— Internet Protocol Security
is a security protocol implemented within the Internet
Protocol layer of the networking stack. IPsec
is defined for
both IPv4 and IPv6 (inet(4)
is a set of protocols, ESP (for Encapsulating Security
Payload) AH (for Authentication Header), and IPComp (for IP Payload
Compression Protocol) that provide security services for IP datagrams. AH both
authenticates and guarantees the integrity of an IP packet by attaching a
cryptographic checksum computed using one-way hash functions. ESP, in
addition, prevents unauthorized parties from reading the payload of an IP
packet by also encrypting it. IPComp tries to increase communication
performance by compressing IP payload, thus reducing the amount of data sent.
This will help nodes on slow links but with enough computing power.
operates in one of two modes: transport mode or tunnel
mode. Transport mode is used to protect peer-to-peer communication between end
nodes. Tunnel mode encapsulates IP packets within other IP packets and is
designed for security gateways such as VPN endpoints.
System configuration requires the crypto(4)
The packets can be passed to a virtual enc(4)
perform packet filtering before outbound encryption and after decapsulation
To properly filter on the inner packets of an IPsec
with firewalls, you can change the values of the following sysctls
is controlled by a key management and policy engine,
that reside in the operating system kernel. Key management is the process of
associating keys with security associations, also know as SAs. Policy
management dictates when new security associations created or destroyed.
The key management engine can be accessed from userland by using
socket API is defined in RFC2367.
The policy engine is controlled by an extension to the
operations, and sysctl(3)
interface. The kernel implements
an extended version of the
interface and allows
the programmer to define IPsec policies which are similar to the per-packet
filters. The setsockopt(2)
interface is used to define
per-socket behavior, and sysctl(3)
interface is used to
define host-wide default behavior.
The kernel code does not implement a dynamic encryption key exchange protocol
such as IKE (Internet Key Exchange). Key exchange protocols are beyond what is
necessary in the kernel and should be implemented as daemon processes which
call the APIs.
IPsec policies can be managed in one of two ways, either by configuring
per-socket policies using the setsockopt(2)
system calls, or
by configuring kernel level packet filter-based policies using the
interface, via the setkey(8)
you can define IPsec policies against packets using rules similar to packet
filtering rules. Refer to setkey(8)
on how to use it.
When setting policies using the setkey(8)
” option instructs the system to
use its default policy, as explained below, for processing packets. The
following sysctl variables are available for configuring the system's IPsec
behavior. The variables can have one of two values. A
which means that if there is a security association then use it but if there
is not then the packets are not processed by IPsec. The value
is synonymous with
”, which requires that a security
association must exist for the packets to move, and not be dropped. These
terms are defined in ipsec_set_policy(8)
If the kernel does not find a matching, system wide, policy then the default
value is applied. The system wide default policy is specified by the following
” which asks the kernel to drop
When the IPsec
protocols are configured for use, all protocols
are included in the system. To selectively enable/disable protocols, use
In addition the following variables are accessible via
, for tweaking the kernel's IPsec behavior:
The variables are interpreted as follows:
- Li ipsec.ah_cleartos
- If set to non-zero, the kernel clears the type-of-service
field in the IPv4 header during AH authentication data computation. This
variable is used to get current systems to inter-operate with devices that
implement RFC1826 AH. It should be set to non-zero (clear the
type-of-service field) for RFC2402 conformance.
- During AH authentication data computation, the kernel will
include a 16bit fragment offset field (including flag bits) in the IPv4
header, after computing logical AND with the variable. The variable is
used for inter-operating with devices that implement RFC1826 AH. It should
be set to zero (clear the fragment offset field during computation) for
- This variable configures the kernel behavior on IPv4 IPsec
tunnel encapsulation. If set to 0, the DF bit on the outer IPv4 header
will be cleared while 1 means that the outer DF bit is set regardless from
the inner DF bit and 2 indicates that the DF bit is copied from the inner
header to the outer one. The variable is supplied to conform to RFC2401
- If set to non-zero, IPv4 IPsec tunnel
encapsulation/decapsulation behavior will be friendly to ECN (explicit
congestion notification), as documented in
gif(4) talks more about the behavior.
- If set to non-zero, debug messages will be generated via
Variables under the
tree have similar
meanings to those described above.
protocol acts as a plug-in to the
therefore supports most of the protocols defined upon those IP-layer
protocols. The icmp(4)
protocols may behave differently with IPsec
can prevent icmp(4)
routines from looking into the IP payload.
S. Kent and R.
Atkinson, IP Authentication Header,
S. Kent and R.
Atkinson, IP Encapsulating Security Payload
(ESP), RFC 2406.
Daniel L. McDonald,
Craig Metz, and Bao G. Phan,
PF_KEY Key Management API, Version 2,
D. L. McDonald, A
Simple IP Security API Extension to BSD Sockets,
work in progress material.
The original IPsec
implementation appeared in the WIDE/KAME
For FreeBSD 5.0
a fully locked IPsec implementation
called fast_ipsec was brought in. The protocols drew heavily on the
implementation of the IPsec protocols. The
policy management code was derived from the KAME implementation found in their
IPsec protocols. The fast_ipsec implementation lacked ip6(4)
support but made use of the crypto(4)
For FreeBSD 7.0 ip6(4)
added to fast_ipsec. After this the old KAME IPsec implementation was dropped
and fast_ipsec became what now is the only IPsec
implementation in FreeBSD
There is no single standard for the policy engine API, so the policy engine API
described herein is just for this implementation.
AH and tunnel mode encapsulation may not work as you might expect. If you
configure inbound “require” policy with an AH tunnel or any IPsec
encapsulating policy with AH (like “
”), tunnelled packets will be rejected.
This is because the policy check is enforced on the inner packet on reception,
and AH authenticates encapsulating (outer) packet, not the encapsulated
(inner) packet (so for the receiving kernel there is no sign of authenticity).
The issue will be solved when we revamp our policy engine to keep all the
packet decapsulation history.
When a large database of security associations or policies is present in the
sockets may fail due to lack of space.
Increasing the socket buffer size may alleviate this problem.
The IPcomp protocol may occasionally error because of zlib(3)
This documentation needs more review.