Through shell, adding an user with a slash path ("/") as directory : ssh/sftp impossible, even after re-creation of it with a named directory - linux

Question

First, what I'm trying to do is to jail (chroot) the user in a specfic directory for sftp access. I found a great tutorial that made the job, you can find it here : http://www.thegeekstuff.com/2012/03/chroot-sftp-setup/. You should read it first (it's very short) to understand the rest.
With the example in the link, I've no problem to create sftpusers group, guestuser user and lock it to the /sftp/guestuser/incoming directory. Here is how it's done :
groupadd sftpusers
useradd -g sftpusers -d /incoming -s /sbin/nologin guestuser
passwd guestuser
chown guestuser:sftpusers /sftp/guestuser/incoming
service ssh restart
What I've tried to do after is to lock another user (we call it "test") from the same group in /sftp/test. So I've created user like this :
useradd -g sftpusers -d / -s /sbin/nologin test
You see, instead /incoming as directory, I've just put "/", then I've changed rights to /sftp/test and restart ssh. But connection fails and give me this error : Write failed: Broken pipe
I thought it was due to the wrong path "/", so I decided to delete user and re-create it with an /incoming directory :
userdel test
useradd -g sftpusers -d /incoming -s /sbin/nologin test
passwd test
chown test:sftpusers /sftp/test/incoming
service ssh restart
But even after, the test user seems to be compromise for ever, because I have the same error when I try to connect through sftp : Write failed: Broken pipe Couldn't read packet: Connection reset by peer
EDIT :
Here is the log file for sshd :
Jan 24 12:46:20 ns sshd[13786]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key
Jan 24 12:46:23 ns sshd[13786]: Accepted password for test from xxx.xxx.xxx.xxx port 37838 ssh2
Jan 24 12:46:23 ns sshd[13786]: pam_unix(sshd:session): session opened for user test by (uid=0)
Jan 24 12:46:24 ns sshd[13949]: fatal: bad ownership or modes for chroot directory "/sftp/test"
Jan 24 12:46:24 ns sshd[13786]: pam_unix(sshd:session): session closed for user test
Jan 24 12:48:17 ns sshd[14103]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key
Jan 24 12:48:19 ns sshd[14103]: Invalid user brad from xxx.xxx.xxx.xxx
Jan 24 12:48:19 ns sshd[14103]: input_userauth_request: invalid user brad [preauth]
Jan 24 12:48:19 ns sshd[14103]: pam_unix(sshd:auth): check pass; user unknown
Jan 24 12:48:19 ns sshd[14103]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=xxx.xxx.xxx.xxx
Jan 24 12:48:22 ns sshd[14103]: Failed password for invalid user brad from xxx.xxx.xxx.xxx port 42756 ssh2
Jan 24 12:48:22 ns sshd[14103]: Received disconnect from xxx.xxx.xxx.xxx: 11: Bye Bye [preauth]

Answer 1

Check the debug log for sshd. You'll likely find it complain about the home directory for user test not being secure.
The sshd is very strict about access to the user's credentials in $HOME/.ssh and will disconnect when it cannot access the directory, or when it detects permissions that would allow any other account to access it.
sshd[13949]: fatal: bad ownership or modes for chroot directory "/sftp/test"
Make sure that the owner of the directory is the only account that can modify any file in $HOME and below. Pay attention specifically to the group settings