Restrict SSH User Access to Certain Directory Using Chrooted Jail

There are several reasons to restrict a SSH user session to a particular directory, especially on web servers, but the obvious one is a system security. In order to lock SSH users in a certain directory, we can use chroot mechanism.

change root (chroot) in Unix-like systems such as Linux, is a means of separating specific user operations from the rest of the Linux system; changes the apparent root directory for the current running user process and its child process with new root directory called a chrooted jail.

In this tutorial, we’ll show you how to restrict a SSH user access to a given directory in Linux. Note that we’ll run the all the commands as root, use the sudo command if you are logged into server as a normal user.

Step 1: Create SSH Chroot Jail

1. Start by creating the chroot jail using the mkdir command below:

# mkdir -p /home/test

2. Next, identify required files, according to the sshd_config man page, the ChrootDirectory option specifies the pathname of the directory to chroot to after authentication. The directory must contain the necessary files and directories to support a user’s session.

For an interactive session, this requires at least a shell, commonly sh, and basic /dev nodes such as null, zero, stdin, stdout, stderr, and tty devices:

# ls -l /dev/{null,zero,stdin,stdout,stderr,random,tty}
Listing Required Files
Listing Required Files

3. Now, create the /dev files as follows using the mknod command. In the command below, the -m flag is used to specify the file permissions bits, c means character file and the two numbers are major and minor numbers that the files point to.

# mkdir -p /home/test/dev/		
# cd /home/test/dev/
# mknod -m 666 null c 1 3
# mknod -m 666 tty c 5 0
# mknod -m 666 zero c 1 5
# mknod -m 666 random c 1 8
Create /dev and Required Files
Create /dev and Required Files

4. Afterwards, set the appropriate permission on the chroot jail. Note that the chroot jail and its subdirectories and subfiles must be owned by root user, and not writable by any normal user or group:

# chown root:root /home/test
# chmod 0755 /home/test
# ls -ld /home/test
Set Permissions on Directory
Set Permissions on Directory

Step 2: Setup Interactive Shell for SSH Chroot Jail

5. First, create the bin directory and then copy the /bin/bash files into the bin directory as follows:

# mkdir -p /home/test/bin
# cp -v /bin/bash /home/test/bin/
Copy Files to bin Directory
Copy Files to bin Directory

6. Now, identify bash required shared libs, as below and copy them into the lib directory:

# ldd /bin/bash
# mkdir -p /home/test/lib64
# cp -v /lib64/{libtinfo.so.5,libdl.so.2,libc.so.6,ld-linux-x86-64.so.2} /home/test/lib64/
Copy Shared Library Files
Copy Shared Library Files

Step 3: Create and Configure SSH User

7. Now, create the SSH user with the useradd command and set a secure password for the user:

# useradd tecmint
# passwd tecmint

8. Create the chroot jail general configurations directory, /home/test/etc and copy the updated account files (/etc/passwd and /etc/group) into this directory as follows:

# mkdir /home/test/etc
# cp -vf /etc/{passwd,group} /home/test/etc/
Copy Password Files
Copy Password Files

Note: Each time you add more SSH users to the system, you will need to copy the updated account files into the /home/test/etc directory.

Step 4: Configure SSH to Use Chroot Jail

9. Now, open the sshd_config file.

# vi /etc/ssh/sshd_config

and add/modify the lines below in the file.

#define username to apply chroot jail to
Match User tecmint
#specify chroot jail
ChrootDirectory /home/test
Configure SSH Chroot Jail
Configure SSH Chroot Jail

Save the file and exit, and restart the SSHD services:

# systemctl restart sshd
OR
# service sshd restart

Step 5: Testing SSH with Chroot Jail

10. At this point, test if the chroot jail setup is working as expected:

# ssh [email protected]
-bash-4.1$ ls
-bash-4.1$ date
-bash-4.1$ uname
Testing SSH User Chroot Jail
Testing SSH User Chroot Jail

From the screenshot above, we can see that the SSH user is locked in the chrooted jail, and can’t run any external commands (ls, date, uname etc).

The user can only execute bash and its builtin commands such as(pwd, history, echo etc) as seen below:

# ssh [email protected]
-bash-4.1$ pwd
-bash-4.1$ echo "Tecmint - Fastest Growing Linux Site"
-bash-4.1$ history
SSH Built-in Commands
SSH Built-in Commands

Step 6. Create SSH User’s Home Directory and Add Linux Commands

11. From the previous step, we can notice that the user is locked in the root directory, we can create a home directory for the the SSH user like so (do this for all future users):

# mkdir -p /home/test/home/tecmint
# chown -R tecmint:tecmint /home/test/home/tecmint
# chmod -R 0700 /home/test/home/tecmint
Create SSH User Home Directory
Create SSH User Home Directory

12. Next, install a few user commands such as ls, date, mkdir in the bin directory:

# cp -v /bin/ls /home/test/bin/
# cp -v /bin/date /home/test/bin/
# cp -v /bin/mkdir /home/test/bin/
Add Commands to SSH User
Add Commands to SSH User

13. Next, check the shared libraries for the commands above and move them into the chrooted jail libraries directory:

# ldd /bin/ls
# cp -v /lib64/{libselinux.so.1,libcap.so.2,libacl.so.1,libc.so.6,libpcre.so.1,libdl.so.2,ld-linux-x86-64.so.2,libattr.so.1,libpthread.so.0} /home/test/lib64/
Copy Shared Libraries
Copy Shared Libraries

Step 7. Testing SFTP with Chroot Jail

14. Do a final test using sftp; check if the commands you have just installed are working.

Add the line below in the /etc/ssh/sshd_config file:

#Enable sftp to chrooted jail 
ForceCommand internal-sftp

Save the file and exit. Then restart the SSHD services:

# systemctl restart sshd
OR
# service sshd restart

15. Now, test using SSH, you’ll get the following error:

# ssh [email protected]
Test SSH Chroot Jail
Test SSH Chroot Jail

Try using SFTP as follows:

# sftp [email protected]
Testing sFTP SSH User
Testing sFTP SSH User

Suggested Read: Restrict SFTP Users to Home Directories Using chroot Jail

That’s it for now!. In this article, we showed you how to restrict a SSH user in a given directory (chrooted jail) in Linux. Use the comment section below to offer us your thoughts about this guide.

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55 thoughts on “Restrict SSH User Access to Certain Directory Using Chrooted Jail”

  1. Hello, thanks for this tutorial. However I have an issue when i want to test the /home/test with user via ssh.

    -sh: error while loading shared libraries: libc.so.6: cannot open shared object file: No such file or directory

    I tried to find a solution, but all issues on Google are on x86 Ubuntu. I’m on x64 16.04.

    Any suggestion ?

    regards

    Reply
  2. Hello John,

    I can not say for sure it is possible (because I do not has a such case), but I guess, that is possible, because scponly is only a shell like bash. But if you can describe your test case I will try to give more help.

    Reply
  3. Can cronjobs or scripts run for the user configured to use scp only. The user is configured to use sftp/scp only and ssh is not allowed.
    Thank you.

    Reply
  4. Thx. @Aaron, I appreciate your remarks!

    The link shared by you for Ahmed could be not useful in these days. At least me, I can not find likewise-open in the default repos for Linux-mint (last version). Maybe I am wrong ;) But for sure Ahmed can use SSSD: “The System Security Services Daemon (SSSD) provides access to different identity and authentication providers”:

    # apt install sssd-ldap sssd-ad sssd-krb5 sssd-ipa
    

    After that he can integrate any LINUX desktop/server, in any LDAP/AD(ldap)/IPA(ldap), and maybe more others …. ! Then the rest of the tutorial(without the likewise-open part) can be used. And with SSSD, you can also have cache credential for any authenticated user, even if the AD/LDAP server is DOWN for some time. As a final word, I think that likewise-open is discontinued (if I remember correctly). In my case likewise-open has fail many years ago!

    Reply
  5. Hello, this tutorial is ok to show how a chroot can be use. But from practical point of view is complicated, and does not scale.

    For a scp run in a jail, is more simple to use scponly. For your test case, a webserver, we can use any container technology (lxc is one possibility ), or even better kvm. But you know, each solution have good points, and bad points.

    Any Linux admin must think what is the best for his particular case. This is the most important for me is ok the solution A or B? How I can reduce the risk for A and for B? I have the skills for A/B? I have the proper resources for A/B (time, servers, storage, and so on)?

    Reply
    • @lulian

      Your are right, from a practical point of view, implementing this may by be complicated especially when used with ssh, scp and other related commands. And also when you need to install additional commands for users and create a PATH for them to run commands without specifying the absolute path to the commands.

      Therefore, it would effectively and reliably work in test cases for testing certain programs in an isolated environment on the system. Thanks for sharing your thoughts with us.

      Reply
  6. When I test SFTP connection.

    "subsystem request failed on channel 0
    Couldn't read packet: Connection reset by peer"
    

    logs – /var/log/secure:

    "error: subsystem: cannot stat /usr/libexec/openssh/sftp-server: No such file or directory
    subsystem request for sftp failed, subsystem not found"
    

    When I change sshd_config: ( https://www.tecmint.com/restrict-sftp-user-home-directories-using-chroot/ )

    "# override default of no subsystems
    Subsystem	sftp	internal-sftp
    #/usr/libexec/openssh/sftp-server
    ForceCommand internal-sftp"
    

    It’s work for me. But, why user see all folder/files from jail ? ex: bin/etc/dev ?
    “cd /” move him to /home/test.

    And How Can I run this Jail with LDAP/geten passwd user from LDAP ?

    Reply

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