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VPN Overview

A virtual private network (VPN) provides a means for securely communicating among remote computers across a public WAN such as the Internet.

A VPN connection can link two LANs (site-to-site VPN) or a remote dial-up user and a LAN. The traffic that flows between these two points passes through shared resources such as routers, switches, and other network equipment that make up the public WAN. To secure VPN communication while passing through the WAN, the two participants create an IP Security (IPsec) tunnel.

Note: The term tunnel does not denote tunnel mode (see Packet Processing in Tunnel Mode). Instead, it refers to the IPsec connection.

IPsec is a suite of related protocols for cryptographically securing communications at the IP Packet Layer. IPsec also provides methods for the manual and automatic negotiation of security associations (SAs) and key distribution, all the attributes for which are gathered in a Domain of Interpretation (DOI). The IPsec DOI is a document containing definitions for all the security parameters required for the successful negotiation of a VPN tunnel—essentially, all the attributes required for SA and IKE negotiations. See RFC 2407 and RFC 2408 for more information.

This topic includes the following sections:

Security Associations

A security association (SA) is a unidirectional agreement between the VPN participants regarding the methods and parameters to use in securing a communication channel. Full bidirectional communication requires at least two SAs, one for each direction. Through the SA, an IPsec tunnel can provide the following security functions:

The security functions you employ depend on your needs. If you need only to authenticate the IP packet source and content integrity, you can authenticate the packet without applying any encryption. On the other hand, if you are concerned only with preserving privacy, you can encrypt the packet without applying any authentication mechanisms. Optionally, you can both encrypt and authenticate the packet. Most network security designers choose to encrypt, authenticate, and replay-protect their VPN traffic.

An IPsec tunnel consists of a pair of unidirectional SAs—one at each end of the tunnel—that specify the security parameter index (SPI), destination IP address, and security protocol (Authentication Header or Encapsulating Security Payload) employed. An SA groups together the following components for securing communications:

For inbound traffic, JUNOS Software looks up the SA by using the following triplet:

For outbound VPN traffic, the policy invokes the SA associated with the VPN tunnel.

IPsec Key Management

The distribution and management of keys are critical to using VPNs successfully. JUNOS Software supports IPsec technology for creating VPN tunnels with three kinds of key creation mechanisms:

You can choose your key creation mechanism—also called authentication method—during Phase 1 and Phase 2 proposal configuration. For more information, see IPsec Tunnel Negotiation.

Note: Manual key creation and AutoKey IKE with certificates are not supported with the dynamic VPN feature at this time.

This topic includes the following sections:

Manual Key

With manual keys, administrators at both ends of a tunnel configure all the security parameters. This is a viable technique for small, static networks where the distribution, maintenance, and tracking of keys are not difficult. However, safely distributing manual-key configurations across great distances poses security issues. Aside from passing the keys face-to-face, you cannot be completely sure that the keys have not been compromised while in transit. Also, whenever you want to change the key, you are faced with the same security issues as when you initially distributed it.

AutoKey IKE

When you need to create and manage numerous tunnels, you need a method that does not require you to configure every element manually. IPsec supports the automated generation and negotiation of keys and security associations using the Internet Key Exchange (IKE) protocol. JUNOS Software refers to such automated tunnel negotiation as AutoKey IKE and supports AutoKey IKE with preshared keys and AutoKey IKE with certificates.

Diffie-Hellman Exchange

A Diffie-Hellman (DH) exchange allows the participants to produce a shared secret value. The strength of the technique is that it allows the participants to create the secret value over an unsecured medium without passing the secret value through the wire. There are five DH groups; JUNOS Software supports groups 1, 2, and 5. The size of the prime modulus used in each group's calculation differs as follows:

Note: The strength of DH Group 1 security has depreciated; therefore we do not recommend its use.

The larger the modulus, the more secure the generated key is considered to be; however, the larger the modulus, the longer the key-generation process takes. Because the modulus for each DH group is a different size, the participants must agree to use the same group.

Note: If you configure multiple (up to four) proposals for Phase 1 negotiations, use the same DH group in all proposals. The same guideline applies to multiple proposals for Phase 2 negotiations.

IPsec Security Protocols

IPsec uses two protocols to secure communications at the IP layer:

You can choose your security protocols—also called authentication and encryption algorithms—during Phase 1 and Phase 2 proposal configuration. For more information, see IPsec Tunnel Negotiation.

This topic includes the following sections:

AH Protocol

The Authentication Header (AH) protocol provides a means to verify the authenticity/integrity of the content and origin of a packet. You can authenticate the packet by the checksum calculated through a Hash Message Authentication Code (HMAC) using a secret key and either MD5 or SHA-1 hash functions.

Note: For more information on MD5 hashing algorithms, see RFC 1321 and RFC 2403. For more information on SHA hashing algorithms, see RFC 2404. For more information on HMAC, see RFC 2104.

ESP Protocol

The Encapsulating Security Payload (ESP) protocol provides a means to ensure privacy (encryption) and source authentication and content integrity (authentication). ESP in tunnel mode encapsulates the entire IP packet (header and payload) and then appends a new IP header to the now-encrypted packet. This new IP header contains the destination address needed to route the protected data through the network. (For more information about tunnel mode, see Packet Processing in Tunnel Mode.)

With ESP, you can both encrypt and authenticate, encrypt only, or authenticate only. For encryption, you can choose one of the following encryption algorithms:

For authentication, you can use either the MD5 or the SHA-1 algorithm.

Note: Even though is possible to select NULL for encryption, it has been demonstrated that IPsec might be vulnerable to attack under such circumstances. Therefore, we suggest that you choose an encryption algorithm for maximum security.

IPsec Tunnel Negotiation

To establish an AutoKey IKE IPsec tunnel, two phases of negotiation are required:

For a manual key IPsec tunnel, because all of the SA parameters have been previously defined, there is no need to negotiate which SAs to use. In essence, the tunnel has already been established. When traffic matches a policy using that manual key tunnel or when a route involves the tunnel, the Juniper Networks device simply encrypts and authenticates the data, as you determined, and forwards it to the destination gateway.

Distributed VPNs in SRX Series Services Gateways

In the SRX3000 and SRX5000 lines, the IKE provides tunnel management for IPsec and authenticates end entities. The IKE performs a Diffie-Hellman (DH) key exchange to generate an IPsec tunnel between network devices. The IPsec tunnels generated by IKE are used to encrypt, decrypt, and authenticate user traffic between the network devices at the IP layer.

The VPN is created by distributing the IKE and IPsec workload among the multiple Security Processing Units (SPUs) of the platform. The IKE workload is distributed based on a key generated from the IKE packet's 4 tuples (source IP address, destination IP addresses, and UDP ports). The workload is distributed by assigning anchoring SPUs logically and mapping the logical SPUs to physical SPU-based on the composition at that given time. This distribution prevents any change in the number and composition of SPUs in the device, which may happen due to hot swap or SPC failure. The SPU in a device communicates with the Routing Engine to create a distributed VPN.

In IPsec, the workload is distributed by the same algorithm that distributes the IKE. The Phase 2 SA for a given VPN tunnel termination points pair is exclusively owned by a particular SPU, and all IPsec packets belonging to this Phase 2 SA are forwarded to the anchoring SPU of that security association for IPsec processing.

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