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.

Debian                                          February 6, 2017                                        IPSEC(4)