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

     ipsec -- Internet Protocol	Security protocol

     options IPSEC
     options IPSEC_SUPPORT
     device crypto

     #include <sys/types.h>
     #include <netinet/in.h>
     #include <netipsec/ipsec.h>
     #include <netipsec/ipsec6.h>

     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 Encapsu-
     lating 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 in-
     crease 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 com-
     munication	between	end nodes.  Tunnel mode	encapsulates IP	packets	within
     other IP packets and is designed for security gateways such as VPN	end-

     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 in-

     To	properly filter	on the inner packets of	an ipsec tunnel	with fire-
     walls, 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 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) 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 as-
     sociation then use	it but if there	is not then the	packets	are not	pro-
     cessed by IPsec.  The value 2 is synonymous with "require", which re-
     quires 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
     When the ipsec protocols are configured for use, all protocols are	in-
     cluded in the system.  To selectively enable/disable protocols, use

     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:

	     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

	     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.

	     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.

     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), crypto(4), enc(4), if_ipsec(4),
     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 original ipsec	implementation appeared	in the WIDE/KAME IPv6/IPsec

     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 im-
     plementation lacked ip6(4)	support	but made use of	the crypto(4) subsys-

     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.

     There is no single	standard for the policy	engine API, so the policy en-
     gine 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 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.

     The IPcomp	protocol may occasionally error	because	of zlib(3) problems.

     This documentation	needs more review.

BSD			       February	6, 2017				   BSD


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