IPSEC(4)

NAME
ipsec - IP security protocol

SYNOPSIS
options IPSEC
options IPSEC_DEBUG

DESCRIPTION
This manual page 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 three 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.

IP Payload Compression Protocol (IPComp)
    increases the communication performance by compressing the datagrams.

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.

  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.

  Cryptographic operations
The current IPsec implementation, formerly called Fast IPsec, uses the opencrypto(9) subsystem to carry out cryptographic operations. This means, in particular, that cryptographic hardware devices are employed whenever possible to optimize the performance of sub-protocols.

System configuration requires the opencrypto(9) subsystem. When the Fast IPsec protocols are configured for use, all protocols are included in the system. To selectively enable/disable protocols, use sysctl(8).

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), icmp6(4), intro(4), ip6(4), ipsecif(4), racoon(8), setkey(8), sysctl(8)

STANDARDS
Daniel L. McDonald, Craig Metz, Bao G. Phan, PF_KEY Key Management API, Version 2, RFC, 2367.

HISTORY
The protocols draw heavily on the OpenBSD implementation of the IPsec protocols. The policy management code is derived from the KAME implementation found in their IPsec protocols. The Fast IPsec protocols are based on code which appeared in FreeBSD 4.7. The NetBSD version is a close copy of the FreeBSD original, and first appeared in NetBSD 2.0.

Support for IPv6 and IPcomp protocols has been added in NetBSD 4.0.

Support for Network Address Translator Traversal as described in RFCs 3947 and 3948 has been added in NetBSD 5.0.

Since NetBSD 6.0, the IPsec implementation formerly known as Fast IPsec is used.

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 conditions, truncated results may be raised from the kernel against SADB_DUMP and SADB_SPDDUMP operations on PF_KEY sockets. 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.

Certain legacy authentication algorithms are not supported because of issues with the opencrypto(9) subsystem.