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newuidmap failed to write mapping means the host could not apply the UID map requested for the container’s user namespace. It does not, by itself, identify why. The most useful checks are which account starts the container, whether that account’s subordinate UID and GID ranges authorize the requested host IDs, and whether the complete generated map is valid. The exact error may say not allowed or Invalid argument; investigate the full mapping request in either case.
What the error means
LXC must map IDs inside a container to IDs on the host when it sets up a user namespace. If the host rejects that mapping, startup can stop with a message such as newuidmap failed to write mapping, often followed by a log entry saying the ID map could not be set up.
The detailed failure matters. A message like newuidmap: uid range ... not allowed indicates a rejected range in the reported configuration. A message like newuidmap: write to uid_map failed: Invalid argument is a different reported failure. Neither phrase alone proves a single cause: compare the complete requested mapping with the host’s effective configuration.
How to diagnose the mapping failure
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Record the complete error
Keep the guest start ID, host start ID, count, and exact error text for every mapping segment. Do not copy a numeric range from someone else’s error as a fix: one forum report involved a very large requested range that was not authorized by that system’s displayed configuration, and its values do not establish the correct range for your host. See the Linux Containers mapping-failure discussion.
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Identify the account that launches the container
Determine whether the caller is root, a regular user, or a service or daemon account. Check that account against the owners and ranges in
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Compare every map segment with the delegated range
For each
lxc.idmapline, compare the guest start ID, host start ID, and count. Verify that the requested host IDs fall within the subordinate range assigned to the account doing the mapping. Check the entire map, not only a recently added line: adding a custom mapping for one guest identity changes the surrounding segments too. A Linux Containers maintainer identified a custom multi-segment map as incorrect in one report, while another example showed a requested host ID outside the displayed allocation: custom mapping example and discussion.Rank #2
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Validate UID and GID maps separately
A valid UID allocation does not establish that the GID allocation is valid. Review
/etc/subuid,/etc/subgid, and the correspondinguandgmapping lines together. Do not assume that one correct map makes the other correct, or reuse a numeric range without confirming it is delegated on your host. -
For LXD, inspect the existing instance’s effective map
A community discussion reports an existing instance retaining an earlier mapping after LXD configuration changed, while a newly created instance used the updated map. Check the actual state of the affected instance rather than assuming a changed default or daemon restart updated it. The report is a reason to inspect state, not a recommendation to delete or recreate an important container: LXD instance-state discussion.
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For Incus or other managed setups, inspect the generated map
Review the map the manager actually generated, including all segments, and compare it with the host’s subordinate allocations. An Incus issue documents a
uid_mapwrite failing withInvalid argumentfor a generated multi-segment map: Incus uid_map write report. A separate report describes isolated ID-map generation that appeared inconsistent with/etc/subuid; it was marked incomplete, so it should not be treated as proof of a general Incus bug or of a universally fixed issue: Incus isolated ID-map report.
Why common fixes can fail
- “The subordinate-ID files exist.” Their presence alone does not show that the correct account owns the needed range or that the requested host IDs and count fit within it.
- “I changed one custom mapping line.” A custom line affects how the full guest-to-host map is divided. Recheck every segment and its start IDs and count.
- “I changed the default and restarted the service.” A managed instance may still have a previously stored map. Inspect its effective configuration before deciding how to apply a change.
- “The error says Invalid argument, so the cause is obvious.” That wording is a clue, not a complete diagnosis. Match the full generated request against the runtime configuration, just as you would for
not allowed.
What to include when asking for help
To make the problem diagnosable, provide the exact failure line and all requested map segments, the identity that starts the container, the relevant subordinate UID and GID allocations, and whether the setup is direct LXC or managed by LXD or Incus. For a managed instance, include its effective map and distinguish an existing instance from a newly created one. Redact unrelated sensitive information, but preserve the numeric ranges and error wording.
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