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 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.) 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. 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.) 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 $
ls -l /proc/$$/ns | awk '{print 1,ドル 9,ドル 10,ドル
11ドル}' Bind mounting
(see mount(2)) one of the files in this directory to
somewhere else in the filesystem keeps the corresponding
namespace of the process specified by pid alive even
if all processes currently in the namespace terminate. Opening one of
the files in this directory (or a file that is bind mounted
to one of these files) returns a file handle for the
corresponding namespace of the process specified by
pid. As long as this file descriptor remains open,
the namespace will remain alive, even if all processes in
the namespace terminate. The file descriptor can be passed
to setns(2). In Linux 3.7
and earlier, these files were visible as hard links. Since
Linux 3.8, they appear as symbolic links. If two processes
are in the same namespace, then the device IDs and inode
numbers of their /proc/[pid]/ns/xxx symbolic links
will be the same; an application can check this using the
stat.st_dev and stat.st_ino fields returned by
stat(2). The content of this symbolic link is a
string containing the namespace type and inode number as in
the following example: $
readlink /proc/$$/ns/uts The symbolic
links in this subdirectory are as follows: 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 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 * 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. 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 5.09 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/.
As well as various /proc files described below, the
namespaces API includes the following system calls:
clone(2)
Each process has a /proc/[pid]/ns/ subdirectory
containing one entry for each namespace that supports being
manipulated by setns(2):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]
uts:[4026531838]
/proc/[pid]/ns/cgroup (since Linux 4.6)
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
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:EXAMPLES
SEE ALSO
COLOPHON