A Primer On IPSec VPN

Introduction
Initially VMware Cloud on AWS will only support IPSec VPN as a method for connecting a SDDC to an on-premises network. Although IPSec VPN has been around for many years, the technology is often times complex and not fully understood by those setting it up. Due to this complexity, there are several potential pitfalls which must be considered when setting up IPSec VPN. Additionally, the administrator must have a certain minimal level of knowledge about the technology in order to troubleshoot if and when there are issues.

This document will provide an overview of IPSec with the intent of providing a simplified explanation of a very complex set of protocols. The discussion will be mostly limited to aspects of IPSec which are relevant to the VMware Cloud on AWS service and will focus on details which will help the administrator troubleshoot when issues arise.

An Overview of IPSec Components
IPSec is not a single protocol, but rather a suite of protocols designed to provide confidentiality, authentication, and integrity for a VPN. In order to accomplish this IPSec uses Internet Key Exchange (IKE) to manage the connection to a peer, defines security associations used to secure and validate data exchanges, and defines security protocols used to carry IP traffic over the VPN. Lets take a closer look at each of these components in more detail.

Security Associations
A Security Association (SA) is a basic component of IPSec and contains information about the security parameters negotiated between peers. There are two types of SAs: IKE (or ISAKMP) SA and IPSec SA.

The IKE SA is used for the control plane of the VPN and contains a combination of mandatory and optional values. These include:

  • Encryption Algorithm (mandatory)
  • Hash Algorithm (mandatory)
  • Authentication Method (mandatory)
  • Diffie-Hellman Group (mandatory)
  • Lifetime (optional)
  • IPSec SAs are used for the data plane of the VPN and are stored internally within the Security Association Database (SADB). The SADB is keyed in such a way as to allow the VPN device to match traffic destined to, or received from a VPN to a specific entry within. Each entry stores a number of parameters along side the SA which define how packets matching the entry should be handled. These include:

  • Sequence number for packets sent/received on the tunnel
  • Anti-replay window for the VPN
  • SA lifetime
  • Mode (tunnel or transport)
  • AH or ESP settings
  • Internet Key Exchange
    Internet Key Exchange (IKE) is used as a control plane protocol for the VPN tunnel. IKE operates in two phases:

    Phase 1 provides mutual authentication between peers and establishes the session key for later exchanges. The result of phase 1 is an ISAKMP SA.

    Phase 2 uses the ISAKMP SA resulting from phase 1 in order to establish the IPSec SAs used to carry IP traffic through the VPN.
    The details of IKE will be covered in a later section.

    Security Protocols
    IPSec defines two security protocols which determine how data plane traffic is sent through the VPN tunnel. These are:

  • Authentication Header (AH)
  • Encapsulating Security Payload (ESP)
  • IPSec also defines two modes of operation: transport and tunnel.

    In transport mode the AH or ESP header is inserted immediately after the original IP header of a packet and the IP protocol number of the packet is changed to either 50 or 51. For this reason it is only useful when the VPN is carrying traffic between the peers themselves.

    In tunnel mode the VPN peers encapsulates data plane packets within a new IP packet. This outer packet uses the source/destination addresses of the VPN peers themselves, contains the AH or ESP header, and uses an IP protocol of either 50 or 51. It is this encapsulation which gives tunnel mode the ability to carry IP traffic for hosts other than the VPN peers themselves.

    Lets take a look at the differences between AH and ESP and their structures when used in transport vs tunnel mode.

    Authentication Header provides for authentication, data integrity, and anti-replay services. It does not provided confidentiality (encryption). The following diagram illustrates the structure of an AH packet in both transport and tunnel modes:

    AH packets are identified by IP protocol 51. The AH header provides a security parameter index and sequence number which allow the VPN peers to associate received packets with an appropriate SA and enables anti-replay services. It also contains an authentication digest which is used to authenticate the packet and ensures data integrity for the entire packet. This is an important point; the digest is for the entire packet including the AH header and all immutable fields of the outermost IP header. It is because of this digest that AH will not work if the VPN peers are behind a NAT gateway. Since NAT, by definition, makes changes to the IP and TCP/UDP headers the digest will always be incorrect post NAT and the receiving peer will reject the packet. In short, AH will not work behind a NAT gateway.

    Encapsulating Security Payload provides for authentication, data integrity, confidentiality (encryption), and anti-replay services. The following diagram illustrates the structure of an ESP packet in both transport and tunnel modes:

    ESP packets are identified by IP protocol 50. Like the AH header, the ESP header provides both a security parameter index and sequence number which serve the same purpose as with AH. Since ESP provides encryption, the header also contains several fields required by the encryption protocols. Additionally, the ESP header provides an authentication digest which is calculated, post encryption, on the contained data. It is important to note that this digest covers only the encrypted portion of the packet and does not include the outer IP or ESP headers. Unlike with AH, NAT and ESP can coexist in certain situations. When using transport mode ESP will only function behind a NAT gateway if the NAT translation is 1:1. When operating in tunnel mode ESP can function without issue behind a NAT gateway.

    Diffie-Hellman Key Exchange
    Diffie-Hellman (DH) is the public key exchange algorithm used by IKE phase 1 to calculate the shared session key. DH defines a number of groups, which refer to the Oakley protocol, and define the overall strength (security) of the key exchange. The DH group used between peers is determined from the proposal exchange as part of IKE phase 1.

    IKE Operations
    At present the VMware Cloud on AWS service only supports IKE version 1. This section will be limited to describing this protocol version.

    As mentioned previously, IKE is used as a control protocol for IPSec and operates in two phases. The first phase of IKE is used to create an encrypted control channel and the second phase is used to negotiate IPSec SAs. IKE communications are performed using UDP with both the source and destination ports being set to 500. Several parameters are required for IKE and are defined as part of the configuration on each peer.

    The below sections describe the operations of IKE phase 1 and phase 2.

    Phase 1
    The purpose of phase 1 is to establish a secure channel for control plane traffic. The result of a successful phase 1 operation is the establishment of an ISAKMP SA which is then used to encrypt and verify all further IKE communications. Phase 1 can operate in two modes: main and aggressive.

    Main mode is the more verbose of the two modes and consists of 6 messages sent between peers.

    The 1st pair of messages are used to exchange security proposals for IKE. The initiator will offer a one or more sets of parameters which it is willing to support and the receiver will then select a parameter set and echo it back to the initiator. If the receiver cannot support any of the offered parameter sets then it will instead respond with a “reject” message and the phase 1 operation will be considered failed.

    The 2nd pair of messages are used to exchange keying materials needed to generate a shared session key. Internally, each peer will used this data to generate and store this shared session key as part of the ISAKMP SA. It is this exchange which makes use of the authentication method parameter since both peers need a mutually agreed upon authentication key in order to derive the session key. Although IKE supports multiple authentication methods, the most common (and easiest to implement) is to use pre-shared keys between peers.

    The last pair of messages are encrypted and authenticated using the ISAKMP SA. The function of this exchange is to perform final authentication of the IKE peer.

    Aggressive mode performs the same function as main mode but in 3 messages instead of 6. This exchange opts for speed at the cost of some security and is generally less preferred than main mode. As such, we will not cover the details of aggressive mode in this document.

    Phase 2
    IKE phase 2 establishes IPSec SAs (one in each direction) for the VPN connection, and is referred to as Quick Mode. At the conclusion of phase 2 each peer will be ready to pass data plane traffic through the VPN. Quick mode consists of 3 messages sent between peers (with an optional 4th message). All messages in phase 2 are secured using the ISAKMP SA established in phase 1.

    The 1st message is sent by the initiator and is used to advertise sets of parameters which it is willing to support for the IPSec SA. The receiver will select a set of parameters and will echo them back to the initiator as part of the 2nd message (or rejects if it finds no acceptable proposals). The 3rd message is used by the initiator to verify the channel, and an optional 4th message is used by the receiver to signal that it is ready to receive data plane traffic through the VPN. This “signal when ready” mechanism is used to allow the receiver time to complete the setup on its end prior to accepting any data. If the receiver wishes to implement this feature then it signals this intention as part of the 2nd message.

    IKE and NAT
    NAT can cause problems with IKE due to the fact that NAT devices tend to modify source IP addresses and TCP/UDP ports. As mentioned previously, IKE requires that peers exchange messages using UDP 500 for both source and destination port. Additionally, IKE authentication is often performed using the source IP of the peer and this authentication tends to fail when modified by NAT. In order to get around these issues IKE makes use of NAT Traversal (NAT-T) when the initiator is located behind a NAT device.

    NAT-T performs three functions:

  • It determines if the remote peer supports NAT-T
  • It detects the presence of a NAT device between peers
  • It uses UDP encapsulation to adjust for the presence of a NAT device
  • The 1st and 2nd functions are performed as part of the phase 1 negotiation. The first function is simple and is advertised as a parameter within the phase 1 proposal exchange. The second function is a bit more complex and relies on each peer sending a series of NAT-D payloads (during the 3rd & 4th message exchange) which includes a hash of the source/destination IP and source UDP port used by the sender. This hash will then be recomputed and checked by each receiving peer. If a NAT device modifies these addresses then the computed hash will not match the advertised hash and the peers will be alerted to the presence of a NAT device.

    If a NAT device is detected then the initiator must change ports of all subsequent exchanges to UDP 4500 (for both source and destination). The receiver will also adjust to use UDP a destination port of 4500 on packets destined for the initiator. As part of the NAT-T function, all data plane packets sent through the VPN will be encapsulated using UDP 4500 between peers. Typically peers of a NAT-T VPN will also send periodic keepalive messages in order to keep the NAT translation for the VPN from expiring.

    A Note on Perfect Forward Secrecy
    Perfect Forward Secrecy (PFS) is used by IPSec in order to force IKE phase 1 to regenerate a new shared key whenever the IPSec SA lifetimes expire. PFS is used to add an extra layer of security to an IPSec VPN by ensuring that if a session key (and thus the associated IPSec SAs) is compromised that it will only be relevant to data which was passed during the lifetime of the compromised IPSec SAs. The use of PFS is generally recommended for IPSec VPNs.

    Summary
    IPSec is a complex suite of protocols, however if an administrator maintains a basic understanding of its operations then the process of troubleshooting will be made much easier. In particular an understanding of the phases of IKE, the UDP ports used by IKE, and how IPSec can be impacted by a NAT gateway can go a long way in formulating a step-by-step process for troubleshooting a broken VPN.

    References
    https://tools.ietf.org/html/rfc2407

    https://tools.ietf.org/html/rfc2408

    https://tools.ietf.org/html/rfc2409

    http://www.ciscopress.com/store/ipsec-vpn-design-9781587051111

    About the Authors

    Dustin Spinhirne

    Solutions Architect Principal at VMware

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