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Adding shell commands to pod containers

Due to the restrictive nature of kubernetes and CoreOS, containerized OpenStack services do not have access to the same shell commands available when they run in an Enterprise Linux environment. For example, common debug tools like ping and ncat are not available in containers running in a podified controlplane.

However, it is possible to install commands into podified containers. Here is an example that installs the ip command into the cinder-backup container in the cinder-backup-0 pod.

Prerequisites

The command's executable should be installed from a source OS that matches the one associated with the container. For example, when the container's image is based on CentOS Stream 9, then you should install CS-9's version of the command. In the following example, the host machine (on which the installation instructions are executed) and the cinder-backup container are based on the same OS. Other OS combinations may work, but your mileage may vary.

Procedure

  1. Find the absolute path of the command you wish to install
realpath $(which ip)

The answer is /usr/sbin/ip. Some commands like nc are actually symlinks, and it's necessary to install the absolute file.

  1. Use kubectl to copy the file into the container
kubectl cp /usr/sbin/ip cinder-backup-0:/usr/sbin/

If necessary, you can include "-c container" to specify a particular container within the pod.

  1. Check to see if additional libraries need to be installed
oc rsh cinder-backup-0 ldd /usr/sbin/ip

Look for any "libXXX => not found" lines, seen here:

        linux-vdso.so.1 (0x00007fff583f9000)
        libbpf.so.1 => not found
        libelf.so.1 => /lib64/libelf.so.1 (0x00007f459e007000)
        libmnl.so.0 => not found
        libcap.so.2 => /lib64/libcap.so.2 (0x00007f459dffd000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f459ddf4000)
        libz.so.1 => /lib64/libz.so.1 (0x00007f459ddda000)
        libzstd.so.1 => /lib64/libzstd.so.1 (0x00007f459dd01000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f459e18e000)
  1. Install any missing libraries

In this example, libbpf.so.1 and libmnl.so.0 were not found, so the host files in /lib64 need to be installed. However, these are just symlinks, and it's necessary to install both the symlink and absolute file from the host. Note that kubectl copies symlinks intact. It creates a corresponding symlink in the container, and does not follow the source's symlink.

# Install the symlinks from the ldd output
kubectl cp /lib64/libbpf.so.1 cinder-backup-0:/lib64/
kubectl cp /lib64/libmnl.so.0 cinder-backup-0:/lib64/

# Copy the absolute files referenced by the symlinks
kubectl cp /lib64/libbpf.so.1.2.0 cinder-backup-0:/lib64/
kubectl cp /lib64/libmnl.so.0.2.0 cinder-backup-0:/lib64/

The ip command is now available for use within in the cinder-backup container.