Jail-Shell

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Jail-shell is a Linux security tool mainly using chroot, namespaces technologies, limiting users to perform specific commands, and access specific directories.

Users can login through SSH, SCP, SFTP, Telnet, terminals, etc. and be restricted to a secure operating enviroment.

Jail-shell can be used for webhost ssh access control, enterprise Administrator's privilege hierarchy management.

Features

• Easy to use

Through the configuration file jail-shell automatically generates the chroot running environment. Through jail-shell management commands it's very easy to add, list, delete, restrict users, and easy to install, delete chroot running environment.

• Chroot technology limits user access

The Linux chroot technology is used to restrict the user's directory access, to avoid users accessing restricted directories and to prevent users from destroying the system.

• Directory read-only protection

The chroot running enviroment is readonly, this prevents users from deleting protected directories and files, creating device files, and accessing restricted files.

• Namespace limit user Visible range

Use Linux namespace technology, limit the visible range of user PID, Mount directories, and avoid information leakage.

• System command channel

Provides a system command-and-proxy channel that allows users to execute a real system's restricted command in a chroot environment, protecting the system in the event that it provides the necessary functionality.

• Automatic processing of chroot Environment command library dependencies

Only a list of commands is required to automatically copy the dynamic library that the command relies on to the chroot environment, avoiding the cumbersome work of copying the dynamic library manually.

• Capabilities Restrictions

Discard critical capabilities privileges to avoid the system, and the chroot running environment, being cracked by rootkit.

• Multi-Linux operating system support

Supports Redhat, SLEs, Debian and their derivative operating systems.

Architecture

Jail-shell contains 3 parts, Pam Plugins, jail-cmd command agents, Jail-shell command tools.

• pam_jail_shell Plugins

Mainly control the login of users. according to the configuration list, use chroot and namespace technology to restrict the login users to a specific restricted directory.

• jail-cmd command-and-proxy

It forwards specific command to the real system, such as passwd, or other user-related business commands, and it also prevents command injection.

• jail-shell commandline tool

Mainly provides the ability to manage the restricted security shell, making it easier for administrators to use, including user's add, delete, shell's configuration, installation, deletion, etc.

Instructions

1. According to the configuration, pam_jail_shell limits users to the specified chroot enviroment.
2. Administrators use jail-shell command to manage the list of restricted users, manage the list of commands for the chroot enviroment, and manage the access range of directories.
3. Jail-cmd proxies specific command, to help implement the necessary business functions.

Compile and install

Compile

git clone https://github.com/pymumu/jail-shell.git
cd jail-shell
make 

Install

sudo make install

Uninstall

sudo /usr/local/jail-shell/install -u

Usage

After installation, you can use jail-shell command to manage jails, jail-shell -h for help.
In use, the steps are as follows:

1. Use useradd username command to add user to the system.
2. Use jail-shell jail command to create a chroot enviroment.
3. Use jail-shell user command to add user to the jails.

Example

The following is an example of adding user test to a jail named test-jail.

1. add user test，and set password

   sudo useradd test -s /bin/bash
   sudo passwd test

2. create chroot enviroment

   sudo jail-shell jail -e test-jail

   After executing the above command, a new jail configuration will be created from the template, and it is opened by vi, you can edit it, after that, remember to save the configuration with vi command :w!.

3. install chroot enviroment

   sudo jail-shell jail -i test-jail

4. add user test to jail test-jail

   sudo jail-shell user -a test -j test-jail

5. connect and test whether test is jailed.

   ssh test@127.0.0.1

   

Jail Config file format description

The jail config file is located at /etc/jail-shell/jail-config/, and file suffix is .cfg
The configuration supports the following commands:

• dir

  • DESC:
    create a directory into jail
  • COMMAND:
    dir PATH MODE OWNER
  • EXAMPLE:
    dir /bin/ 0755 root:root

• file:

  • DESC:
    copy a file into jail
  • COMMAND:
    file SRC DEST MODE OWNER
  • EXAMPLE:
    file /etc/nsswitch.conf /etc/nsswitch.conf 0644 root:root

• hlink:

  • DESC:
    create a hardlink file into jail
  • COMMAND:
    file SRC DEST MODE OWNER
  • EXAMPLE:
    file /etc/nsswitch.conf /etc/nsswitch.conf 0644 root:root

• slink:

  • DESC:
    create a symbolic link into jail
  • COMMAND:
    slink TARGET LINKNAME
  • EXAMPLE:
    slink /bin/bash /bin/sh

• clink:

  • DESC:
    Try to create hardlinks instead of copying the files. If linking fails it falls back to copying
  • COMMAND:
    clink TARGET LINKNAME
  • EXAMPLE:
    clink /etc/localtime /etc/localtime

• node:

  • DESC:
    create device file.
  • COMMAND:
    node PATH TYPE MAJON MINOR MODE OWNER
  • EXAMPLE:
    node /dev/null c 1 3 666 root:root
  • NOTE: security tips
    should avoid adding block device files.

• bind:

  • DESC:
    bind a directory to jail
  • COMMAND:
    bind [SRC] DEST OPTION
  • OPTION: rw,ro,dev,nodev,exec,noexec, refer to (man mount) for the parameter description
    %u in path '[SRC] DEST' will be replaced as user name
  • EXAMPLE:
    bind / ro,nodev,nosuid
bind /opt/ /opt/ ro,nodev,noexec
bind /opt/upload /opt/upload rw,nodev,noexec,nosuid
bind /opt/%u /opt/upload ro,nodev,noexec,nosuid

• cmd:

  • DESC:
    executes commands within the system which outside jail.
  • COMMAND:
    cmd SRC DEST RUN_AS_USER
  • RUN_AS_USER: User who executes system commands, -:- means user in jail
  • EXAMPLE:
    cmd /usr/bin/passwd /usr/bin/passwd -:-
cmd /some/root/command /some/root/command root:root
cmd /some/user/command /some/user/command user:user
  • NOTE: security tips This channel may lead users to escape jail, should avoid adding command which can be shell-inject,
    For example, read the commands entered by the user

Security Tips

When using jail-shell, the minimum security authorization principle should be adopted. In the premise of ensuring the use of functions, reduce user rights.

1. bind tips

   • Except /dev directory, it is recommended to add nodev parameters, /dev directory must set to ro, noexec (read-only, disable executable) permissions.
   • For the chroot environment directory, it is recommended to set ro, nodev, nosuid (read only, prohibit device files, and prohibit suid files) permissions.
   • For writable bind directories, it is recommended to set nodev, noexec, nosuid (disable device files, disable executable files, disable suid files) permissions.

2. avoid commands

   • avoid: debug commands such as gdb, mount, strace, etc..

File Directory Description

Debugging the chroot environment

When you copy a command to the chroot environment, if the copy command fails, you need to debug to find the missing dependent files, and add them to the chroot environment.
Copy the strace command into the chroot environment, and then use strace to execute the commands that need to be debugged to find the missing dependent files.
The following debugging commands are as follows

strace -F -eopen command

-eopen represents a list of files that the trace process opens.
After executing the above command, troubleshoot to find the open file list.

open ("/etc/ld.so.preload", "O_RDONLY") = -1 ENOENT (No, such, file, or, directory)

As indicated above, the /etc/ld-so.preload file does not exist when reading, and may need to add the above files to the chroot environment. At this point, you can use the clink, file command to add missing files to the chroot environment.

License

Jail-shell using GPL-V2 License.

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Similar tools

jailkit https://olivier.sessink.nl/jailkit/
rshell https://en.wikipedia.org/wiki/Restricted_shell
firejail https://github.com/netblue30/firejail