That's ... exactly how IPv6 works?
Look at the default prefix table at https://en.wikipedia.org/wiki/IPv6_address#Default_address_s... .
Or did you mean something else? You still need a dual stack configuration though, there's nothing getting around that when you change the address space. Hence "happy eyeballs" and all that.
Yes there is, at least outside of the machine. All you need to do is have an internal network (100.64/16, 169.254/16, wherever) local to the machine. If you machine is on say 2001::1, then when an application attempts to listen on an ipv4 address it opens a socket listening on 2001::1 instead, and when an application writes a packet to 1.0.0.1, your OS translates it to ::ffff:100:1. This can be even more hidden than things like internal docker networks.
Your network then has a route to ::ffff:0:0/96 via a gateway (typically just the default router), with a source of 2001::1
When the packet arrives at a router with v6 and v4 on (assume your v4 address is 2.2.2.2), that does a 6:4 translation, just like a router does v4:v4 nat
The packet then runs over the v4 network until it reaches 1.0.0.1 with a source of 2.2.2.2, and a response is sent back to 2.2.2.2 where it is de-natted to a destination of 2001:1 and source of ::ffff:100.1
That way you don't need to change any application unless you want to reach ipv6 only devices, you don't need to run separate ipv4 and ipv6 stacks on your routers, and you can migrate easilly, with no more overhead than a typical 44 nat for rfc1918 devices.
Likewise you can serve on your ipv6 only devices by listening on 2001::1 port 80, and having a nat which port forwards traffic coming to 2.2.2.2:80 to 2001::1 port 80 with a source of ::ffff:(whatever)
(using colons as a deliminator wasn't great either, you end up with http://[2001::1]:80/ which is horrible)
That is horrible, but you do no longer have any possibility of confusion between an IP address and a hostname/domain-name/whatever-it's-called. So, yeah, benefits and detriments.
> Your network then has a route to ::ffff:0:0/96 via a gateway...
I keep forgetting about IPv4-mapped addresses. Thanks for reminding me of them with this writeup. I should really get around to playing with them some day soon.
> If you machine is on say 2001::1, then when an application attempts to listen on an ipv4 address it opens a socket listening on 2001::1 instead, and when an application writes a packet to 1.0.0.1, your OS translates it to ::ffff:100:1. ...
> Your network then has a route to ::ffff:0:0/96 via a gateway (typically just the default router), with a source of 2001::1
What's the name of this translation mechanism that you're talking about? It seems to be the important part of the system.
I ask because when I visit [0] in Firefox on a Linux system with both globally-routable IPv6 and locally-routable IPv4 addresses configured, I see a TCP conversation with the remote IPv4 address 192.168.2.2. When I remove the IPv4 address (and the IPv4 default route) from the local host, I get immediate failures... neither v4 nor v6 traffic is made.
When I add the route it looks like you suggested I add
ip route add ::ffff:0:0/96 dev eth0 via <$DEFAULT_IPV6_GATEWAY_IP>
Based on my testing, it looks like this is only a way to represent IPv4 addresses as IPv6 addresses, as ::ffff:192.168.2.2 gets translated into ::ffff:c0a8:202, but the OS uses that to create IPv4 traffic. If your system doesn't have an IPv4 address configured on it, then this doesn't seem to help you at all. What am I missing?
Could have used 2001~1001~~1 instead of 2001:1001::1, which looks weird today, but wouldn't have done if that had been chosen all those years ago.
(unless : as an ipv6 separator predates its use as a separator for tcp/udp ports, in which case tcp/udp should have used ~. Other symbols are available)