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IPSEC(4)		 BSD Kernel Interfaces Manual		      IPSEC(4)

     ipsec -- IP security protocol

     #include <sys/types.h>
     #include <netinet/in.h>
     #include <netinet6/ipsec.h>

     ipsec is a	security protocol implemented within the Internet Protocol
     layer of the TCP/IP stack.	 ipsec is defined for both IPv4	and IPv6
     (inet(4) and inet6(4)).  ipsec contains two protocols, ESP, the encapsu-
     lated security payload protocol and AH, the authentication	header proto-
     col.  ESP prevents	unauthorized parties from reading the payload of an IP
     packet by encrypting it using secret key cryptography algorithms.	AH
     both authenticates	guarantees the integrity of an IP packet by attaching
     a cryptographic checksum computed using one-way hash functions.  ipsec
     has operates in one of two	modes: transport mode or tunnel	mode.  Trans-
     port 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.

   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 associat-
     ing 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 pro-
     tocol such	as IKE (Internet Key Exchange).	 Key exchange protocols	are
     beyond what is necessary in the kernel and	should be implemented as dae-
     mon 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 configur-
     ing kernel	level packet filter-based policies using the PF_KEY interface,
     via the setkey(8) command.	 In either case, IPsec policies	must be	speci-
     fied using	the syntax described in	ipsec_set_policy(3).  Please refer to
     the setkey(8) man page for	instructions on	its use.

     When setting policies using the setkey(8) command the "default" option
     you can have the system use its default policy, explained below, for pro-
     cessing packets.  The following sysctl variables are available for	con-
     figuring 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 associ-
     ation 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 de-
     fault value is applied.  The system wide default policy is	specified by
     the following sysctl(8) variables.	 0 means "discard" which asks the ker-
     nel 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	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.inet6.ipsec6.ecn		  integer	yes
     net.inet6.ipsec6.debug		  integer	yes

     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	in-
	     clude 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	re-
	     gardless 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 notifica-
	     tion), as documented in draft-ietf-ipsec-ecn-02.txt.  gif(4)
	     talks more	about the behavior.

	     If	set to non-zero, debug messages	will be	generated via

     Variables under the net.inet6.ipsec6 tree have similar meanings to	those
     described above.

     The ipsec protocol	acts as	a plug-in to the inet(4) and inet6(4) proto-
     cols and therefore	supports most of the protocols defined upon those IP-
     layer protocols.  The icmp(4) and icmp6(4)	protocols may behave differ-
     ently with	ipsec because ipsec can	prevent	icmp(4)	or icmp6(4) routines
     from looking into the IP payload.

     ioctl(2), socket(2), ipsec_set_policy(3), icmp6(4), intro(4), ip6(4),
     setkey(8),	sysctl(8)

     S.	Kent and R. Atkinson, IP Authentication	Header,	RFC 2404.

     S.	Kent and R. Atkinson, IP Encapsulating Security	Payload	(ESP), RFC

     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

     The implementation	described herein appeared in WIDE/KAME IPv6/IPsec

     The IPsec support is subject to change as the IPsec protocols develop.

     There is no single	standard for the policy	engine API, so the policy en-
     gine API described	herein is just for KAME	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 en-
     capsulating 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 authen-
     ticity).  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 al-
     leviate this problem.

BSD			       February	14, 2006			   BSD


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