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.

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}

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

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

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/

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/

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

# useradd tuxi
# passwd tuxi

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/

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.