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IPsec - Internet Protocol Security protocol
IPsec is a security protocol implemented within the Internet Protocol
layer of the networking stack. IPsec is defined for both IPv4 and IPv6
(inet(4) and inet6(4)). IPsec 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. IPsec 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) subsystem.
The packets can be passed to a virtual enc(4) interface, to perform
packet filtering before outbound encryption and after decapsulation
To properly filter on the inner packets of an IPsec tunnel with
firewalls, you can change the values of the following sysctls
Name Default Enable
net.inet.ipsec.filtertunnel 0 1
net.inet6.ipsec6.filtertunnel 0 1
IPsec 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 PF_KEY
sockets. The PF_KEY socket API is defined in RFC2367.
The policy engine is controlled by an extension to the PF_KEY API,
setsockopt(2) operations, and sysctl(3) interface. The kernel implements
an extended version of the PF_KEY 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 PF_KEY
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) command, the “default” 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 1 means “use”, 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 2 is synonymous with “require”, 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).
Name Type Changeable
net.inet.ipsec.esp_trans_deflev integer yes
net.inet.ipsec.esp_net_deflev integer yes
net.inet.ipsec.ah_trans_deflev integer yes
net.inet.ipsec.ah_net_deflev integer yes
net.inet6.ipsec6.esp_trans_deflev integer yes
net.inet6.ipsec6.esp_net_deflev integer yes
net.inet6.ipsec6.ah_trans_deflev integer yes
net.inet6.ipsec6.ah_net_deflev integer yes
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 sysctl(8) variables. 0 means “discard” which asks the
kernel to drop the packet. 1 means “none”.
Name Type Changeable
net.inet.ipsec.def_policy integer yes
net.inet6.ipsec6.def_policy integer yes
Miscellaneous sysctl variables
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 sysctl(8), for
tweaking the kernel’s IPsec behavior:
Name Type Changeable
net.inet.ipsec.ah_cleartos integer yes
net.inet.ipsec.ah_offsetmask integer yes
net.inet.ipsec.dfbit integer yes
net.inet.ipsec.ecn integer yes
net.inet.ipsec.debug integer yes
net.inet6.ipsec6.ecn integer yes
net.inet6.ipsec6.debug integer yes
The variables are interpreted as follows:
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 RFC2402
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 chapter 6.1.
If set to non-zero, IPv4 IPsec tunnel encapsulation/decapsulation
behavior will be friendly to ECN (explicit congestion
notification), as documented in draft-ietf-ipsec-ecn-02.txt.
gif(4) talks more about the behavior.
If set to non-zero, debug messages will be generated via
Variables under the net.inet6.ipsec6 tree have similar meanings to those
The IPsec protocol acts as a plug-in to the inet(4) and inet6(4)
protocols and therefore supports most of the protocols defined upon those
IP-layer protocols. The icmp(4) and icmp6(4) protocols may behave
differently with IPsec because IPsec can prevent icmp(4) or icmp6(4)
routines from looking into the IP payload.
ioctl(2), socket(2), ipsec_set_policy(3), crypto(4), enc(4), icmp6(4),
intro(4), ip6(4), setkey(8), sysctl(8)
S. Kent and R. Atkinson, IP Authentication Header, RFC 2404.
S. Kent and R. Atkinson, IP Encapsulating Security Payload (ESP), RFC
Daniel L. McDonald, Craig Metz, and Bao G. Phan, PF_KEY Key Management
API, Version 2, RFC, 2367.
D. L. McDonald, A Simple IP Security API Extension to BSD Sockets,
internet draft, draft-mcdonald-simple-ipsec-api-03.txt, work in progress
The original IPsec implementation appeared in the WIDE/KAME IPv6/IPsec
For FreeBSD 5.0 a fully locked IPsec implementation called fast_ipsec was
brought in. The protocols drew heavily on the OpenBSD 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) support was 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 “esp/tunnel/A-B/use
ah/transport/A-B/require”), 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 kernel the SADB_DUMP and SADB_SPDDUMP operations on PF_KEY sockets
may fail due to lack of space. Increasing the socket buffer size may
alleviate this problem.
The IPcomp protocol support is currently broken.
This documentation needs more review.