IPSEC(4) BSD Kernel Interfaces Manual IPSEC(4)NAMEipsec — IP security protocol
DESCRIPTIONipsec is a security protocol in Internet Protocol layer. ipsec is
defined for both IPv4 and IPv6 (inet(4) and inet6(4)). ipsec consists of
two sub-protocols, namely ESP (encapsulated security payload) and AH
(authentication header). ESP protects IP payload from wire-tapping by
encrypting it by secret key cryptography algorithms. AH guarantees
integrity of IP packet and protects it from intermediate alteration or
impersonation, by attaching cryptographic checksum computed by one-way
hash functions. ipsec has two operation modes: transport mode and tunnel
mode. Transport mode is for protecting peer-to-peer communication
between end nodes. Tunnel mode includes IP-in-IP encapsulation operation
and is designed for security gateways, like VPN configurations.
ipsec is controlled by key management engine and policy engine, in the
operating system kernel.
Key management engine can be accessed from the userland by using PF_KEY
sockets. The PF_KEY socket API is defined in RFC2367.
Policy engine can be controlled by extended part of PF_KEY API,
setsockopt(2) operations, and sysctl(3) interface. The kernel implements
extended version of PF_KEY interface, and allows you to define IPsec pol‐
icy like per-packet filters. setsockopt(2) interface is used to define
per-socket behavior, and sysctl(3) interface is used to define host-wide
The kernel code does not implement dynamic encryption key exchange proto‐
col like IKE (Internet Key Exchange). That should be implemented as
userland programs (usually as daemons), by using the above described
The kernel implements experimental policy management code. You can man‐
age the IPsec policy in two ways. One is to configure per-socket policy
using setsockopt(2). The other is to configure kernel packet filter-
based policy using PF_KEY interface, via setkey(8). In both cases, IPsec
policy must be specified with syntax described in ipsec_set_policy(3).
With setsockopt(2), you can define IPsec policy in per-socket basis. You
can enforce particular IPsec policy onto packets that go through particu‐
With setkey(8) you can define IPsec policy against packets, using sort of
packet filtering rule. Refer to setkey(8) on how to use it.
In the latter case, “default” policy is allowed for use with setkey(8).
By configuring policy to default, you can refer system-wide sysctl(8)
variable for default settings. The following variables are available. 1
means “use”, and 2 means “require” in the syntax.
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 kernel finds no matching policy system wide default value is applied.
System wide default is specified by the following sysctl(8) variables. 0
means “discard” which asks the kernel to drop the packet. 1 means
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 kernel
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 type-of-service field in
the IPv4 header during AH authentication data computation. The
variable is for tweaking AH behavior to interoperate 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 16bit fragment offset field (including flag bits) in IPv4
header, after computing logical AND with the variable. The vari‐
able is for tweaking AH behavior to interoperate with devices
that implement RFC1826 AH. It should be set to zero (clear the
fragment offset field during computation) for RFC2402 confor‐
The variable configures the kernel behavior on IPv4 IPsec tunnel
encapsulation. If set to 0, DF bit on the outer IPv4 header will
be cleared. 1 means that the outer DF bit is set regardless from
the inner DF bit. 2 means that the DF bit is copied from the
inner header to the outer. 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 notifica‐
tion), 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 net.inet6.ipsec6 tree has similar meaning as the
The ipsec protocol works like plug-in to inet(4) and inet6(4) protocols.
Therefore, ipsec supports most of the protocols defined upon those IP-
layer protocols. Some of the protocols, like icmp(4) or icmp6(4), may
behave differently with ipsec. This is because ipsec can prevent icmp(4)
or icmp6(4) routines from looking into IP payload.
SEE ALSOioctl(2), socket(2), ipsec_set_policy(3), icmp6(4), intro(4), ip6(4),
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 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 against 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 we enforce policy check on inner packet on reception, and AH
authenticates encapsulating (outer) packet, not the encapsulated (inner)
packet (so for the receiving kernel there's no sign of authenticity).
The issue will be solved when we revamp our policy engine to keep all the
packet decapsulation history.
Under certain condition, truncated result may be raised from the kernel
against SADB_DUMP and SADB_SPDDUMP operation on PF_KEY socket. This
occurs if there are too many database entries in the kernel and socket
buffer for the PF_KEY socket is insufficient. If you manipulate many
IPsec key/policy database entries, increase the size of socket buffer.
BSD January 29, 1999 BSD