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bin/51669: hosts.allow syntax for IPv6 literals is ... unclear

>Number:         51669
>Category:       bin
>Synopsis:       hosts.allow syntax for IPv6 literals is ... unclear
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Nov 29 09:35:00 +0000 2016
>Release:        NetBSD 7.0
System: NetBSD 7.0 NetBSD 7.0 (GENERIC.201509250726Z) amd64
Architecture: x86_64
Machine: amd64
	The manual page for hosts.allow on NetBSD has this to say
	about how one can specify an address range in IPv6:

    o An expression of the form `ipv6-addr/ipv6-mask' is interpreted
      as masked IPv6 address match, just like masked IPv4 address
      match (see above).  Note that `ipv6-mask' portion must always be

	This doesn't quite add up with the implementation, it seems.
	First off, the thing behind the / isn't an ipv6-mask, it's a
	prefix length.  And the implementation only (?) accepts
	`[ipv6-net/prefixlen]', and the above does not say that the
	brackets are a required part of the syntax.

	Furthermore, this also deviates from the syntax used by both
	FreeBSD and Linux, which specify `[ipv6-net]/prefixlen',
	i.e. the prefixlen is only accepted outside of the brackets.
	The FreeBSD man page is a lot clearer on this specific point:

    o An  expression  of the form `[n:n:n:n:n:n:n:n]/m'	is interpreted
      as a `[net]/prefixlen' pair. A IPv6 host address is  matched  if
      `prefixlen'  bits	 of  `net' is equal to the `prefixlen' bits of
      the  address.   For   example,   the   [net]/prefixlen   pattern
      `[3ffe:505:2:1::]/64'   matches	every  address	in  the	 range
      `3ffe:505:2:1::' through `3ffe:505:2:1:ffff:ffff:ffff:ffff'.

	This appears, modulo an a/an correction, to be identical to
	the documentation on Debian Linux as well.

	Experiment with variations of the above patterns, and see that
	you can only do `[ipv6-net/prefixlen]' syntax on NetBSD.

	Inspect the man pages of those other projects.

	Not supplied here...
	There's of course a compatibility issue if we're going to
	start accepting '[ipv6-net]/prefixlen' syntax as well, the old
	syntax should probably continue to be supported.

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