NAME¶
ipsec
—
Internet Protocol Security protocol
SYNOPSIS¶
options IPSEC
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.
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:
Sy 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:
- Li 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).
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),
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.