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

     ipsec -- IP security protocol

     #include <&lt;sys/types.h>&gt;
     #include <&lt;netinet/in.h>&gt;
     #include <&lt;netinet6/ipsec.h>&gt;

     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 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
     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 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
             include 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
             inner 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
     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 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
     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