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NAME
namespaces - overview of Linux namespaces
DESCRIPTION
A namespace wraps a global system resource in an abstraction that makes it appear to the
processes within the namespace that they have their own isolated instance of the global
resource. Changes to the global resource are visible to other processes that are members
of the namespace, but are invisible to other processes. One use of namespaces is to
implement containers.
This page provides pointers to information on the various namespace types, describes the
associated /proc files, and summarizes the APIs for working with namespaces.
Namespace types
The following table shows the namespace types available on Linux. The second column of
the table shows the flag value that is used to specify the namespace type in various APIs.
The third column identifies the manual page that provides details on the namespace type.
The last column is a summary of the resources that are isolated by the namespace type.
Namespace Flag Page Isolates
Cgroup CLONE_NEWCGROUP cgroup_namespaces(7) Cgroup root
directory
IPC CLONE_NEWIPC ipc_namespaces(7) System V IPC, POSIX
message queues
Network CLONE_NEWNET network_namespaces(7) Network devices,
stacks, ports, etc.
Mount CLONE_NEWNS mount_namespaces(7) Mount points
PID CLONE_NEWPID pid_namespaces(7) Process IDs
Time CLONE_NEWTIME time_namespaces(7) Boot and monotonic
clocks
User CLONE_NEWUSER user_namespaces(7) User and group IDs
UTS CLONE_NEWUTS uts_namespaces(7) Hostname and NIS
domain name
The namespaces API
As well as various /proc files described below, the namespaces API includes the following
system calls:
clone(2)
The clone(2) system call creates a new process. If the flags argument of the call
specifies one or more of the CLONE_NEW* flags listed above, then new namespaces are
created for each flag, and the child process is made a member of those namespaces.
(This system call also implements a number of features unrelated to namespaces.)
setns(2)
The setns(2) system call allows the calling process to join an existing namespace.
The namespace to join is specified via a file descriptor that refers to one of the
/proc/pid/ns files described below.
unshare(2)
The unshare(2) system call moves the calling process to a new namespace. If the
flags argument of the call specifies one or more of the CLONE_NEW* flags listed
above, then new namespaces are created for each flag, and the calling process is
made a member of those namespaces. (This system call also implements a number of
features unrelated to namespaces.)
ioctl(2)
Various ioctl(2) operations can be used to discover information about namespaces.
These operations are described in ioctl_ns(2).
Creation of new namespaces using clone(2) and unshare(2) in most cases requires the
CAP_SYS_ADMIN capability, since, in the new namespace, the creator will have the power to
change global resources that are visible to other processes that are subsequently created
in, or join the namespace. User namespaces are the exception: since Linux 3.8, no
privilege is required to create a user namespace.
The /proc/[pid]/ns/ directory
Each process has a /proc/pid/ns/ subdirectory containing one entry for each namespace that
supports being manipulated by setns(2):
$ ls -l /proc/$$/ns | awk '{print $1, $9, $10, $11}'
total 0
lrwxrwxrwx. cgroup -> cgroup:[4026531835]
lrwxrwxrwx. ipc -> ipc:[4026531839]
lrwxrwxrwx. mnt -> mnt:[4026531840]
lrwxrwxrwx. net -> net:[4026531969]
lrwxrwxrwx. pid -> pid:[4026531836]
lrwxrwxrwx. pid_for_children -> pid:[4026531834]
lrwxrwxrwx. time -> time:[4026531834]
lrwxrwxrwx. time_for_children -> time:[4026531834]
lrwxrwxrwx. user -> user:[4026531837]
lrwxrwxrwx. uts -> uts:[4026531838]
Bind mounting (see mount(2)) one of the files in this directory to somewhere else in the
filesystem keeps the corresponding namespace of the process specified by pid alive even if
all processes currently in the namespace terminate.
Opening one of the files in this directory (or a file that is bind mounted to one of these
files) returns a file handle for the corresponding namespace of the process specified by
pid. As long as this file descriptor remains open, the namespace will remain alive, even
if all processes in the namespace terminate. The file descriptor can be passed to
setns(2).
In Linux 3.7 and earlier, these files were visible as hard links. Since Linux 3.8, they
appear as symbolic links. If two processes are in the same namespace, then the device IDs
and inode numbers of their /proc/pid/ns/xxx symbolic links will be the same; an
application can check this using the stat.st_dev and stat.st_ino fields returned by
stat(2). The content of this symbolic link is a string containing the namespace type and
inode number as in the following example:
$ readlink /proc/$$/ns/uts
uts:[4026531838]
The symbolic links in this subdirectory are as follows:
/proc/pid/ns/cgroup (since Linux 4.6)
This file is a handle for the cgroup namespace of the process.
/proc/pid/ns/ipc (since Linux 3.0)
This file is a handle for the IPC namespace of the process.
/proc/pid/ns/mnt (since Linux 3.8)
This file is a handle for the mount namespace of the process.
/proc/pid/ns/net (since Linux 3.0)
This file is a handle for the network namespace of the process.
/proc/pid/ns/pid (since Linux 3.8)
This file is a handle for the PID namespace of the process. This handle is
permanent for the lifetime of the process (i.e., a process's PID namespace
membership never changes).
/proc/pid/ns/pid_for_children (since Linux 4.12)
This file is a handle for the PID namespace of child processes created by this
process. This can change as a consequence of calls to unshare(2) and setns(2) (see
pid_namespaces(7)), so the file may differ from /proc/pid/ns/pid. The symbolic
link gains a value only after the first child process is created in the namespace.
(Beforehand, readlink(2) of the symbolic link will return an empty buffer.)
/proc/pid/ns/time (since Linux 5.6)
This file is a handle for the time namespace of the process.
/proc/pid/ns/time_for_children (since Linux 5.6)
This file is a handle for the time namespace of child processes created by this
process. This can change as a consequence of calls to unshare(2) and setns(2) (see
time_namespaces(7)), so the file may differ from /proc/pid/ns/time.
/proc/pid/ns/user (since Linux 3.8)
This file is a handle for the user namespace of the process.
/proc/pid/ns/uts (since Linux 3.0)
This file is a handle for the UTS namespace of the process.
Permission to dereference or read (readlink(2)) these symbolic links is governed by a
ptrace access mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
The /proc/sys/user directory
The files in the /proc/sys/user directory (which is present since Linux 4.9) expose limits
on the number of namespaces of various types that can be created. The files are as
follows:
max_cgroup_namespaces
The value in this file defines a per-user limit on the number of cgroup namespaces
that may be created in the user namespace.
max_ipc_namespaces
The value in this file defines a per-user limit on the number of ipc namespaces
that may be created in the user namespace.
max_mnt_namespaces
The value in this file defines a per-user limit on the number of mount namespaces
that may be created in the user namespace.
max_net_namespaces
The value in this file defines a per-user limit on the number of network namespaces
that may be created in the user namespace.
max_pid_namespaces
The value in this file defines a per-user limit on the number of PID namespaces
that may be created in the user namespace.
max_time_namespaces (since Linux 5.7)
The value in this file defines a per-user limit on the number of time namespaces
that may be created in the user namespace.
max_user_namespaces
The value in this file defines a per-user limit on the number of user namespaces
that may be created in the user namespace.
max_uts_namespaces
The value in this file defines a per-user limit on the number of uts namespaces
that may be created in the user namespace.
Note the following details about these files:
• The values in these files are modifiable by privileged processes.
• The values exposed by these files are the limits for the user namespace in which the
opening process resides.
• The limits are per-user. Each user in the same user namespace can create namespaces up
to the defined limit.
• The limits apply to all users, including UID 0.
• These limits apply in addition to any other per-namespace limits (such as those for PID
and user namespaces) that may be enforced.
• Upon encountering these limits, clone(2) and unshare(2) fail with the error ENOSPC.
• For the initial user namespace, the default value in each of these files is half the
limit on the number of threads that may be created (/proc/sys/kernel/threads-max). In
all descendant user namespaces, the default value in each file is MAXINT.
• When a namespace is created, the object is also accounted against ancestor namespaces.
More precisely:
• Each user namespace has a creator UID.
• When a namespace is created, it is accounted against the creator UIDs in each of the
ancestor user namespaces, and the kernel ensures that the corresponding namespace
limit for the creator UID in the ancestor namespace is not exceeded.
• The aforementioned point ensures that creating a new user namespace cannot be used
as a means to escape the limits in force in the current user namespace.
Namespace lifetime
Absent any other factors, a namespace is automatically torn down when the last process in
the namespace terminates or leaves the namespace. However, there are a number of other
factors that may pin a namespace into existence even though it has no member processes.
These factors include the following:
• An open file descriptor or a bind mount exists for the corresponding /proc/pid/ns/*
file.
• The namespace is hierarchical (i.e., a PID or user namespace), and has a child
namespace.
• It is a user namespace that owns one or more nonuser namespaces.
• It is a PID namespace, and there is a process that refers to the namespace via a
/proc/pid/ns/pid_for_children symbolic link.
• It is a time namespace, and there is a process that refers to the namespace via a
/proc/pid/ns/time_for_children symbolic link.
• It is an IPC namespace, and a corresponding mount of an mqueue filesystem (see
mq_overview(7)) refers to this namespace.
• It is a PID namespace, and a corresponding mount of a proc(5) filesystem refers to this
namespace.
EXAMPLES
See clone(2) and user_namespaces(7).
SEE ALSO
nsenter(1), readlink(1), unshare(1), clone(2), ioctl_ns(2), setns(2), unshare(2), proc(5),
capabilities(7), cgroup_namespaces(7), cgroups(7), credentials(7), ipc_namespaces(7),
network_namespaces(7), pid_namespaces(7), user_namespaces(7), uts_namespaces(7), lsns(8),
switch_root(8)