Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
IPSEC(4)               FreeBSD 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 implemented within the Internet Protocol
     layer of the TCP/IP stack.  ipsec is defined for both IPv4 and IPv6
     (inet(4) and inet6(4)).  ipsec contains two protocols, ESP, the encapsu-
     lated security payload protocol and AH, the authentication header proto-
     col.  ESP prevents unauthorized parties from reading the payload of an IP
     packet by encrypting it using secret key cryptography algorithms.  AH
     both authenticates guarantees the integrity of an IP packet by attaching
     a cryptographic checksum computed using one-way hash functions.  ipsec
     has operates in one of two modes: transport mode or tunnel mode.  Trans-
     port mode is used to protect peer-to-peer communication between end
     nodes.  Tunnel mode encapsulates IP packets within other IP packets and
     is designed for security gateways such as VPN endpoints.

   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) command.  In either case, IPsec policies must be speci-
     fied using the syntax described in ipsec_set_policy(3).  Please refer to
     the setkey(8) man page for instructions on its use.

     When setting policies using the setkey(8) command the ``default'' option
     you can have the system use its default policy, explained below, for pro-
     cessing packets.  The following sysctl variables are available for con-
     figuring 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 asso-
     ciation 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
     requires 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
     default value is applied.  The system wide default policy 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 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.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
             include 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 imple-
             ment 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
             regardless 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

     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), 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 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 the 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 with an 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 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
     alleviate this problem.

FreeBSD 6.2                    February 14, 2006                   FreeBSD 6.2


Want to link to this manual page? Use this URL:

home | help