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I'm getting started with running Docker 20.10.14 in rootless mode on Linux Mint 20 (based on Ubuntu 20.04). I've created a user named dockerd for this, UID 127 (with group dockerd, GID 139) and added the following to both /etc/subuid and /etc/subgid

dockerd:200000:65536

Now, inside a Docker container run by this dockerd user I create a file on the host file system (via a bind-mounted directory).

  • If I create it as UID 1000 in the container it maps to user 200999 on the host.
  • If I create it as UID 1 (user daemon) in the container it has UID 200000 on the host.
  • If I create it as UID 0 (user root) in the container it has UID 127 on the host.

So it appears that the sub-UID/sub-GID bindings are "1-based" and do not include UID 0. Is this how it's supposed to work or am I doing something wrong? Shouldn't I be able to map the root user in the container to as UID of my choice on the host? If so, how do I do that?

From https://unix.stackexchange.com/a/397168/107961

dockremap:165536:65536

165536 is the system UID to start the UID mapping at (Which will be UID 0 in the container)

... it sounds like the first UID in the mapping should map to UID 0, but that's not what I'm seeing.

Another weird thing is that a file owner by UID 0 on the host is owned by UID 65534 (nobody) in the container. Is that supposed to happen?

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  • Which version of Docker? Commented Apr 11, 2022 at 10:19
  • 20.10.14. Edited that in. Commented Apr 11, 2022 at 11:38
  • So is the uid of dockerd 127? in suck case it fits: 1+65536=65537 uids are mapped. Commented Apr 11, 2022 at 12:53
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    @A.B. yes, the UID of dockerd is 127, but I'm not following how "it fits" that the root user inside the container is mapped to 127 and not 200000. Could you elaborate? Commented Apr 12, 2022 at 7:22
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    Well, no, I'm not happy with it. :) I would be happy if it mapped the start of the range I specified, 200000, to UID 0. Do you know how to achieve that? Commented Apr 12, 2022 at 12:24

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