is a security protocol implemented within the Internet Protocol layer
of the networking stack.
is defined for both IPv4 and IPv6
(inet4
and
inet6(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)
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
tunnel with firewalls, add
options IPSEC_FILTERTUNNEL
to the kernel configuration file.
Kernel interface
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.
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_policy8.
NameTypeChangeable
"net.inet.ipsec.esp_trans_deflevintegeryes"
"net.inet.ipsec.esp_net_deflevintegeryes"
"net.inet.ipsec.ah_trans_deflevintegeryes"
"net.inet.ipsec.ah_net_deflevintegeryes"
"net.inet6.ipsec6.esp_trans_deflevintegeryes"
"net.inet6.ipsec6.esp_net_deflevintegeryes"
"net.inet6.ipsec6.ah_trans_deflevintegeryes"
"net.inet6.ipsec6.ah_net_deflevintegeryes"
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
''
NameTypeChangeable
"net.inet.ipsec.def_policyintegeryes"
"net.inet6.ipsec6.def_policyintegeryes"
Miscellaneous sysctl variables
When the
protocols are configured for use, all protocols are included in the system.
To selectively enable/disable protocols, use
sysctl(8).
NameDefault
"net.inet.esp.esp_enableOn"
"net.inet.ah.ah_enableOn"
"net.inet.ipcomp.ipcomp_enableOff"
In addition the following variables are accessible via sysctl(8),
for tweaking the kernel's IPsec behavior:
NameTypeChangeable
"net.inet.ipsec.ah_cleartosintegeryes"
"net.inet.ipsec.ah_offsetmaskintegeryes"
"net.inet.ipsec.dfbitintegeryes"
"net.inet.ipsec.ecnintegeryes"
"net.inet.ipsec.debugintegeryes"
"net.inet6.ipsec6.ecnintegeryes"
"net.inet6.ipsec6.debugintegeryes"
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.txtgif(4)
talks more about the behavior.
ipsec.debug
If set to non-zero, debug messages will be generated via
syslog(3).
Variables under the
net.inet6.ipsec6
tree have similar meanings to those described above.
PROTOCOLS
The
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
because
can prevent
icmp(4)
or
icmp6(4)
routines from looking into the IP payload.
Daniel L. McDonald
Craig Metz
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
implementation appeared in the WIDE/KAME IPv6/IPsec stack.
For
Fx 5.0
a fully locked IPsec implementation called fast_ipsec was brought in.
The protocols drew heavily on the
Ox 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
Fx 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
implementation in
Fx .
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.
