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IMPLEMENTATION
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IMPLEMENTATION
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Implementation Note
KAME Project
http://www.kame.net/
$KAME: IMPLEMENTATION,v 1.222 2001/06/01 05:47:01 jinmei Exp $
NOTE: The document tries to describe behaviors/implementation choices
of the latest KAME/*BSD stack (like KAME/NetBSD 1.5). The description
here may not be applicable to KAME-integrated *BSD releases (like stock
NetBSD 1.5.1), as we have certain amount of changes between them. Still,
some of the content can be useful for KAME-integrated *BSD releases.
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
* For supported library functions/kernel APIs, see sys/netinet6/ADVAPI.
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 1.8 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: 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]x
and KAME/BSDI4,
- supported but turned off by default on KAME/NetBSD,
- not supported on KAME/FreeBSD228, KAME/OpenBSD and KAME/BSDI3.
see 1.12 in this document for details.
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))
RFC2766: Network Address Translation - Protocol Translation (NAT-PT)
* Section 4.2 is implemented by totd (see ports/totd, or pkgsrc/net/totd).
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.
draft-ietf-ipngwg-icmp-name-lookups-07: IPv6 Name Lookups Through ICMP
draft-ietf-dhc-dhcpv6-15.txt: DHCPv6
draft-ietf-dhc-dhcpv6exts-12.txt: Extensions for DHCPv6
* kame/dhcp6 has test implementation, which will not be compiled in
default compilation.
* 15/12 drafts are not explicit about padding and string termination.
at IETF48, the author confirmed that there's no padding/termination
(and extensions can appear unaligned). our code follows the comment.
draft-ietf-ipngwg-rfc2553bis-03.txt:
Basic Socket Interface Extensions for IPv6 (revised)
draft-ietf-ipngwg-rfc2292bis-02.txt:
Advanced Sockets API for IPv6 (revised)
* Some of the updates in the draft are not implemented yet. See
TODO.2292bis for more details.
draft-ietf-mobileip-ipv6-13.txt: Mobility Support in IPv6
* See section 6.
draft-ietf-ngtrans-tcpudp-relay-04.txt:
An IPv6-to-IPv4 transport relay translator
* FAITH tcp relay translator (faithd) implements this. See 3.1 for more
details.
draft-ietf-ipngwg-router-selection-01.txt:
Default Router Preferences and More-Specific Routes
* router-side: both router preference and specific routes are supported.
* host-side: only router preferences is supported.
draft-ietf-ipngwg-scoping-arch-02.txt:
The architecture, text representation, and usage of IPv6
scoped addresses.
* some part of the documentation (especially about the routing
model) is not supported yet.
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-itojun-ipv6-tcp-to-anycast-00.txt:
Disconnecting TCP connection toward IPv6 anycast address
draft-itojun-ipv6-transition-abuse-02.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-tclass-api-02.txt: Socket API for IPv6 traffic class field
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
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 Unsolicited Neighbor
Advertisement transmission command as admin 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.
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 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 a inbound packet,
and reject it if it is from the node itself.
You may also want to look at #if condition marked "heuristics" in
sys/netinet6/nd6_nbr.c:nd6_dad_timer() as workaround (note that the code
fragment in "heuristics" section is not spec conformant).
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 behavior in RFC2461 section 5.2
(when default router list is empty), the kernel needs to know the default
outgoing interface. To configure the default outgoing interface, use
commands like "ndp -I de0" as root. Note that the spec misuse the word
"host" and "node" in several places in the section.
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 KAME team and/or modify
nd6_maxndopt in sys/netinet6/nd6.c. If there are high demands we may
provide 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 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 2292bis API, and (2) hints from tcp_input.
It is questionable if they are really trustworthy. For example, a rogue
userland program can use IPV6_REACHCONF to confuse ND process. Neighbor
cache is a system-wide information pool, and it is bad to allow single process
to affect others. Also, tcp_input can be hosed by hijack attempts. It is
wrong to allow hijack attempts to affect ND process.
Starting June 2000, ND code has a protection mechanism against incorrect
upper-layer reachability confirmation. ND code counts subsequent upper-layer
hints. If the number of hints reaches maximum, 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 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 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 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 link 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
draft-ietf-ipngwg-scoping-arch-xx.txt. 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 (both 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.
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 draft-ietf-ipngwg-rfc2292bis-xx) 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 and
draft-ietf-ipngwg-rfc2553bis-xx.txt. 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 draft-ietf-ipngwg-rfc2553bis-xx.txt. 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. Thus, we recommend you to use the extended format described
above.
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 net.inet6.ip6.accept_rtadv
(host autoconfigures itself if it is set to 1).
By autoconfiguration, network address prefix for the receiving interface
(usually global address prefix) is 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 great
for nomadic usage (notebooks/laptops). If one wishes to ignore Router
Advertisements, use sysctl to set net.inet6.ip6.accept_rtadv to 0.
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 single network interface (except loopback)
This is therefore unwise to enable net.inet6.ip6.accept_rtadv on routers,
or multi-interface host. 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 host has single
interface only, autoconfigred host with
multiple interface is out-of-scope)
1 1 invalid, or experimental
(out-of-scope of spec)
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 Source Address Selection
KAME's source address selection takes care of the following
conditions:
- address scope
- outgoing interface
- whether an address is deprecated
- whether an address is temporary (in terms of RFC 3041)
- prefix matching against the destination
Roughly speaking, the selection policy is as follows:
- always use an address that belongs to the same scope zone as the
destination.
- addresses that have equal or larger scope than the scope of the
destination are preferred.
- a deprecated address is not used in new communications if an
alternate (non-deprecated) address is available and has sufficient
scope.
- a temporary address (in terms of RFC 3041 privacy extension) are
preferred to a public address.
- if none of above conditions tie-breaks, addresses assigned on the
outgoing interface are preferred.
- if none of above conditions tie-breaks, one which is longest prefix
matching against the destination is preferred as the last resort.
For instance, ::1 is selected for ff01::1,
fe80::200:f8ff:fe01:6317%ne0 for fe80::2a0:24ff:feab:839b%ne0.
To see how longest-matching works, suppose that
3ffe:501:808:1:200:f8ff:fe01:6317 and 3ffe:2001:9:124:200:f8ff:fe01:6317
are given on the outgoing interface. Then the former is chosen as the
source for the destination 3ffe:501:800::1. Note that even if all
available addresses have smaller scope than the scope of the
destination, we choose one anyway. For example, if we have link-local
and site-local addresses only, we choose a site-local addresses for a
global destination. If the packet is going to break a site boundary,
the boundary router will return an ICMPv6 destination unreachable
error with code 2 - beyond scope of source address.
The precise desripction of the algorithm is quite complicated. To
describe the algorithm, we introduce the following notation:
For a given destination D,
samescope(D): The set of addresses that have the same scope as D.
largerscope(D): The set of addresses that have a larger scope than D.
smallerscope(D): The set of addresses that have a smaller scope than D.
For a given set of addresses A,
DEP(A): the set of deprecated addresses in A.
nonDEP(A): A - DEP(A).
For a given set of addresses A,
tmp(A): the set of preferred temporary-autoconfigured or
manually-configure addresses in A.
Also, the algorithm assumes that the outgoing interface for the
destination D is determined. We call the interface "I".
The algorithm is as follows. Selection proceeds step by step as
described; For example, if an address is selected by item 1, item 2 and
later are not considered at all.
0. If there is no address in the same scope zone as D, just give up;
the packet will not be sent.
1. If we do not prefer temporary addresses, go to 3.
Otherwise, and if tmp(samescope(D)) is not empty,
then choose an address that is on the interface I. If every
address is on I, or every address is on a different interface
from I, choose an arbitrary one provided that an address longest
matching against D is always preferred.
2. If tmp(largerscope(D)) is not empty,
then choose an address that has the smallest scope. If more than one
address has the smallest scope, choose an arbitrary one provided
that addresses on I are always preferred.
3. If nonDEP(samescope(D)) is not empty,
then apply the same logic as of 1.
4. If nonDEP(largerscope(D)) is not empty,
then apply the same logic as of 2.
5. If we do not prefer temporary addresses, go to 7.
Otherwise, and if tmp(DEP(samescope(D))) is not empty,
then choose an address that is on the interface I. If every
address is on I, or every address is on a different interface
from I, choose an arbitrary one provided that an address longest
matching against D is always preferred.
6. If tmp(DEP(largerscope(D))) is not empty,
then choose an address that has the smallest scope. If more than
one address has the smallest scope, choose an arbitrary one provided
that an address on I is always preferred.
7. If DEP(samescope(D)) is not empty,
then apply the same logic as of 5.
8. If DEP(largerscope(D)) is not empty,
then apply the same logic as of 6.
9. If we do not prefer temporary addresses, go to 11.
Otherwise, and if tmp(nonDEP(smallerscope(D))) is not empty,
then choose an address that has the largest scope. If more than
one address has the largest scope, choose an arbitrary one provided
that an address on I is always preferred.
10. If tmp(DEP(smallerscope(D))) is not empty,
then choose an address that has the largest scope. If more than
one address has the largest scope, choose an arbitrary one provided
that an address on I is always preferred.
11. If nonDEP(smallerscope(D)) is not empty,
then apply the same logic as of 9.
12. If DEP(smallerscope(D)) is not empty,
then apply the same logic as of 10.
There exists a document about source address selection
(draft-ietf-ipngwg-default-addr-select-xx.txt). KAME's algorithm
described above takes a similar approach to the document, but there
are some differences. See the document for more details.
There are some cases where we do not use the above rule. One
example is connected TCP session, and we use the address kept in TCP
protocol control block (tcb) as the source.
Another example is source address for Neighbor Advertisement.
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 longest-match 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).
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, RFC2292 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 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 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 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 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