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NAME
ipsec — Internet Protocol Security protocol
SYNOPSIS
options IPSEC
options IPSEC_SUPPORT
device crypto
#include <sys/types.h>
#include <netinet/in.h>
#include <netipsec/ipsec.h>
#include <netipsec/ipsec6.h>
DESCRIPTION
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 inbound.
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
Kernel interface
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.
Policy management
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.
Depending on the socket's address family, IPPROTO_IP or IPPROTO_IPV6 transport level and IP_IPSEC_POLICY or
IPV6_IPSEC_POLICY socket options may be used to configure per-socket security policies. A properly-formed
IPsec policy specification structure can be created using ipsec_set_policy(3) function and used as socket
option value for the setsockopt(2) call.
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 sysctl(8).
Name Default
net.inet.esp.esp_enable On
net.inet.ah.ah_enable On
net.inet.ipcomp.ipcomp_enable On
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.inet.ipsec.natt_cksum_policy integer yes
net.inet.ipsec.check_policy_history integer yes
net.inet6.ipsec6.ecn integer yes
net.inet6.ipsec6.debug integer yes
The variables are interpreted as follows:
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.
ipsec.ah_offsetmask
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 conformance.
ipsec.dfbit
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.
ipsec.ecn
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.
ipsec.debug
If set to non-zero, debug messages will be generated via syslog(3).
ipsec.natt_cksum_policy
Controls how the kernel handles TCP and UDP checksums when ESP in UDP encapsulation is used for
IPsec transport mode. If set to a non-zero value, the kernel fully recomputes checksums for
inbound TCP segments and UDP datagrams after they are decapsulated and decrypted. If set to 0 and
original addresses were configured for corresponding SA by the IKE daemon, the kernel incrementally
recomputes checksums for inbound TCP segments and UDP datagrams. If addresses were not configured,
the checksums are ignored.
ipsec.check_policy_history
Enables strict policy checking for inbound packets. By default, inbound security policies check
that packets handled by IPsec have been decrypted and authenticated. If this variable is set to a
non-zero value, each packet handled by IPsec is checked against the history of IPsec security
associations. The IPsec security protocol, mode, and SA addresses must match.
Variables under the net.inet6.ipsec6 tree have similar meanings to those described above.
PROTOCOLS
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.
SEE ALSO
ioctl(2), socket(2), ipsec_set_policy(3), crypto(4), enc(4), if_ipsec(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 2406.
STANDARDS
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 material.
HISTORY
The original ipsec implementation appeared in the WIDE/KAME IPv6/IPsec stack.
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) subsystem.
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.
BUGS
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 may occasionally error because of zlib(3) problems.
This documentation needs more review.