|IPSEC(4)||Device Drivers Manual||IPSEC(4)|
ipsec — Internet
Protocol Security protocol
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
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
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
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
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 “
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
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
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).
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.
discard” which asks the kernel to
drop the packet.
Miscellaneous sysctl variables¶
ipsec protocols are configured
for use, all protocols are included in the system. To selectively
enable/disable protocols, use sysctl(8).
In addition the following variables are accessible via sysctl(8), for tweaking the kernel's IPsec behavior:
The variables are interpreted as follows:
- 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.
- 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.
- 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.
- If set to non-zero, IPv4 IPsec tunnel encapsulation/decapsulation behavior
will be friendly to ECN (explicit congestion notification), as documented
draft-ietf-ipsec-ecn-02.txt. gif(4) talks more about the behavior.
- If set to non-zero, debug messages will be generated via syslog(3).
- 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.
- 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
have similar meanings to those described above.
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 can prevent icmp(4) or
icmp6(4) routines from looking into the IP payload.
S. Kent and R. Atkinson, IP Authentication Header, RFC 2404.
S. Kent and R. Atkinson, IP Encapsulating Security Payload (ESP), RFC 2406.
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.
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
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
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_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.
|February 6, 2017||Debian|