How to Create a Chroot Jail for Services on FreeBSD

Introduction

A chroot jail is a security feature that isolates a process and its child processes from the rest of the system by changing the apparent root directory. This prevents the service from accessing files outside its designated environment, reducing the risk of system-wide compromise in case of an attack.

FreeBSD offers built-in support for chroot, making it an effective way to enhance the security of services like web servers, FTP servers, and SSH daemons. This article will guide you through setting up a chroot jail for a service on FreeBSD.

Prerequisites

Before setting up a chroot jail, ensure you have:

• A FreeBSD system with root privileges
• Basic knowledge of FreeBSD commands
• Installed service(s) you wish to isolate (e.g., OpenSSH, Apache, or Nginx)

Step 1: Create the Chroot Directory Structure

First, choose a directory to serve as the chroot jail. A common location is /var/chroot.

mkdir -p /var/chroot/myservice

Within this directory, replicate the necessary system directories and files required by the service. Typically, these include:

• /bin - Essential binaries
• /lib and /libexec - Required libraries
• /usr - Additional binaries and libraries
• /etc - Configuration files
• /dev - Device nodes (if needed)

Create the directories:

mkdir -p /var/chroot/myservice/{bin,lib,libexec,usr,etc,dev,var,home,tmp}
chmod 1777 /var/chroot/myservice/tmp

Step 2: Copy Necessary Binaries and Libraries

Identify the binaries your service depends on and copy them into the chroot environment. Use ldd to check for dependencies. For example, if you need to copy sh:

cp /bin/sh /var/chroot/myservice/bin/
ldd /bin/sh

This outputs something like:

/libexec/ld-elf.so.1 => /libexec/ld-elf.so.1 (0x20000000)
libc.so.7 => /lib/libc.so.7 (0x20100000)

Copy these dependencies:

cp /libexec/ld-elf.so.1 /var/chroot/myservice/libexec/
cp /lib/libc.so.7 /var/chroot/myservice/lib/

Repeat for all required binaries.

Step 3: Set Up Configuration Files

Copy essential configuration files. If setting up an SSH jail, copy:

cp /etc/group /var/chroot/myservice/etc/
cp /etc/master.passwd /var/chroot/myservice/etc/
cp /etc/passwd /var/chroot/myservice/etc/
cp /etc/resolv.conf /var/chroot/myservice/etc/
cp /etc/ssh/sshd_config /var/chroot/myservice/etc/

Ensure that sensitive information is not exposed by removing unneeded users from /var/chroot/myservice/etc/passwd and /var/chroot/myservice/etc/group.

Step 4: Create Device Nodes

Some services require access to special device nodes. Use mknod to create them:

mknod /var/chroot/myservice/dev/null c 2 2
chmod 666 /var/chroot/myservice/dev/null
mknod /var/chroot/myservice/dev/zero c 2 12
chmod 666 /var/chroot/myservice/dev/zero

Step 5: Configure and Start the Service

Modify the service configuration to use the chroot jail. For example, if setting up SSH, edit /var/chroot/myservice/etc/ssh/sshd_config:

ChrootDirectory /var/chroot/myservice

Ensure the SSH daemon starts with chroot enabled:

service sshd restart

For other services like Apache, adjust the configuration to point to the chroot directory and restart the service accordingly.

Step 6: Test the Chroot Jail

To verify the chroot environment is functioning properly, use chroot to enter it:

chroot /var/chroot/myservice /bin/sh

Once inside, attempt to navigate outside the chroot jail. If everything is set up correctly, this should be impossible.

Step 7: Automate the Chroot Jail at Boot

Ensure the service runs within the chroot jail on system startup by modifying /etc/rc.conf:

sshd_enable="YES"
sshd_chroot_enable="YES"

For other services, modify their startup scripts accordingly.

Conclusion

Setting up a chroot jail on FreeBSD is an effective way to enhance security by isolating services from the main system. By carefully configuring the environment, copying necessary binaries, and adjusting service configurations, you can run essential services in a controlled, restricted space. This limits the impact of potential security vulnerabilities and provides an additional layer of protection for your system.