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Add daemon documentation on user namespaces feature
Remove the experimental docs for user namespaces and add similar content to the `docker daemon` command documentation. Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
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Phil Estes
Tibor Vass
@ -62,6 +62,7 @@ weight = -1
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--tlscert="~/.docker/cert.pem" Path to TLS certificate file
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--tlskey="~/.docker/key.pem" Path to TLS key file
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--tlsverify Use TLS and verify the remote
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--userns-remap="default" Enable user namespace remapping
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--userland-proxy=true Use userland proxy for loopback traffic
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Options with [] may be specified multiple times.
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@ -632,6 +633,133 @@ For information about how to create an authorization plugin, see [authorization
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plugin](../../extend/authorization.md) section in the Docker extend section of this documentation.
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## Daemon user namespace options
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The Linux kernel [user namespace support](http://man7.org/linux/man-pages/man7/user_namespaces.7.html) provides additional security by enabling
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a process, and therefore a container, to have a unique range of user and
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group IDs which are outside the traditional user and group range utilized by
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the host system. Potentially the most important security improvement is that,
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by default, container processes running as the `root` user will have expected
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administrative privilege (with some restrictions) inside the container but will
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effectively be mapped to an unprivileged `uid` on the host.
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When user namespace support is enabled, Docker creates a single daemon-wide mapping
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for all containers running on the same engine instance. The mappings will
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utilize the existing subordinate user and group ID feature available on all modern
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Linux distributions.
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The [`/etc/subuid`](http://man7.org/linux/man-pages/man5/subuid.5.html) and
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[`/etc/subgid`](http://man7.org/linux/man-pages/man5/subgid.5.html) files will be
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read for the user, and optional group, specified to the `--userns-remap`
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parameter. If you do not wish to specify your own user and/or group, you can
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provide `default` as the value to this flag, and a user will be created on your behalf
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and provided subordinate uid and gid ranges. This default user will be named
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`dockremap`, and entries will be created for it in `/etc/passwd` and
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`/etc/group` using your distro's standard user and group creation tools.
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> **Note**: The single mapping per-daemon restriction is in place for now
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> because Docker shares image layers from its local cache across all
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> containers running on the engine instance. Since file ownership must be
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> the same for all containers sharing the same layer content, the decision
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> was made to map the file ownership on `docker pull` to the daemon's user and
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> group mappings so that there is no delay for running containers once the
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> content is downloaded. This design preserves the same performance for `docker
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> pull`, `docker push`, and container startup as users expect with
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> user namespaces disabled.
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### Starting the daemon with user namespaces enabled
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To enable user namespace support, start the daemon with the
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`--userns-remap` flag, which accepts values in the following formats:
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- uid
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- uid:gid
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- username
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- username:groupname
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If numeric IDs are provided, translation back to valid user or group names
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will occur so that the subordinate uid and gid information can be read, given
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these resources are name-based, not id-based. If the numeric ID information
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provided does not exist as entries in `/etc/passwd` or `/etc/group`, daemon
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startup will fail with an error message.
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*Example: starting with default Docker user management:*
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```
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$ docker daemon --userns-remap=default
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```
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When `default` is provided, Docker will create - or find the existing - user and group
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named `dockremap`. If the user is created, and the Linux distribution has
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appropriate support, the `/etc/subuid` and `/etc/subgid` files will be populated
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with a contiguous 65536 length range of subordinate user and group IDs, starting
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at an offset based on prior entries in those files. For example, Ubuntu will
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create the following range, based on an existing user named `user1` already owning
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the first 65536 range:
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```
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$ cat /etc/subuid
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user1:100000:65536
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dockremap:165536:65536
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```
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> **Note:** On a fresh Fedora install, we had to `touch` the
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> `/etc/subuid` and `/etc/subgid` files to have ranges assigned when users
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> were created. Once these files existed, range assignment on user creation
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> worked properly.
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If you have a preferred/self-managed user with subordinate ID mappings already
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configured, you can provide that username or uid to the `--userns-remap` flag.
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If you have a group that doesn't match the username, you may provide the `gid`
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or group name as well; otherwise the username will be used as the group name
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when querying the system for the subordinate group ID range.
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### Detailed information on `subuid`/`subgid` ranges
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Given potential advanced use of the subordinate ID ranges by power users, the
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following paragraphs define how the Docker daemon currently uses the range entries
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found within the subordinate range files.
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The simplest case is that only one contiguous range is defined for the
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provided user or group. In this case, Docker will use that entire contiguous
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range for the mapping of host uids and gids to the container process. This
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means that the first ID in the range will be the remapped root user, and the
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IDs above that initial ID will map host ID 1 through the end of the range.
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From the example `/etc/subid` content shown above, the remapped root
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user would be uid 165536.
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If the system administrator has set up multiple ranges for a single user or
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group, the Docker daemon will read all the available ranges and use the
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following algorithm to create the mapping ranges:
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1. The range segments found for the particular user will be sorted by *start ID* ascending.
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2. Map segments will be created from each range in increasing value with a length matching the length of each segment. Therefore the range segment with the lowest numeric starting value will be equal to the remapped root, and continue up through host uid/gid equal to the range segment length. As an example, if the lowest segment starts at ID 1000 and has a length of 100, then a map of 1000 -> 0 (the remapped root) up through 1100 -> 100 will be created from this segment. If the next segment starts at ID 10000, then the next map will start with mapping 10000 -> 101 up to the length of this second segment. This will continue until no more segments are found in the subordinate files for this user.
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3. If more than five range segments exist for a single user, only the first five will be utilized, matching the kernel's limitation of only five entries in `/proc/self/uid_map` and `proc/self/gid_map`.
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### User namespace known restrictions
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The following standard Docker features are currently incompatible when
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running a Docker daemon with user namespaces enabled:
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- sharing PID or NET namespaces with the host (`--pid=host` or `--net=host`)
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- sharing a network namespace with an existing container (`--net=container:*other*`)
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- sharing an IPC namespace with an existing container (`--ipc=container:*other*`)
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- A `--readonly` container filesystem (this is a Linux kernel restriction against remounting with modified flags of a currently mounted filesystem when inside a user namespace)
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- external (volume or graph) drivers which are unaware/incapable of using daemon user mappings
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- Using `--privileged` mode flag on `docker run`
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In general, user namespaces are an advanced feature and will require
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coordination with other capabilities. For example, if volumes are mounted from
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the host, file ownership will have to be pre-arranged if the user or
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administrator wishes the containers to have expected access to the volume
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contents.
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Finally, while the `root` user inside a user namespaced container process has
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many of the expected admin privileges that go along with being the superuser, the
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Linux kernel has restrictions based on internal knowledge that this is a user namespaced
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process. The most notable restriction that we are aware of at this time is the
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inability to use `mknod`. Permission will be denied for device creation even as
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container `root` inside a user namespace.
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## Miscellaneous options
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IP masquerading uses address translation to allow containers without a public
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