Roaming ssh over IPv4/v6 HOWTO:

You have a well-connected box "foo" somewhere.

At home, you have proper inbound IPv6, but no inbound IPv4.  One of your boxen
is named "bar" and you want to get to it.

You don't want to degrade to IPv4 when not needed -- 𝑒𝑠𝑝𝑒𝑐𝑖𝑎𝑙𝑙𝑦 when you're
home; going over the 'Net from laptop on the desk to your regular machine under
that very same desk would be comcastically slow.


On your laptop:
.--[ .ssh/config ]
Match host bar !exec "ip -6 route|grep -q default"
    Hostname bar.domain.net
    ProxyJump foo-jump
Match host bar exec "ip -6 route|grep -q default"
    Hostname bar.domain.net
# the above repeated eleventy times, for each host

Host foo-jump
	User jump
	Hostname foo.domain.net
	IdentityFile /home/$USER/.ssh/id_rsa.jump
`----
(unlike the normal key, id_rsa.jump is passwordless)


On the server:
.--[ /etc/passwd ]
jump:x:1003:1003:,,,:/home/jump:/bin/false
`----
.--[ /home/jump/.ssh/authorized_keys ]
ssh-rsa AAAABlahBlahBlahityBlah jump@laptop
`----