ipsec - Internet Protocol Security Architecture


The IP Security Architecture (IPsec) provides protection for IP
datagrams. The protection can include confidentiality, strong integrity
of the data, partial sequence integrity (replay protection), and data
authentication. IPsec is performed inside the IP processing, and it can
be applied with or without the knowledge of an Internet application.

IPsec applies to both IPv4 and IPv6. See ip(4P) and ip6(4P).

Protection Mechanisms

IPsec provides two mechanisms for protecting data. The Authentication
Header (AH) provides strong integrity, replay protection, and data
authentication. AH protects as much of the IP datagram as it can. AH
cannot protect fields that change nondeterministically between sender and

The Encapsulating Security Payload (ESP) provides confidentiality over
what it encapsulates, as well as the services that AH provides, but only
over that which it encapsulates. ESP's authentication services are
optional, which allow ESP and AH to be used together on the same datagram
without redundancy.

Authentication and encryption algorithms are used for IPsec.
Authentication algorithms produce an integrity checksum value or
"digest"based on the data and a key. Encryption algorithms operate on
data in units of a "block size".

NAT Traversal

IPsec's ESP can also encapsulate itself in UDP if IKE (see in.iked(8))
discovers a Network Address Translator (NAT) between two communicating

A UDP socket can be specified as a NAT-Traversal endpoint. See udp(4P)
for details.

Security Associations

AH and ESP use Security Associations (SA). SA's are entities that specify
security properties from one host to another. Two communicating machines
require two SAs (at a minimum) to communicate securely. However,
communicating machines that use multicast can share the same multicast
SA. SAs are managed through the pf_key(4P) interface. For IPv4, automatic
SA management is available through the Internet Key Exchange (IKE), as
implemented by in.iked(8). A command-line front-end is available by means
of ipseckey(8). An IPsec SA is identified by a tuple of <AH or ESP,
destination IP address, and SPI>. The Security Parameters Index (SPI) is
an arbitrary 32-bit value that is transmitted on the wire with an AH or
ESP packet. See ipsecah(4P) or ipsecesp(4P) for an explanation about
where the SPI falls in a protected packet.

Protection Policy and Enforcement Mechanisms

Mechanism and policy are separate. The policy for applying IPsec is
enforced on a system-wide or per-socket level. Configuring system-wide
policy and per-tunnel policy (see Transport Mode and Tunnel Mode
sections) is done via the ipsecconf(8) command. Configuring per-socket
policy is discussed later in this section.

System-wide IPsec policy is applied to incoming and outgoing datagrams.
Some additional rules can be applied to outgoing datagrams because of the
additional data known by the system. Inbound datagrams can be accepted or
dropped. The decision to drop or accept an inbound datagram is based on
several criteria which sometimes overlap or conflict. Conflict resolution
is resolved by which rule is parsed first, with one exception: if a
policy entry states that traffic should bypass all other policy, it is
automatically be accepted. Outbound datagrams are sent with or without
protection. Protection may (or may not) indicate specific algorithms. If
policy normally would protect a datagram, it can be bypassed either by an
exception in system-wide policy or by requesting a bypass in per-socket

Intra-machine traffic policies are enforced, but actual security
mechanisms are not applied. Instead, the outbound policy on an intra-
machine packet translates into an inbound packet with those mechanisms

IPsec policy is enforced in the ip(4P) driver. Several ndd tunables for
/dev/ip affect policy enforcement, including:

If equal to 1 (the default), allow certain
cleartext icmp messages to bypass policy.
For ICMP echo requests ("ping"messages),
protect the response like the request. If
zero, treat icmp messages like other IP

If 1, allow inbound cleartext IGMP messages
to bypass IPsec policy.

If 1, allow inbound cleartext PIM messages
to bypass IPsec policy.

IPsec logs policy failures and errors to
/var/adm/messages. To prevent syslog from
being overloaded, the IPsec kernel modules
limit the rate at which errors can be
logged. You can query/set
ipsec_policy_log_interval using ndd(8). The
value is in milliseconds. Only one message
can be logged per interval.

Transport Mode and Tunnel Mode

If IPsec is used on a tunnel, Tunnel Mode IPsec can be used to protect
distinct flows within a tunnel or to cause packets that do not match per-
tunnel policy to drop. System-wide policy is always Transport Mode. A
tunnel can use Transport Mode IPsec or Tunnel Mode IPsec.

Per-Socket Policy
The IP_SEC_OPT or IPV6_SEC_OPT socket option is used to set per-socket
IPsec policy. The structure used for an IP_SEC_OPT request is:

typedef struct ipsec_req {
uint_t ipsr_ah_req; /* AH request */
uint_t ipsr_esp_req; /* ESP request */
uint_t ipsr_self_encap_req; /* Self-Encap request */
uint8_t ipsr_auth_alg; /* Auth algs for AH */
uint8_t ipsr_esp_alg; /* Encr algs for ESP */
uint8_t ipsr_esp_auth_alg; /* Auth algs for ESP */
} ipsec_req_t;

The IPsec request has fields for both AH and ESP. Algorithms may or may
not be specified. The actual request for AH or ESP services can take one
of the following values:

Bypass all policy. Only the superuser may request
this service.

Regardless of other policy, require the use of the
IPsec service.

The following value can be logically ORed to an IPSEC_PREF_REQUIRED

Regardless of other policy, enforce a unique SA for
traffic originating from this socket.

In the event IP options not normally encapsulated by ESP need to be, the
ipsec_self_encap_req is used to add an additional IP header outside the
original one. Algorithm values from <net/pfkeyv2.h> are as follows:

Uses the MD5-HMAC (RFC 2403) algorithm for

Uses the SHA1-HMAC (RFC 2404) algorithm for

Uses the DES (RFC 2405) algorithm for encryption.

Uses the Triple DES (RFC 2451) algorithm for

Uses the Blowfish (RFC 2451) algorithm for

Uses the Advanced Encryption Standard
algorithm for encryption.

Uses the SHA2 hash algorithms with HMAC (RFC
4868)for authentication.

An application should use either the getsockopt(3SOCKET) or the
setsockopt(3SOCKET) call to manipulate IPsec requests. For example:

#include <sys/socket.h>
#include <netinet/in.h>
#include <net/pfkeyv2.h> /* For SADB_*ALG_* */
/* .... socket setup skipped */
rc = setsockopt(s, IPPROTO_IP, IP_SEC_OPT,
(const char *)&ipsec_req, sizeof (ipsec_req_t));


While IPsec is an effective tool in securing network traffic, it will not
make security problems disappear. Security issues beyond the mechanisms
that IPsec offers may be discussed in similar "Security" or "Security
Consideration" sections within individual reference manual pages.

While a non-root user cannot bypass IPsec, a non-root user can set policy
to be different from the system-wide policy. For ways to prevent this,
consult the ndd(8) variables in /dev/ip.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Committed |


getsockopt(3SOCKET), setsockopt(3SOCKET), inet(4P), ip(4P), ip6(4P),
ipsecah(4P), ipsecesp(4P), pf_key(4P), udp(4P), attributes(7),
in.iked(8), ipsecconf(8), ipseckey(8), ndd(8)

Kent, S., and Atkinson, R., RFC 2401, Security Architecture for the
Internet Protocol, The Internet Society, 1998.

Kent, S. and Atkinson, R., RFC 2406, IP Encapsulating Security Payload
(ESP), The Internet Society, 1998.

Madson, C., and Doraswamy, N., RFC 2405, The ESP DES-CBC Cipher Algorithm
with Explicit IV, The Internet Society, 1998.

Madsen, C. and Glenn, R., RFC 2403, The Use of HMAC-MD5-96 within ESP and
AH, The Internet Society, 1998.

Madsen, C. and Glenn, R., RFC 2404, The Use of HMAC-SHA-1-96 within ESP
and AH, The Internet Society, 1998.

Pereira, R. and Adams, R., RFC 2451, The ESP CBC-Mode Cipher Algorithms,
The Internet Society, 1998.

Kelly, S. and Frankel, S., RFC 4868, Using HMAC-SHA-256, HMAC-SHA-384,
and HMAC-SHA-512 with IPsec, 2007.

Huttunen, A., Swander, B., Volpe, V., DiBurro, L., Stenberg, M., RFC
3948, UDP Encapsulation of IPsec ESP Packets, The Internet Society, 2005.

September 25, 2009 IPSEC(4P)