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Debugging Container ID Cannot Be Mapped to Host ID Error

The error container id 1000000 cannot be mapped to a host id indicates that CircleCI could not start your container because the userns remapping failed.

This document explains what this error means and how to fix it.

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failed to register layer: Error processing tar file(exit status 1): container id 1000000 cannot be mapped to a host id

What is userns?

The user namespace, or sometimes just userns, is a feature of the Linux kernel that adds another layer of security into the Linux container technology. The userns allows a host machine to run containers in different UID/GID namespaces from the host machine. This means all containers can have UID 0 (root account) in their own namespace and run processes without the host machine giving root priviledges to the container.

When a userns is created, the Linux kernel provides a mapping of UID/GID between the container and the host machine. For example, if you start a container and run a process with UID 0 inside the container, the Linux kernel maps the container’s UID 0 to a non-privileged UID such as UID 65534 on the host machine. Because of this mapping, the process inside the container can run as if it’s the root user, but it is actually run by the non-root user on the host machine.

How Docker and CircleCI use userns

CircleCI runs Docker containers with userns enabled in order to run customers’ containers securely. When Docker starts a container, Docker pulls an image and extracts image layers form the image. If a layer contains files created with UID/GID outside the remapping range that CircleCI configured on the host machine, say UID 1000000, Docker cannot remap the UID/GID and fails to start the container. This is exactly what’s happening in the error you see at the top of this document.

What should I do?

When you hit this error, you need to update the files’ UID/GID and re-create the image.

Please contact the image maintainer and report the error details.

If you are an image maintainer, you need to look for which file has high UID/GID and correct it accordingly.

Here is one way to find such a file inside circleci/doc-highid.

First, grab the high UID/GID value from container id XXX cannot be mapped to a host id error message in CircleCI. For example, the value is 1000000 in the error message you see at the top of this document.

Second, start the container and look for which files get the high value. There are multiple ways to do this, but one way is using find command. Note that the following example only works with BSD/GNU find command. If BSD/GNU find is not installed in your container, you may need to use different commands.

# Start a shell inside the container
$ docker run -it circleci/doc-highid sh

# Find the file with high UID/GID
$ find / \( -uid 1000000 \)  -ls 2>/dev/null
     50      0 -rw-r--r--   1 veryhigh veryhigh        0 Jul  9 03:05 /file-with-high-id

# Confirm that the file has high UID/GID
$ ls -ln file-with-high-id
-rw-r--r-- 1 1000000 1000000 0 Jul  9 03:05 file-with-high-id

Once you find which file has a high UID/GID, make sure to change the ownership of the file and re-create the image.