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Filesharing with chrooted SFTP

2014-02-12

Live demo in BSD Now Episode 024 | Originally written by TJ for bsdnow.tv | Last updated: 2014/05/08

So you've followed our SSH tutorial and now you're ready to hand out accounts to your friends, right? Well, there are times when you want to securely share files with them, but don't want them having shell access to your server. OpenSSH includes SFTP, the secure file transfer protocol, which provides both authentication and encryption. Normally, you need to give a user an SSH login for them to be able to transfer files via SFTP, but there’s also a useful option to disallow shell access and only let them transfer files. What's more, you can lock them to a certain directory so they can't browse your filesystem. This tutorial will show you how to do just that. I’m assuming you already have sshd configured and running.

Let's create a new user for them to use and edit the sshd_config file to chroot them.

# adduser

Username: gnub
Full name: Some Random GNUb
Uid (Leave empty for default):
Login group [gnub]:
Login group is gnub. Invite gnub into other groups? []:
Login class [default]:
Shell (sh csh tcsh bash rbash nologin) [sh]: nologin
Home directory [/home/gnub]:
Home directory permissions (Leave empty for default):
Use password-based authentication? [yes]:
Use an empty password? (yes/no) [no]:
Use a random password? (yes/no) [no]:
Enter password:
Enter password again:
Lock out the account after creation? [no]:
Username   : gnub
Password   : *****
Full Name  : Some Random GNUb
Uid     : 1004
Class     :
Groups   : gnub
Home       : /home/gnub
Home Mode  :
Shell     : /usr/sbin/nologin
Locked   : no
OK? (yes/no): yes
adduser: INFO: Successfully added (gnub) to the user database.
Add another user? (yes/no): no
Goodbye!

Take note of the "nologin" shell - that's not the default. Next we edit the SSH configuration:

# vi /etc/ssh/sshd_config

Add something like this to the bottom:

Match User gnub
    PasswordAuthentication yes
    ChrootDirectory %h
    ForceCommand internal-sftp
    AllowTcpForwarding no
    AllowAgentForwarding no
    PermitTunnel no
    PermitTTY no
    X11Forwarding no

Restart the daemon.

# /etc/rc.d/sshd restart

Next, we set some permissions and make a directory for them to actually put files in.

# chown root:gnub /home/gnub
# mkdir /home/gnub/files
# chown gnub:gnub /home/gnub/files

Now you can give your friend the SFTP login and they will be locked in the home directory, but able to upload and download things to and from the "files" directory. If they try to login via SSH to get a shell, they should get the error:

This service allows sftp connections only.

Too bad for them.

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