Everything about Ipsec totally explained
IPsec (
IP security) is a
suite of protocols for securing
Internet Protocol (IP) communications by
authenticating and/or
encrypting each in a
data stream. IPsec also includes protocols for cryptographic
key establishment.
Summary
IPsec protocols operate at the
network layer, layer 3 of the
OSI model. Other Internet security protocols in widespread use, such as
SSL,
TLS and
SSH, operate from the
transport layer up (OSI layers 4 - 7). This makes IPsec more flexible, as it can be used for protecting layer 4 protocols, including both
TCP and
UDP, the most commonly used transport layer protocols. IPsec has an advantage over SSL and other methods that operate at higher layers: an application doesn't need to be designed to use IPsec, whereas the ability to use SSL or another higher-layer protocol must be incorporated into the design of an application.
Security architecture
IPsec is implemented by a set of
cryptographic protocols for (1) securing packet flows, (2)
mutual authentication and (3)
establishing cryptographic parameters.
The IP security architecture uses the concept of a security association as the basis for building security functions into
IP. A security association is simply the bundle of algorithms and parameters (such as keys) that's being used to encrypt and authenticate a particular flow in one direction. Therefore, in normal bi-directional traffic, the flows are secured by a pair of security associations. The actual choice of encryption and authentication algorithms (from a defined list) is left to the IPsec administrator.
In order to decide what protection is to be provided for an outgoing packet, IPsec uses the
Security Parameter Index (SPI), an index to the security association database (SADB), along with the destination address in a packet header, which together uniquely identify a security association for that packet. A similar procedure is performed for an incoming packet, where IPsec gathers decryption and verification keys from the security association database.
For multicast, a security association is provided for the group, and is duplicated across all authorized receivers of the group. There may be more than one security association for a group, using different SPIs, thereby allowing multiple levels and sets of security within a group. Indeed, each sender can have multiple security associations, allowing authentication, since a receiver can only know that someone knowing the keys sent the data. Note that the relevant standard doesn't describe how the association is chosen and duplicated across the group; it's assumed that a responsible party will have made the choice.
Current status as a standard
IPsec implementation is a mandatory part of
IPv6, but isn't an integral part of
IPv4. However, because of the slow uptake of IPv6, IPsec is most commonly used to secure IPv4 traffic. IPsec protocols were originally defined by
RFCs 1825 & 1829, published in 1995. In 1998, these documents were obsoleted by RFC 2401 RFC 2412, which are not compatible with RFC 1825 & RFC 1829, although they're conceptually identical. In December 2005, third-generation documents, RFC 4301 & RFC 4309, were produced. They are largely a superset of RFC 2401 & RFC 2412, but provide a
second Internet Key Exchange standard. These third-generation documents standardized the abbreviation of IPsec to uppercase “IP” and lowercase “sec”. It is unusual to see any product that offers support for RFCs 1825 & 1829. “ESP” generally refers to RFC 2406, while ESPbis refers to RFC 4303.
Design intent
IPsec was intended to provide either
transport mode (end-to-end) security of packet traffic in which the end-point computers do the security processing, or
tunnel mode (portal-to-portal)
communications security in which security of packet traffic is provided to several machines (even to whole
LANs) by a single node.
IPsec can be used to create
Virtual Private Networks (VPN) in either mode, and this is the dominant use. Note, however, that the security implications are quite different between the two operational modes.
End-to-end communication security on an Internet-wide scale has been slower to develop than many had expected. Part of the reason is that no universal, or universally trusted,
Public Key Infrastructure (PKI) has emerged (
DNSSEC was originally envisioned for this); another part is that many users understand neither their needs nor the available options well enough to promote inclusion in vendors' products.
Since the
Internet Protocol doesn't inherently provide any security capabilities, IPsec was introduced to provide security services such as the following:
- Encrypting traffic (so it can't be read by parties other than those for whom it's intended)
- Integrity validation (ensuring traffic hasn't been modified along its path)
- Authenticating the peers (ensuring that traffic is from a trusted party)
- Anti-replay (protecting against replay of the secure session).
Modes
There are two modes of
IPsec operation:
transport mode and
tunnel mode.
Transport mode
In
transport mode, only the payload (the data you transfer) of the IP packet is
encrypted and/or authenticated. The routing is intact, since the IP header is neither modified nor encrypted; however, when the
authentication header is used, the IP addresses can't be
translated, as this will invalidate the
hash value. The
transport and
application layers are always secured by hash, so they can't be modified in any way (for example by
translating the
port numbers).
Transport mode is used for host-to-host communications.
A means to encapsulate IPsec messages for
NAT traversal has been defined by
RFC documents describing the
NAT-T mechanism.
Tunnel mode
In
tunnel mode, the entire IP packet (data plus the message headers) is encrypted and/or authenticated. It must then be encapsulated into a new IP packet for routing to work.
Tunnel mode is used for network-to-network communications (secure tunnels between routers, for example for
VPNs) or host-to-network and host-to-host communications over the
Internet.
Technical details
Two protocols have been developed to provide packet-level security for both
IPv4 and
IPv6:
The IP Authentication Header provides integrity and authentication and non-repudiation, if the appropriate choice of cryptographic algorithms is made.
The IP Encapsulating Security Payload provides confidentiality, along with optional (but strongly recommended) authentication and integrity protection.
Cryptographic algorithms defined for use with IPsec include HMAC-SHA1 for integrity protection, and TripleDES-CBC and AES-CBC for confidentiality. Refer to RFC 4305 for details.
Authentication header (AH)
The AH is intended to guarantee connectionless integrity and data origin authentication of IP datagrams. Further, it can optionally protect against replay attacks by using the sliding window technique and discarding old packets. AH protects the IP payload and all header fields of an IP datagram except for mutable fields, for example those that might be altered in transit. In IPv4, mutable (and therefore unauthenticated) IP header fields include TOS, Flags, Fragment Offset, TTL and Header Checksum. AH operates directly on top of IP, using IP protocol number 51.
An AH packet diagram:
| 0 - 7 bit |
8 - 15 bit |
16 - 23 bit |
24 - 31 bit |
| Next header |
Payload length |
RESERVED |
| Security parameters index (SPI) |
| Sequence number |
|
Authentication data (variable)
|
Field meanings: Next header : Identifies the protocol of the transferred data.
; Payload length : Size of AH packet.
RESERVED : Reserved for future use (all zero until then).
; Security parameters index (SPI) : Identifies the security parameters, which, in combination with the IP address, then identify the security association implemented with this packet.
Sequence number : A monotonically increasing number, used to prevent replay attacks.
; Authentication data : Contains the integrity check value (ICV) necessary to authenticate the packet; it may contain padding.
Encapsulating Security Payload (ESP)
The ESP protocol provides origin authenticity, integrity, and confidentiality protection of a packet. ESP also supports encryption-only and authentication-only configurations, but using encryption without authentication is strongly discouraged because it's insecure.. Unlike AH, the IP packet header isn't protected by ESP. (Although in tunnel mode ESP, protection is afforded to the whole inner IP packet, including the inner header; the outer header remains unprotected.) ESP operates directly on top of IP, using IP protocol number 50.
An ESP packet diagram:
| 0 - 7 bit |
8 - 15 bit |
16 - 23 bit |
24 - 31 bit |
| Security parameters index (SPI) |
| Sequence number |
|
Payload data (variable)
|
| |
Padding (0-255 bytes) |
|
| |
|
Pad Length |
Next Header |
|
Authentication Data (variable)
|
Field meanings: Security parameters index (SPI) : Identifies the security parameters in combination with IP address.
; Sequence number : A monotonically increasing number, used to prevent replay attacks.
Payload data : The data to be transferred.
; Padding : Used with some block ciphers to pad the data to the full length of a block.
Pad length : Size of padding in bytes.
; Next header : Identifies the protocol of the transferred data.
Authentication data : Contains the data used to authenticate the packet.
Implementations
IPsec support is usually implemented in the kernel with key management and ISAKMP/IKE negotiation carried out from user-space. Existing IPsec implementations tend to include both of these functionalities. However, as there's a standard interface for key management, it's possible to control one kernel IPsec stack using key management tools from a different implementation.
Because of this, there's confusion as to the origins of the IPsec implementation that's in the Linux kernel. The FreeS/WAN project made the first complete and open source implementation of IPsec for Linux. It consists of a kernel IPsec stack (KLIPS), as well as a key management daemon (pluto) and many shell scripts. The FreeS/WAN project was disbanded in March 2004. Openswan and strongSwan are continuations of FreeS/WAN. The KAME project also implemented complete IPsec support for NetBSD, FreeBSD. Its key management daemon is called racoon. OpenBSD made its own ISAKMP/IKE daemon, simply named isakmpd (which was also ported to other systems, including Linux).
However, none of these kernel IPsec stacks were integrated into the Linux kernel. Alexey Kuznetsov and David S. Miller wrote a kernel IPsec implementation from scratch for the Linux kernel around the end of 2002. This stack was subsequently released as part of Linux 2.6, and is referred to variously as "native" or "NETKEY".
Therefore, contrary to popular belief, the Linux IPsec stack didn't originate from the KAME project. As it supports the standard PF_KEY protocol (RFC 2367) and the native XFRM interface for key management, the Linux IPsec stack can be used in conjunction with either pluto from Openswan/strongSwan, isakmpd from OpenBSD project, racoon from the KAME project or without any ISAKMP/IKE daemon (using manual keying).
The new architectures of network processors, including multi-core processors with integrated encryption engines, change the way the IPsec stacks are designed. A dedicated Fast Path is used in order to offload the processing of the IPsec processing (SA, SP lookups, encryption, etc.). These Fast Path stacks must be co-integrated on dedicated cores with Linux or RTOS running on other cores. These OS are the control plane that runs ISAKMP/IKE of the Fast Path IPsec stack.
There are a number of implementations of IPsec and ISAKMP/IKE protocols. These include:
6WINDGate
, Network processor MPU Fast Path IPsec stack
NRL (External Link) IPsec, one of the original sources of IPsec code (External Link)
OpenBSD, with its own code derived from NRL IPsec
the KAME stack, that's included in Mac OS X, NetBSD and FreeBSD
"IPsec" in Cisco IOS Software (External Link)
"IPsec" in Microsoft Windows, including Windows XP(External Link) (External Link), Windows 2000(External Link), Windows 2003(External Link), and Windows Vista (External Link)
SafeNet QuickSec toolkits (External Link)
IPsec in Solaris (External Link)
IBM AIX operating system
IBM z/OS
IPsec and IKE in HP-UX (HP-UX IPSec)
"IPsec and IKE" in VxWorks (External Link)
Further Information
Get more info on 'Ipsec'.
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