For review: namespaces(7) man page
Michael Kerrisk (man-pages)
mtk.manpages at gmail.com
Wed Aug 20 23:38:05 UTC 2014
Hello Eric et al.
Here is the current draft of the namespaces(7) man page, which
gives an overview of namespaces and the namespaces API. The rendered
version is below, and the source is attached.
Review comments/suggestions for improvements / bug fixes welcome.
Cheers,
Michael
==
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.
Linux provides the following namespaces:
Namespace Constant Isolates
IPC CLONE_IPC System V IPC, POSIX mesage queues
Network CLONE_NEWNET Network devices, stacks, ports, etc.
Mount CLONE_NEWNS Mount points
PID CLONE_NEWPID Process IDs
User CLONE_NEWUSER User and group IDs
UTS CLONE_NEWUTS Hostname and NIS domain name
This page describes the various namespaces and the associated
/proc files, and summarizes the APIs for working with namespaces.
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 imple‐
ments 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 speci‐
fies 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.)
Creation of new namespaces using clone(2) and unshare(2) in most
cases requires the CAP_SYS_ADMIN capability. User namespaces are
the exception: since Linux 3.8, no privilege is required to cre‐
ate 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/$$/ns
total 0
lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 ipc -> ipc:[4026531839]
lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 mnt -> mnt:[4026531840]
lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 net -> net:[4026531956]
lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 pid -> pid:[4026531836]
lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 user -> user:[4026531837]
lrwxrwxrwx. 1 mtk mtk 0 Jan 14 01:20 uts -> uts:[4026531838]
Bind mounting (see mount(2)) one of the files in this directory
to somewhere else in the filesystem keeps the corresponding
namespace of the process specified by pid alive even if all pro‐
cesses currently in the namespace terminate.
Opening one of the files in this directory (or a file that is
bind mounted to one of these files) returns a file handle for the
corresponding namespace of the process specified by pid. As long
as this file descriptor remains open, the namespace will remain
alive, even if all processes in the namespace terminate. The
file descriptor can be passed to setns(2).
In Linux 3.7 and earlier, these files were visible as hard links.
Since Linux 3.8, they appear as symbolic links. If two processes
are in the same namespace, then the inode numbers of their
/proc/[pid]/ns/xxx symbolic links will be the same; an applica‐
tion can check this using the stat.st_ino field returned by
stat(2). The content of this symbolic link is a string contain‐
ing the namespace type and inode number as in the following exam‐
ple:
$ readlink /proc/$$/ns/uts
uts:[4026531838]
The files in this subdirectory are as follows:
/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.
/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 IPC namespace of the
process.
IPC namespaces (CLONE_NEWIPC)
IPC namespaces isolate certain IPC resources, namely, System V
IPC objects (see svipc(7)) and (since Linux 2.6.30) POSIX message
queues (see mq_overview(7). The common characteristic of these
IPC mechanisms is that IPC objects are identified by mechanisms
other than filesystem pathnames.
Each IPC namespace has its own set of System V IPC identifiers
and its own POSIX message queue filesystem. Objects created in
an IPC namespace are visible to all other processes that are mem‐
bers of that namespace, but are not visible to processes in other
IPC namespaces.
The following /proc interfaces are distinct in each IPC names‐
pace:
* The POSIX message queue interfaces in /proc/sys/fs/mqueue.
* The System V IPC interfaces in /proc/sys/kernel, namely: msg‐
max, msgmnb, msgmni, sem, shmall, shmmax, shmmni, and
shm_rmid_forced.
* The System V IPC interfaces in /proc/sysvipc.
When an IPC namespace is destroyed (i.e., when the last process
that is a member of the namespace terminates), all IPC objects in
the namespace are automatically destroyed.
Use of IPC namespaces requires a kernel that is configured with
the CONFIG_IPC_NS option.
Network namespaces (CLONE_NEWNET)
Network namespaces provide isolation of the system resources
associated with networking: network devices, IPv4 and IPv6 proto‐
col stacks, IP routing tables, firewalls, the /proc/net direc‐
tory, the /sys/class/net directory, port numbers (sockets), and
so on. A physical network device can live in exactly one network
namespace. A virtual network device ("veth") pair provides a
pipe-like abstraction that can be used to create tunnels between
network namespaces, and can be used to create a bridge to a phys‐
ical network device in another namespace.
When a network namespace is freed (i.e., when the last process in
the namespace terminates), its physical network devices are moved
back to the initial network namespace (not to the parent of the
process).
Use of network namespaces requires a kernel that is configured
with the CONFIG_NET_NS option.
Mount namespaces (CLONE_NEWNS)
Mount namespaces isolate the set of filesystem mount points,
meaning that processes in different mount namespaces can have
different views of the filesystem hierarchy. The set of mounts
in a mount namespace is modified using mount(2) and umount(2).
The /proc/[pid]/mounts file (present since Linux 2.4.19) lists
all the filesystems currently mounted in the process's mount
namespace. The format of this file is documented in fstab(5).
Since kernel version 2.6.15, this file is pollable: after opening
the file for reading, a change in this file (i.e., a filesystem
mount or unmount) causes select(2) to mark the file descriptor as
readable, and poll(2) and epoll_wait(2) mark the file as having
an error condition.
The /proc/[pid]/mountstats file (present since Linux 2.6.17)
exports information (statistics, configuration information) about
the mount points in the process's mount namespace. This file is
only readable by the owner of the process. Lines in this file
have the form:
device /dev/sda7 mounted on /home with fstype ext3 [statistics]
( 1 ) ( 2 ) (3 ) (4)
The fields in each line are:
(1) The name of the mounted device (or "nodevice" if
there is no corresponding device).
(2) The mount point within the filesystem tree.
(3) The filesystem type.
(4) Optional statistics and configuration information.
Currently (as at Linux 2.6.26), only NFS filesystems
export information via this field.
PID namespaces (CLONE_NEWPID)
See pid_namespaces(7).
User namespaces (CLONE_NEWUSER)
See user_namespaces(7).
UTS namespaces (CLONE_NEWUTS)
UTS namespaces provide isolation of two system identifiers: the
hostname and the NIS domain name. These identifiers are set
using sethostname(2) and setdomainname(2), and can be retrieved
using uname(2), gethostname(2), and getdomainname(2).
Use of UTS namespaces requires a kernel that is configured with
the CONFIG_UTS_NS option.
CONFORMING TO
Namespaces are a Linux-specific feature.
EXAMPLE
See user_namespaces(7).
SEE ALSO
nsenter(1), readlink(1), unshare(1), clone(2), setns(2),
unshare(2), proc(5), credentials(7), capabilities(7), pid_names‐
paces(7), user_namespaces(7), switch_root(8)
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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