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NAME | DESCRIPTION | EXAMPLES | SEE ALSO | COLOPHON |
NAMESPACES(7) Linux Programmer's Manual NAMESPACES(7)
namespaces - overview of Linux namespaces
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 below, 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 below, 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/$$/ns | awk '{print $1, $9, $10, $11}'
total 0
lrwxrwxrwx. cgroup -> cgroup:[4026531835]
lrwxrwxrwx. ipc -> ipc:[4026531839]
lrwxrwxrwx. mnt -> mnt:[4026531840]
lrwxrwxrwx. net -> net:[4026531969]
lrwxrwxrwx. pid -> pid:[4026531836]
lrwxrwxrwx. pid_for_children -> pid:[4026531834]
lrwxrwxrwx. time -> time:[4026531834]
lrwxrwxrwx. time_for_children -> time:[4026531834]
lrwxrwxrwx. user -> user:[4026531837]
lrwxrwxrwx. uts -> uts:[4026531838]
Bind mounting (see mount(2)) one of the files in this directory to
somewhere else in the filesystem keeps the corresponding namespace of
the process specified by pid alive even if all processes currently in
the namespace terminate.
Opening one of the files in this directory (or a file that is bind
mounted to one of these files) returns a file handle for the corre‐
sponding namespace of the process specified by pid. As long as this
file descriptor remains open, the namespace will remain alive, even
if all processes in the namespace terminate. The file descriptor can
be passed to setns(2).
In Linux 3.7 and earlier, these files were visible as hard links.
Since Linux 3.8, they appear as symbolic links. If two processes are
in the same namespace, then the device IDs and inode numbers of their
/proc/[pid]/ns/xxx symbolic links will be the same; an application
can check this using the stat.st_dev and stat.st_ino fields returned
by stat(2). The content of this symbolic link is a string containing
the namespace type and inode number as in the following example:
$ readlink /proc/$$/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 cre‐
ated in the namespace. (Beforehand, readlink(2) of the sym‐
bolic 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 pro‐
cesses created by this process. This can change as a conse‐
quence 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 names‐
pace.
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 names‐
pace.
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 cre‐
ated (/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 cre‐
ator UIDs in each of the ancestor user namespaces, and the ker‐
nel ensures that the corresponding namespace limit for the cre‐
ator 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 correspond‐
ing /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 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.
See clone(2) and user_namespaces(7).
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),
pam_namespace(8), switch_root(8)
This page is part of release 5.08 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-04-11 NAMESPACES(7)
Pages that refer to this page: nsenter(1) , procps(1) , ps(1) , systemd-detect-virt(1) , unshare(1) , clone2(2) , __clone2(2) , clone(2) , clone3(2) , getdomainname(2) , gethostname(2) , ioctl_ns(2) , setdomainname(2) , sethostname(2) , setns(2) , unshare(2) , proc(5) , procfs(5) , systemd.exec(5) , cgroup_namespaces(7) , cgroups(7) , credentials(7) , ipc_namespaces(7) , mount_namespaces(7) , mq_overview(7) , network_namespaces(7) , pid_namespaces(7) , time_namespaces(7) , user_namespaces(7) , uts_namespaces(7) , lsns(8) , rdma-system(8)
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