NAME
ipsec —
IP security protocol
DESCRIPTION
This manual pages describes the IPsec protocol. For the network device driver
please see
ipsecif(4).
ipsec is a security protocol in the Internet Protocol (IP)
layer.
ipsec is defined for both IPv4 and IPv6
(
inet(4) and
inet6(4)).
ipsec consists of two sub-protocols:
- Encapsulated Security Payload (ESP)
- protects IP payloads from wire-tapping (interception) by
encrypting them with secret key cryptography algorithms.
- Authentication Header (AH)
- guarantees the integrity of IP packets and protects them
from intermediate alteration or impersonation, by attaching cryptographic
checksums computed by one-way hash functions.
ipsec has two operation modes:
- 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, as in Virtual Private Network (VPN)
configurations.
Since version 6,
NetBSD uses the IPsec implementation
formerly known as FAST_IPSEC. Its specifics and kernel options are described
in the
fast_ipsec(4) manual
page.
Kernel interface
ipsec is controlled by two engines in the kernel: one for key
management and one for policy.
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 can be controlled through the
PF_KEY
API,
setsockopt(2)
operations, and the
sysctl(3)
interface. The kernel implements an extended version of the
PF_KEY
interface and allows you to define IPsec policy
like per-packet filters.
setsockopt(2) is used to
define per-socket behavior, and
sysctl(3) is used to define
host-wide default behavior.
The kernel does not implement dynamic encryption key exchange protocols like IKE
(Internet Key Exchange). That should be done in userland (usually as a
daemon), using the APIs described above.
Policy management
The kernel implements experimental policy management code. You can manage 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 the
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 on a per-socket basis. You can enforce particular IPsec
policy on packets that go through a particular socket.
With
setkey(8) you can define
IPsec policy for packets using a form of packet filtering rules. See
setkey(8) for details.
In the latter case, “
default
” policy is
allowed for use with
setkey(8).
By configuring policy to
default
, you can refer to
system-wide
sysctl(8) variables
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 the kernel finds no matching policy, the system-wide default value is
applied. System-wide defaults are 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 kernel
IPsec behavior:
Name |
Type |
Changeable |
net.inet.ipsec.ah_cleartos |
integer |
yes |
net.inet.ipsec.ah_offsetmask |
integer |
yes |
net.inet.ipsec.crypto_support |
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:
-
-
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. 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.
-
-
ipsec.ah_offsetmask
- During AH authentication data computation, the kernel will
include a 16 bit fragment offset field (including flag bits) in the IPv4
header, after computing logical AND with the variable. The variable 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 conformance.
-
-
ipsec.crypto_support
- This variable configures the kernel behavior for selecting
encryption drivers. If set to > 0, the kernel will select a hardware
encryption driver first. If set to < 0, the kernel will select a
software encryption driver first. If set to 0, the kernel will select
either a hardware or software driver.
-
-
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. 1 means that the outer DF bit is set 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.
-
-
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 their
net.inet.ipsec
counterparts.
PROTOCOLS
The
ipsec protocol works like a 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 ALSO
ioctl(2),
socket(2),
ipsec_set_policy(3),
fast_ipsec(4),
icmp6(4),
intro(4),
ip6(4),
ipsecif(4),
racoon(8),
setkey(8),
sysctl(8)
STANDARDS
Daniel L. McDonald,
Craig Metz, and Bao G. Phan,
PF_KEY Key Management API, Version 2,
RFC, 2367.
BUGS
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 the version introduced by KAME.
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
”), tunneled 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 or use
sysctl(8) interface.