mjhjmtu

Whilst creating a testuser, I was 99% sure that the user path would have been its home directory, and in theory nothing else could have been seen when connecting regardless.

I was wrong, so in the attempt to limit the SSH of a user to a certain folder and only that up I read some guidelines and SE answers (e.g. this one) to found most of them pointing to something like

Match User testuser
 ChrootDirectory /home/testuser
 ForceCommand internal-sftp

But when I attempted to SSH, an error message saying "broken pipe" is arise and I couldn't figure out to resolve.

Any idea where I can continue to read and/or setup?

Thanks

Your problem likely is because user's home directory is writable for user in question (testuser). SSH does not allow chrooting into (user) writable directory and terminates the connection. You should have a log message in your server's logs.

sshd_config man page says about ChrootDirectory:

 ChrootDirectory
 Specifies the pathname of a directory to chroot(2) to after
 authentication. At session startup sshd(8) checks that all
 components of the pathname are root-owned directories which are
 not writable by any other user or group. ...

Your options depend on your requirements. You could change the home directory owner to root and remove other write permissions and have the (writable) sub-directories owned by the user. For example:

mkdir /home/testuser_tmp
mv /home/testuser /home/testuser_tmp/
mv /home/testuser_tmp /testuser_tmp

If the user also has shell access, you probably should update the home directory entry in /etc/passwd to point to the writable sub-directory.