1. Chroot Jail Ssh
2. Ssh Chroot Server

With the above, user joe can ssh in and will be restricted to the chroot. Unfortunately, this doesn't do much, but it gives you an idea of how it can be set up. Depending on what you want to. Jan 07, 2018 Multiple -t options force tty allocation, even if ssh has no local tty. In this example, change password for user called nixcraft on server1.cyberciti.biz by issuing the passwd command over ssh. The chrooted SSH will be installed in such a way that it will still use the configuration files of the standard OpenSSH Debian package which are in /etc/ssh/, and you will be able to use the standard OpenSSH Debian init script /etc/init.d/ssh. Therefore you do not have to create your own init script and configuration file. Sftp performs all operations over an encrypted ssh Connection. It has the features of using ssh public key authentication and more as like ssh. If a user only allowed to access his files without ssh shell access we can create a chroot environment for those user’s. While chroot enabled user’s will be jailed into there own home directory.

A chroot on Unixoperating systems is an operation that changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name (and therefore normally cannot access) files outside the designated directory tree. The term 'chroot' may refer to the chroot(2)system call or the chroot(8) wrapper program. The modified environment is called a chroot jail.

History[edit]

The chroot works, but unlike what the man says After the chroot, sshd (8) changes the working directory to the user's home directory the working dir of the user after login is /home/sftp, not /home/sftp/allu, and my guess is that because /home/sftp/allu belongs to guacam and not sftpu (while the sftpg group has rwx access in allu and could write.

The chroot system call was introduced during development of Version 7 Unix in 1979. One source suggests that Bill Joy added it on 18 March 1982 – 17 months before 4.2BSD was released – in order to test its installation and build system.^[1] All versions of BSD that had a kernel have chroot(2).^[2][3] An early use of the term 'jail' as applied to chroot comes from Bill Cheswick creating a honeypot to monitor a cracker in 1991.^[4]

The first article about a jailbreak has been discussed on the security column of SunWorld Online which is written by Carole Fennelly; the August 1999 and January 1999 editions cover most of the chroot() topics.^[5]

To make it useful for virtualization, FreeBSD expanded the concept and in its 4.0 release in 2000 introduced the jail command.^[6]

By 2002, an article written by Nicolas Boiteux described how to create a jail on Linux ^[7]

By 2003, first internet microservices providers with Linux jails provide SAAS/PAAS (shell containers, proxy, ircd, bots, ...) services billed for consumption into the jail by usage^[8]

By 2005, Sun released Solaris Containers (also known as Solaris Zones), described as 'chroot on steroids.'^[9]

By 2008, LXC (upon which Docker was later built) adopted the 'container' terminology^[10] and gained popularity in 2013 due to inclusion into Linux kernel 3.8 of user namespaces.^[11]

Uses[edit]

A chroot environment can be used to create and host a separate virtualized copy of the software system. This can be useful for:

Testing and development

A test environment can be set up in the chroot for software that would otherwise be too risky to deploy on a production system.

Dependency control

Software can be developed, built and tested in a chroot populated only with its expected dependencies. This can prevent some kinds of linkage skew that can result from developers building projects with different sets of program libraries installed.

Compatibility

Legacy software or software using a different ABI must sometimes be run in a chroot because their supporting libraries or data files may otherwise clash in name or linkage with those of the host system.

Recovery

Should a system be rendered unbootable, a chroot can be used to move back into the damaged environment after bootstrapping from an alternate root file system (such as from installation media, or a Live CD).

Privilege separation

Programs are allowed to carry open file descriptors (for files, pipelines and network connections) into the chroot, which can simplify jail design by making it unnecessary to leave working files inside the chroot directory. This also simplifies the common arrangement of running the potentially vulnerable parts of a privileged program in a sandbox, in order to pre-emptively contain a security breach. Note that chroot is not necessarily enough to contain a process with root privileges.

Limitations[edit]

Chroot Jail Ssh

The chroot mechanism is not intended to defend against intentional tampering by privileged (root) users. On most systems, chroot contexts do not stack properly and chrooted programs with sufficient privileges may perform a second chroot to break out. To mitigate the risk of this security weakness, chrooted programs should relinquish root privileges as soon as practical after chrooting, or other mechanisms – such as FreeBSD jails – should be used instead. Note that some systems, such as FreeBSD, take precautions to prevent the second chroot attack.^[12]

On systems that support device nodes on ordinary filesystems, a chrooted root user can still create device nodes and mount the file systems on them; thus, the chroot mechanism is not intended by itself to be used to block low-level access to system devices by privileged users. It is not intended to restrict the use of resources like I/O, bandwidth, disk space or CPU time. Most Unixes are not completely file system-oriented and leave potentially disruptive functionality like networking and process control available through the system call interface to a chrooted program.

At startup, programs expect to find scratch space, configuration files, device nodes and shared libraries at certain preset locations. For a chrooted program to successfully start, the chroot directory must be populated with a minimum set of these files. This can make chroot difficult to use as a general sandboxing mechanism.

Only the root user can perform a chroot. This is intended to prevent users from putting a setuid program inside a specially crafted chroot jail (for example, with a fake /etc/passwd and /etc/shadow file) that would fool it into a privilege escalation.

Some Unixes offer extensions of the chroot mechanism to address at least some of these limitations (see Implementations of operating system-level virtualization technology).

Graphical applications on chroot[edit]

It is possible to run graphical applications on a chrooted environment, using methods such as:^[13][14]

• Use xhost (or copy the secret from .Xauthority)
• Nested X servers like Xnest or the more modern Xephyr (or start a real X server from inside the jail)
• Accessing the chroot via SSH using the X11 forwarding (ssh -X) feature
• xchroot an extended version of chroot for users and Xorg/X11 forwarding (socat/mount)
• An X11 VNC server and connecting a VNC client outside the environment.

Notable applications[edit]

The Postfix mail transfer agent operates as a pipeline of individually chrooted helper programs.

Like 4.2BSD before it, the Debian and Ubuntu internal package-building farms use chroots extensively to catch unintentional build dependencies between packages. SUSE uses a similar method with its build program. Fedora, Red Hat, and various RPM-based distributions build all RPMs using a chroot tool such as mock.

Many FTP servers for POSIX systems use the chroot mechanism to sandbox untrusted FTP clients. This may be done by forking a process to handle an incoming connection, then chrooting the child (to avoid having to populate the chroot with libraries required for program startup).

If privilege separation is enabled, the OpenSSH daemon will chroot an unprivileged helper process into an empty directory to handle pre-authentication network traffic for each client. The daemon can also sandbox SFTP and shell sessions in a chroot (from version 4.9p1 onwards).^[15]

Chrome OS can use a chroot to run a Linux instance using Crouton,^[16] providing an otherwise thin OS with access to hardware resources. The security implications related in this article apply here.

Linux host kernel virtual file systems and configuration files[edit]

To have a functional chroot environment in Linux, the kernel virtual file systems and configuration files also have to be mounted/copied from host to chroot.

See also[edit]

References[edit]

1. ^'jail, section 9'. docs.freebsd.org.
2. ^Losh, Warner (February 2, 2000). 'Warner's Random Hacking Blog: Whither chroot?'.
3. ^'Data Infrastructures for the rest of us - III - software'.
4. ^Cheswick, Bill (1991). 'An Evening with Berferd: In Which a Cracker is Lured, Endured, and Studied'(PDF). USENIX Summer Conference Proceedings, Volume 1. USENIX. San Francisco, California: The Association. p. 163.
5. ^Carole, Fennelly. 'Escap Chroot'. SunWorld Online. Carole Fennelly. Archived from the original on 2000-01-09.
6. ^Riondato, Matteo. 'FreeBSD Handbook 'Jails' Chapter'. freebsd.org. The FreeBSD Project. Retrieved 2018-10-30.
7. ^Nicolas, Boiteux. 'chroot shell'. lycos.fr. Nicolas Boiteux. Archived from the original on 2002-10-14. Retrieved 24 March 2018.
8. ^'Girafon'. girafon.org. girafon. Archived from the original on 2004-06-12. Retrieved 24 March 2018.
9. ^Schmidt, Klaus (2006-09-02). High Availability and Disaster Recovery: Concepts, Design, Implementation. Springer Science & Business Media. p. 186. ISBN9783540345824. Retrieved 2014-08-21.
10. ^'SourceForge LXC Download Files'. sourceforge.net. Retrieved 2014-08-21.
11. ^Rosen, Rami (2014-03-26). 'Linux Containers and the Future Cloud'(PDF). Retrieved 2014-08-21.
12. ^'chroot(2)'. www.freebsd.org.
13. ^'Development/Howto/Chroot'. Mandriva Wiki. 25 July 2011. Archived from the original on 2014-03-26.
14. ^'Archived copy'. Archived from the original on 2011-08-31. Retrieved 2011-10-13.CS1 maint: archived copy as title (link)
15. ^'sshd_config(5) manual page'. 2017-10-26. Retrieved 2018-02-04.
16. ^'Chromium OS Universal Chroot Environment (on github)'. Retrieved 2016-12-17.

External links[edit]

• chroot(2): change root directory – FreeBSD System Calls Manual
• chroot(8): change root directory – FreeBSD System Manager's Manual
• chroot(2): change root directory – Linux Programmer's Manual – System Calls

You may have seen the recent commit message from djm@ about the new feature in OpenSSH: ChrootDirectory

Ssh Chroot Server

Damien Miller (djm@), who worked on this new feature with Markus Friedl (markus@), offers more details about ChrootDirectory:

This commit adds a chroot(2) facility to sshd, controlled by a new sshd_config(5) option 'ChrootDirectory'. This can be used to 'jail' users into a limited view of the filesystem, such as their home directory, rather than letting them see the full filesystem.

More from Damien follows.

Unfortunately, setting up a chroot(2) environment is complicated, fragile and annoying to maintain. The most frequent reason our users have given when asking for chroot support in sshd is so they can set up file servers that limit semi-trusted users to be able to access certain files only. Because of this, we have made this particular case very easy to configure.

In a previous commit, markus@ implemented an 'in-process' sftp server in sshd, basically linking sftp-server(8) into sshd(8). When the in-process sftp server is used, sshd does not need any special chroot configuration (no /dev nodes, no libraries, no statically-linked sftp-server) so the chroot setup and maintenance burden is eliminated. The chroot support does work for login and command-execution sessions too, but administrators will need to configure the chroot environment manually.

To set up a restricted sftp server one should use the 'ForceCommand' and 'ChrootDirectory' directives in sshd_config. Presumably most people will not want to restrict every user, so they should also use the 'Match' directive to select a user or group to apply the restrictions to. For example:

This will cause the user 'djm' to be chrooted to the '/chroot' directory at login, and the use of the in-process sftp server will be forced for all connections. I.e. the user will not be able to login interactively, or run arbitrary commands - the login will only be useful for sftp transfers. Note that the user's home directory may exist under the '/chroot' directory above (e.g. '/chroot/home/djm') and sshd will try to chdir to it before starting to serve files, but it doesn't matter if it does not exist.

Setting up a safe chroot jail is somewhat tricky, and it is quite easy to make to compromise one's security. To reduce this risk, sshd ensures the ChrootDirectory and each of its components is root-owned and not writable by other users, but it is still possible for administrators to break their own setups by doing dumb things (e.g. leaving /dev nodes for the physical drives in a chroot, executing scripts inside the chroot from cron(8) or elsewhere, etc.).

A limitation of the chroot support is that the in-process sftp server does not support scp(1) transfers. scp is a really busted protocol and it would be a fair bit more work to build it in in the way we have built in sftp. It is still possible to support chrooted scp, but administrators will need to populate the chroot environment manually. Please use sftp instead.

To make the internal-sftp chroot work for me, I made the following changes to /etc/ssh/sshd_config:

The full commit message:

Thanks to Damien Miller for taking the time to explain the ChrootDirectory feature.