. The VPN device at the receiving end will verify the integrity of the

incoming packet by computing its own hash value, and complain that the hash value appended to

the received packet doesn't match. The VPN device at the receiving end doesn't know about the

NAT in the middle, so it assumes that the data has been maliciously altered.

IPSec using

ESP

in

Tunnel

mode encapsulates the entire original packet (including headers) in a

new IP packet. The new IP packet's source address is the outbound address of the sending VPN

gateway, and its destination address is the inbound address of the VPN device at the receiving end.

When using

ESP

protocol with authentication, the packet contents (in this case, the entire original

packet) are encrypted. The encrypted contents, but not the new headers, are signed with a hash

value appended to the packet.

Tunnel

mode

ESP

with authentication is compatible with NAT because integrity checks are

performed over the combination of the "original header plus original payload," which is unchanged

by a NAT device.

Transport

mode

ESP

with authentication is not compatible with NAT.

18.7.6

VPN, NAT, and NAT Traversal

NAT is incompatible with the AH protocol in both transport

and tunnel

mode. An IPSec VPN using

the AH protocol digitally signs the outbound packet, both data payload and headers, with a hash

value appended to the packet, but a NAT device between the IPSec endpoints rewrites the source or

destination address. As a result, the VPN device at the receiving end finds a mismatch between the

hash value and the data and assumes that the data has been maliciously altered.

NAT is not normally compatible with ESP in transport mode either, but the NBG5715’s

NAT

Traversal

feature provides a way to handle this. NAT traversal allows you to set up an IKE SA when

there are NAT routers between the two IPSec routers.

Table 58

VPN and NAT

SECURITY PROTOCOL

MODE

NAT

AH

Transport

N

AH

Tunnel

N

ESP

Transport

N

ESP

Tunnel

Y

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Figure 86

NAT Router Between IPSec Routers

Normally you cannot set up an IKE SA with a NAT router between the two IPSec routers because

the NAT router changes the header of the IPSec packet. NAT traversal solves the problem by adding

a UDP port 500 header to the IPSec packet. The NAT router forwards the IPSec packet with the UDP

port 500 header unchanged. In the above figure, when IPSec router

A

tries to establish an IKE SA,

IPSec router

B

checks the UDP port 500 header, and IPSec routers

A

and

B

build the IKE SA.

For NAT traversal to work, you must:

•

Use ESP security protocol (in either transport or tunnel mode).

•

Use IKE keying mode.

•

Enable NAT traversal on both IPSec endpoints.

•

Set the NAT router to forward UDP port 500 to IPSec router

A

.

Finally, NAT is compatible with ESP in tunnel mode because integrity checks are performed over the

combination of the "original header plus original payload," which is unchanged by a NAT device. The

compatibility of AH and ESP with NAT in tunnel and transport modes is summarized in the following

table.

Y* - This is supported in the NBG5715 if you enable NAT traversal.

18.7.7

ID Type and Content

With aggressive negotiation mode (see

Section 18.7.4 on page 138

), the NBG5715 identifies

incoming SAs by ID type and content since this identifying information is not encrypted. This

enables the NBG5715 to distinguish between multiple rules for SAs that connect from remote IPSec

routers that have dynamic WAN IP addresses.

Regardless of the ID type and content configuration, the NBG5715 does not allow you to save

multiple active rules with overlapping local and remote IP addresses.

With main mode (see

Section 18.7.4 on page 138

), the ID type and content are encrypted to

provide identity protection. In this case the NBG5715 can only distinguish between up to 12

different incoming SAs that connect from remote IPSec routers that have dynamic WAN IP

addresses. The NBG5715 can distinguish up to 48 incoming SAs because you can select between

three encryption algorithms (DES, 3DES and AES), two authentication algorithms (MD5 and SHA1)

and eight key groups when you configure a VPN rule (see

Section 18.4 on page 123

). The ID type

and content act as an extra level of identification for incoming SAs.

Table 59

VPN and NAT

SECURITY PROTOCOL

MODE

NAT

AH

Transport

N

AH

Tunnel

N

ESP

Transport

Y*

ESP

Tunnel

Y

A

B

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