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Re: ssh issue that I don't understand

On 06/07/2026 01:02, Jeffrey Walton wrote:

On Sun, Jul 5, 2026 at 6:35 PM Simon Hollenbach <ionpowered@gmail.com> wrote:

On Sun, 5 Jul 2026 at 21:40, Chris Green <cl@isbd.net> wrote:

I have a VPS called isbd.biz and I access it using ssh.  My user
account on both my local machine (desktop and laptop) is 'chris' and
my user account on isbd.biz is also 'chris'.

I have account 'chris' on isbd.biz set up to allow login only using
public key authentication.

The ip address of isbd.biz is 213.171.194.64.

If I log in with 'ssh 213.171.194.64' I, correctly, get prompted for
my local system's public key passphrase and I can log in successfully.

However if I try 'ssh isbd.biz' I get prompted for "chris@isbd.biz's
password" which doesn't exist and I can't log in.

Your domain also has an AAAA record, could it be that ssh is trying to
reach that instead of the IP you tested above?
You should be able to test this by forcing SSH to use IPv4, using `ssh
-4 isbd.biz`. Sprinkle in some `--verbose` flags if you still don't
understand what's going on, you can repeat `--verbose` or use multiple
`-v`s in a row, e.g. `ssh -vvv [...]` to increase verbosity further.

That does not make sense (to me).  Isn't that like saying you connect
to one host using IPv4, and another host using IPv6, under the same
DNS name.  Why would someone do that???
I think there's an old adage that "hosts don't have IP addresses, interfaces do". It's easy to think that the isbd.biz box has two IP addresses, but in practice there's always going to be _some_ sort of split between IPv4 and IPv6, even if it's just down to the "network stack" being different.
The target here is to make a connection to the SSH daemon. So if that's only listening on one interface that MIGHT not be the one with the IPv6 address on (maybe IPv6 is tunnel, as an example). Even if there's only one interface, SSHd could still be listening on "0.0.0.0" rather than "::" (I think the default can be changed, but in practice "::" means "everything IPv4 *and* IPv6). 

For most purposes, IPv4 and IPv6 do look very similar to "different hosts".





'host isbd.biz' returns the correct IP address for isbd.biz so why
does ssh not work using the name rather than the IP address?

Do you have some custom ssh client config on the machines connecting
to your VPS? It should be in ~/.ssh/config or /etc/ssh/ssh_config ,
See `man 5 ssh_config`. If so, does any of it concern the host you're
connecting to?

Jeff
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