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Create a chrooted ssh user

This tutorial explains how to install and configure a chroot enviroment for an ssh user. This setup enables you to give out ssh accounts without having to fear that this user can see all files on the system.

Installing ssh

First you need to have a patched version of the sshd server. Luckily these patches can be enabled with the use flag "chroot" in the sshd use flags.

#echo "net-misc/openssh chroot" >> /etc/portage/package.use
#emerge openssh

creating the chroot enviroment

We will create our chroot enviroment in /home/chroot.
To make the chroot work, run the following commands to make the needed directories and devices for the chrooted user.

mkdir /home/chroot/
mkdir /home/chroot/home/
cd /home/chroot
mkdir etc
mkdir bin
mkdir lib
mkdir usr
mkdir usr/bin
mkdir dev
mknod dev/null c 1 3
mknod dev/zero c 1 5

Now we need to populate the directories with some binaries.
copy the following script into a file. If you need more apps, add them
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"
for prog in $APPS;  do
        cp $prog ./$prog

        # obtain a list of related libraries
        ldd $prog > /dev/null
        if [ "$?" = 0 ] ; then
                LIBS=`ldd $prog | awk '{ print $3 }'`
                for l in $LIBS; do
                        mkdir ./`dirname $l` > /dev/null 2>&1
                        cp $l ./$l
                done
        fi
done

After you have run the script, your chroot enviroment is almost done.
run


 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
 touch etc/passwd
 grep /etc/passwd -e "^root" > etc/passwd

to copy some libraries and user information into the chroot.
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:

/etc/init.d/ssh restart

Creating chrooted users

ssh decides which user should be chrooted and which not by the "home directory" entry in the /etc/passwd.
Example for a non-chrooted user:
user_a:x:2002:100:User A:/home/user_a:/bin/bash
This user will be chrooted:
user_b:x:2003:100:User B:/home/chroot/./home/user_b:/bin/bash
Now lets add a testuser to the chrooted user list:

useradd -s /bin/bash -m -d /home/chroot/./home/testuser -c "testuser" -g users testuser

Then we give testuser a password:

passwd testuser

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

Now log in as testuser and see if everything worked.

Have fun

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