1NAMESPACES(7)              Linux Programmer's Manual             NAMESPACES(7)
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NAME

6       namespaces - overview of Linux namespaces
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DESCRIPTION

9       A namespace wraps a global system resource in an abstraction that makes
10       it appear to the processes within the namespace that  they  have  their
11       own  isolated  instance  of the global resource.  Changes to the global
12       resource are visible to other processes that are members of  the  name‐
13       space,  but are invisible to other processes.  One use of namespaces is
14       to implement containers.
15
16       This page provides pointers to information  on  the  various  namespace
17       types,  describes  the  associated /proc files, and summarizes the APIs
18       for working with namespaces.
19
20   Namespace types
21       The following table shows the namespace types available on Linux.   The
22       second column of the table shows the flag value that is used to specify
23       the namespace type in various APIs.  The third  column  identifies  the
24       manual page that provides details on the namespace type.  The last col‐
25       umn is a summary of the resources that are isolated  by  the  namespace
26       type.
27
28       Namespace Flag            Page                  Isolates
29       Cgroup    CLONE_NEWCGROUP cgroup_namespaces(7)  Cgroup root directory
30       IPC       CLONE_NEWIPC    ipc_namespaces(7)     System V IPC,
31                                                       POSIX message queues
32       Network   CLONE_NEWNET    network_namespaces(7) Network devices,
33                                                       stacks, ports, etc.
34       Mount     CLONE_NEWNS     mount_namespaces(7)   Mount points
35       PID       CLONE_NEWPID    pid_namespaces(7)     Process IDs
36       Time      CLONE_NEWTIME   time_namespaces(7)    Boot and monotonic
37                                                       clocks
38       User      CLONE_NEWUSER   user_namespaces(7)    User and group IDs
39       UTS       CLONE_NEWUTS    uts_namespaces(7)     Hostname and NIS
40                                                       domain name
41
42   The namespaces API
43       As  well as various /proc files described below, the namespaces API in‐
44       cludes the following system calls:
45
46       clone(2)
47              The clone(2) system call creates a new process.   If  the  flags
48              argument  of  the  call  specifies one or more of the CLONE_NEW*
49              flags listed below, then new namespaces  are  created  for  each
50              flag,  and  the  child  process  is made a member of those name‐
51              spaces.  (This system call also implements a number of  features
52              unrelated to namespaces.)
53
54       setns(2)
55              The  setns(2)  system call allows the calling process to join an
56              existing namespace.  The namespace to join is  specified  via  a
57              file  descriptor  that refers to one of the /proc/[pid]/ns files
58              described below.
59
60       unshare(2)
61              The unshare(2) system call moves the calling process  to  a  new
62              namespace.   If  the flags argument of the call specifies one or
63              more of the CLONE_NEW* flags listed below, then  new  namespaces
64              are  created  for  each  flag, and the calling process is made a
65              member of those namespaces.  (This system call also implements a
66              number of features unrelated to namespaces.)
67
68       ioctl(2)
69              Various  ioctl(2) operations can be used to discover information
70              about   namespaces.    These   operations   are   described   in
71              ioctl_ns(2).
72
73       Creation  of new namespaces using clone(2) and unshare(2) in most cases
74       requires the CAP_SYS_ADMIN capability, since, in the new namespace, the
75       creator will have the power to change global resources that are visible
76       to other processes that are subsequently created in, or join the  name‐
77       space.   User  namespaces are the exception: since Linux 3.8, no privi‐
78       lege is required to create a user namespace.
79
80   The /proc/[pid]/ns/ directory
81       Each process has a /proc/[pid]/ns/ subdirectory  containing  one  entry
82       for each namespace that supports being manipulated by setns(2):
83
84           $ ls -l /proc/$$/ns | awk '{print $1, $9, $10, $11}'
85           total 0
86           lrwxrwxrwx. cgroup -> cgroup:[4026531835]
87           lrwxrwxrwx. ipc -> ipc:[4026531839]
88           lrwxrwxrwx. mnt -> mnt:[4026531840]
89           lrwxrwxrwx. net -> net:[4026531969]
90           lrwxrwxrwx. pid -> pid:[4026531836]
91           lrwxrwxrwx. pid_for_children -> pid:[4026531834]
92           lrwxrwxrwx. time -> time:[4026531834]
93           lrwxrwxrwx. time_for_children -> time:[4026531834]
94           lrwxrwxrwx. user -> user:[4026531837]
95           lrwxrwxrwx. uts -> uts:[4026531838]
96
97       Bind  mounting  (see  mount(2))  one  of the files in this directory to
98       somewhere else in the filesystem keeps the corresponding  namespace  of
99       the  process  specified by pid alive even if all processes currently in
100       the namespace terminate.
101
102       Opening one of the files in this directory (or  a  file  that  is  bind
103       mounted  to  one  of  these files) returns a file handle for the corre‐
104       sponding namespace of the process specified by pid.  As  long  as  this
105       file  descriptor remains open, the namespace will remain alive, even if
106       all processes in the namespace terminate.  The file descriptor  can  be
107       passed to setns(2).
108
109       In  Linux  3.7  and  earlier,  these  files were visible as hard links.
110       Since Linux 3.8, they appear as symbolic links.  If two  processes  are
111       in  the  same namespace, then the device IDs and inode numbers of their
112       /proc/[pid]/ns/xxx symbolic links will be the same; an application  can
113       check  this  using  the  stat.st_dev and stat.st_ino fields returned by
114       stat(2).  The content of this symbolic link is a string containing  the
115       namespace type and inode number as in the following example:
116
117           $ readlink /proc/$$/ns/uts
118           uts:[4026531838]
119
120       The symbolic links in this subdirectory are as follows:
121
122       /proc/[pid]/ns/cgroup (since Linux 4.6)
123              This file is a handle for the cgroup namespace of the process.
124
125       /proc/[pid]/ns/ipc (since Linux 3.0)
126              This file is a handle for the IPC namespace of the process.
127
128       /proc/[pid]/ns/mnt (since Linux 3.8)
129              This file is a handle for the mount namespace of the process.
130
131       /proc/[pid]/ns/net (since Linux 3.0)
132              This file is a handle for the network namespace of the process.
133
134       /proc/[pid]/ns/pid (since Linux 3.8)
135              This  file  is  a  handle  for the PID namespace of the process.
136              This handle is permanent for the lifetime of the process  (i.e.,
137              a process's PID namespace membership never changes).
138
139       /proc/[pid]/ns/pid_for_children (since Linux 4.12)
140              This  file  is a handle for the PID namespace of child processes
141              created by this process.  This can change as  a  consequence  of
142              calls to unshare(2) and setns(2) (see pid_namespaces(7)), so the
143              file may differ  from  /proc/[pid]/ns/pid.   The  symbolic  link
144              gains  a  value only after the first child process is created in
145              the namespace.  (Beforehand, readlink(2) of  the  symbolic  link
146              will return an empty buffer.)
147
148       /proc/[pid]/ns/time (since Linux 5.6)
149              This file is a handle for the time namespace of the process.
150
151       /proc/[pid]/ns/time_for_children (since Linux 5.6)
152              This  file is a handle for the time namespace of child processes
153              created by this process.  This can change as  a  consequence  of
154              calls  to  unshare(2)  and setns(2) (see time_namespaces(7)), so
155              the file may differ from /proc/[pid]/ns/time.
156
157       /proc/[pid]/ns/user (since Linux 3.8)
158              This file is a handle for the user namespace of the process.
159
160       /proc/[pid]/ns/uts (since Linux 3.0)
161              This file is a handle for the UTS namespace of the process.
162
163       Permission to dereference or read (readlink(2)) these symbolic links is
164       governed  by  a  ptrace access mode PTRACE_MODE_READ_FSCREDS check; see
165       ptrace(2).
166
167   The /proc/sys/user directory
168       The files in the /proc/sys/user directory (which is present since Linux
169       4.9)  expose  limits  on the number of namespaces of various types that
170       can be created.  The files are as follows:
171
172       max_cgroup_namespaces
173              The value in this file defines a per-user limit on the number of
174              cgroup namespaces that may be created in the user namespace.
175
176       max_ipc_namespaces
177              The value in this file defines a per-user limit on the number of
178              ipc namespaces that may be created in the user namespace.
179
180       max_mnt_namespaces
181              The value in this file defines a per-user limit on the number of
182              mount namespaces that may be created in the user namespace.
183
184       max_net_namespaces
185              The value in this file defines a per-user limit on the number of
186              network namespaces that may be created in the user namespace.
187
188       max_pid_namespaces
189              The value in this file defines a per-user limit on the number of
190              PID namespaces that may be created in the user namespace.
191
192       max_time_namespaces (since Linux 5.7)
193              The value in this file defines a per-user limit on the number of
194              time namespaces that may be created in the user namespace.
195
196       max_user_namespaces
197              The value in this file defines a per-user limit on the number of
198              user namespaces that may be created in the user namespace.
199
200       max_uts_namespaces
201              The value in this file defines a per-user limit on the number of
202              uts namespaces that may be created in the user namespace.
203
204       Note the following details about these files:
205
206       *  The values in these files are modifiable by privileged processes.
207
208       *  The values exposed by these files are the limits for the user  name‐
209          space in which the opening process resides.
210
211       *  The  limits  are per-user.  Each user in the same user namespace can
212          create namespaces up to the defined limit.
213
214       *  The limits apply to all users, including UID 0.
215
216       *  These limits apply in addition to  any  other  per-namespace  limits
217          (such as those for PID and user namespaces) that may be enforced.
218
219       *  Upon  encountering  these  limits, clone(2) and unshare(2) fail with
220          the error ENOSPC.
221
222       *  For the initial user namespace, the default value in each  of  these
223          files is half the limit on the number of threads that may be created
224          (/proc/sys/kernel/threads-max).  In all descendant user  namespaces,
225          the default value in each file is MAXINT.
226
227       *  When  a  namespace  is created, the object is also accounted against
228          ancestor namespaces.  More precisely:
229
230          +  Each user namespace has a creator UID.
231
232          +  When a namespace is created, it is accounted against the  creator
233             UIDs  in each of the ancestor user namespaces, and the kernel en‐
234             sures that the corresponding namespace limit for the creator  UID
235             in the ancestor namespace is not exceeded.
236
237          +  The  aforementioned  point ensures that creating a new user name‐
238             space cannot be used as a means to escape the limits in force  in
239             the current user namespace.
240
241   Namespace lifetime
242       Absent  any  other factors, a namespace is automatically torn down when
243       the last process in the namespace terminates or leaves  the  namespace.
244       However,  there  are a number of other factors that may pin a namespace
245       into existence even though it has no member processes.   These  factors
246       include the following:
247
248       *  An open file descriptor or a bind mount exists for the corresponding
249          /proc/[pid]/ns/* file.
250
251       *  The namespace is hierarchical (i.e., a PID or user  namespace),  and
252          has a child namespace.
253
254       *  It is a user namespace that owns one or more nonuser namespaces.
255
256       *  It  is  a  PID  namespace, and there is a process that refers to the
257          namespace via a /proc/[pid]/ns/pid_for_children symbolic link.
258
259       *  It is a time namespace, and there is a process that  refers  to  the
260          namespace via a /proc/[pid]/ns/time_for_children symbolic link.
261
262       *  It  is  an  IPC  namespace,  and  a corresponding mount of an mqueue
263          filesystem (see mq_overview(7)) refers to this namespace.
264
265       *  It is a PID namespace,  and  a  corresponding  mount  of  a  proc(5)
266          filesystem refers to this namespace.
267

EXAMPLES

269       See clone(2) and user_namespaces(7).
270

SEE ALSO

272       nsenter(1),  readlink(1),  unshare(1), clone(2), ioctl_ns(2), setns(2),
273       unshare(2), proc(5), capabilities(7), cgroup_namespaces(7), cgroups(7),
274       credentials(7),   ipc_namespaces(7),  network_namespaces(7),  pid_name‐
275       spaces(7), user_namespaces(7),  uts_namespaces(7),  lsns(8),  pam_name‐
276       space(8), switch_root(8)
277

COLOPHON

279       This  page  is  part of release 5.10 of the Linux man-pages project.  A
280       description of the project, information about reporting bugs,  and  the
281       latest     version     of     this    page,    can    be    found    at
282       https://www.kernel.org/doc/man-pages/.
283
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286Linux                             2020-11-01                     NAMESPACES(7)
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