Re: Cygwin/Win32 utility function to convert "raw" IPv6 address string into *.ipv6-literal.net string ?

[Jeffrey Altman <jaltman@secure-endpoints.com>]

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On 9/28/2023 1:56 PM, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via Cygwin wrote:
>> What do you think that output is - the PTR is resolved to "localhost."
> You obviously did not get the point that I was making.  Using ip6.arpa *is* the standard
> way to get around with "DNS-like" IPv6 addresses, as it would be "understood".  Using
> the "ipv6-literal.net" domain is not portable and would result in NXDOMAIN anywhere
> but Windows (where the resolver seems to intercept and convert them internally).

The ip6.arpa names are used to lookup PTR records which contain hostnames.

The ipv6-literal.net names are used to simulate AAAA records and map the 
name to an IPv6 address (with an optional address scope).

For those that are unaware of the history[1][2][3], UNC names support 
the use of IPv4 addresses as an alternative to SMB server names or DNS 
host names. End users expect to be able to specify a UNC path such as \\2001:db8:85a3:8d3:1319:8a2e:370:7348\share

or

   \\[2001:db8:85a3:8d3:1319:8a2e:370:7348]\share

However, colons are illegal in UNC paths and therefore standard IPv6 
representations cannot be used. The <encoded-ipv6-addr>.ipv6-literal.net 
server name is a method of representing an IPv6 address as a UNC server 
name such that it can be locally translated by getaddrinfo() into an 
IPv6 address without querying DNS. \\2001-db8-85a3-8d3-1319-8a2e-370-7348.ipv6-literal.net\share

If the ipv6.arpa representation\\8.4.3.7.0.7.3.0.e.2.a.8.9.1.3.1.3.d.8.0.3.a.5.8.8.b.d.0.1.0.0.2.ip6.arpa\path

was used as a server name and treated as a DNS AAAA lookup that would be harder for
humans to construct and no more portable.   The ip6.arpa representation would also
be unable to represent IPv6 address scopes which are supported by ipv6-literal.net
names.

I am unaware of any Microsoft Windows APIs that can be called to translate from
an IPv6 address to an ipv6-literal.net string.

[1] 
https://en.wikipedia.org/wiki/IPv6_address#Literal_IPv6_addresses_in_UNC_path_names 
[2] https://devblogs.microsoft.com/oldnewthing/20100915-00/?p=12863 [3] 
https://learn.microsoft.com/en-us/windows/win32/api/winnetwk/nf-winnetwk-wnetaddconnection3a 



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