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.
