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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 in Internet Protocol layer.  ipsec is	de-
     fined for both IPv4 and IPv6 (inet(4) and inet6(4)).  ipsec consists of
     two sub-protocols,	namely ESP (encapsulated security payload) and AH (au-
     thentication header).  ESP	protects IP payload from wire-tapping by en-
     crypting it by secret key cryptography algorithms.	 AH guarantees integ-
     rity of IP	packet and protects it from intermediate alteration or imper-
     sonation, by attaching cryptographic checksum computed by one-way hash
     functions.	 ipsec has two operation modes:	transport mode and tunnel
     mode.  Transport mode is for protecting peer-to-peer communication	be-
     tween end nodes.  Tunnel mode includes IP-in-IP encapsulation operation
     and is designed for security gateways, like VPN configurations.

   Kernel interface
     ipsec is controlled by key	management engine and policy engine, in	the
     operating system kernel.

     Key management engine can be accessed from	the userland by	using PF_KEY
     sockets.  The PF_KEY socket API is	defined	in RFC2367.

     Policy engine can be controlled by	extended part of PF_KEY	API,
     setsockopt(2) operations, and sysctl(3) interface.	 The kernel implements
     extended version of PF_KEY	interface, and allows you to define IPsec pol-
     icy like per-packet filters.  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	dynamic	encryption key exchange	proto-
     col like IKE (Internet Key	Exchange).  That should	be implemented as
     userland programs (usually	as daemons), by	using the above	described

   Policy management
     The kernel	implements experimental	policy management code.	 You can man-
     age 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	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 in	per-socket basis.  You
     can enforce particular IPsec policy onto packets that go through particu-
     lar socket.

     With setkey(8) you	can define IPsec policy	against	packets, using sort of
     packet filtering rule.  Refer to setkey(8)	on how to use it.

     In	the latter case, "default" policy is allowed for use with setkey(8).
     By	configuring policy to default, you can refer system-wide sysctl(8)
     variable 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	kernel finds no	matching policy	system wide default value is applied.
     System wide default is specified by the following sysctl(8) variables.  0
     means "discard" which asks	the kernel to drop the packet.	1 means

     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.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 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.

	     During AH authentication data computation,	the kernel will	in-
	     clude 16bit fragment offset field (including flag bits) in	IPv4
	     header, after computing logical AND with the variable.  The vari-
	     able 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 confor-

	     The variable configures the kernel	behavior on IPv4 IPsec tunnel
	     encapsulation.  If	set to 0, DF bit on the	outer IPv4 header will
	     be	cleared.  1 means that the outer DF bit	is set regardless from
	     the inner DF bit.	2 means	that the DF bit	is copied from the in-
	     ner header	to the outer.  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 net.inet6.ipsec6 tree has similar meaning as the
     net.inet.ipsec counterpart.

     The ipsec protocol	works like 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.

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

     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 policy engine API,	so the policy engine
     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 against 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 we	enforce	policy check on	inner packet on	reception, and AH au-
     thenticates 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 condition, truncated	result may be raised from the kernel
     against SADB_DUMP and SADB_SPDDUMP	operation on PF_KEY socket.  This oc-
     curs if there are too many	database entries in the	kernel and socket buf-
     fer for the PF_KEY	socket is insufficient.	 If you	manipulate many	IPsec
     key/policy	database entries, increase the size of socket buffer.

BSD			       January 29, 1999				   BSD


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