Ubuntu Manpage: ipsec — Internet Protocol Security protocol

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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.

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.