add ipv6 doc

diff --git a/doc/misc/ipv6 b/doc/misc/ipv6
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+$Id: ipv6,v 1.1 2000/05/23 22:28:09 explorer Exp $
+
+Currently, there are multiple interesting problems with ipv6
+implementations on various platforms.  These problems range from not
+being able to use ipv6 with bind9 (or in particular the ISC socket
+library, contained in libisc) to listen-on lists not being respected,
+to strange warnings but seemingly correct behavior of named.
+
+
+COMPILE-TIME ISSUES
+-------------------
+
+The socket library requires a certain level of support from the
+operating system.  In particular, it must follow the advanced ipv6
+socket API to be usable.  The systems which do not follow this will
+currently not get any warnings or errors, but ipv6 will simply not
+function on them.
+
+These systems currently include, but are not limited to:
+
+       AIX 3.4 (with ipv6 patches)
+
+
+RUN-TIME ISSUES
+---------------
+
+In the original drafts of the ipv6 RFC documents, binding an ipv6
+socket to the ipv6 wildcard address would also cause the socket to
+accept ipv4 connections and datagrams.  When an ipv4 packet is
+received on these systems, it is mapped into an ipv6 address.  For
+example, 1.2.3.4 would be mapped into ffff::1.2.3.4.  The intent of
+this mapping was to make transition from an ipv4-only application into
+ipv6 easier, by only requiring one socket to be open on a given port.
+
+Later, it was discovered that this was generally a bad idea.  For one,
+many firewalls will block connection to 1.2.3.4, but will let through
+ffff::1.2.3.4.  This, of course, is bad.  Also, access controll lists
+written to accept only ipv4 addresses were suddenly ignored unless
+they were rewritten to handle the ipv6 mapped addresses as well.
+
+In bind9, we always bind to the ipv6 wildcard port for both TCP and
+UDP, and specific addresses for ipv4 sockets.  This causes some
+interesting behavior depending on the system implementation of ipv6.
+
+
+IPV6 Accepts IPV4, Specific IPV4 Addresses Bindings Fail
+--------------------------------------------------------
+
+The only OS which seems to do this is linux.  If an ipv6 socket is
+bound to the ipv6 wildcard socket, and a specific ipv4 socket is
+later bound (say, to 10.2.3.4 port 53) the ipv4 binding will fail.
+
+What this means to bind9 is that the application will log warnings
+about being unable to bind to a socket because the address is already
+in use.  Since the ipv6 socket will accept ipv4 packets and map them,
+however, the ipv4 addresses continue to function.
+
+The effect is that the config file listen-on directive will not be
+respected on these systems.
+
+
+IPV6 Accepts IPV4, Specific IPV4 Address Bindings Succeed
+---------------------------------------------------------
+
+In this case, the system allows opening an ipv6 wildcard address
+socket and then binding to a more specific ipv4 address later.  An
+example of this type of system is Digital Unix with ipv6 patches
+applied.
+
+What this means to bind9 is that the application will respect
+listen-on in regards to ipv4 sockets, but it will use mapped ipv6
+addresses for any that do not match a listen-on address.  This, in
+effect, makes listen-on useless for these machines as well.
+
+
+IPV6 Does Not Accept IPV4
+-------------------------
+
+On these systems, opening an IPV6 socket does not implicitly open any
+ipv4 sockets.  An example of these systems are NetBSD-current with the
+latest KAME patch, and other systems which use the latest KAME patches
+as their ipv6 implementation.
+
+On these sytems, listen-on is fully functional, as the ipv6 socket
+only accepts ipv6 packets, and the ipv4 sockets will handle the ipv4
+packets.
+
+
+RELEVANT RFCs
+-------------
+
+2373:  IP Version 6 Addressing Architecture
+
+2553:  Basic Socket Interface Extensions for IPv6
+
+draft-ietf-ipngwg-rfc2292bis-01:  Advanced Sockets API for IPv6 (draft)