man7/namespaces.7

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  27 .TH NAMESPACES 7 2016-03-15 "Linux" "Linux Programmer's Manual"
  28 .SH NAME
  29 namespaces \- overview of Linux namespaces
  30 .SH DESCRIPTION
  31 A namespace wraps a global system resource in an abstraction that
  32 makes it appear to the processes within the namespace that they
  33 have their own isolated instance of the global resource.
  34 Changes to the global resource are visible to other processes
  35 that are members of the namespace, but are invisible to other processes.
  36 One use of namespaces is to implement containers.
  37
  38 Linux provides the following namespaces:
  39 .TS
  40 lB lB lB
  41 l lB l.
  42 Namespace       Constant        Isolates
  43 Cgroup  CLONE_NEWCGROUP Cgroup root directory
  44 IPC     CLONE_NEWIPC    System V IPC, POSIX message queues
  45 Network CLONE_NEWNET    Network devices, stacks, ports, etc.
  46 Mount   CLONE_NEWNS     Mount points
  47 PID     CLONE_NEWPID    Process IDs
  48 User    CLONE_NEWUSER   User and group IDs
  49 UTS     CLONE_NEWUTS    Hostname and NIS domain name
  50 .TE
  51
  52 This page describes the various namespaces and the associated
  53 .I /proc
  54 files, and summarizes the APIs for working with namespaces.
  55 .\"
  56 .\" ==================== The namespaces API ====================
  57 .\"
  58 .SS The namespaces API
  59 As well as various
  60 .I /proc
  61 files described below,
  62 the namespaces API includes the following system calls:
  63 .TP
  64 .BR clone (2)
  65 The
  66 .BR clone (2)
  67 system call creates a new process.
  68 If the
  69 .I flags
  70 argument of the call specifies one or more of the
  71 .B CLONE_NEW*
  72 flags listed below, then new namespaces are created for each flag,
  73 and the child process is made a member of those namespaces.
  74 (This system call also implements a number of features
  75 unrelated to namespaces.)
  76 .TP
  77 .BR setns (2)
  78 The
  79 .BR setns (2)
  80 system call allows the calling process to join an existing namespace.
  81 The namespace to join is specified via a file descriptor that refers to
  82 one of the
  83 .IR /proc/[pid]/ns
  84 files described below.
  85 .TP
  86 .BR unshare (2)
  87 The
  88 .BR unshare (2)
  89 system call moves the calling process to a new namespace.
  90 If the
  91 .I flags
  92 argument of the call specifies one or more of the
  93 .B CLONE_NEW*
  94 flags listed below, then new namespaces are created for each flag,
  95 and the calling process is made a member of those namespaces.
  96 (This system call also implements a number of features
  97 unrelated to namespaces.)
  98 .PP
  99 Creation of new namespaces using
 100 .BR clone (2)
 101 and
 102 .BR unshare (2)
 103 in most cases requires the
 104 .BR CAP_SYS_ADMIN
 105 capability.
 106 User namespaces are the exception: since Linux 3.8,
 107 no privilege is required to create a user namespace.
 108 .\"
 109 .\" ==================== The /proc/[pid]/ns/ directory ====================
 110 .\"
 111 .SS The /proc/[pid]/ns/ directory
 112 Each process has a
 113 .IR /proc/[pid]/ns/
 114 .\" See commit 6b4e306aa3dc94a0545eb9279475b1ab6209a31f
 115 subdirectory containing one entry for each namespace that
 116 supports being manipulated by
 117 .BR setns (2):
 118
 119 .in +4n
 120 .nf
 121 $ \fBls -l /proc/$$/ns\fP
 122 total 0
 123 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 cgroup -> cgroup:[4026531835]
 124 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc -> ipc:[4026531839]
 125 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt -> mnt:[4026531840]
 126 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net -> net:[4026531969]
 127 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid -> pid:[4026531836]
 128 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user -> user:[4026531837]
 129 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts -> uts:[4026531838]
 130 .fi
 131 .in
 132
 133 Bind mounting (see
 134 .BR mount (2))
 135 one of the files in this directory
 136 to somewhere else in the filesystem keeps
 137 the corresponding namespace of the process specified by
 138 .I pid
 139 alive even if all processes currently in the namespace terminate.
 140
 141 Opening one of the files in this directory
 142 (or a file that is bind mounted to one of these files)
 143 returns a file handle for
 144 the corresponding namespace of the process specified by
 145 .IR pid .
 146 As long as this file descriptor remains open,
 147 the namespace will remain alive,
 148 even if all processes in the namespace terminate.
 149 The file descriptor can be passed to
 150 .BR setns (2).
 151
 152 In Linux 3.7 and earlier, these files were visible as hard links.
 153 Since Linux 3.8, they appear as symbolic links.
 154 If two processes are in the same namespace, then the inode numbers of their
 155 .IR /proc/[pid]/ns/xxx
 156 symbolic links will be the same; an application can check this using the
 157 .I stat.st_ino
 158 field returned by
 159 .BR stat (2).
 160 The content of this symbolic link is a string containing
 161 the namespace type and inode number as in the following example:
 162
 163 .in +4n
 164 .nf
 165 $ \fBreadlink /proc/$$/ns/uts\fP
 166 uts:[4026531838]
 167 .fi
 168 .in
 169
 170 The files in this subdirectory are as follows:
 171 .TP
 172 .IR /proc/[pid]/ns/cgroup " (since Linux 4.6)"
 173 This file is a handle for the cgroup namespace of the process.
 174 .TP
 175 .IR /proc/[pid]/ns/ipc " (since Linux 3.0)"
 176 This file is a handle for the IPC namespace of the process.
 177 .TP
 178 .IR /proc/[pid]/ns/mnt " (since Linux 3.8)"
 179 This file is a handle for the mount namespace of the process.
 180 .TP
 181 .IR /proc/[pid]/ns/net " (since Linux 3.0)"
 182 This file is a handle for the network namespace of the process.
 183 .TP
 184 .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
 185 This file is a handle for the PID namespace of the process.
 186 .TP
 187 .IR /proc/[pid]/ns/user " (since Linux 3.8)"
 188 This file is a handle for the user namespace of the process.
 189 .TP
 190 .IR /proc/[pid]/ns/uts " (since Linux 3.0)"
 191 This file is a handle for the UTS namespace of the process.
 192 .\"
 193 .\" ==================== Cgroup namespaces ====================
 194 .\"
 195 .SS Cgroup namespaces (CLONE_NEWCGROUP)
 196 See
 197 .BR cgroup_namespaces (7).
 198 .\"
 199 .\" ==================== IPC namespaces ====================
 200 .\"
 201 .SS IPC namespaces (CLONE_NEWIPC)
 202 IPC namespaces isolate certain IPC resources,
 203 namely, System V IPC objects (see
 204 .BR svipc (7))
 205 and (since Linux 2.6.30)
 206 .\" commit 7eafd7c74c3f2e67c27621b987b28397110d643f
 207 .\" https://lwn.net/Articles/312232/
 208 POSIX message queues (see
 209 .BR mq_overview (7)).
 210 The common characteristic of these IPC mechanisms is that IPC
 211 objects are identified by mechanisms other than filesystem
 212 pathnames.
 213
 214 Each IPC namespace has its own set of System V IPC identifiers and
 215 its own POSIX message queue filesystem.
 216 Objects created in an IPC namespace are visible to all other processes
 217 that are members of that namespace,
 218 but are not visible to processes in other IPC namespaces.
 219
 220 The following
 221 .I /proc
 222 interfaces are distinct in each IPC namespace:
 223 .IP * 3
 224 The POSIX message queue interfaces in
 225 .IR /proc/sys/fs/mqueue .
 226 .IP *
 227 The System V IPC interfaces in
 228 .IR /proc/sys/kernel ,
 229 namely:
 230 .IR msgmax ,
 231 .IR msgmnb  ,
 232 .IR msgmni ,
 233 .IR sem ,
 234 .IR shmall ,
 235 .IR shmmax ,
 236 .IR shmmni ,
 237 and
 238 .IR shm_rmid_forced .
 239 .IP *
 240 The System V IPC interfaces in
 241 .IR /proc/sysvipc .
 242 .PP
 243 When an IPC namespace is destroyed
 244 (i.e., when the last process that is a member of the namespace terminates),
 245 all IPC objects in the namespace are automatically destroyed.
 246
 247 Use of IPC namespaces requires a kernel that is configured with the
 248 .B CONFIG_IPC_NS
 249 option.
 250 .\"
 251 .\" ==================== Network namespaces ====================
 252 .\"
 253 .SS Network namespaces (CLONE_NEWNET)
 254 Network namespaces provide isolation of the system resources associated
 255 with networking: network devices, IPv4 and IPv6 protocol stacks,
 256 IP routing tables, firewalls, the
 257 .I /proc/net
 258 directory, the
 259 .I /sys/class/net
 260 directory, port numbers (sockets), and so on.
 261 A physical network device can live in exactly one
 262 network namespace.
 263 A virtual network device ("veth") pair provides a pipe-like abstraction
 264 .\" FIXME Add pointer to veth(4) page when it is eventually completed
 265 that can be used to create tunnels between network namespaces,
 266 and can be used to create a bridge to a physical network device
 267 in another namespace.
 268
 269 When a network namespace is freed
 270 (i.e., when the last process in the namespace terminates),
 271 its physical network devices are moved back to the
 272 initial network namespace (not to the parent of the process).
 273
 274 Use of network namespaces requires a kernel that is configured with the
 275 .B CONFIG_NET_NS
 276 option.
 277 .\"
 278 .\" ==================== Mount namespaces ====================
 279 .\"
 280 .SS Mount namespaces (CLONE_NEWNS)
 281 Mount namespaces isolate the set of filesystem mount points,
 282 meaning that processes in different mount namespaces can
 283 have different views of the filesystem hierarchy.
 284 The set of mounts in a mount namespace is modified using
 285 .BR mount (2)
 286 and
 287 .BR umount (2).
 288
 289 The
 290 .IR /proc/[pid]/mounts
 291 file (present since Linux 2.4.19)
 292 lists all the filesystems currently mounted in the
 293 process's mount namespace.
 294 The format of this file is documented in
 295 .BR fstab (5).
 296 Since kernel version 2.6.15, this file is pollable:
 297 after opening the file for reading, a change in this file
 298 (i.e., a filesystem mount or unmount) causes
 299 .BR select (2)
 300 to mark the file descriptor as having an exceptional condition, and
 301 .BR poll (2)
 302 and
 303 .BR epoll_wait (2)
 304 mark the file as having a priority event
 305 .RB ( POLLPRI ).
 306 (Before Linux 2.6.30,
 307 a change in this file was indicated by the file descriptor
 308 being marked as readable for
 309 .BR select (2),
 310 and being marked as having an error condition for
 311 .BR poll (2)
 312 and
 313 .BR epoll_wait (2).)
 314 .RE
 315 .\"
 316 .\" ==================== PID namespaces ====================
 317 .\"
 318 .SS PID namespaces (CLONE_NEWPID)
 319 See
 320 .BR pid_namespaces (7).
 321 .\"
 322 .\" ==================== User namespaces ====================
 323 .\"
 324 .SS User namespaces (CLONE_NEWUSER)
 325 See
 326 .BR user_namespaces (7).
 327 .\"
 328 .\" ==================== UTS namespaces ====================
 329 .\"
 330 .SS UTS namespaces (CLONE_NEWUTS)
 331 UTS namespaces provide isolation of two system identifiers:
 332 the hostname and the NIS domain name.
 333 These identifiers are set using
 334 .BR sethostname (2)
 335 and
 336 .BR setdomainname (2),
 337 and can be retrieved using
 338 .BR uname (2),
 339 .BR gethostname (2),
 340 and
 341 .BR getdomainname (2).
 342
 343 Use of UTS namespaces requires a kernel that is configured with the
 344 .B CONFIG_UTS_NS
 345 option.
 346 .SH CONFORMING TO
 347 Namespaces are a Linux-specific feature.
 348 .SH EXAMPLE
 349 See
 350 .BR user_namespaces (7).
 351 .SH SEE ALSO
 352 .BR lsns (1),
 353 .BR nsenter (1),
 354 .BR readlink (1),
 355 .BR unshare (1),
 356 .BR clone (2),
 357 .BR setns (2),
 358 .BR unshare (2),
 359 .BR proc (5),
 360 .BR capabilities (7),
 361 .BR cgroup_namespaces (7),
 362 .BR cgroups (7),
 363 .BR credentials (7),
 364 .BR pid_namespaces (7),
 365 .BR user_namespaces (7),
 366 .BR switch_root (8)