How do I limit an sFTP user to one directory?

OpenSSH (4.8p1 for the GNU/Linux port) and up feature a configuration option: ChrootDirectory. This has been made possible by a new SFTP subsystem statically linked to sshd.

This makes it easy to replace a basic FTP service without the hassle of configuring encryption and/or bothering with FTP passive and active modes when operating through a NAT router. This is also simpler than packages such as rssh or other patches because it does not require setting up and maintaining (i.e. security updates) a chroot environment.

To enable it, you obviously need the new version 4.8p1. I personaly use the cvs version and the debian/ directory of the sid package to build a well integrated Debian package 4.8p1~cvs-1.

In /etc/ssh/sshd_config:

You need to configure OpenSSH to use its internal SFTP subsystem.

Subsystem sftp internal-sftp

Then, I configured chroot()ing in a match rule.

Match group sftponly
ChrootDirectory /home/%u
X11Forwarding no
AllowTcpForwarding no
ForceCommand internal-sftp

The directory in which to chroot() must be owned by root. After the call to chroot(), sshd changes directory to the home directory relative to the new root directory. That is why I use / as home directory.

$ chown root.root /home/user
$ usermod -d / user
$ adduser user sftponly

This seems to work as expected:

$ sftp user@host
Connecting to host...
user@host's password:
sftp> ls
build               cowbuildinall       incoming            johnbuilderclean
sftp> pwd
Remote working directory: /
sftp> cd ..
sftp> ls
build               cowbuildinall       incoming            johnbuilderclean

We inlined the original instructions from https://debian-administration.org/article/590/OpenSSH_SFTP_chroot_with_ChrootDirectory⁠ as at the time of writing that website no longer functions.