NAMESPACES
Section: Linux Programmer's Manual (7)
Updated: 2021-03-22
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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.
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/$$/ns | awk '{print $1, $9, $10, $11}'
total 0
lrwxrwxrwx. cgroup -> cgroup:[4026531835]
lrwxrwxrwx. ipc -> ipc:[4026531839]
lrwxrwxrwx. mnt -> mnt:[4026531840]
lrwxrwxrwx. net -> net:[4026531969]
lrwxrwxrwx. pid -> pid:[4026531836]
lrwxrwxrwx. pid_for_children -> pid:[4026531834]
lrwxrwxrwx. time -> time:[4026531834]
lrwxrwxrwx. time_for_children -> time:[4026531834]
lrwxrwxrwx. user -> user:[4026531837]
lrwxrwxrwx. uts -> uts:[4026531838]
Bind mounting (see
mount(2))
one of the files in this directory
to somewhere else in the filesystem keeps
the corresponding namespace of the process specified by
pid
alive even if all processes currently in the namespace terminate.
Opening one of the files in this directory
(or a file that is bind mounted to one of these files)
returns a file handle for
the corresponding namespace of the process specified by
pid.
As long as this file descriptor remains open,
the namespace will remain alive,
even if all processes in the namespace terminate.
The file descriptor can be passed to
setns(2).
In Linux 3.7 and earlier, these files were visible as hard links.
Since Linux 3.8,
they appear as symbolic links.
If two processes are in the same namespace,
then the device IDs and inode numbers of their
/proc/[pid]/ns/xxx
symbolic links will be the same; an application can check this using the
stat.st_dev
and
stat.st_ino
fields returned by
stat(2).
The content of this symbolic link is a string containing
the namespace type and inode number as in the following example:
$ readlink /proc/$$/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),
pam_namespace(8),
switch_root(8)
COLOPHON
This page is part of release 5.11 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/.
Index
- NAME
-
- DESCRIPTION
-
- Namespace types
-
- The namespaces API
-
- The /proc/[pid]/ns/ directory
-
- The /proc/sys/user directory
-
- Namespace lifetime
-
- EXAMPLES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:49 GMT, May 09, 2021