IPv4 in IPv6 in AAAA records

Colm MacCarthaigh colm.maccarthaigh at heanet.ie
Tue Aug 24 09:45:08 UTC 2004

On Mon, Aug 23, 2004 at 06:33:15PM -0500, Phil Howard wrote:
> An address expressed like ::ffff: could be used on a system
> that has only IPv6 implemented, or only has IPv6 reachability, or has a
> LAN that is limited to IPv6, and such an address can be converted to IPv4
> at some point between that machine's stack (inclusive) to that network's
> gateway (NAT), and go out over the rest of the net as IPv4.

Absolutely not! As Itojun sais, ::ffff addresses are supposed to be
local to a host only, they are never to appear on the wire, see
his ID for reasons why this is a bad thing:


What you are describing is almost like a relay translator, see:


But this is a layer 3 device, doing it in layer 2 won't really work
reliabily (because of header incompatibilities). And it uses the C6::/64

> Getting back to DNS, it's also a way to query a single record type once
> and get an address that says "Use IPv4 instead, and here's the address".
> Should any of what I describe not be done, or be done some other way?

You're just using the wrong prefix is all :) ::ffff is for host-only
translation, a well-configured host should deny any packets with this
prefix to come in over the wire.

Colm MacCárthaigh  /  HEAnet, Teach Brooklawn,  / Innealtóir Ghréasáin
+353 1 6609040    / Bóthar Shelbourne, BÁC, IE /   http://www.hea.net/

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