The IPSEC protocols in Linux

This section provides details of the IPSEC protocols which FreeS/WAN implements

The basic idea of IPSEC is to provide security functions, authentication and encryption , at the IP (Internet Protocol) level. This requires a higher-level protocol (IKE) to set things up for the IP-level services (ESP and AH).

Three protocols are used in an IPSEC implementation:

ESP, Encapsulating Security Payload
Encrypts and/or authenticates data
AH, Authentication Header
Provides a packet authentication service
IKE, Internet Key Exchange
Negotiates connection parameters, including keys, for the other two
The term "IPSEC" is slightly ambiguous. In some contexts, it includes all three of the above but in other contexts it refers only to AH and ESP.

Applying IPSEC
Authentication and encryption functions for network data can, of course, be provided at other levels. Many security protocols work at levels above IP.

PGP encrypts and authenticates mail messages
SSH authenticates remote logins and then encrypts the session
SSL or TLS provides security at the sockets layer, e.g. for secure web browsing
and so on. Other techniques work at levels below IP. For example, data on a communications circuit or an entire network can be encrypted by specialised hardware. This is common practice in high-security applications.

Advantages of IPSEC
There are, however, advantages to doing it at the IP level instead of, or as well as, at other levels.

IPSEC is the most general way to provide these services for the Internet.

Higher-level services protect a single protocol; for example PGP protects mail.
Lower level services protect a single medium; for example a pair of encryption boxes on the ends of a line make wiretaps on that line useless unless the attacker is capable of breaking the encryption.
IPSEC, however, can protect any protocol running above IP and any medium which IP runs over. More to the point, it can protect a mixture of application protocols running over a complex combination of media. This is the normal situation for Internet communication; IPSEC is the only general solution.

IPSEC can also provide some security services "in the background", with no visible impact on users. To use PGP encryption and signatures on mail, for example, the user must at least:

remember his or her passphrase,
keep it secure
follow procedures to validate correspondents keys
These systems can be designed so that the burden on users is not onerous, but any system will place some requirements on users. No such system can hope to be secure if users are sloppy about meeting those requirements. The author has seen username and password stuck on terminals with post-it notes in an allegedly secure environment, for example.

Limitations of IPSEC
IPSEC is designed to secure IP links between machines. It does that well, but it is important to remember that there are many things it does not do. Some of the important limitations are:

IPSEC cannot be secure if your system isnt
System security on IPSEC gateway machines is an essential requirement if IPSEC is to function as designed. No system can be trusted if the underlying machine has been subverted. See books on Unix security such as Garfinkel and Spafford or our web references for Linux security or more general computer security.
Of course, there is another side to this. IPSEC can be a powerful tool for improving system and network security. For example, requiring packet authentication makes various spoofing attacks harder and IPSEC tunnels can be extremely useful for s