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ipsec - IP security protocol
ipsec is a security protocol implemented within the Internet Protocol
layer of the TCP/IP stack. ipsec is defined for both IPv4 and IPv6
(inet(4) and inet6(4)). ipsec contains two protocols, ESP, the
encapsulated security payload protocol and AH, the authentication header
protocol. ESP prevents unauthorized parties from reading the payload of
an IP packet by encrypting it using secret key cryptography algorithms.
AH both authenticates guarantees the integrity of an IP packet by
attaching a cryptographic checksum computed using one-way hash functions.
ipsec has 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.
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) command. In either case, IPsec policies
must be specified using the syntax described in ipsec_set_policy(3).
Please refer to the setkey(8) man page for instructions on its use.
When setting policies using the setkey(8) command the “default” option
you can have the system use its default policy, 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
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), icmp6(4), intro(4), ip6(4),
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 implementation described herein appeared in WIDE/KAME IPv6/IPsec
The IPsec support is subject to change as the IPsec protocols develop.
There is no single standard for the policy engine API, so the policy
engine API described herein is just for KAME 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.