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[FreeBSD/releng/10.0.git] / share / doc / IPv6 / IMPLEMENTATION

        Implementation Note

        KAME Project
        http://www.kame.net/
        $KAME: IMPLEMENTATION,v 1.216 2001/05/25 07:43:01 jinmei Exp $
        $FreeBSD$

NOTE: The document tries to describe behaviors/implementation choices
of the latest KAME/*BSD stack.  The description here may not be
applicable to KAME-integrated *BSD releases, as we have certain amount
of changes between them.  Still, some of the content can be useful for
KAME-integrated *BSD releases.

Table of Contents

        1. IPv6
        1.1 Conformance
        1.2 Neighbor Discovery
        1.3 Scope Zone Index
        1.3.1 Kernel internal
        1.3.2 Interaction with API
        1.3.3 Interaction with users (command line)
        1.4 Plug and Play
        1.4.1 Assignment of link-local, and special addresses
        1.4.2 Stateless address autoconfiguration on hosts
        1.4.3 DHCPv6
        1.5 Generic tunnel interface
        1.6 Address Selection
        1.6.1 Source Address Selection
        1.6.2 Destination Address Ordering
        1.7 Jumbo Payload
        1.8 Loop prevention in header processing
        1.9 ICMPv6
        1.10 Applications
        1.11 Kernel Internals
        1.12 IPv4 mapped address and IPv6 wildcard socket
        1.12.1 KAME/BSDI3 and KAME/FreeBSD228
        1.12.2 KAME/FreeBSD[34]x
        1.12.2.1 KAME/FreeBSD[34]x, listening side
        1.12.2.2 KAME/FreeBSD[34]x, initiating side
        1.12.3 KAME/NetBSD
        1.12.3.1 KAME/NetBSD, listening side
        1.12.3.2 KAME/NetBSD, initiating side
        1.12.4 KAME/BSDI4
        1.12.4.1 KAME/BSDI4, listening side
        1.12.4.2 KAME/BSDI4, initiating side
        1.12.5 KAME/OpenBSD
        1.12.5.1 KAME/OpenBSD, listening side
        1.12.5.2 KAME/OpenBSD, initiating side
        1.12.6 More issues
        1.12.7 Interaction with SIIT translator
        1.13 sockaddr_storage
        1.14 Invalid addresses on the wire
        1.15 Node's required addresses
        1.15.1 Host case
        1.15.2 Router case
        1.16 Advanced API
        1.17 DNS resolver
        2. Network Drivers
        2.1 FreeBSD 2.2.x-RELEASE
        2.2 BSD/OS 3.x
        2.3 NetBSD
        2.4 FreeBSD 3.x-RELEASE
        2.5 FreeBSD 4.x-RELEASE
        2.6 OpenBSD 2.x
        2.7 BSD/OS 4.x
        3. Translator
        3.1 FAITH TCP relay translator
        3.2 IPv6-to-IPv4 header translator
        4. IPsec
        4.1 Policy Management
        4.2 Key Management
        4.3 AH and ESP handling
        4.4 IPComp handling
        4.5 Conformance to RFCs and IDs
        4.6 ECN consideration on IPsec tunnels
        4.7 Interoperability
        4.8 Operations with IPsec tunnel mode
        4.8.1 RFC2401 IPsec tunnel mode approach
        4.8.2 draft-touch-ipsec-vpn approach
        5. ALTQ
        6. Mobile IPv6
        6.1 KAME node as correspondent node
        6.2 KAME node as home agent/mobile node
        6.3 Old Mobile IPv6 code
        7. Coding style
        8. Policy on technology with intellectual property right restriction

1. IPv6

1.1 Conformance

The KAME kit conforms, or tries to conform, to the latest set of IPv6
specifications.  For future reference we list some of the relevant documents
below (NOTE: this is not a complete list - this is too hard to maintain...).
For details please refer to specific chapter in the document, RFCs, manpages
come with KAME, or comments in the source code.

Conformance tests have been performed on past and latest KAME STABLE kit,
at TAHI project.  Results can be viewed at http://www.tahi.org/report/KAME/.
We also attended Univ. of New Hampshire IOL tests (http://www.iol.unh.edu/)
in the past, with our past snapshots.

RFC1639: FTP Operation Over Big Address Records (FOOBAR)
    * RFC2428 is preferred over RFC1639.  ftp clients will first try RFC2428,
      then RFC1639 if failed.
RFC1886: DNS Extensions to support IPv6
RFC1933: (see RFC2893)
RFC1981: Path MTU Discovery for IPv6
RFC2080: RIPng for IPv6
    * KAME-supplied route6d, bgpd and hroute6d support this.
RFC2283: Multiprotocol Extensions for BGP-4
    * so-called "BGP4+".
    * KAME-supplied bgpd supports this.
RFC2292: Advanced Sockets API for IPv6
    * see RFC3542
RFC2362: Protocol Independent Multicast-Sparse Mode (PIM-SM)
    * RFC2362 defines the packet formats and the protcol of PIM-SM.
RFC2373: IPv6 Addressing Architecture
    * KAME supports node required addresses, and conforms to the scope
      requirement.
RFC2374: An IPv6 Aggregatable Global Unicast Address Format
    * KAME supports 64-bit length of Interface ID.
RFC2375: IPv6 Multicast Address Assignments
    * Userland applications use the well-known addresses assigned in the RFC.
RFC2428: FTP Extensions for IPv6 and NATs
    * RFC2428 is preferred over RFC1639.  ftp clients will first try RFC2428,
      then RFC1639 if failed.
RFC2460: IPv6 specification
RFC2461: Neighbor discovery for IPv6
    * See 1.2 in this document for details.
RFC2462: IPv6 Stateless Address Autoconfiguration
    * See 1.4 in this document for details.
RFC2463: ICMPv6 for IPv6 specification
    * See 1.9 in this document for details.
RFC2464: Transmission of IPv6 Packets over Ethernet Networks
RFC2465: MIB for IPv6: Textual Conventions and General Group
    * Necessary statistics are gathered by the kernel.  Actual IPv6 MIB
      support is provided as patchkit for ucd-snmp.
RFC2466: MIB for IPv6: ICMPv6 group
    * Necessary statistics are gathered by the kernel.  Actual IPv6 MIB
      support is provided as patchkit for ucd-snmp.
RFC2467: Transmission of IPv6 Packets over FDDI Networks
RFC2472: IPv6 over PPP
RFC2492: IPv6 over ATM Networks
    * only PVC is supported.
RFC2497: Transmission of IPv6 packet over ARCnet Networks
RFC2545: Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
RFC2553: (see RFC3493)
RFC2671: Extension Mechanisms for DNS (EDNS0)
    * see USAGE for how to use it.
    * not supported on kame/freebsd4 and kame/bsdi4.
RFC2673: Binary Labels in the Domain Name System
    * KAME/bsdi4 supports A6, DNAME and binary label to some extent.
    * KAME apps/bind8 repository has resolver library with partial A6, DNAME
      and binary label support.
RFC2675: IPv6 Jumbograms
    * See 1.7 in this document for details.
RFC2710: Multicast Listener Discovery for IPv6
RFC2711: IPv6 router alert option
RFC2732: Format for Literal IPv6 Addresses in URL's
    * The spec is implemented in programs that handle URLs
      (like freebsd ftpio(3) and fetch(1), or netbsd ftp(1))
RFC2874: DNS Extensions to Support IPv6 Address Aggregation and Renumbering
    * KAME/bsdi4 supports A6, DNAME and binary label to some extent.
    * KAME apps/bind8 repository has resolver library with partial A6, DNAME
      and binary label support.
RFC2893: Transition Mechanisms for IPv6 Hosts and Routers
    * IPv4 compatible address is not supported.
    * automatic tunneling (4.3) is not supported.
    * "gif" interface implements IPv[46]-over-IPv[46] tunnel in a generic way,
      and it covers "configured tunnel" described in the spec.
      See 1.5 in this document for details.
RFC2894: Router renumbering for IPv6
RFC3041: Privacy Extensions for Stateless Address Autoconfiguration in IPv6
RFC3056: Connection of IPv6 Domains via IPv4 Clouds
    * So-called "6to4".
    * "stf" interface implements it.  Be sure to read
      draft-itojun-ipv6-transition-abuse-01.txt
      below before configuring it, there can be security issues.
RFC3142: An IPv6-to-IPv4 transport relay translator
    * FAITH tcp relay translator (faithd) implements this.  See 3.1 for more
      details.
RFC3152: Delegation of IP6.ARPA
    * libinet6 resolvers contained in the KAME snaps support to use
      the ip6.arpa domain (with the nibble format) for IPv6 reverse
      lookups.
RFC3484: Default Address Selection for IPv6
    * the selection algorithm for both source and destination addresses
      is implemented based on the RFC, though some rules are still omitted.
RFC3493: Basic Socket Interface Extensions for IPv6
    * IPv4 mapped address (3.7) and special behavior of IPv6 wildcard bind
      socket (3.8) are,
        - supported and turned on by default on KAME/FreeBSD[34]
          and KAME/BSDI4,
        - supported but turned off by default on KAME/NetBSD and KAME/FreeBSD5,
        - not supported on KAME/FreeBSD228, KAME/OpenBSD and KAME/BSDI3.
      see 1.12 in this document for details.
    * The AI_ALL and AI_V4MAPPED flags are not supported.
RFC3542: Advanced Sockets API for IPv6 (revised)
    * For supported library functions/kernel APIs, see sys/netinet6/ADVAPI.
    * Some of the updates in the draft are not implemented yet.  See
      TODO.2292bis for more details.
RFC4007: IPv6 Scoped Address Architecture
    * some part of the documentation (especially about the routing
      model) is not supported yet.
    * zone indices that contain scope types have not been supported yet.

draft-ietf-ipngwg-icmp-name-lookups-09: IPv6 Name Lookups Through ICMP
draft-ietf-ipv6-router-selection-07.txt:
        Default Router Preferences and More-Specific Routes
    * router-side: both router preference and specific routes are supported.
    * host-side: only router preference is supported.
draft-ietf-pim-sm-v2-new-02.txt
        A revised version of RFC2362, which includes the IPv6 specific
        packet format and protocol descriptions.
draft-ietf-dnsext-mdns-00.txt: Multicast DNS
    * kame/mdnsd has test implementation, which will not be built in
      default compilation.  The draft will experience a major change in the
      near future, so don't rely upon it.
draft-ietf-ipngwg-icmp-v3-02.txt: ICMPv6 for IPv6 specification (revised)
    * See 1.9 in this document for details.
draft-itojun-ipv6-tcp-to-anycast-01.txt:
        Disconnecting TCP connection toward IPv6 anycast address
draft-ietf-ipv6-rfc2462bis-06.txt: IPv6 Stateless Address
        Autoconfiguration (revised)
draft-itojun-ipv6-transition-abuse-01.txt:
        Possible abuse against IPv6 transition technologies (expired)
    * KAME does not implement RFC1933/2893 automatic tunnel.
    * "stf" interface implements some address filters.  Refer to stf(4)
      for details.  Since there's no way to make 6to4 interface 100% secure,
      we do not include "stf" interface into GENERIC.v6 compilation.
    * kame/openbsd completely disables IPv4 mapped address support.
    * kame/netbsd makes IPv4 mapped address support off by default.
    * See section 1.12.6 and 1.14 for more details.
draft-itojun-ipv6-flowlabel-api-01.txt: Socket API for IPv6 flow label field
    * no consideration is made against the use of routing headers and such.

1.2 Neighbor Discovery

Our implementation of Neighbor Discovery is fairly stable.  Currently
Address Resolution, Duplicated Address Detection, and Neighbor
Unreachability Detection are supported.  In the near future we will be
adding an Unsolicited Neighbor Advertisement transmission command as
an administration tool.

Duplicated Address Detection (DAD) will be performed when an IPv6 address
is assigned to a network interface, or the network interface is enabled
(ifconfig up).  It is documented in RFC2462 5.4.
If DAD fails, the address will be marked "duplicated" and message will be
generated to syslog (and usually to console).  The "duplicated" mark
can be checked with ifconfig.  It is administrators' responsibility to check
for and recover from DAD failures.  We may try to improve failure recovery
in future KAME code.

A successor version of RFC2462 (called rfc2462bis) clarifies the
behavior when DAD fails (i.e., duplicate is detected): if the
duplicate address is a link-local address formed from an interface
identifier based on the hardware address which is supposed to be
uniquely assigned (e.g., EUI-64 for an Ethernet interface), IPv6
operation on the interface should be disabled.  The KAME
implementation supports this as follows: if this type of duplicate is
detected, the kernel marks "disabled" in the ND specific data
structure for the interface.  Every IPv6 I/O operation in the kernel
checks this mark, and the kernel will drop packets received on or
being sent to the "disabled" interface.  Whether the IPv6 operation is
disabled or not can be confirmed by the ndp(8) command.  See the man
page for more details.

DAD procedure may not be effective on certain network interfaces/drivers.
If a network driver needs long initialization time (with wireless network
interfaces this situation is popular), and the driver mistakingly raises
IFF_RUNNING before the driver becomes ready, DAD code will try to transmit
DAD probes to not-really-ready network driver and the packet will not go out
from the interface.  In such cases, network drivers should be corrected.

Some of network drivers loop multicast packets back to themselves,
even if instructed not to do so (especially in promiscuous mode).  In
such cases DAD may fail, because the DAD engine sees inbound NS packet
(actually from the node itself) and considers it as a sign of
duplicate.  In this case, drivers should be corrected to honor
IFF_SIMPLEX behavior.  For example, you may need to check source MAC
address on an inbound packet, and reject it if it is from the node
itself.

Neighbor Discovery specification (RFC2461) does not talk about neighbor
cache handling in the following cases:
(1) when there was no neighbor cache entry, node received unsolicited
    RS/NS/NA/redirect packet without link-layer address
(2) neighbor cache handling on medium without link-layer address
    (we need a neighbor cache entry for IsRouter bit)
For (1), we implemented workaround based on discussions on IETF ipngwg mailing
list.  For more details, see the comments in the source code and email
thread started from (IPng 7155), dated Feb 6 1999.

IPv6 on-link determination rule (RFC2461) is quite different from
assumptions in BSD IPv4 network code.  To implement the behavior in
RFC2461 section 6.3.6 (3), the kernel needs to know the default
outgoing interface.  To configure the default outgoing interface, use
commands like "ndp -I de0" as root.  Then the kernel will have a
"default" route to the interface with the cloning "C" bit being on.
This default route will cause to make a neighbor cache entry for every
destination that does not match an explicit route entry.

Note that we intentionally disable configuring the default interface
by default.  This is because we found it sometimes caused inconvenient
situation while it was rarely useful in practical usage.  For example,
consider a destination that has both IPv4 and IPv6 addresses but is
only reachable via IPv4.  Since our getaddrinfo(3) prefers IPv6 by
default, an (TCP) application using the library with PF_UNSPEC first
tries to connect to the IPv6 address.  If we turn on RFC 2461 6.3.6
(3), we have to wait for quite a long period before the first attempt
to make a connection fails.  If we turn it off, the first attempt will
immediately fail with EHOSTUNREACH, and then the application can try
the next, reachable address.

The notion of the default interface is also disabled when the node is
acting as a router.  The reason is that routers tend to control all
routes stored in the kernel and the default route automatically
installed would rather confuse the routers.  Note that the spec misuse
the word "host" and "node" in several places in Section 5.2 of RFC
2461.  We basically read the word "node" in this section as "host,"
and thus believe the implementation policy does not break the
specification.

To avoid possible DoS attacks and infinite loops, KAME stack will accept
only 10 options on ND packet.  Therefore, if you have 20 prefix options
attached to RA, only the first 10 prefixes will be recognized.
If this troubles you, please contact the KAME team and/or modify
nd6_maxndopt in sys/netinet6/nd6.c.  If there are high demands we may
provide a sysctl knob for the variable.

Proxy Neighbor Advertisement support is implemented in the kernel.
For instance, you can configure it by using the following command:
        # ndp -s fe80::1234%ne0 0:1:2:3:4:5 proxy
where ne0 is the interface which attaches to the same link as the
proxy target.
There are certain limitations, though:
- It does not send unsolicited multicast NA on configuration.  This is MAY
  behavior in RFC2461.
- It does not add random delay before transmission of solicited NA.  This is
  SHOULD behavior in RFC2461.
- We cannot configure proxy NDP for off-link address.  The target address for
  proxying must be link-local address, or must be in prefixes configured to
  node which does proxy NDP.
- RFC2461 is unclear about if it is legal for a host to perform proxy ND.
  We do not prohibit hosts from doing proxy ND, but there will be very limited
  use in it.

Starting mid March 2000, we support Neighbor Unreachability Detection
(NUD) on p2p interfaces, including tunnel interfaces (gif).  NUD is
turned on by default.  Before March 2000 the KAME stack did not
perform NUD on p2p interfaces.  If the change raises any
interoperability issues, you can turn off/on NUD by per-interface
basis.  Use "ndp -i interface -nud" to turn it off.  Consult ndp(8)
for details.

RFC2461 specifies upper-layer reachability confirmation hint.  Whenever
upper-layer reachability confirmation hint comes, ND process can use it
to optimize neighbor discovery process - ND process can omit real ND exchange
and keep the neighbor cache state in REACHABLE.
We currently have two sources for hints: (1) setsockopt(IPV6_REACHCONF)
defined by the RFC3542 API, and (2) hints from tcp(6)_input.

It is questionable if they are really trustworthy.  For example, a
rogue userland program can use IPV6_REACHCONF to confuse the ND
process.  Neighbor cache is a system-wide information pool, and it is
bad to allow a single process to affect others.  Also, tcp(6)_input
can be hosed by hijack attempts.  It is wrong to allow hijack attempts
to affect the ND process.

Starting June 2000, the ND code has a protection mechanism against
incorrect upper-layer reachability confirmation.  The ND code counts
subsequent upper-layer hints.  If the number of hints reaches the
maximum, the ND code will ignore further upper-layer hints and run
real ND process to confirm reachability to the peer.  sysctl
net.inet6.icmp6.nd6_maxnudhint defines the maximum # of subsequent
upper-layer hints to be accepted.
(from April 2000 to June 2000, we rejected setsockopt(IPV6_REACHCONF) from
non-root process - after a local discussion, it looks that hints are not
that trustworthy even if they are from privileged processes)

If inbound ND packets carry invalid values, the KAME kernel will
drop these packet and increment statistics variable.  See
"netstat -sn", icmp6 section.  For detailed debugging session, you can
turn on syslog output from the kernel on errors, by turning on sysctl MIB
net.inet6.icmp6.nd6_debug.  nd6_debug can be turned on at bootstrap
time, by defining ND6_DEBUG kernel compilation option (so you can
debug behavior during bootstrap).  nd6_debug configuration should
only be used for test/debug purposes - for a production environment,
nd6_debug must be set to 0.  If you leave it to 1, malicious parties
can inject broken packet and fill up /var/log partition.

1.3 Scope Zone Index

IPv6 uses scoped addresses.  It is therefore very important to
specify the scope zone index (link index for a link-local address, or
site index for a site-local address) with an IPv6 address.  Without a
zone index, a scoped IPv6 address is ambiguous to the kernel, and
the kernel would not be able to determine the outbound zone for a
packet to the scoped address.  KAME code tries to address the issue in
several ways.

The entire architecture of scoped addresses is documented in RFC4007.
One non-trivial point of the architecture is that the link scope is
(theoretically) larger than the interface scope.  That is, two
different interfaces can belong to a same single link.  However, in a
normal operation, we can assume that there is 1-to-1 relationship
between links and interfaces.  In other words, we can usually put
links and interfaces in the same scope type.  The current KAME
implementation assumes the 1-to-1 relationship.  In particular, we use
interface names such as "ne1" as unique link identifiers.  This would
be much more human-readable and intuitive than numeric identifiers,
but please keep your mind on the theoretical difference between links
and interfaces.

Site-local addresses are very vaguely defined in the specs, and both
the specification and the KAME code need tons of improvements to
enable its actual use.  For example, it is still very unclear how we
define a site, or how we resolve host names in a site.  There is work
underway to define behavior of routers at site border, but, we have
almost no code for site boundary node support (neither forwarding nor
routing) and we bet almost noone has.  We recommend, at this moment,
you to use global addresses for experiments - there are way too many
pitfalls if you use site-local addresses.

1.3.1 Kernel internal

In the kernel, the link index for a link-local scope address is
embedded into the 2nd 16bit-word (the 3rd and 4th bytes) in the IPv6
address.
For example, you may see something like:
        fe80:1::200:f8ff:fe01:6317
in the routing table and the interface address structure (struct
in6_ifaddr).  The address above is a link-local unicast address which
belongs to a network link whose link identifier is 1 (note that it
eqauls to the interface index by the assumption of our
implementation).  The embedded index enables us to identify IPv6
link-local addresses over multiple links effectively and with only a
little code change.

The use of the internal format must be limited inside the kernel.  In
particular, addresses sent by an application should not contain the
embedded index (except via some very special APIs such as routing
sockets).  Instead, the index should be specified in the sin6_scope_id
field of a sockaddr_in6 structure.  Obviously, packets sent to or
received from must not contain the embedded index either, since the
index is meaningful only within the sending/receiving node.

In order to deal with the differences, several kernel routines are
provided.  These are available by including <netinet6/scope_var.h>.
Typically, the following functions will be most generally used:

- int sa6_embedscope(struct sockaddr_in6 *sa6, int defaultok);
  Embed sa6->sin6_scope_id into sa6->sin6_addr.  If sin6_scope_id is
  0, defaultok is non-0, and the default zone ID (see RFC4007) is
  configured, the default ID will be used instead of the value of the
  sin6_scope_id field.  On success, sa6->sin6_scope_id will be reset
  to 0.

  This function returns 0 on success, or a non-0 error code otherwise.
 
- int sa6_recoverscope(struct sockaddr_in6 *sa6);
  Extract embedded zone ID in sa6->sin6_addr and set
  sa6->sin6_scope_id to that ID.  The embedded ID will be cleared with
  0.

  This function returns 0 on success, or a non-0 error code otherwise.

- int in6_clearscope(struct in6_addr *in6);
  Reset the embedded zone ID in 'in6' to 0.  This function never fails, and
  returns 0 if the original address is intact or non 0 if the address is
  modified.  The return value doesn't matter in most cases; currently, the
  only point where we care about the return value is ip6_input() for checking
  whether the source or destination addresses of the incoming packet is in
  the embedded form.

- int in6_setscope(struct in6_addr *in6, struct ifnet *ifp,
                   u_int32_t *zoneidp);
  Embed zone ID determined by the address scope type for 'in6' and the
  interface 'ifp' into 'in6'.  If zoneidp is non NULL, *zoneidp will
  also have the zone ID.

  This function returns 0 on success, or a non-0 error code otherwise.

The typical usage of these functions is as follows:

sa6_embedscope() will be used at the socket or transport layer to
convert a sockaddr_in6 structure passed by an application into the
kernel-internal form.  In this usage, the second argument is often the
'ip6_use_defzone' global variable.

sa6_recoverscope() will also be used at the socket or transport layer
to convert an in6_addr structure with the embedded zone ID into a
sockaddr_in6 structure with the corresponding ID in the sin6_scope_id
field (and without the embedded ID in sin6_addr).

in6_clearscope() will be used just before sending a packet to the wire
to remove the embedded ID.  In general, this must be done at the last
stage of an output path, since otherwise the address would lose the ID
and could be ambiguous with regard to scope.

in6_setscope() will be used when the kernel receives a packet from the
wire to construct the kernel internal form for each address field in
the packet (typical examples are the source and destination addresses
of the packet).  In the typical usage, the third argument 'zoneidp'
will be NULL.  A non-NULL value will be used when the validity of the
zone ID must be checked, e.g., when forwarding a packet to another
link (see ip6_forward() for this usage).

An application, when sending a packet, is basically assumed to specify
the appropriate scope zone of the destination address by the
sin6_scope_id field (this might be done transparently from the
application with getaddrinfo() and the extended textual format - see
below), or at least the default scope zone(s) must be configured as a
last resort.  In some cases, however, an application could specify an
ambiguous address with regard to scope, expecting it is disambiguated
in the kernel by some other means.  A typical usage is to specify the
outgoing interface through another API, which can disambiguate the
unspecified scope zone.  Such a usage is not recommended, but the
kernel implements some trick to deal with even this case.

A rough sketch of the trick can be summarized as the following
sequence.

   sa6_embedscope(dst, ip6_use_defzone);
   in6_selectsrc(dst, ..., &ifp, ...);
   in6_setscope(&dst->sin6_addr, ifp, NULL);

sa6_embedscope() first tries to convert sin6_scope_id (or the default
zone ID) into the kernel-internal form.  This can fail with an
ambiguous destination, but it still tries to get the outgoing
interface (ifp) in the attempt of determining the source address of
the outgoing packet using in6_selectsrc().  If the interface is
detected, and the scope zone was originally ambiguous, in6_setscope()
can finally determine the appropriate ID with the address itself and
the interface, and construct the kernel-internal form.  See, for
example, comments in udp6_output() for more concrete example.

In any case, kernel routines except ones in netinet6/scope6.c MUST NOT
directly refer to the embedded form.  They MUST use the above
interface functions.  In particular, kernel routines MUST NOT have the
following code fragment:

        /* This is a bad practice.  Don't do this */
        if (IN6_IS_ADDR_LINKLOCAL(&sin6->sin6_addr))
                sin6->sin6_addr.s6_addr16[1] = htons(ifp->if_index);

This is bad for several reasons.  First, address ambiguity is not
specific to link-local addresses (any non-global multicast addresses
are inherently ambiguous, and this is particularly true for
interface-local addresses).  Secondly, this is vulnerable to future
changes of the embedded form (the embedded position may change, or the
zone ID may not actually be the interface index).  Only scope6.c
routines should know the details.

The above code fragment should thus actually be as follows:

        /* This is correct. */
        in6_setscope(&sin6->sin6_addr, ifp, NULL);
        (and catch errors if possible and necessary)

1.3.2 Interaction with API

There are several candidates of API to deal with scoped addresses
without ambiguity.

The IPV6_PKTINFO ancillary data type or socket option defined in the
advanced API (RFC2292 or RFC3542) can specify
the outgoing interface of a packet.  Similarly, the IPV6_PKTINFO or
IPV6_RECVPKTINFO socket options tell kernel to pass the incoming
interface to user applications.

These options are enough to disambiguate scoped addresses of an
incoming packet, because we can uniquely identify the corresponding
zone of the scoped address(es) by the incoming interface.  However,
they are too strong for outgoing packets.  For example, consider a
multi-sited node and suppose that more than one interface of the node
belongs to a same site.  When we want to send a packet to the site,
we can only specify one of the interfaces for the outgoing packet with
these options; we cannot just say "send the packet to (one of the
interfaces of) the site."

Another kind of candidates is to use the sin6_scope_id member in the
sockaddr_in6 structure, defined in RFC2553.  The KAME kernel
interprets the sin6_scope_id field properly in order to disambiguate scoped
addresses.  For example, if an application passes a sockaddr_in6
structure that has a non-zero sin6_scope_id value to the sendto(2)
system call, the kernel should send the packet to the appropriate zone
according to the sin6_scope_id field.  Similarly, when the source or
the destination address of an incoming packet is a scoped one, the
kernel should detect the correct zone identifier based on the address
and the receiving interface, fill the identifier in the sin6_scope_id
field of a sockaddr_in6 structure, and then pass the packet to an
application via the recvfrom(2) system call, etc.

However, the semantics of the sin6_scope_id is still vague and on the
way to standardization.  Additionally, not so many operating systems
support the behavior above at this moment.

In summary,
- If your target system is limited to KAME based ones (i.e. BSD
  variants and KAME snaps), use the sin6_scope_id field assuming the
  kernel behavior described above.
- Otherwise, (i.e. if your program should be portable on other systems
  than BSDs)
  + Use the advanced API to disambiguate scoped addresses of incoming
    packets.
  + To disambiguate scoped addresses of outgoing packets,
    * if it is okay to just specify the outgoing interface, use the
      advanced API.  This would be the case, for example, when you
      should only consider link-local addresses and your system
      assumes 1-to-1 relationship between links and interfaces.
    * otherwise, sorry but you lose.  Please rush the IETF IPv6
      community into standardizing the semantics of the sin6_scope_id
      field.

Routing daemons and configuration programs, like route6d and ifconfig,
will need to manipulate the "embedded" zone index.  These programs use
routing sockets and ioctls (like SIOCGIFADDR_IN6) and the kernel API
will return IPv6 addresses with the 2nd 16bit-word filled in.  The
APIs are for manipulating kernel internal structure.  Programs that
use these APIs have to be prepared about differences in kernels
anyway.

getaddrinfo(3) and getnameinfo(3) support an extended numeric IPv6
syntax, as documented in RFC4007.  You can specify the outgoing link,
by using the name of the outgoing interface as the link, like
"fe80::1%ne0" (again, note that we assume there is 1-to-1 relationship
between links and interfaces.)  This way you will be able to specify a
link-local scoped address without much trouble.

Other APIs like inet_pton(3) and inet_ntop(3) are inherently
unfriendly with scoped addresses, since they are unable to annotate
addresses with zone identifier.

1.3.3 Interaction with users (command line)

Most of user applications now support the extended numeric IPv6
syntax.  In this case, you can specify outgoing link, by using the name
of the outgoing interface like "fe80::1%ne0" (sorry for the duplicated
notice, but please recall again that we assume 1-to-1 relationship
between links and interfaces).  This is even the case for some
management tools such as route(8) or ndp(8).  For example, to install
the IPv6 default route by hand, you can type like
        # route add -inet6 default fe80::9876:5432:1234:abcd%ne0
(Although we suggest you to run dynamic routing instead of static
routes, in order to avoid configuration mistakes.)

Some applications have command line options for specifying an
appropriate zone of a scoped address (like "ping6 -I ne0 ff02::1" to
specify the outgoing interface).  However, you can't always expect such
options.  Additionally, specifying the outgoing "interface" is in
theory an overspecification as a way to specify the outgoing "link"
(see above).  Thus, we recommend you to use the extended format
described above.  This should apply to the case where the outgoing
interface is specified.

In any case, when you specify a scoped address to the command line,
NEVER write the embedded form (such as ff02:1::1 or fe80:2::fedc),
which should only be used inside the kernel (see Section 1.3.1), and 
is not supposed to work.

1.4 Plug and Play

The KAME kit implements most of the IPv6 stateless address
autoconfiguration in the kernel.
Neighbor Discovery functions are implemented in the kernel as a whole.
Router Advertisement (RA) input for hosts is implemented in the
kernel.  Router Solicitation (RS) output for endhosts, RS input
for routers, and RA output for routers are implemented in the
userland.

1.4.1 Assignment of link-local, and special addresses

IPv6 link-local address is generated from IEEE802 address (ethernet MAC
address).  Each of interface is assigned an IPv6 link-local address
automatically, when the interface becomes up (IFF_UP).  Also, direct route
for the link-local address is added to routing table.

Here is an output of netstat command:

Internet6:
Destination                   Gateway                   Flags      Netif Expire
fe80::%ed0/64                 link#1                    UC           ed0
fe80::%ep0/64                 link#2                    UC           ep0

Interfaces that has no IEEE802 address (pseudo interfaces like tunnel
interfaces, or ppp interfaces) will borrow IEEE802 address from other
interfaces, such as ethernet interfaces, whenever possible.
If there is no IEEE802 hardware attached, last-resort pseudorandom value,
which is from MD5(hostname), will be used as source of link-local address.
If it is not suitable for your usage, you will need to configure the
link-local address manually.

If an interface is not capable of handling IPv6 (such as lack of multicast
support), link-local address will not be assigned to that interface.
See section 2 for details.

Each interface joins the solicited multicast address and the
link-local all-nodes multicast addresses (e.g.  fe80::1:ff01:6317
and ff02::1, respectively, on the link the interface is attached).
In addition to a link-local address, the loopback address (::1) will be
assigned to the loopback interface.  Also, ::1/128 and ff01::/32 are
automatically added to routing table, and loopback interface joins
node-local multicast group ff01::1.

1.4.2 Stateless address autoconfiguration on hosts

In IPv6 specification, nodes are separated into two categories:
routers and hosts.  Routers forward packets addressed to others, hosts does
not forward the packets.  net.inet6.ip6.forwarding defines whether this
node is a router or a host (router if it is 1, host if it is 0).

It is NOT recommended to change net.inet6.ip6.forwarding while the node
is in operation.  IPv6 specification defines behavior for "host" and "router"
quite differently, and switching from one to another can cause serious
troubles.  It is recommended to configure the variable at bootstrap time only.

The first step in stateless address configuration is Duplicated Address
Detection (DAD).  See 1.2 for more detail on DAD.

When a host hears Router Advertisement from the router, a host may
autoconfigure itself by stateless address autoconfiguration.  This
behavior can be controlled by the net.inet6.ip6.accept_rtadv sysctl
variable and a per-interface flag managed in the kernel.  The latter,
which we call "if_accept_rtadv" here, can be changed by the ndp(8)
command (see the manpage for more details).  When the sysctl variable
is set to 1, and the flag is set, the host autoconfigures itself.  By
autoconfiguration, network address prefixes for the receiving
interface (usually global address prefix) are added.  The default
route is also configured.

Routers periodically generate Router Advertisement packets.  To
request an adjacent router to generate RA packet, a host can transmit
Router Solicitation.  To generate an RS packet at any time, use the
"rtsol" command.  The "rtsold" daemon is also available. "rtsold"
generates Router Solicitation whenever necessary, and it works greatly
for nomadic usage (notebooks/laptops).  If one wishes to ignore Router
Advertisements, use sysctl to set net.inet6.ip6.accept_rtadv to 0.
Additionally, ndp(8) command can be used to control the behavior
per-interface basis.

To generate Router Advertisement from a router, use the "rtadvd" daemon.

Note that the IPv6 specification assumes the following items and that
nonconforming cases are left unspecified:
- Only hosts will listen to router advertisements
- Hosts have a single network interface (except loopback)
This is therefore unwise to enable net.inet6.ip6.accept_rtadv on routers,
or multi-interface hosts.  A misconfigured node can behave strange
(KAME code allows nonconforming configuration, for those who would like
to do some experiments).

To summarize the sysctl knob:
        accept_rtadv    forwarding      role of the node
        ---             ---             ---
        0               0               host (to be manually configured)
        0               1               router
        1               0               autoconfigured host
                                        (spec assumes that hosts have a single
                                        interface only, autoconfigred hosts
                                        with multiple interfaces are
                                        out-of-scope)
        1               1               invalid, or experimental
                                        (out-of-scope of spec)

The if_accept_rtadv flag is referred only when accept_rtadv is 1 (the
latter two cases).  The flag does not have any effects when the sysctl
variable is 0.

See 1.2 in the document for relationship between DAD and autoconfiguration.

1.4.3 DHCPv6

We supply a tiny DHCPv6 server/client in kame/dhcp6. However, the
implementation is premature (for example, this does NOT implement
address lease/release), and it is not in default compilation tree on
some platforms. If you want to do some experiment, compile it on your
own.

DHCPv6 and autoconfiguration also needs more work.  "Managed" and "Other"
bits in RA have no special effect to stateful autoconfiguration procedure
in DHCPv6 client program ("Managed" bit actually prevents stateless
autoconfiguration, but no special action will be taken for DHCPv6 client).

1.5 Generic tunnel interface

GIF (Generic InterFace) is a pseudo interface for configured tunnel.
Details are described in gif(4) manpage.
Currently
        v6 in v6
        v6 in v4
        v4 in v6
        v4 in v4
are available.  Use "gifconfig" to assign physical (outer) source
and destination address to gif interfaces.
Configuration that uses same address family for inner and outer IP
header (v4 in v4, or v6 in v6) is dangerous.  It is very easy to
configure interfaces and routing tables to perform infinite level
of tunneling.  Please be warned.

gif can be configured to be ECN-friendly.  See 4.5 for ECN-friendliness
of tunnels, and gif(4) manpage for how to configure.

If you would like to configure an IPv4-in-IPv6 tunnel with gif interface,
read gif(4) carefully.  You may need to remove IPv6 link-local address
automatically assigned to the gif interface.

1.6 Address Selection

1.6.1 Source Address Selection

The KAME kernel chooses the source address for an outgoing packet
sent from a user application as follows:

1. if the source address is explicitly specified via an IPV6_PKTINFO
   ancillary data item or the socket option of that name, just use it.
   Note that this item/option overrides the bound address of the
   corresponding (datagram) socket.

2. if the corresponding socket is bound, use the bound address.

3. otherwise, the kernel first tries to find the outgoing interface of
   the packet.  If it fails, the source address selection also fails.
   If the kernel can find an interface, choose the most appropriate
   address based on the algorithm described in RFC3484.

   The policy table used in this algorithm is stored in the kernel.
   To install or view the policy, use the ip6addrctl(8) command.  The
   kernel does not have pre-installed policy.  It is expected that the
   default policy described in the draft should be installed at the
   bootstrap time using this command.

   This draft allows an implementation to add implementation-specific
   rules with higher precedence than the rule "Use longest matching
   prefix."  KAME's implementation has the following additional rules
   (that apply in the appeared order):

   - prefer addresses on alive interfaces, that is, interfaces with
     the UP flag being on.  This rule is particularly useful for
     routers, since some routing daemons stop advertising prefixes
    (addresses) on interfaces that have become down.

   - prefer addresses on "preferred" interfaces.  "Preferred"
     interfaces can be specified by the ndp(8) command.  By default,
     no interface is preferred, that is, this rule does not apply.
     Again, this rule is particularly useful for routers, since there
     is a convention, among router administrators, of assigning
     "stable" addresses on a particular interface (typically a
     loopback interface).

   In any case, addresses that break the scope zone of the
   destination, or addresses whose zone do not contain the outgoing
   interface are never chosen.

When the procedure above fails, the kernel usually returns
EADDRNOTAVAIL to the application.

In some cases, the specification explicitly requires the
implementation to choose a particular source address.  The source
address for a Neighbor Advertisement (NA) message is an example.
Under the spec (RFC2461 7.2.2) NA's source should be the target
address of the corresponding NS's target.  In this case we follow the
spec rather than the above rule.

If you would like to prohibit the use of deprecated address for some
reason, configure net.inet6.ip6.use_deprecated to 0.  The issue
related to deprecated address is described in RFC2462 5.5.4 (NOTE:
there is some debate underway in IETF ipngwg on how to use
"deprecated" address).

As documented in the source address selection document, temporary
addresses for privacy extension are less preferred to public addresses
by default.  However, for administrators who are particularly aware of
the privacy, there is a system-wide sysctl(3) variable
"net.inet6.ip6.prefer_tempaddr".  When the variable is set to
non-zero, the kernel will rather prefer temporary addresses.  The
default value of this variable is 0.

1.6.2 Destination Address Ordering

KAME's getaddrinfo(3) supports the destination address ordering
algorithm described in RFC3484.  Getaddrinfo(3) needs to know the
source address for each destination address and policy entries
(described in the previous section) for the source and destination
addresses.  To get the source address, the library function opens a
UDP socket and tries to connect(2) for the destination.  To get the
policy entry, the function issues sysctl(3).

1.7 Jumbo Payload

KAME supports the Jumbo Payload hop-by-hop option used to send IPv6
packets with payloads longer than 65,535 octets.  But since currently
KAME does not support any physical interface whose MTU is more than
65,535, such payloads can be seen only on the loopback interface(i.e.
lo0).

If you want to try jumbo payloads, you first have to reconfigure the
kernel so that the MTU of the loopback interface is more than 65,535
bytes; add the following to the kernel configuration file:
        options         "LARGE_LOMTU"           #To test jumbo payload
and recompile the new kernel.

Then you can test jumbo payloads by the ping6 command with -b and -s
options.  The -b option must be specified to enlarge the size of the
socket buffer and the -s option specifies the length of the packet,
which should be more than 65,535.  For example, type as follows; 
        % ping6 -b 70000 -s 68000 ::1

The IPv6 specification requires that the Jumbo Payload option must not
be used in a packet that carries a fragment header.  If this condition
is broken, an ICMPv6 Parameter Problem message must be sent to the
sender.  KAME kernel follows the specification, but you cannot usually
see an ICMPv6 error caused by this requirement.

If KAME kernel receives an IPv6 packet, it checks the frame length of
the packet and compares it to the length specified in the payload
length field of the IPv6 header or in the value of the Jumbo Payload
option, if any.  If the former is shorter than the latter, KAME kernel
discards the packet and increments the statistics.  You can see the
statistics as output of netstat command with `-s -p ip6' option:
        % netstat -s -p ip6
        ip6:
                (snip)
                1 with data size < data length

So, KAME kernel does not send an ICMPv6 error unless the erroneous
packet is an actual Jumbo Payload, that is, its packet size is more
than 65,535 bytes.  As described above, KAME kernel currently does not
support physical interface with such a huge MTU, so it rarely returns an
ICMPv6 error.

TCP/UDP over jumbogram is not supported at this moment.  This is because
we have no medium (other than loopback) to test this.  Contact us if you
need this.

IPsec does not work on jumbograms.  This is due to some specification twists
in supporting AH with jumbograms (AH header size influences payload length,
and this makes it real hard to authenticate inbound packet with jumbo payload
option as well as AH).

There are fundamental issues in *BSD support for jumbograms.  We would like to
address those, but we need more time to finalize the task.  To name a few:
- mbuf pkthdr.len field is typed as "int" in 4.4BSD, so it cannot hold
  jumbogram with len > 2G on 32bit architecture CPUs.  If we would like to
  support jumbogram properly, the field must be expanded to hold 4G +
  IPv6 header + link-layer header.  Therefore, it must be expanded to at least
  int64_t (u_int32_t is NOT enough).
- We mistakingly use "int" to hold packet length in many places.  We need
  to convert them into larger numeric type.  It needs a great care, as we may
  experience overflow during packet length computation.
- We mistakingly check for ip6_plen field of IPv6 header for packet payload
  length in various places.  We should be checking mbuf pkthdr.len instead.
  ip6_input() will perform sanity check on jumbo payload option on input,
  and we can safely use mbuf pkthdr.len afterwards.
- TCP code needs careful updates in bunch of places, of course.

1.8 Loop prevention in header processing

IPv6 specification allows arbitrary number of extension headers to
be placed onto packets.  If we implement IPv6 packet processing
code in the way BSD IPv4 code is implemented, kernel stack may
overflow due to long function call chain.  KAME sys/netinet6 code
is carefully designed to avoid kernel stack overflow.  Because of
this, KAME sys/netinet6 code defines its own protocol switch
structure, as "struct ip6protosw" (see netinet6/ip6protosw.h).

In addition to this, we restrict the number of extension headers
(including the IPv6 header) in each incoming packet, in order to
prevent a DoS attack that tries to send packets with a massive number
of extension headers.  The upper limit can be configured by the sysctl
value net.inet6.ip6.hdrnestlimit.  In particular, if the value is 0,
the node will allow an arbitrary number of headers. As of writing this
document, the default value is 50.

IPv4 part (sys/netinet) remains untouched for compatibility.
Because of this, if you receive IPsec-over-IPv4 packet with massive
number of IPsec headers, kernel stack may blow up.  IPsec-over-IPv6 is okay.

1.9 ICMPv6

After RFC2463 was published, IETF ipngwg has decided to disallow ICMPv6 error
packet against ICMPv6 redirect, to prevent ICMPv6 storm on a network medium.
KAME already implements this into the kernel.

RFC2463 requires rate limitation for ICMPv6 error packets generated by a
node, to avoid possible DoS attacks.  KAME kernel implements two rate-
limitation mechanisms, tunable via sysctl:
- Minimum time interval between ICMPv6 error packets
        KAME kernel will generate no more than one ICMPv6 error packet,
        during configured time interval.  net.inet6.icmp6.errratelimit
        controls the interval (default: disabled).
- Maximum ICMPv6 error packet-per-second
        KAME kernel will generate no more than the configured number of
        packets in one second.  net.inet6.icmp6.errppslimit controls the
        maximum packet-per-second value (default: 200pps)
Basically, we need to pick values that are suitable against the bandwidth
of link layer devices directly attached to the node.  In some cases the
default values may not fit well.  We are still unsure if the default value
is sane or not.  Comments are welcome.

1.10 Applications

For userland programming, we support IPv6 socket API as specified in
RFC2553/3493, RFC3542 and upcoming internet drafts.

TCP/UDP over IPv6 is available and quite stable.  You can enjoy "telnet",
"ftp", "rlogin", "rsh", "ssh", etc.  These applications are protocol
independent.  That is, they automatically chooses IPv4 or IPv6
according to DNS.

1.11 Kernel Internals

 (*) TCP/UDP part is handled differently between operating system platforms.
     See 1.12 for details.

The current KAME has escaped from the IPv4 netinet logic.  While
ip_forward() calls ip_output(), ip6_forward() directly calls
if_output() since routers must not divide IPv6 packets into fragments.

ICMPv6 should contain the original packet as long as possible up to
1280.  UDP6/IP6 port unreach, for instance, should contain all
extension headers and the *unchanged* UDP6 and IP6 headers.
So, all IP6 functions except TCP6 never convert network byte
order into host byte order, to save the original packet.

tcp6_input(), udp6_input() and icmp6_input() can't assume that IP6
header is preceding the transport headers due to extension
headers.  So, in6_cksum() was implemented to handle packets whose IP6
header and transport header is not continuous.  TCP/IP6 nor UDP/IP6
header structure don't exist for checksum calculation.

To process IP6 header, extension headers and transport headers easily,
KAME requires network drivers to store packets in one internal mbuf or
one or more external mbufs.  A typical old driver prepares two
internal mbufs for 100 - 208 bytes data, however, KAME's reference
implementation stores it in one external mbuf.

"netstat -s -p ip6" tells you whether or not your driver conforms
KAME's requirement.  In the following example, "cce0" violates the
requirement. (For more information, refer to Section 2.)

        Mbuf statistics:
                317 one mbuf
                two or more mbuf::
                        lo0 = 8
                        cce0 = 10
                3282 one ext mbuf
                0 two or more ext mbuf

Each input function calls IP6_EXTHDR_CHECK in the beginning to check
if the region between IP6 and its header is
continuous.  IP6_EXTHDR_CHECK calls m_pullup() only if the mbuf has
M_LOOP flag, that is, the packet comes from the loopback
interface.  m_pullup() is never called for packets coming from physical
network interfaces.

TCP6 reassembly makes use of IP6 header to store reassemble
information.  IP6 is not supposed to be just before TCP6, so
ip6tcpreass structure has a pointer to TCP6 header.  Of course, it has
also a pointer back to mbuf to avoid m_pullup().

Like TCP6, both IP and IP6 reassemble functions never call m_pullup().

xxx_ctlinput() calls in_mrejoin() on PRC_IFNEWADDR.  We think this is
one of 4.4BSD implementation flaws.  Since 4.4BSD keeps ia_multiaddrs
in in_ifaddr{}, it can't use multicast feature if the interface has no
unicast address.  So, if an application joins to an interface and then
all unicast addresses are removed from the interface, the application
can't send/receive any multicast packets.  Moreover, if a new unicast
address is assigned to the interface, in_mrejoin() must be called.
KAME's interfaces, however, have ALWAYS one link-local unicast
address.  These extensions have thus not been implemented in KAME.

1.12 IPv4 mapped address and IPv6 wildcard socket

RFC2553/3493 describes IPv4 mapped address (3.7) and special behavior
of IPv6 wildcard bind socket (3.8).  The spec allows you to:
- Accept IPv4 connections by AF_INET6 wildcard bind socket.
- Transmit IPv4 packet over AF_INET6 socket by using special form of
  the address like ::ffff:10.1.1.1.
but the spec itself is very complicated and does not specify how the
socket layer should behave.
Here we call the former one "listening side" and the latter one "initiating
side", for reference purposes.

Almost all KAME implementations treat tcp/udp port number space separately
between IPv4 and IPv6.  You can perform wildcard bind on both of the address
families, on the same port.

There are some OS-platform differences in KAME code, as we use tcp/udp
code from different origin.  The following table summarizes the behavior.

                listening side          initiating side
                (AF_INET6 wildcard      (connection to ::ffff:10.1.1.1)
                socket gets IPv4 conn.)
                ---                     ---
KAME/BSDI3      not supported           not supported
KAME/FreeBSD228 not supported           not supported
KAME/FreeBSD3x  configurable            supported
                default: enabled
KAME/FreeBSD4x  configurable            supported
                default: enabled
KAME/NetBSD     configurable            supported
                default: disabled
KAME/BSDI4      enabled                 supported
KAME/OpenBSD    not supported           not supported

The following sections will give you more details, and how you can
configure the behavior.

Comments on listening side:

It looks that RFC2553/3493 talks too little on wildcard bind issue,
specifically on (1) port space issue, (2) failure mode, (3) relationship
between AF_INET/INET6 wildcard bind like ordering constraint, and (4) behavior
when conflicting socket is opened/closed.  There can be several separate
interpretation for this RFC which conform to it but behaves differently.
So, to implement portable application you should assume nothing
about the behavior in the kernel.  Using getaddrinfo() is the safest way.
Port number space and wildcard bind issues were discussed in detail
on ipv6imp mailing list, in mid March 1999 and it looks that there's
no concrete consensus (means, up to implementers).  You may want to
check the mailing list archives.
We supply a tool called "bindtest" that explores the behavior of
kernel bind(2).  The tool will not be compiled by default.

If a server application would like to accept IPv4 and IPv6 connections,
it should use AF_INET and AF_INET6 socket (you'll need two sockets).
Use getaddrinfo() with AI_PASSIVE into ai_flags, and socket(2) and bind(2)
to all the addresses returned.
By opening multiple sockets, you can accept connections onto the socket with
proper address family.  IPv4 connections will be accepted by AF_INET socket,
and IPv6 connections will be accepted by AF_INET6 socket (NOTE: KAME/BSDI4
kernel sometimes violate this - we will fix it).

If you try to support IPv6 traffic only and would like to reject IPv4
traffic, always check the peer address when a connection is made toward
AF_INET6 listening socket.  If the address is IPv4 mapped address, you may
want to reject the connection.  You can check the condition by using
IN6_IS_ADDR_V4MAPPED() macro.  This is one of the reasons the author of
the section (itojun) dislikes special behavior of AF_INET6 wildcard bind.

Comments on initiating side:

Advise to application implementers: to implement a portable IPv6 application
(which works on multiple IPv6 kernels), we believe that the following
is the key to the success:
- NEVER hardcode AF_INET nor AF_INET6.
- Use getaddrinfo() and getnameinfo() throughout the system.
  Never use gethostby*(), getaddrby*(), inet_*() or getipnodeby*().
- If you would like to connect to destination, use getaddrinfo() and try
  all the destination returned, like telnet does.
- Some of the IPv6 stack is shipped with buggy getaddrinfo().  Ship a minimal
  working version with your application and use that as last resort.

If you would like to use AF_INET6 socket for both IPv4 and IPv6 outgoing
connection, you will need tweaked implementation in DNS support libraries,
as documented in RFC2553/3493 6.1.  KAME libinet6 includes the tweak in
getipnodebyname().  Note that getipnodebyname() itself is not recommended as
it does not handle scoped IPv6 addresses at all.  For IPv6 name resolution
getaddrinfo() is the preferred API.  getaddrinfo() does not implement the
tweak.

When writing applications that make outgoing connections, story goes much
simpler if you treat AF_INET and AF_INET6 as totally separate address family.
{set,get}sockopt issue goes simpler, DNS issue will be made simpler.  We do
not recommend you to rely upon IPv4 mapped address.

1.12.1 KAME/BSDI3 and KAME/FreeBSD228

The platforms do not support IPv4 mapped address at all (both listening side
and initiating side).  AF_INET6 and AF_INET sockets are totally separated.

Port number space is totally separate between AF_INET and AF_INET6 sockets. 

It should be noted that KAME/BSDI3 and KAME/FreeBSD228 are not conformant
to RFC2553/3493 section 3.7 and 3.8.  It is due to code sharing reasons.

1.12.2 KAME/FreeBSD[34]x

KAME/FreeBSD3x and KAME/FreeBSD4x use shared tcp4/6 code (from
sys/netinet/tcp*) and shared udp4/6 code (from sys/netinet/udp*).
They use unified inpcb/in6pcb structure.

1.12.2.1 KAME/FreeBSD[34]x, listening side

The platform can be configured to support IPv4 mapped address/special
AF_INET6 wildcard bind (enabled by default).  There is no kernel compilation
option to disable it.  You can enable/disable the behavior with sysctl
(per-node), or setsockopt (per-socket).

Wildcard AF_INET6 socket grabs IPv4 connection if and only if the following 
conditions are satisfied:
- there's no AF_INET socket that matches the IPv4 connection
- the AF_INET6 socket is configured to accept IPv4 traffic, i.e.
  getsockopt(IPV6_V6ONLY) returns 0.

(XXX need checking)

1.12.2.2 KAME/FreeBSD[34]x, initiating side

KAME/FreeBSD3x supports outgoing connection to IPv4 mapped address
(::ffff:10.1.1.1), if the node is configured to accept IPv4 connections
by AF_INET6 socket.

(XXX need checking)

1.12.3 KAME/NetBSD

KAME/NetBSD uses shared tcp4/6 code (from sys/netinet/tcp*) and shared
udp4/6 code (from sys/netinet/udp*).  The implementation is made differently
from KAME/FreeBSD[34]x.  KAME/NetBSD uses separate inpcb/in6pcb structures,
while KAME/FreeBSD[34]x uses merged inpcb structure.

It should be noted that the default configuration of KAME/NetBSD is not
conformant to RFC2553/3493 section 3.8.  It is intentionally turned off by
default for security reasons.

The platform can be configured to support IPv4 mapped address/special AF_INET6
wildcard bind (disabled by default).  Kernel behavior can be summarized as
follows:
- default: special support code will be compiled in, but is disabled by
  default.  It can be controlled by sysctl (net.inet6.ip6.v6only),
  or setsockopt(IPV6_V6ONLY).
- add "INET6_BINDV6ONLY": No special support code for AF_INET6 wildcard socket
  will be compiled in.  AF_INET6 sockets and AF_INET sockets are totally
  separate.  The behavior is similar to what described in 1.12.1.

sysctl setting will affect per-socket configuration at in6pcb creation time
only.  In other words, per-socket configuration will be copied from sysctl
configuration at in6pcb creation time.  To change per-socket behavior, you
must perform setsockopt or reopen the socket.  Change in sysctl configuration
will not change the behavior or sockets that are already opened.

1.12.3.1 KAME/NetBSD, listening side

Wildcard AF_INET6 socket grabs IPv4 connection if and only if the following 
conditions are satisfied:
- there's no AF_INET socket that matches the IPv4 connection
- the AF_INET6 socket is configured to accept IPv4 traffic, i.e.
  getsockopt(IPV6_V6ONLY) returns 0.

You cannot bind(2) with IPv4 mapped address.  This is a workaround for port
number duplicate and other twists.

1.12.3.2 KAME/NetBSD, initiating side

When getsockopt(IPV6_V6ONLY) is 0 for a socket, you can make an outgoing
traffic to IPv4 destination over AF_INET6 socket, using IPv4 mapped
address destination (::ffff:10.1.1.1).

When getsockopt(IPV6_V6ONLY) is 1 for a socket, you cannot use IPv4 mapped
address for outgoing traffic.

1.12.4 KAME/BSDI4

KAME/BSDI4 uses NRL-based TCP/UDP stack and inpcb source code,
which was derived from NRL IPv6/IPsec stack.  We guess it supports IPv4 mapped
address and speical AF_INET6 wildcard bind.  The implementation is, again,
different from other KAME/*BSDs.

1.12.4.1 KAME/BSDI4, listening side

NRL inpcb layer supports special behavior of AF_INET6 wildcard socket.
There is no way to disable the behavior.

Wildcard AF_INET6 socket grabs IPv4 connection if and only if the following 
condition is satisfied:
- there's no AF_INET socket that matches the IPv4 connection

1.12.4.2 KAME/BSDI4, initiating side

KAME/BSDi4 supports connection initiation to IPv4 mapped address
(like ::ffff:10.1.1.1).

1.12.5 KAME/OpenBSD

KAME/OpenBSD uses NRL-based TCP/UDP stack and inpcb source code,
which was derived from NRL IPv6/IPsec stack.

It should be noted that KAME/OpenBSD is not conformant to RFC2553/3493 section
3.7 and 3.8.  It is intentionally omitted for security reasons.

1.12.5.1 KAME/OpenBSD, listening side

KAME/OpenBSD disables special behavior on AF_INET6 wildcard bind for
security reasons (if IPv4 traffic toward AF_INET6 wildcard bind is allowed,
access control will become much harder).  KAME/BSDI4 uses NRL-based TCP/UDP
stack as well, however, the behavior is different due to OpenBSD's security
policy.

As a result the behavior of KAME/OpenBSD is similar to KAME/BSDI3 and
KAME/FreeBSD228 (see 1.12.1 for more detail).

1.12.5.2 KAME/OpenBSD, initiating side

KAME/OpenBSD does not support connection initiation to IPv4 mapped address
(like ::ffff:10.1.1.1).

1.12.6 More issues

IPv4 mapped address support adds a big requirement to EVERY userland codebase.
Every userland code should check if an AF_INET6 sockaddr contains IPv4
mapped address or not.  This adds many twists:

- Access controls code becomes harder to write.
  For example, if you would like to reject packets from 10.0.0.0/8,
  you need to reject packets to AF_INET socket from 10.0.0.0/8,
  and to AF_INET6 socket from ::ffff:10.0.0.0/104.
- If a protocol on top of IPv4 is defined differently with IPv6, we need to be
  really careful when we determine which protocol to use.
  For example, with FTP protocol, we can not simply use sa_family to determine
  FTP command sets.  The following example is incorrect:
        if (sa_family == AF_INET)
                use EPSV/EPRT or PASV/PORT;     /*IPv4*/
        else if (sa_family == AF_INET6)
                use EPSV/EPRT or LPSV/LPRT;     /*IPv6*/
        else
                error;
  The correct code, with consideration to IPv4 mapped address, would be:
        if (sa_family == AF_INET)
                use EPSV/EPRT or PASV/PORT;     /*IPv4*/
        else if (sa_family == AF_INET6 && IPv4 mapped address)
                use EPSV/EPRT or PASV/PORT;     /*IPv4 command set on AF_INET6*/
        else if (sa_family == AF_INET6 && !IPv4 mapped address)
                use EPSV/EPRT or LPSV/LPRT;     /*IPv6*/
        else
                error;
  It is too much to ask for every body to be careful like this.
  The problem is, we are not sure if the above code fragment is perfect for
  all situations.
- By enabling kernel support for IPv4 mapped address (outgoing direction),
  servers on the kernel can be hosed by IPv6 native packet that has IPv4
  mapped address in IPv6 header source, and can generate unwanted IPv4 packets.
  draft-itojun-ipv6-transition-abuse-01.txt, draft-cmetz-v6ops-v4mapped-api-
  harmful-00.txt, and draft-itojun-v6ops-v4mapped-harmful-01.txt
  has more on this scenario.

Due to the above twists, some of KAME userland programs has restrictions on
the use of IPv4 mapped addresses:
- rshd/rlogind do not accept connections from IPv4 mapped address.
  This is to avoid malicious use of IPv4 mapped address in IPv6 native
  packet, to bypass source-address based authentication.
- ftp/ftpd assume that you are on dual stack network.  IPv4 mapped address
  will be decoded in userland, and will be passed to AF_INET sockets
  (in other words, ftp/ftpd do not support SIIT environment).

1.12.7 Interaction with SIIT translator

SIIT translator is specified in RFC2765.  KAME node cannot become a SIIT
translator box, nor SIIT end node (a node in SIIT cloud).

To become a SIIT translator box, we need to put additional code for that.
We do not have the code in our tree at this moment.

There are multiple reasons that we are unable to become SIIT end node.
(1) SIIT translators require end nodes in the SIIT cloud to be IPv6-only.
Since we are unable to compile INET-less kernel, we are unable to become
SIIT end node.  (2) As presented in 1.12.6, some of our userland code assumes
dual stack network.  (3) KAME stack filters out IPv6 packets with IPv4
mapped address in the header, to secure non-SIIT case (which is much more
common).  Effectively KAME node will reject any packets via SIIT translator
box.  See section 1.14 for more detail about the last item.

There are documentation issues too - SIIT document requires very strange
things.  For example, SIIT document asks IPv6-only (meaning no IPv4 code)
node to be able to construct IPv4 IPsec headers.  If a node knows how to
construct IPv4 IPsec headers, that is not an IPv6-only node, it is a dual-stack
node.  The requirements imposed in SIIT document contradict with the other
part of the document itself.

1.13 sockaddr_storage

When RFC2553 was about to be finalized, there was discussion on how struct
sockaddr_storage members are named.  One proposal is to prepend "__" to the
members (like "__ss_len") as they should not be touched.  The other proposal
was that don't prepend it (like "ss_len") as we need to touch those members
directly.  There was no clear consensus on it.

As a result, RFC2553 defines struct sockaddr_storage as follows:
        struct sockaddr_storage {
                u_char  __ss_len;       /* address length */
                u_char  __ss_family;    /* address family */
                /* and bunch of padding */
        };
On the contrary, XNET draft defines as follows:
        struct sockaddr_storage {
                u_char  ss_len;         /* address length */
                u_char  ss_family;      /* address family */
                /* and bunch of padding */
        };

In December 1999, it was agreed that RFC2553bis (RFC3493) should pick the
latter (XNET) definition.

KAME kit prior to December 1999 used RFC2553 definition.  KAME kit after
December 1999 (including December) will conform to XNET definition,
based on RFC3493 discussion.

If you look at multiple IPv6 implementations, you will be able to see
both definitions.  As an userland programmer, the most portable way of
dealing with it is to:
(1) ensure ss_family and/or ss_len are available on the platform, by using
    GNU autoconf,
(2) have -Dss_family=__ss_family to unify all occurrences (including header
    file) into __ss_family, or
(3) never touch __ss_family.  cast to sockaddr * and use sa_family like:
        struct sockaddr_storage ss;
        family = ((struct sockaddr *)&ss)->sa_family

1.14 Invalid addresses on the wire

Some of IPv6 transition technologies embed IPv4 address into IPv6 address.
These specifications themselves are fine, however, there can be certain
set of attacks enabled by these specifications.  Recent specification
documents covers up those issues, however, there are already-published RFCs
that does not have protection against those (like using source address of
::ffff:127.0.0.1 to bypass "reject packet from remote" filter).

To name a few, these address ranges can be used to hose an IPv6 implementation,
or bypass security controls:
- IPv4 mapped address that embeds unspecified/multicast/loopback/broadcast
  IPv4 address (if they are in IPv6 native packet header, they are malicious)
        ::ffff:0.0.0.0/104      ::ffff:127.0.0.0/104
        ::ffff:224.0.0.0/100    ::ffff:255.0.0.0/104 
- 6to4 (RFC3056) prefix generated from unspecified/multicast/loopback/
  broadcast/private IPv4 address
        2002:0000::/24          2002:7f00::/24          2002:e000::/24
        2002:ff00::/24          2002:0a00::/24          2002:ac10::/28  
        2002:c0a8::/32
- IPv4 compatible address that embeds unspecified/multicast/loopback/broadcast
  IPv4 address (if they are in IPv6 native packet header, they are malicious).
  Note that, since KAME doe snot support RFC1933/2893 auto tunnels, KAME nodes
  are not vulnerable to these packets.
        ::0.0.0.0/104   ::127.0.0.0/104 ::224.0.0.0/100 ::255.0.0.0/104 

Also, since KAME does not support RFC1933/2893 auto tunnels, seeing IPv4
compatible is very rare.  You should take caution if you see those on the wire.

If we see IPv6 packets with IPv4 mapped address (::ffff:0.0.0.0/96) in the
header in dual-stack environment (not in SIIT environment), they indicate
that someone is trying to impersonate IPv4 peer.  The packet should be dropped.

IPv6 specifications do not talk very much about IPv6 unspecified address (::)
in the IPv6 source address field.  Clarification is in progress.
Here are couple of comments:
- IPv6 unspecified address can be used in IPv6 source address field, if and
  only if we have no legal source address for the node.  The legal situations
  include, but may not be limited to, (1) MLD while no IPv6 address is assigned
  to the node and (2) DAD.
- If IPv6 TCP packet has IPv6 unspecified address, it is an attack attempt.
  The form can be used as a trigger for TCP DoS attack.  KAME code already
  filters them out.
- The following examples are seemingly illegal.  It seems that there's general
  consensus among ipngwg for those.  (1) Mobile IPv6 home address option,
  (2) offlink packets (so routers should not forward them).
  KAME implements (2) already.

KAME code is carefully written to avoid such incidents.  More specifically,
KAME kernel will reject packets with certain source/destination address in IPv6
base header, or IPv6 routing header.  Also, KAME default configuration file
is written carefully, to avoid those attacks.

draft-itojun-ipv6-transition-abuse-01.txt, draft-cmetz-v6ops-v4mapped-api-
harmful-00.txt and draft-itojun-v6ops-v4mapped-harmful-01.txt has more on
this issue.

1.15 Node's required addresses

RFC2373 section 2.8 talks about required addresses for an IPv6
node.  The section talks about how KAME stack manages those required
addresses.

1.15.1 Host case

The following items are automatically assigned to the node (or the node will
automatically joins the group), at bootstrap time:
- Loopback address
- All-nodes multicast addresses (ff01::1)

The following items will be automatically handled when the interface becomes
IFF_UP:
- Its link-local address for each interface
- Solicited-node multicast address for link-local addresses
- Link-local allnodes multicast address (ff02::1)

The following items need to be configured manually by ifconfig(8) or prefix(8).
Alternatively, these can be autoconfigured by using stateless address
autoconfiguration.
- Assigned unicast/anycast addresses
- Solicited-Node multicast address for assigned unicast address

Users can join groups by using appropriate system calls like setsockopt(2).

1.15.2 Router case

In addition to the above, routers needs to handle the following items.

The following items need to be configured manually by using ifconfig(8).
o The subnet-router anycast addresses for the interfaces it is configured
  to act as a router on (prefix::/64)
o All other anycast addresses with which the router has been configured

The router will join the following multicast group when rtadvd(8) is available
for the interface.
o All-Routers Multicast Addresses (ff02::2)

Routing daemons will join appropriate multicast groups, as necessary,
like ff02::9 for RIPng.

Users can join groups by using appropriate system calls like setsockopt(2).

1.16 Advanced API

Current KAME kernel implements RFC3542 API.  It also implements RFC2292 API,
for backward compatibility purposes with *BSD-integrated codebase.
KAME tree ships with RFC3542 headers.
*BSD-integrated codebase implements either RFC2292, or RFC3542, API.
see "COVERAGE" document for detailed implementation status.

Here are couple of issues to mention:
- *BSD-integrated binaries, compiled for RFC2292, will work on KAME kernel.
  For example, OpenBSD 2.7 /sbin/rtsol will work on KAME/openbsd kernel.
- KAME binaries, compiled using RFC3542, will not work on *BSD-integrated
  kenrel.  For example, KAME /usr/local/v6/sbin/rtsol will not work on
  OpenBSD 2.7 kernel.
- RFC3542 API is not compatible with RFC2292 API.  RFC3542 #define symbols
  conflict with RFC2292 symbols.  Therefore, if you compile programs that
  assume RFC2292 API, the compilation itself goes fine, however, the compiled
  binary will not work correctly.  The problem is not KAME issue, but API
  issue.  For example, Solaris 8 implements RFC3542 API.  If you compile
  RFC2292-based code on Solaris 8, the binary can behave strange.

There are few (or couple of) incompatible behavior in RFC2292 binary backward
compatibility support in KAME tree.  To enumerate:
- Type 0 routing header lacks support for strict/loose bitmap.
  Even if we see packets with "strict" bit set, those bits will not be made
  visible to the userland.
  Background: RFC2292 document is based on RFC1883 IPv6, and it uses
  strict/loose bitmap.  RFC3542 document is based on RFC2460 IPv6, and it has
  no strict/loose bitmap (it was removed from RFC2460).  KAME tree obeys
  RFC2460 IPv6, and lacks support for strict/loose bitmap.

The RFC3542 documents leave some particular cases unspecified.  The
KAME implementation treats them as follows:
- The IPV6_DONTFRAG and IPV6_RECVPATHMTU socket options for TCP
  sockets are ignored.  That is, the setsocktopt() call will succeed
  but the specified value will have no effect.

1.17 DNS resolver

KAME ships with modified DNS resolver, in libinet6.a.
libinet6.a has a couple of extensions against libc DNS resolver:
- Can take "options insecure1" and "options insecure2" in /etc/resolv.conf,
  which toggles RES_INSECURE[12] option flag bit.
- EDNS0 receive buffer size notification support.  It can be enabled by
  "options edns0" in /etc/resolv.conf.  See USAGE for details.
- IPv6 transport support (queries/responses over IPv6).  Most of BSD official
  releases now has it already.
- Partial A6 chain chasing/DNAME/bit string label support (KAME/BSDI4).


2. Network Drivers

KAME requires three items to be added into the standard drivers:

(1) (freebsd[234] and bsdi[34] only) mbuf clustering requirement.
    In this stable release, we changed MINCLSIZE into MHLEN+1 for all the
    operating systems in order to make all the drivers behave as we expect.  

(2) multicast.  If "ifmcstat" yields no multicast group for a
    interface, that interface has to be patched.

To avoid troubles, we suggest you to comment out the device drivers
for unsupported/unnecessary cards, from the kernel configuration file.
If you accidentally enable unsupported drivers, some of the userland
tools may not work correctly (routing daemons are typical example).

In the following sections, "official support" means that KAME developers
are using that ethernet card/driver frequently.

(NOTE: In the past we required all pcmcia drivers to have a call to
in6_ifattach().  We have no such requirement any more)

2.1 FreeBSD 2.2.x-RELEASE

Here is a list of FreeBSD 2.2.x-RELEASE drivers and its conditions:

        driver  mbuf(1)         multicast(2)    official support?
        ---     ---             ---             ---
        (Ethernet)
        ar      looks ok        -               -
        cnw     ok              ok              yes (*)
        ed      ok              ok              yes
        ep      ok              ok              yes
        fe      ok              ok              yes
        sn      looks ok        -               -   (*)
        vx      looks ok        -               -
        wlp     ok              ok              -   (*)
        xl      ok              ok              yes
        zp      ok              ok              -
        (FDDI)
        fpa     looks ok        ?               -
        (ATM)
        en      ok              ok              yes
        (Serial)
        lp      ?               -               not work
        sl      ?               -               not work
        sr      looks ok        ok              -   (**)

You may want to add an invocation of "rtsol" in "/etc/pccard_ether",
if you are using notebook computers and PCMCIA ethernet card.

(*) These drivers are distributed with PAO (http://www.jp.freebsd.org/PAO/).

(**) There was some report says that, if you make sr driver up and down and
then up, the kernel may hang up.  We have disabled frame-relay support from
sr driver and after that this looks to be working fine.  If you need
frame-relay support to come back, please contact KAME developers.

2.2 BSD/OS 3.x

The following lists BSD/OS 3.x device drivers and its conditions:

        driver  mbuf(1)         multicast(2)    official support?
        ---     ---             ---             ---
        (Ethernet)
        cnw     ok              ok              yes
        de      ok              ok              -
        df      ok              ok              -
        eb      ok              ok              -
        ef      ok              ok              yes
        exp     ok              ok              -
        mz      ok              ok              yes
        ne      ok              ok              yes
        we      ok              ok              -
        (FDDI)
        fpa     ok              ok              -
        (ATM)
        en      maybe           ok              -
        (Serial)
        ntwo    ok              ok              yes
        sl      ?               -               not work
        appp    ?               -               not work

You may want to use "@insert" directive in /etc/pccard.conf to invoke
"rtsol" command right after dynamic insertion of PCMCIA ethernet cards.

2.3 NetBSD

The following table lists the network drivers we have tried so far.

        driver          mbuf(1) multicast(2)    official support?
        ---             ---     ---             ---
        (Ethernet)
        awi pcmcia/i386 ok      ok              -
        bah zbus/amiga  NG(*)
        cnw pcmcia/i386 ok      ok              yes
        ep pcmcia/i386  ok      ok              -
        fxp pci/i386    ok(*2)  ok              -
        tlp pci/i386    ok      ok              -
        le sbus/sparc   ok      ok              yes
        ne pci/i386     ok      ok              yes
        ne pcmcia/i386  ok      ok              yes
        rtk pci/i386    ok      ok              -
        wi pcmcia/i386  ok      ok              yes
        (ATM)
        en pci/i386     ok      ok              -

(*) This may need some fix, but I'm not sure what arcnet interfaces assume...

2.4 FreeBSD 3.x-RELEASE

Here is a list of FreeBSD 3.x-RELEASE drivers and its conditions:

        driver  mbuf(1)         multicast(2)    official support?
        ---     ---             ---             ---
        (Ethernet)
        cnw     ok              ok              -(*)
        ed      ?               ok              -
        ep      ok              ok              -
        fe      ok              ok              yes
        fxp     ?(**)
        lnc     ?               ok              -
        sn      ?               ?               -(*)
        wi      ok              ok              yes
        xl      ?               ok              -

(*) These drivers are distributed with PAO as PAO3
    (http://www.jp.freebsd.org/PAO/).
(**) there were trouble reports with multicast filter initialization.

More drivers will just simply work on KAME FreeBSD 3.x-RELEASE but have not
been checked yet.

2.5 FreeBSD 4.x-RELEASE

Here is a list of FreeBSD 4.x-RELEASE drivers and its conditions:

        driver          multicast
        ---             ---
        (Ethernet)
        lnc/vmware      ok

2.6 OpenBSD 2.x

Here is a list of OpenBSD 2.x drivers and its conditions:

        driver          mbuf(1)         multicast(2)    official support?
        ---             ---             ---             ---
        (Ethernet)
        de pci/i386     ok              ok              yes
        fxp pci/i386    ?(*)
        le sbus/sparc   ok              ok              yes
        ne pci/i386     ok              ok              yes
        ne pcmcia/i386  ok              ok              yes
        wi pcmcia/i386  ok              ok              yes

(*) There seem to be some problem in driver, with multicast filter
configuration.  This happens with certain revision of chipset on the card.
Should be fixed by now by workaround in sys/net/if.c, but still not sure.

2.7 BSD/OS 4.x

The following lists BSD/OS 4.x device drivers and its conditions:

        driver  mbuf(1)         multicast(2)    official support?
        ---     ---             ---             ---
        (Ethernet)
        de      ok              ok              yes
        exp     (*)

You may want to use "@insert" directive in /etc/pccard.conf to invoke
"rtsol" command right after dynamic insertion of PCMCIA ethernet cards.

(*) exp driver has serious conflict with KAME initialization sequence.
A workaround is committed into sys/i386/pci/if_exp.c, and should be okay by now.


3. Translator

We categorize IPv4/IPv6 translator into 4 types.

Translator A --- It is used in the early stage of transition to make
it possible to establish a connection from an IPv6 host in an IPv6
island to an IPv4 host in the IPv4 ocean.

Translator B --- It is used in the early stage of transition to make
it possible to establish a connection from an IPv4 host in the IPv4
ocean to an IPv6 host in an IPv6 island.

Translator C --- It is used in the late stage of transition to make it
possible to establish a connection from an IPv4 host in an IPv4 island
to an IPv6 host in the IPv6 ocean.

Translator D --- It is used in the late stage of transition to make it
possible to establish a connection from an IPv6 host in the IPv6 ocean
to an IPv4 host in an IPv4 island.

KAME provides an TCP relay translator for category A.  This is called
"FAITH".  We also provide IP header translator for category A.

3.1 FAITH TCP relay translator

FAITH system uses TCP relay daemon called "faithd" helped by the KAME kernel.
FAITH will reserve an IPv6 address prefix, and relay TCP connection
toward that prefix to IPv4 destination.

For example, if the reserved IPv6 prefix is 3ffe:0501:0200:ffff::, and
the IPv6 destination for TCP connection is 3ffe:0501:0200:ffff::163.221.202.12,
the connection will be relayed toward IPv4 destination 163.221.202.12.

        destination IPv4 node (163.221.202.12)
          ^
          | IPv4 tcp toward 163.221.202.12
        FAITH-relay dual stack node
          ^
          | IPv6 TCP toward 3ffe:0501:0200:ffff::163.221.202.12
        source IPv6 node

faithd must be invoked on FAITH-relay dual stack node.

For more details, consult kame/kame/faithd/README and RFC3142.

3.2 IPv6-to-IPv4 header translator

(to be written)


4. IPsec

IPsec is implemented as the following three components.

(1) Policy Management
(2) Key Management
(3) AH, ESP and IPComp handling in kernel

Note that KAME/OpenBSD does NOT include support for KAME IPsec code,
as OpenBSD team has their home-brew IPsec stack and they have no plan
to replace it.  IPv6 support for IPsec is, therefore, lacking on KAME/OpenBSD.

http://www.netbsd.org/Documentation/network/ipsec/ has more information
including usage examples.

4.1 Policy Management

The kernel implements experimental policy management code.  There are two ways
to manage security policy.  One is to configure per-socket policy using
setsockopt(3).  In this cases, policy configuration is described in
ipsec_set_policy(3).  The other is to configure kernel packet filter-based
policy using PF_KEY interface, via setkey(8).

The policy entry will be matched in order.  The order of entries makes
difference in behavior.

4.2 Key Management

The key management code implemented in this kit (sys/netkey) is a
home-brew PFKEY v2 implementation.  This conforms to RFC2367.

The home-brew IKE daemon, "racoon" is included in the kit (kame/kame/racoon,
or usr.sbin/racoon).
Basically you'll need to run racoon as daemon, then setup a policy
to require keys (like ping -P 'out ipsec esp/transport//use').
The kernel will contact racoon daemon as necessary to exchange keys.

In IKE spec, there's ambiguity about interpretation of "tunnel" proposal.
For example, if we would like to propose the use of following packet:
        IP AH ESP IP payload
some implementation proposes it as "AH transport and ESP tunnel", since
this is more logical from packet construction point of view.  Some
implementation proposes it as "AH tunnel and ESP tunnel".
Racoon follows the latter route (previously it followed the former, and
the latter interpretation seems to be popular/consensus).
This raises real interoperability issue.  We hope this to be resolved quickly.

racoon does not implement byte lifetime for both phase 1 and phase 2
(RFC2409 page 35, Life Type = kilobytes).

4.3 AH and ESP handling

IPsec module is implemented as "hooks" to the standard IPv4/IPv6
processing.  When sending a packet, ip{,6}_output() checks if ESP/AH
processing is required by checking if a matching SPD (Security
Policy Database) is found.  If ESP/AH is needed,
{esp,ah}{4,6}_output() will be called and mbuf will be updated
accordingly.  When a packet is received, {esp,ah}4_input() will be
called based on protocol number, i.e. (*inetsw[proto])().
{esp,ah}4_input() will decrypt/check authenticity of the packet,
and strips off daisy-chained header and padding for ESP/AH.  It is
safe to strip off the ESP/AH header on packet reception, since we
will never use the received packet in "as is" form.

By using ESP/AH, TCP4/6 effective data segment size will be affected by
extra daisy-chained headers inserted by ESP/AH.  Our code takes care of
the case.

Basic crypto functions can be found in directory "sys/crypto".  ESP/AH
transform are listed in {esp,ah}_core.c with wrapper functions.  If you
wish to add some algorithm, add wrapper function in {esp,ah}_core.c, and
add your crypto algorithm code into sys/crypto.

Tunnel mode works basically fine, but comes with the following restrictions:
- You cannot run routing daemon across IPsec tunnel, since we do not model
  IPsec tunnel as pseudo interfaces.
- Authentication model for AH tunnel must be revisited.  We'll need to
  improve the policy management engine, eventually.
- Path MTU discovery does not work across IPv6 IPsec tunnel gateway due to
  insufficient code.

AH specification does not talk much about "multiple AH on a packet" case.
We incrementally compute AH checksum, from inside to outside.  Also, we
treat inner AH to be immutable.
For example, if we are to create the following packet:
        IP AH1 AH2 AH3 payload
we do it incrementally.  As a result, we get crypto checksums like below:
        AH3 has checksum against "IP AH3' payload".
                where AH3' = AH3 with checksum field filled with 0.
        AH2 has checksum against "IP AH2' AH3 payload".
        AH1 has checksum against "IP AH1' AH2 AH3 payload",
Also note that AH3 has the smallest sequence number, and AH1 has the largest
sequence number.

To avoid traffic analysis on shorter packets, ESP output logic supports
random length padding.  By setting net.inet.ipsec.esp_randpad (or
net.inet6.ipsec6.esp_randpad) to positive value N, you can ask the kernel
to randomly pad packets shorter than N bytes, to random length smaller than
or equal to N.  Note that N does not include ESP authentication data length.
Also note that the random padding is not included in TCP segment
size computation.  Negative value will turn off the functionality.
Recommended value for N is like 128, or 256.  If you use a too big number
as N, you may experience inefficiency due to fragmented packets.

4.4 IPComp handling

IPComp stands for IP payload compression protocol.  This is aimed for
payload compression, not the header compression like PPP VJ compression.
This may be useful when you are using slow serial link (say, cell phone)
with powerful CPU (well, recent notebook PCs are really powerful...).
The protocol design of IPComp is very similar to IPsec, though it was
defined separately from IPsec itself.

Here are some points to be noted:
- IPComp is treated as part of IPsec protocol suite, and SPI and
  CPI space is unified.  Spec says that there's no relationship
  between two so they are assumed to be separate in specs.
- IPComp association (IPCA) is kept in SAD.
- It is possible to use well-known CPI (CPI=2 for DEFLATE for example),
  for outbound/inbound packet, but for indexing purposes one element from
  SPI/CPI space will be occupied anyway.
- pfkey is modified to support IPComp.  However, there's no official
  SA type number assignment yet.  Portability with other IPComp
  stack is questionable (anyway, who else implement IPComp on UN*X?).
- Spec says that IPComp output processing must be performed before AH/ESP
  output processing, to achieve better compression ratio and "stir" data
  stream before encryption.  The most meaningful processing order is:
  (1) compress payload by IPComp, (2) encrypt payload by ESP, then (3) attach
  authentication data by AH.
  However, with manual SPD setting, you are able to violate the ordering
  (KAME code is too generic, maybe).  Also, it is just okay to use IPComp
  alone, without AH/ESP.
- Though the packet size can be significantly decreased by using IPComp, no
  special consideration is made about path MTU (spec talks nothing about MTU
  consideration).  IPComp is designed for serial links, not ethernet-like
  medium, it seems.
- You can change compression ratio on outbound packet, by changing
  deflate_policy in sys/netinet6/ipcomp_core.c.  You can also change outbound
  history buffer size by changing deflate_window_out in the same source code.
  (should it be sysctl accessible, or per-SAD configurable?)
- Tunnel mode IPComp is not working right.  KAME box can generate tunnelled
  IPComp packet, however, cannot accept tunneled IPComp packet.
- You can negotiate IPComp association with racoon IKE daemon.
- KAME code does not attach Adler32 checksum to compressed data.
  see ipsec wg mailing list discussion in Jan 2000 for details.

4.5 Conformance to RFCs and IDs

The IPsec code in the kernel conforms (or, tries to conform) to the
following standards:
    "old IPsec" specification documented in rfc182[5-9].txt
    "new IPsec" specification documented in:
        rfc240[1-6].txt rfc241[01].txt rfc2451.txt rfc3602.txt
    IPComp:
        RFC2393: IP Payload Compression Protocol (IPComp)
IKE specifications (rfc240[7-9].txt) are implemented in userland
as "racoon" IKE daemon.

Currently supported algorithms are:
    old IPsec AH
        null crypto checksum (no document, just for debugging)
        keyed MD5 with 128bit crypto checksum (rfc1828.txt)
        keyed SHA1 with 128bit crypto checksum (no document)
        HMAC MD5 with 128bit crypto checksum (rfc2085.txt)
        HMAC SHA1 with 128bit crypto checksum (no document)
        HMAC RIPEMD160 with 128bit crypto checksum (no document)
    old IPsec ESP
        null encryption (no document, similar to rfc2410.txt)
        DES-CBC mode (rfc1829.txt)
    new IPsec AH
        null crypto checksum (no document, just for debugging)
        keyed MD5 with 96bit crypto checksum (no document)
        keyed SHA1 with 96bit crypto checksum (no document)
        HMAC MD5 with 96bit crypto checksum (rfc2403.txt
        HMAC SHA1 with 96bit crypto checksum (rfc2404.txt)
        HMAC SHA2-256 with 96bit crypto checksum (draft-ietf-ipsec-ciph-sha-256-00.txt)
        HMAC SHA2-384 with 96bit crypto checksum (no document)
        HMAC SHA2-512 with 96bit crypto checksum (no document)
        HMAC RIPEMD160 with 96bit crypto checksum (RFC2857)
        AES XCBC MAC with 96bit crypto checksum (RFC3566)
    new IPsec ESP
        null encryption (rfc2410.txt)
        DES-CBC with derived IV
                (draft-ietf-ipsec-ciph-des-derived-01.txt, draft expired)
        DES-CBC with explicit IV (rfc2405.txt)
        3DES-CBC with explicit IV (rfc2451.txt)
        BLOWFISH CBC (rfc2451.txt)
        CAST128 CBC (rfc2451.txt)
        RIJNDAEL/AES CBC (rfc3602.txt)
        AES counter mode (rfc3686.txt)

        each of the above can be combined with new IPsec AH schemes for
        ESP authentication.
    IPComp
        RFC2394: IP Payload Compression Using DEFLATE

The following algorithms are NOT supported:
    old IPsec AH
        HMAC MD5 with 128bit crypto checksum + 64bit replay prevention
                (rfc2085.txt)
        keyed SHA1 with 160bit crypto checksum + 32bit padding (rfc1852.txt)

The key/policy management API is based on the following document, with fair
amount of extensions:
        RFC2367: PF_KEY key management API

4.6 ECN consideration on IPsec tunnels

KAME IPsec implements ECN-friendly IPsec tunnel, described in
draft-ietf-ipsec-ecn-02.txt.
Normal IPsec tunnel is described in RFC2401.  On encapsulation,
IPv4 TOS field (or, IPv6 traffic class field) will be copied from inner
IP header to outer IP header.  On decapsulation outer IP header
will be simply dropped.  The decapsulation rule is not compatible
with ECN, since ECN bit on the outer IP TOS/traffic class field will be
lost.
To make IPsec tunnel ECN-friendly, we should modify encapsulation
and decapsulation procedure.  This is described in
draft-ietf-ipsec-ecn-02.txt, chapter 3.3.

KAME IPsec tunnel implementation can give you three behaviors, by setting
net.inet.ipsec.ecn (or net.inet6.ipsec6.ecn) to some value:
- RFC2401: no consideration for ECN (sysctl value -1)
- ECN forbidden (sysctl value 0)
- ECN allowed (sysctl value 1)
Note that the behavior is configurable in per-node manner, not per-SA manner
(draft-ietf-ipsec-ecn-02 wants per-SA configuration, but it looks too much
for me).

The behavior is summarized as follows (see source code for more detail):

                encapsulate                     decapsulate
                ---                             ---
RFC2401         copy all TOS bits               drop TOS bits on outer
                from inner to outer.            (use inner TOS bits as is)

ECN forbidden   copy TOS bits except for ECN    drop TOS bits on outer
                (masked with 0xfc) from inner   (use inner TOS bits as is)
                to outer.  set ECN bits to 0.

ECN allowed     copy TOS bits except for ECN    use inner TOS bits with some
                CE (masked with 0xfe) from      change.  if outer ECN CE bit
                inner to outer.                 is 1, enable ECN CE bit on
                set ECN CE bit to 0.            the inner.

General strategy for configuration is as follows:
- if both IPsec tunnel endpoint are capable of ECN-friendly behavior,
  you'd better configure both end to "ECN allowed" (sysctl value 1).
- if the other end is very strict about TOS bit, use "RFC2401"
  (sysctl value -1).
- in other cases, use "ECN forbidden" (sysctl value 0).
The default behavior is "ECN forbidden" (sysctl value 0).

For more information, please refer to:
        draft-ietf-ipsec-ecn-02.txt
        RFC2481 (Explicit Congestion Notification)
        KAME sys/netinet6/{ah,esp}_input.c

(Thanks goes to Kenjiro Cho <kjc@csl.sony.co.jp> for detailed analysis)

4.7 Interoperability

IPsec, IPComp (in kernel) and IKE (in userland as "racoon") has been tested
at several interoperability test events, and it is known to interoperate
with many other implementations well.  Also, KAME IPsec has quite wide
coverage for IPsec crypto algorithms documented in RFC (we do not cover
algorithms with intellectual property issues, though).

Here are (some of) platforms we have tested IPsec/IKE interoperability
in the past, no particular order.  Note that both ends (KAME and
others) may have modified their implementation, so use the following
list just for reference purposes.
        6WIND, ACC, Allied-telesis, Altiga, Ashley-laurent (vpcom.com),
        BlueSteel, CISCO IOS, Checkpoint FW-1, Compaq Tru54 UNIX
        X5.1B-BL4, Cryptek, Data Fellows (F-Secure), Ericsson,
        F-Secure VPN+ 5.40, Fitec, Fitel, FreeS/WAN, HITACHI, HiFn,
        IBM AIX 5.1, III, IIJ (fujie stack), Intel Canada, Intel
        Packet Protect, MEW NetCocoon, MGCS, Microsoft WinNT/2000/XP,
        NAI PGPnet, NEC IX5000, NIST (linux IPsec + plutoplus),
        NetLock, Netoctave, Netopia, Netscreen, Nokia EPOC, Nortel
        GatewayController/CallServer 2000 (not released yet),
        NxNetworks, OpenBSD isakmpd on OpenBSD, Oullim information
        technologies SECUREWORKS VPN gateway 3.0, Pivotal, RSA,
        Radguard, RapidStream, RedCreek, Routerware, SSH, SecGo
        CryptoIP v3, Secure Computing, Soliton, Sun Solaris 8,
        TIS/NAI Gauntret, Toshiba, Trilogy AdmitOne 2.6, Trustworks
        TrustedClient v3.2, USAGI linux, VPNet, Yamaha RT series,
        ZyXEL

Here are (some of) platforms we have tested IPComp/IKE interoperability
in the past, in no particular order.
        Compaq, IRE, SSH, NetLock, FreeS/WAN, F-Secure VPN+ 5.40

VPNC (vpnc.org) provides IPsec conformance tests, using KAME and OpenBSD
IPsec/IKE implementations.  Their test results are available at
http://www.vpnc.org/conformance.html, and it may give you more idea
about which implementation interoperates with KAME IPsec/IKE implementation.

4.8 Operations with IPsec tunnel mode

First of all, IPsec tunnel is a very hairy thing.  It seems to do a neat thing
like VPN configuration or secure remote accesses, however, it comes with lots
of architectural twists.

RFC2401 defines IPsec tunnel mode, within the context of IPsec.  RFC2401
defines tunnel mode packet encapsulation/decapsulation on its own, and
does not refer other tunnelling specifications.  Since RFC2401 advocates
filter-based SPD database matches, it would be natural for us to implement
IPsec tunnel mode as filters - not as pseudo interfaces.

There are some people who are trying to separate IPsec "tunnel mode" from
the IPsec itself.  They would like to implement IPsec transport mode only,
and combine it with tunneling pseudo devices.  The prime example is found
in draft-touch-ipsec-vpn-01.txt.  However, if you really define pseudo
interfaces separately from IPsec, IKE daemons would need to negotiate
transport mode SAs, instead of tunnel mode SAs.  Therefore, we cannot
really mix RFC2401-based interpretation and draft-touch-ipsec-vpn-01.txt
interpretation.

The KAME stack implements can be configured in two ways.  You may need
to recompile your kernel to switch the behavior.
- RFC2401 IPsec tunnel mode approach (4.8.1)
- draft-touch-ipsec-vpn approach (4.8.2)
        Works in all kernel configuration, but racoon(8) may not interoperate.

There are pros and cons on these approaches:

RFC2401 IPsec tunnel mode (filter-like) approach
        PRO: SPD lookup fits nicely with packet filters (if you integrate them)
        CON: cannot run routing daemons across IPsec tunnels
        CON: it is very hard to control source address selection on originating
                cases
        ???: IPv6 scope zone is kept the same
draft-touch-ipsec-vpn (transportmode + Pseudo-interface) approach
        PRO: run routing daemons across IPsec tunnels
        PRO: source address selection can be done normally, by looking at
                IPsec tunnel pseudo devices
        CON: on outbound, possibility of infinite loops if routing setup
                is wrong
        CON: due to differences in encap/decap logic from RFC2401, it may not
                interoperate with very picky RFC2401 implementations
                (those who check TOS bits, for example)
        CON: cannot negotiate IKE with other IPsec tunnel-mode devices
                (the other end has to implement 
        ???: IPv6 scope zone is likely to be different from the real ethernet
                interface

The recommendation is different depending on the situation you have:
- use draft-touch-ipsec-vpn if you have the control over the other end.
  this one is the best in terms of simplicity.
- if the other end is normal IPsec device with RFC2401 implementation,
  you need to use RFC2401, otherwise you won't be able to run IKE.
- use RFC2401 approach if you just want to forward packets back and forth
  and there's no plan to use IPsec gateway itself as an originating device.

4.8.1 RFC2401 IPsec tunnel mode approach

To configure your device as RFC2401 IPsec tunnel mode endpoint, you will
use "tunnel" keyword in setkey(8) "spdadd" directives.  Let us assume the
following topology (A and B could be a network, like prefix/length):

        ((((((((((((The internet))))))))))))
          |                       |
          |C (global)             |D
        your device             peer's device
          |A (private)            |B
        ==+===== VPN net        ==+===== VPN net

The policy configuration directive is like this.  You will need manual
SAs, or IKE daemon, for actual encryption:

        # setkey -c <<EOF
        spdadd A B any -P out ipsec esp/tunnel/C-D/use;
        spdadd B A any -P in ipsec esp/tunnel/D-C/use;
        ^D

The inbound/outbound traffic is monitored/captured by SPD engine, which works
just like packet filters.

With this, forwarding case should work flawlessly.  However, troubles arise
when you have one of the following requirements:
- When you originate traffic from your VPN gateway device to VPN net on the
  other end (like B), you want your source address to be A (private side)
  so that the traffic would be protected by the policy.
  With this approach, however, the source address selection logic follows
  normal routing table, and C (global side) will be picked for any outgoing
  traffic, even if the destination is B.  The resulting packet will be like
  this:
        IP[C -> B] payload
  and will not match the policy (= sent in clear).
- When you want to run routing protocols on top of the IPsec tunnel, it is
  not possible.  As there is no pseudo device that identifies the IPsec tunnel,
  you cannot identify where the routing information came from.  As a result,
  you can't run routing daemons.

4.8.2 draft-touch-ipsec-vpn approach

With this approach, you will configure gif(4) tunnel interfaces, as well as
IPsec transport mode SAs.

        # gifconfig gif0 C D
        # ifconfig gif0 A B
        # setkey -c <<EOF
        spdadd C D any -P out ipsec esp/transport//use;
        spdadd D C any -P in ipsec esp/transport//use;
        ^D

Since we have a pseudo-interface "gif0", and it affects the routes and
the source address selection logic, we can have source address A, for
packets originated by the VPN gateway to B (and the VPN cloud).
We can also exchange routing information over the tunnel (gif0), as the tunnel
is represented as a pseudo interface (dynamic routes points to the
pseudo interface).

There is a big drawbacks, however; with this, you can use IKE if and only if
the other end is using draft-touch-ipsec-vpn approach too.  Since racoon(8)
grabs phase 2 IKE proposals from the kernel SPD database, you will be
negotiating IPsec transport-mode SAs with the other end, not tunnel-mode SAs.
Also, since the encapsulation mechanism is different from RFC2401, you may not
be able to interoperate with a picky RFC2401 implementations - if the other
end checks certain outer IP header fields (like TOS), you will not be able to
interoperate.


5. ALTQ

KAME kit includes ALTQ, which supports FreeBSD3, FreeBSD4, FreeBSD5
NetBSD.  OpenBSD has ALTQ merged into pf and its ALTQ code is not
compatible with other platforms so that KAME's ALTQ is not used for
OpenBSD.  For BSD/OS, ALTQ does not work.
ALTQ in KAME supports IPv6.
(actually, ALTQ is developed on KAME repository since ALTQ 2.1 - Jan 2000)

ALTQ occupies single character device number.  For FreeBSD, it is officially
allocated.  For OpenBSD and NetBSD, we use the number which is not
currently allocated (will eventually get an official number).
The character device is enabled for i386 architecture only.  To enable and
compile ALTQ-ready kernel for other architectures, take the following steps:
- assume that your architecture is FOOBAA.
- modify sys/arch/FOOBAA/FOOBAA/conf.c (or somewhere that defines cdevsw),
  to include a line for ALTQ.  look at sys/arch/i386/i386/conf.c for
  example.  The major number must be same as i386 case.
- copy kernel configuration file (like ALTQ.v6 or GENERIC.v6) from i386,
  and modify accordingly.
- build a kernel.
- before building userland, change netbsd/{lib,usr.sbin,usr.bin}/Makefile
  (or openbsd/foobaa) so that it will visit altq-related sub directories.


6. Mobile IPv6

6.1 KAME node as correspondent node

Default installation recognizes home address option (in destination
options header).  No sub-options are supported.  Interaction with
IPsec, and/or 2292bis API, needs further study.

6.2 KAME node as home agent/mobile node

KAME kit includes Ericsson mobile-ip6 code.  The integration is just started
(in Feb 2000), and we will need some more time to integrate it better.

See kame/mip6config/{QUICKSTART,README_MIP6.txt} for more details.

The Ericsson code implements revision 09 of the mobile-ip6 draft.  There
are other implementations available:
        NEC: http://www.6bone.nec.co.jp/mipv6/internal-dist/ (-13 draft)
        SFC: http://neo.sfc.wide.ad.jp/~mip6/ (-13 draft)

7. Coding style

The KAME developers basically do not make a bother about coding
style.  However, there is still some agreement on the style, in order
to make the distributed development smooth.

- follow *BSD KNF where possible.  note: there are multiple KNF standards.
- the tab character should be 8 columns wide (tabstops are at 8, 16, 24, ...
  column).  With vi, use ":set ts=8 sw=8".
  With GNU Emacs 20 and later, the easiest way is to use the "bsd" style of
  cc-mode with the variable "c-basic-offset" being 8;
  (add-hook 'c-mode-common-hook
            (function
             (lambda ()
               (c-set-style "bsd")
               (setq c-basic-offset 8)  ; XXX for Emacs 20 only
               )))
  The "bsd" style in GNU Emacs 21 sets the variable to 8 by default,
  so the line marked by "XXX" is not necessary if you only use GNU
  Emacs 21.
- each line should be within 80 characters.
- keep a single open/close bracket in a comment such as in the following
  line:
        putchar('(');   /* ) */
  without this, some vi users would have a hard time to match a pair of
  brackets.  Although this type of bracket seems clumsy and is even
  harmful for some other type of vi users and Emacs users, the
  agreement in the KAME developers is to allow it.
- add the following line to the head of every KAME-derived file:
  /*    (dollar)KAME(dollar)    */
  where "(dollar)" is the dollar character ($), and around "$" are tabs.
  (this is for C.  For other language, you should use its own comment
  line.)
  Once committed to the CVS repository, this line will contain its
  version number (see, for example, at the top of this file).  This
  would make it easy to report a bug.
- when creating a new file with the WIDE copyright, tap "make copyright.c" at
  the top-level, and use copyright.c as a template.  KAME RCS tag will be
  included automatically.
- when editing a third-party package, keep its own coding style as
  much as possible, even if the style does not follow the items above.
- it is recommended to always wrap an expression containing
  bitwise operators by parentheses, especially when the expression is
  combined with relational operators, in order to avoid unintentional
  mismatch of operators.  Thus, we should write
        if ((a & b) == 0)       /* (A) */
  or
        if (a & (b == 0))       /* (B) */
  instead of
        if (a & b == 0)         /* (C) */
  even if the programmer's intention was (C), which is equivalent to
  (B) according to the grammar of the language C.
  Thus, we should write a code to test if a bit-flag is set for a
  given variable as follows:
        if ((flag & FLAG_A) == 0)       /* (D) the FLAG_A is NOT set */
        if ((flag & FLAG_A) != 0)       /* (E) the FLAG_A is set */
  Some developers in the KAME project rather prefer the following style:
        if (!(flag & FLAG_A))   /* (F) the FLAG_A is NOT set */
        if ((flag & FLAG_A))    /* (G) the FLAG_A is set */
  because it would be more intuitive in terms of the relationship
  between the negation operator (!) and the semantics of the
  condition.  The KAME developers have discussed the style, and have
  agreed that all the styles from (D) to (G) are valid.  So, when you
  see styles like (D) and (E) in the KAME code and feel a bit strange,
  please just keep them.  They are intentional.
- When inserting a separate block just to define some intra-block
  variables, add the level of indentation as if the block was in a
  control statement such as if-else, for, or while.  For example,
        foo ()
        {
                int a;

                {
                        int internal_a;
                        ...
                }
        }
  should be used, instead of
        foo ()
        {
                int a;

            {
                int internal_a;
                ...
             }
        }
- Do not use printf() or log() in the packet input path of the kernel code.
  They can make the system vulnerable to packet flooding attacks (results in
  /var overflow).
- (not a style issue)
  To disable a module that is mistakenly imported (by CVS), just
  remove the source tree in the repository.  Note, however, that the
  removal might annoy other developers who have already checked the
  module out, so you should announce the removal as soon as possible.
  Also, be 100% sure not to remove other modules.

When you want to contribute something to the KAME project, and if *you
do not mind* the agreement, it would be helpful for the project to
keep these rules.  Note, however, that we would never intend to force
you to adopt our rules.  We would rather regard your own style,
especially when you have a policy about the style.


8. Policy on technology with intellectual property right restriction

There are quite a few IETF documents/whatever which has intellectual property
right (IPR) restriction.  KAME's stance is stated below.

    The goal of KAME is to provide freely redistributable, BSD-licensed,
    implementation of Internet protocol technologies.
    For this purpose, we implement protocols that (1) do not need license
    contract with IPR holder, and (2) are royalty-free.
    The reason for (1) is, even if KAME contracts with the IPR holder in
    question, the users of KAME stack (usually implementers of some other
    codebase) would need to make a license contract with the IPR holder.
    It would damage the "freely redistributable" status of KAME codebase.

    By doing so KAME is (implicitly) trying to advocate no-license-contract,
    royalty-free, release of IPRs.

Note however, as documented in README, we do not guarantee that KAME code
is free of IPR infringement, you MUST check it if you are to integrate
KAME into your product (or whatever):
    READ CAREFULLY: Several countries have legal enforcement for
    export/import/use of cryptographic software.  Check it before playing
    with the kit.  We do not intend to be your legalese clearing house
    (NO WARRANTY).  If you intend to include KAME stack into your product,
    you'll need to check if the licenses on each file fit your situations,
    and/or possible intellectual property right issues.

                                                 <end of IMPLEMENTATION>