Categories
Admin CentOS Security

How to Set up a Secure sftp-only Service

Intro
Updated Jan, 2015.

Usually I post a document because I think I have something to add. This time I found a link that covers the topic better than I could. I just wanted to have it covered here. What if you want to offer an sftp-only jailed account? Can you do that? How do you do it?

The Answer
Well, it used to be all here: http://blog.swiftbyte.com/linux/allowing-sftp-access-while-chrooting-the-user-and-denying-shell-access/. But that link is no longer valid.

I tried it, appropriately modified for CentOS and it worked perfectly. A few notes. Presumably you will already have ssh installed. Who can imagine a server without it? So there’s typically no need to install openssh-server.

I was leery mucking with subsystem sftp. What if it prevented me from doing sftp to my own account and having full access like I’m used to? Turns out it does no harm in that regard.

Very minor point. His documentation might be good for Ubuntu. To restart the ssh daemon in CentOS/Fedora, I recommend a sudo service sshd restart. Do you wonder if that will knock you out of your own ssh session? I did. It does not. Not sure why not!

These groupadd/useradd/usermod functions are “cute.” I’m old school and used to editing the darn files by hand (/etc/passwd, /etc/group). I suppose it’s safer to use the cute functions – less chance a typo could render your server inoperable (yup, done that).

Let’s call my sftp-only user is joerg.

I did the chown root:root thing, but initially the files weren’t accessible to the joerg user. The permissions were 700 on the home directory, now owned by root. That produces this error when you try to sftp:

$ sftp joerg@localhost
sftp> dir

Couldn't get handle: Permission denied

That’s no good, so I liberalized the permissions:

$ sudo chmod go+rx /home/joerg

My /etc/passwd line for this user looks like this:

joerg:x:1004:901:Joerg, etherip author:/home/joerg:/bin/false

So note the unusual shell, /bin/false. That’s the key to locking things down.

In /etc/group I have this;

joerg:x:1004:

If you want to add the entries by hand to passwd and group then if I recall correctly you run a pwconv to generate an appropriate entry for it in /etc/shadow, and a sudo passwd joerg to set up a desired password.

Does it work? Yeah, it really does.

$ sftp joerg@localhost
Connecting to localhost…
sftponlyuser@localhost’s password:
sftp> pwd
Remote working directory: /
sftp> cd ..
sftp> pwd
Remote working directory: /
sftp> cd /etc
Couldn’t canonicalise: No such file or directory
sftp> ls -l
[shows files in /home/joerg]

Moreover, ssh really is shut out:

$ ssh joerg@localhost
joerg@localhost’s password:

This hangs and never returns with a prompt!

Cool, huh?

Locking out this same account
Now suppose you only intended joerg to temporarily have access and you want to lock the account out without actually removing it. This can be done with:

$ sudo passwd -l joerg

This puts an invalid character in that account’s shadow file entry.

Conclusion
We have an easy prescription to make a jailed sftp-only account that we tested and found really works. Regular accounts were not affected. The base article on which I embellished is now kaput so I’ve added a few more details to make up for that.

Categories
Linux

An SSH Terminal App for the HP Touchpad

October, 2016 Update
Needless to say, the HP Touchpad never caught on and mine is collecting dust.

What I just got is the new 8″ Amazon Fire HD Tablet. You can get a free good-quality ssh client for it called serverauditor. The keyboard emulation (linux CLI needs all those unusual keys pretty badly). The battery life is genuinely good – better than the Touchpad. It’s $89.

I’ll keep the blog post below online for historical purposes.

Updated Version
My previous post got out-of-date so rapidly that I have to start this topic all over! DO NOT follow my previous advice.

The Bluetooth Keyboard – It’s Worth It
My Bluetooth keyboard came in. It’s really awesome. I advise to get it if you want to treat your Touchpad (the cognoscenti prefer TP) like a Netbook from time-to-time, namely, by having the ability to type rapidly and comfortably. Get the HP one made for the Touchpad because:
– it’s small like you’d expect as a companion for a small tablet computer, yet the keys are full size
– it has some really convenient shortcut keys so you’re not spending too much time shifting your hand from keyboard to screen, namely:
— volume controls
— screen on/off
— even a key that shows your cards
– plus some keys that do stuff that’s harder with just a TP
— Ctrl (control) key, yeah!
— arrow keys
–mute key
–screen brightness/dimmer keys
–plus other keys I haven’t tested yet
– and the : and / keys are primary keys like they should be

So far I’m missing a
-Home/End key and if I ever get my terminal working again
– an ESC (escape) key

All-in-all I’d say the Bluetooth keyboard is an obviously well-engineered product – a perfect pairing for the TP.

It’s $45 at Amazon. And yes, I am writing this blog entry on my new keyboard!

I also bought an off-brand display stand. By Mivizu. It’s better than NOT having one, but it’s kind of flimsy and awkward. In no way a fun and beautifully engineered companion to the TP, unlike the IPad case that everyone likes to play with.

What About that SSH Terminal?
I probably messed things up with Preware alpha/beta software. They have released an Xterm, but I arrived at it from various previous upgrades and either Xecutah or the XServer is not working for me. The XServer does not launch a new card like it should. See below.

I will probably have to Web Dr my device (start from a factory install state), which they warn you should be prepared to do when using test software. Live and learn. I have not had time to do that yet, but I wanted to delete my old post and get the new facts out here before others went down the wrong path.

So, briefly, an ssh, bash, xterm to your underlying Linux on your TP are all available from the webos-internals.org site.

Sep 29th I saw an upgrade for Xecutah, Servers and xterm – to v 0.9.3. I did the upgrade and, to my surprise, I am back in business again! The xterm launches once again and so I do not have to Web Dr my Touchpad.

I thought I owed it to the community to experiment, so I decided to change root’s shell to bash! That’s right, the shell is that old /bin/sh by default. Once you’ve installed it, bash appears in /opt/bin/bash. Well…that worked too. I now have a comfortable shell that launches for me when I fire up my xterm, or xterms. Of course I brought over my .bashrc file – using sftp of course – with its familiar prompt definition and convenience aliases such as the universal “ll” for ls -l. To make really sure I hadn’t blown up my Touchpad, I rebooted. Yes, reboot from your shell really does work to reboot your TP! And yes, it came back with flying colors.

Esc key in the xterm for real Keyboard Users
I don’t think the HP keyboard has an escape key, not that I can find. So you’re in a bit of a bind if you use it for your xterm during a vi editing session. What you can do is momentarily bring up the virtual keyboard by hitting the, um, keyboard key. Xecutah now comes with instructions on how to generate the escape key on the virtual keyboard (hold t, choose right-most character, then “[” as your next character) which work. Then, when you’ve got your Esc, which you don’t need to often anyways, hit that keyboard key again to recommended using your comfy real keyboard.

So I am a happier camper once again. I even contributed to webos-internals. You should, too, if you think they’re providing a valued service as I do.

To be continued.