Portable OpenSSH with Linux namespace support

Add NamespacePIDFile in sshd_config for enter into namespace of the given process. It is useful to isolate user in a container and can be used as replacement for ChrootDirectory. For example:

$ cat  << EOF > Dockerfile
FROM ubuntu:focal
RUN useradd -m -u $(id -u) $(id -un)
CMD sleep infinity
EOF
$ IMAGE_ID=$(docker build -q .)
$ CONTAINER_ID=$(docker run -d $IMAGE_ID)
$ sudo mkdir /run/namespace
$ docker inspect -f '{{.State.Pid}}' $CONTAINER_ID | sudo tee /run/namespace/$(id -un).pid
$ sudo tee -a /etc/ssh/sshd_config > /dev/null << EOF
Match User $(id -un)
NamespacePIDFile /run/namespace/%u.pid
EOF
$ systemctl restart sshd
$ ssh -l $(id -un) localhost        # using the same host just for test purposes

Voilà! You are inside the container.

For enter in the container, the setns system call is used to join the same namespace as process given, just like using the nsenter command:

$ sudo nsenter -a -t $(cat /run/namespace/$(id -un).pid) su - $(id -un)

Portable OpenSSH

OpenSSH is a complete implementation of the SSH protocol (version 2) for secure remote login, command execution and file transfer. It includes a client ssh and server sshd, file transfer utilities scp and sftp as well as tools for key generation (ssh-keygen), run-time key storage (ssh-agent) and a number of supporting programs.

This is a port of OpenBSD's OpenSSH to most Unix-like operating systems, including Linux, OS X and Cygwin. Portable OpenSSH polyfills OpenBSD APIs that are not available elsewhere, adds sshd sandboxing for more operating systems and includes support for OS-native authentication and auditing (e.g. using PAM).

Documentation

The official documentation for OpenSSH are the man pages for each tool:

Stable Releases

Stable release tarballs are available from a number of download mirrors. We recommend the use of a stable release for most users. Please read the release notes for details of recent changes and potential incompatibilities.

Building Portable OpenSSH

Dependencies

Portable OpenSSH is built using autoconf and make. It requires a working C compiler, standard library and headers, and zlib. libcrypto from either LibreSSL or OpenSSL may also be used, but OpenSSH may be built without it supporting a subset of crypto algorithms.

FIDO security token support need libfido2 and its dependencies. Also, certain platforms and build-time options may require additional dependencies, see README.platform for details.

Building a release

Releases include a pre-built copy of the configure script and may be built using:

tar zxvf openssh-X.YpZ.tar.gz
cd openssh
./configure # [options]
make && make tests

See the Build-time Customisation section below for configure options. If you plan on installing OpenSSH to your system, then you will usually want to specify destination paths.

Building from git

If building from git, you'll need autoconf installed to build the configure script. The following commands will check out and build portable OpenSSH from git:

git clone https://github.com/openssh/openssh-portable # or https://anongit.mindrot.org/openssh.git
cd openssh-portable
autoreconf
./configure
make && make tests

Build-time Customisation

There are many build-time customisation options available. All Autoconf destination path flags (e.g. --prefix) are supported (and are usually required if you want to install OpenSSH).

For a full list of available flags, run configure --help but a few of the more frequently-used ones are described below. Some of these flags will require additional libraries and/or headers be installed.

Flag	Meaning
`--with-pam`	Enable PAM support. OpenPAM, Linux PAM and Solaris PAM are supported.
`--with-libedit`	Enable libedit support for sftp.
`--with-kerberos5`	Enable Kerberos/GSSAPI support. Both Heimdal and MIT Kerberos implementations are supported.
`--with-selinux`	Enable SELinux support.
`--with-security-key-builtin`	Include built-in support for U2F/FIDO2 security keys. This requires libfido2 be installed.

Development

Portable OpenSSH development is discussed on the openssh-unix-dev mailing list (archive mirror). Bugs and feature requests are tracked on our Bugzilla.

Reporting bugs

Non-security bugs may be reported to the developers via Bugzilla or via the mailing list above. Security bugs should be reported to openssh@openssh.com.