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OpenDarwin 7.2.1 - man page for ipsec (opendarwin section 4)

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

NAME
     ipsec -- IP security protocol

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

DESCRIPTION
     ipsec is a security protocol in Internet Protocol layer.  ipsec is defined for both IPv4 and
     IPv6 (inet(4) and inet6(4)).  ipsec consists of two sub-protocols, namely ESP (encapsulated
     security payload) and AH (authentication header).	ESP protects IP payload from wire-tapping
     by encrypting it by secret key cryptography algorithms.  AH guarantees integrity of IP
     packet and protects it from intermediate alteration or impersonation, by attaching crypto-
     graphic checksum computed by one-way hash functions.  ipsec has two operation modes: trans-
     port mode and tunnel mode.  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, like VPN configurations.

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

     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 policy 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 protocol like IKE (Inter-
     net Key Exchange).  That should be implemented as userland programs (usually as daemons), by
     using the above described APIs.

   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 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 par-
     ticular IPsec policy onto packets that go through particular 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 ``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.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 type-of-service field in the IPv4 header dur-
	     ing 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 16bit fragment
	     offset field (including flag bits) in 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.dfbit
	     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 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 net.inet6.ipsec6 tree has similar meaning as the net.inet.ipsec counterpart.

PROTOCOLS
     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 proto-
     cols, 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), setkey(8), sysctl(8)

STANDARDS
     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 material.

HISTORY
     The implementation described herein appeared in WIDE/KAME IPv6/IPsec stack.

BUGS
     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 encapsulating 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 authenticates encap-
     sulating (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 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.

BSD					 January 29, 1999				      BSD


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