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.
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 |
User |
CLONE_NEWUSER |
user_namespaces(7) |
User and group IDs |
UTS |
CLONE_NEWUTS |
uts_namespaces(7) |
Hostname and NIS domain name |
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.
Each process has a
/proc/[pid]/ns/ subdirectory containing one entry for
each namespace that supports being manipulated by
setns(2):
$ ls -l /proc/$$/ns
total 0
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 cgroup -> cgroup:[4026531835]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc -> ipc:[4026531839]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt -> mnt:[4026531840]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net -> net:[4026531969]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid -> pid:[4026531836]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user -> user:[4026531837]
lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts -> uts:[4026531838]
Bind mounting (see
mount(2)) one of the files in this directory to
somewhere else in the filesystem keeps the corresponding namespace of the
process specified by
pid alive even if all processes currently in the
namespace terminate.
Opening one of the files in this directory (or a file that is bind mounted to
one of these files) returns a file handle for the corresponding namespace of
the process specified by
pid. As long as this file descriptor remains
open, the namespace will remain alive, even if all processes in the namespace
terminate. The file descriptor can be passed to
setns(2).
In Linux 3.7 and earlier, these files were visible as hard links. Since Linux
3.8, they appear as symbolic links. If two processes are in the same
namespace, then the device IDs and inode numbers of their
/proc/[pid]/ns/xxx symbolic links will be the same; an application can
check this using the
stat.st_dev and
stat.st_ino fields returned
by
stat(2). The content of this symbolic link is a string containing
the namespace type and inode number as in the following example:
$ readlink /proc/$$/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/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 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_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.
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 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)