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Install Apptainer

Apptainer (formerly known as Singularity, see announcement) is a tool that creates docker-like process containers but without giving extra privileges to unprivileged users. It is used by pilot jobs (which are submitted by per-collaboration workload management systems) to isolate user jobs from the pilot's files and processes and from other users' files and processes. It also supplies a chroot environment in order to run user jobs in different operating system images under one Linux kernel.

Apptainer works either by making use of unprivileged user namespaces or with a setuid-root assist program. By default it does not install the setuid-root assist program and it uses only unprivileged user namespaces. Unprivileged user namespaces are available on all OS versions that OSG supports, although it is not enabled by default on EL 7; instructions to enable it are below. The feature is enabled by default on EL 8.

Kernel vs. Userspace Security

Enabling unprivileged user namespaces increases the risk to the kernel. However, the kernel is much more widely reviewed than Apptainer and the additional capability given to users is more limited. OSG Security considers the non-setuid, kernel-based method to have a lower security risk, and they also recommend disabling network namespaces as detailed below.

The OSG has installed Apptainer in OASIS, so most sites will not need to install Apptainer locally unless they have non-OSG users that need it.

This document is intended for system administrators that wish to enable, install, and/or configure Apptainer.

Before Starting

As with all OSG Software installations, there are some one-time (per host) steps to prepare in advance:

  • Ensure the host has a supported operating system
  • Obtain root access to the host
  • If you intend to install Apptainer locally, then prepare the required Yum repositories. Note that the apptainer RPM comes from the EPEL Yum repository. OSG validates that distribution, and detailed instructions are still here.

In addition, this is highly recommended for image distribution and for access to Apptainer itself:

Choosing whether or not to install Apptainer

There are two sets of instructions on this page:

OSG VOs all support running apptainer directly from CVMFS, when CVMFS is available and unprivileged user namespaces are enabled. Unprivileged user namespaces are enabled by default on EL 8, and OSG recommends that system administrators enable it on EL 7 worker nodes. When unprivileged user namespaces are enabled, OSG recommends that sites not install Apptainer unless they have non-OSG users that require it.

Sites that do want to install apptainer locally have two choices on how to do it. They can install it with a script which creates an unprivileged relocatable installation directory, or they can install it by RPM. Sites that install the RPM will by default still only get a non-setuid installation that makes use of unprivileged user namespaces and will need to install an additional apptainer-suid RPM if they want a setuid installation that does not require unprivileged user namespaces.

Enabling Unprivileged Apptainer

The instructions in this section are for enabling Apptainer to run unprivileged by enabling unprivileged user namespaces.

  1. Enable user namespaces via sysctl on EL 7:

    If the operating system is an EL 7, enable unprivileged Apptainer with the following steps. This step is not needed on EL 8 because it is enabled by default.

    root@host # echo "user.max_user_namespaces = 15000" \
        > /etc/sysctl.d/90-max_user_namespaces.conf
    root@host # sysctl -p /etc/sysctl.d/90-max_user_namespaces.conf
  2. (Recommended) Disable network namespaces:

    root@host # echo "user.max_net_namespaces = 0" \
        > /etc/sysctl.d/90-max_net_namespaces.conf
    root@host # sysctl -p /etc/sysctl.d/90-max_net_namespaces.conf

    OSG VOs do not need network namespaces with Apptainer, and disabling them significantly lowers the risk profile of enabling user namespaces and reduces the frequency of needing to apply urgent updates. Most of the kernel vulnerabilities related to unprivileged user namespaces over the last few years have been in combination with network namespaces.

    Network namespaces are, however, utilized by other software, such as Docker or Podman. Disabling network namespaces may break other software, or limit its capabilities (such as requiring the --net=host option in Docker or Podman).

    Disabling network namespaces blocks the systemd PrivateNetwork feature, which is a feature that is used by some EL 8 services. It is also configured for some EL 7 services but they are all disabled by default. To check them all, look for PrivateNetwork in /lib/systemd/system/*.service and see which of those services are enabled but failed to start. The only default such service on EL 8 is systemd-hostnamed, and a popular non-default such service is mlocate-updatedb. The PrivateNetwork feature can be turned off for a service without modifying an RPM-installed file through a <service>.d/*.conf file, for example for systemd-hostnamed:

    root@host # cd /etc/systemd/system
    root@host # mkdir -p systemd-hostnamed.service.d
    root@host # (echo "[Service]"; echo "PrivateNetwork=no") \
    root@host # systemctl daemon-reload
    root@host # systemctl start systemd-hostnamed
    root@host # systemctl status systemd-hostnamed

Configuring Docker to work with Apptainer

If docker is being used to run jobs, the following options are recommended to allow unprivileged Apptainer to run (it does not need --privileged or any added capabilities):

--security-opt seccomp=unconfined --security-opt systempaths=unconfined

--security-opt seccomp=unconfined enables unshare to be called (which is needed to create namespaces), and --security-opt systempaths=unconfined allows /proc to be mounted in an unprivileged process namespace (as is done by apptainer exec -p). --security-opt systempaths=unconfined requires Docker 19.03 or later. The options are secure as long as the system administrator controls the images and does not allow user code to run as root, and are generally more secure than adding capabilities. If at this point no setuid or setcap programs needs to be run within the container, adding the following option will improve security by preventing any privilege escalation (Apptainer uses the same feature on its containers):

--security-opt no-new-privileges

In addition, the following option is recommended for allowing unprivileged fuse mounts:


Configuring Unprivileged Apptainer

When unprivileged user namespaces are enabled and VOs run apptainer from CVMFS, the Apptainer configuration file also comes from CVMFS so local sites have no control over changing the configuration. However, the most common local configuration change to the apptainer RPM is to add additional local "bind path" options to map extra local file paths into containers. This can instead be accomplished by setting the APPTAINER_BINDPATH variable in the environment of jobs, for example through configuration on your compute entrypoint. This is a comma-separated list of paths to bind, following the syntax of the apptainer exec --bind option. In order to be backward compatible with Singularity, also set SINGULARITY_BINDPATH to the same value. Apptainer also recognizes that variable but it prints a deprecation warning if only a SINGULARITY_ variable is set without the corresponding APPTAINER_ variable.

There are also other environment variables that can affect Apptainer operation; see the Apptainer documentation for details.

Validating Unprivileged Apptainer in CVMFS

If you will not be installing Apptainer locally and you haven't yet installed CVMFS, please do so. Alternatively, use the cvmfsexec package configured for osg as an unprivileged user and mount the and repositories.

Then as an unprivileged user verify that Apptainer in CVMFS works with this command:

user@host $ /cvmfs/ \
                exec --contain --ipc --pid --bind /cvmfs \
                /cvmfs/ \
                ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
user           1       0  0 10:51 console  00:00:00 appinit
user          11       1  0 10:51 console  00:00:00 /usr/bin/ps -ef

Installing Apptainer

The instructions in this section are for installing a local copy of Apptainer, either an unprivileged installation or an RPM installation.

Installing Apptainer via unprivileged script

To install a relocatable unprivileged installation of Apptainer, follow the instructions in the upstream documentation.

Installing Apptainer via RPM

To install the apptainer RPM, make sure that your host is up to date before installing the required packages:

  1. Clean yum cache:

    root@host # yum clean all --enablerepo=*
  2. Update software:

    root@host # yum update

    This command will update all packages

  3. Install Apptainer

    root@host # yum install apptainer
  4. If you choose to install the (not recommended) setuid-root portion of Apptainer, that can be done by instead doing this:

    root@host # yum install apptainer-suid

Configuring Apptainer RPM

Generally Apptainer requires no configuration, but if you install it by RPM the primary configuration is done in /etc/apptainer/apptainer.conf.


If you modify /etc/apptainer/apptainer.conf, be careful with your upgrade procedures. RPM will not automatically merge your changes with new upstream configuration keys, which may cause a broken install or inadvertently change the site configuration. Apptainer changes its default configuration file more frequently than typical OSG software.

Look for apptainer.conf.rpmnew after upgrades and merge in any changes to the defaults.

Upgrading from Singularity RPM

When upgrading from Singularity to Apptainer, any local changes that were made to /etc/singularity/singularity.conf need to be manually migrated to /etc/apptainer/apptainer.conf and the /etc/singularity directory needs to be deleted.

See the Apptainer Migrating from Singularity guide and its explanation of Singularity compatibility for more details.

Limiting Image Types with Setuid Installation

If the RPM installation is setuid, consider the following.

Images based on loopback devices carry an inherently higher exposure to unknown kernel exploits compared to directory-based images distributed via CVMFS. See this article for further discussion.

In setuid mode, the SIF images produced by default by Apptainer are mounted with loopback devices. However, OSG VOs only need directory-based images, and Apptainer can also mount SIF images using unprivileged user namespaces. Hence, it is reasonable to disable the loopback-based images by setting the following option in /etc/apptainer/apptainer.conf:

    max loop devices = 0

While reasonable for some sites, this is not required as there are currently no public kernel exploits for this issue; any exploits are patched by Red Hat when they are discovered. If loopback devices are disabled but unprivileged user namespaces are enabled, then users can run Apptainer with the --userns option (which is the same thing as the default in a non-setuid installation) and still be able to mount images unprivileged, although they will get an error if they don't use the option.

Validating Apptainer installation

After apptainer is installed, as an ordinary user run the following command to verify it:

user@host $ apptainer exec --contain --ipc --pid docker://centos:7 ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
user           1       0  0 11:07 console  00:00:00 appinit
user          12       1  0 11:07 console  00:00:00 /usr/bin/ps -ef

Starting and Stopping Services

Apptainer has no services to start or stop.


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