Chrooted SSH HowTo - Page 2
3 Create The Chroot Environment
Next I create a chroot environment under /home/chroot. This is the directory that all chrooted SSH users will get jailed in, i.e. they will not be able to see any files/directories outside /home/chroot.
I have to create some directories in /home/chroot, and I have to copy a few binaries like /bin/bash, /bin/ls, etc. as well as the libraries on which these binaries depend into the chroot environment so that they are available to any chrooted user.
mknod dev/null c 1 3
mknod dev/zero c 1 5
Now that we have created the necessary directories, we are goning to copy some binaries and all the libraries on which they depend into the chroot environment. This is an excerpt of a script that I found on http://mail.incredimail.com/howto/openssh/create_chroot_env that does this. Just copy and paste the following lines into your shell, and hit Return. If you want to make more programs available to your chrooted users, just add these programs to the APPS line:
APPS="/bin/bash /bin/ls /bin/mkdir /bin/mv /bin/pwd /bin/rm /usr/bin/id /usr/bin/ssh /bin/ping /usr/bin/dircolors"
Then we do this:
cp /lib/libnss_compat.so.2 /lib/libnsl.so.1 /lib/libnss_files.so.2 ./lib/
echo '#!/bin/bash' > usr/bin/groups
echo "id -Gn" >> usr/bin/groups
grep /etc/passwd -e "^root" > etc/passwd
You should also copy the line of the group in which you will create new users from /etc/group to /home/chroot/etc/group. In this tutorial we will create users in the group users, so we do this:
grep /etc/group -e "^root" -e "^users" > etc/group
and restart SSH:
4 Create A Chrooted User
Even with the chrooted SSH that we have just installed you can log in without being chrooted (which makes sense if you log in as root, for example). Now, how does the chrooted SSH decide whom to chroot and whom not? That's easy: the chrooted SSH looks up the user who is trying to log in in /etc/passwd. If the user's home directory in /etc/passwd has a . in it, then the user is going to be chrooted.
Example (from /etc/passwd):
This user will not be chrooted.
This user will be chrooted.
Now we create the user testuser with the home directory /home/chroot/./home/testuser and the group users (which is the default group for users on Debian so you do not have to specify it explicitly):
useradd -s /bin/bash -m -d /home/chroot/./home/testuser -c "testuser" -g users testuser
Then we give testuser a password:
Finally, we have to copy the line for testuser in /etc/passwd to /home/chroot/etc/passwd:
grep /etc/passwd -e "^testuser" >> /home/chroot/etc/passwd
We have already copied the users group line from /etc/group to /home/chroot/etc/group so we do not have to do this here again. If you create a chrooted user in another group than users, add this group to /home/chroot/etc/group:
grep /etc/group -e "^othergroup" >> /home/chroot/etc/group
Now try to log in to SSH as testuser. You should be chrooted and not be able to browse files/directories outside /home/chroot.