Network virtualization

Singularity provides full integration with cni , to make network virtualization easy. The following options can be used with the the action commands (exec, run, and shell) to create and configure a virtualized network for a container.

Note

Many of these options are available only to the root user by default. Unrestricted ability to configure networking for containers would allow users to affect networking on the host, or in a cluster.

Singularity 3.8 allows the administrator to permit a list of unprivileged users and/or groups to use specified network configurations. This is accomplished through settings in singularity.conf. See the administrator guide for details.

--dns

The --dns option allows you to specify a comma separated list of DNS servers to add to the /etc/resolv.conf file.

$ nslookup sylabs.io | grep Server
Server:             127.0.0.53

$ sudo singularity exec --dns 8.8.8.8 ubuntu.sif nslookup sylabs.io | grep Server
Server:             8.8.8.8

$ sudo singularity exec --dns 8.8.8.8 ubuntu.sif cat /etc/resolv.conf
nameserver 8.8.8.8

--hostname

The --hostname option accepts a string argument to change the hostname within the container.

$ hostname
ubuntu-bionic

$ sudo singularity exec --hostname hal-9000 my_container.sif hostname
hal-9000

--net

Passing the --net flag will cause the container to join a new network namespace when it initiates. New in Singularity 3.0, a bridge interface will also be set up by default.

$ hostname -I
10.0.2.15

$ sudo singularity exec --net my_container.sif hostname -I
10.22.0.4

--network

The --network option can only be invoked in combination with the --net flag. It accepts a comma delimited string of network types. Each entry will bring up a dedicated interface inside container.

$ hostname -I
172.16.107.251 10.22.0.1

$ sudo singularity exec --net --network ptp ubuntu.sif hostname -I
10.23.0.6

$ sudo singularity exec --net --network bridge,ptp ubuntu.sif hostname -I
10.22.0.14 10.23.0.7

When invoked, the --network option searches the singularity configuration directory (commonly /usr/local/etc/singularity/network/) for the cni configuration file corresponding to the requested network type(s). Several configuration files are installed with Singularity by default corresponding to the following network types:

  • bridge

  • ptp

  • ipvlan

  • macvlan

  • none (must be used alone)

By default, none is the only network configuration that can be used by non-privileged users. It isolates the container network from the host network with a loopback interface.

Administrators can permit certain users or groups to request other network configurations through options in singularity.conf. Additional cni configuration files can be added to the network configuration directory as required, and Singularity’s provided configurations may also be modified.

--network-args

The --network-args option provides a convenient way to specify arguments to pass directly to the cni plugins. It must be used in conjunction with the --net flag.

For instance, let’s say you want to start an NGINX server on port 80 inside of the container, but you want to map it to port 8080 outside of the container:

$ sudo singularity instance start --writable-tmpfs \
    --net --network-args "portmap=8080:80/tcp" docker://nginx web2

The above command will start the Docker Hub official NGINX image running in a background instance called web2. The NGINX instance will need to be able to write to disk, so we’ve used the --writable-tmpfs argument to allocate some space in memory. The --net flag is necessary when using the --network-args option, and specifying the portmap=8080:80/tcp argument which will map port 80 inside of the container to 8080 on the host.

Now we can start NGINX inside of the container:

$ sudo singularity exec instance://web2 nginx

And the curl command can be used to verify that NGINX is running on the host port 8080 as expected.

$ curl localhost:8080
10.22.0.1 - - [16/Oct/2018:09:34:25 -0400] "GET / HTTP/1.1" 200 612 "-" "curl/7.58.0" "-"
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
    body {
        width: 35em;
        margin: 0 auto;
        font-family: Tahoma, Verdana, Arial, sans-serif;
    }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>

<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>

<p><em>Thank you for using nginx.</em></p>
</body>
</html>

For more information about cni, check the cni specification.