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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 defined for both IPv4 and IPv6
     (inet(4) and inet6(4)).  ipsec consists of two sub-protocols, namely ESP (encapsulated security pay-load) payload)
     load) 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 interme-diate intermediate
     diate alteration or impersonation, by attaching cryptographic checksum computed by one-way hash func-tions. functions.
     tions.  ipsec has two operation modes: transport mode and tunnel mode.  Transport mode is for protect-ing protecting
     ing peer-to-peer communication between end nodes.  Tunnel mode includes IP-in-IP encapsulation opera-tion operation
     tion 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
     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 (Internet 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 particular 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 speci-fied specified
     fied 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:

             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 variable
             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-mance. conformance.

             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 regard-less regardless
             less 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 notification), 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 syslog(3).

     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, draft-mcdonaldsimple-ipsec-api-03.txt,
     simple-ipsec-api-03.txt, work in progress material.

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

     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 authentic-ity). authenticity).
     ity).  The issue will be solved when we revamp our policy engine to keep all the packet decapsulation

     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/pol-icy key/policy
     icy database entries, increase the size of socket buffer.

BSD                            January 29, 1999                            BSD

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