man7/namespaces.7

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  27 .TH NAMESPACES 7 2017-05-03 "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/$$/ns\fP
 122 total 0
 123 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 cgroup \-> cgroup:[4026531835]
 124 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
 125 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
 126 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
 127 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
 128 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children \-> pid:[4026531834]
 129 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
 130 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
 131 .fi
 132 .in
 133
 134 Bind mounting (see
 135 .BR mount (2))
 136 one of the files in this directory
 137 to somewhere else in the filesystem keeps
 138 the corresponding namespace of the process specified by
 139 .I pid
 140 alive even if all processes currently in the namespace terminate.
 141
 142 Opening one of the files in this directory
 143 (or a file that is bind mounted to one of these files)
 144 returns a file handle for
 145 the corresponding namespace of the process specified by
 146 .IR pid .
 147 As long as this file descriptor remains open,
 148 the namespace will remain alive,
 149 even if all processes in the namespace terminate.
 150 The file descriptor can be passed to
 151 .BR setns (2).
 152
 153 In Linux 3.7 and earlier, these files were visible as hard links.
 154 Since Linux 3.8,
 155 .\" commit bf056bfa80596a5d14b26b17276a56a0dcb080e5
 156 they appear as symbolic links.
 157 If two processes are in the same namespace, then the inode numbers of their
 158 .IR /proc/[pid]/ns/xxx
 159 symbolic links will be the same; an application can check this using the
 160 .I stat.st_ino
 161 field returned by
 162 .BR stat (2).
 163 The content of this symbolic link is a string containing
 164 the namespace type and inode number as in the following example:
 165
 166 .in +4n
 167 .nf
 168 $ \fBreadlink /proc/$$/ns/uts\fP
 169 uts:[4026531838]
 170 .fi
 171 .in
 172
 173 The symbolic links in this subdirectory are as follows:
 174 .TP
 175 .IR /proc/[pid]/ns/cgroup " (since Linux 4.6)"
 176 This file is a handle for the cgroup namespace of the process.
 177 .TP
 178 .IR /proc/[pid]/ns/ipc " (since Linux 3.0)"
 179 This file is a handle for the IPC namespace of the process.
 180 .TP
 181 .IR /proc/[pid]/ns/mnt " (since Linux 3.8)"
 182 .\" commit 8823c079ba7136dc1948d6f6dcb5f8022bde438e
 183 This file is a handle for the mount namespace of the process.
 184 .TP
 185 .IR /proc/[pid]/ns/net " (since Linux 3.0)"
 186 This file is a handle for the network namespace of the process.
 187 .TP
 188 .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
 189 .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
 190 This file is a handle for the PID namespace of the process.
 191 This handle is permanent for the lifetime of the process
 192 (i.e., a process's PID namespace membership never changes).
 193 .TP
 194 .IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
 195 .\" commit eaa0d190bfe1ed891b814a52712dcd852554cb08
 196 This file is a handle for the PID namespace of
 197 child processes created by this process.
 198 This can change as a consequence of calls to
 199 .BR unshare (2)
 200 and
 201 .BR setns (2)
 202 (see
 203 .BR pid_namespaces (7)),
 204 so the file may differ from
 205 .IR /proc/[pid]/ns/pid .
 206 .TP
 207 .IR /proc/[pid]/ns/user " (since Linux 3.8)"
 208 .\" commit cde1975bc242f3e1072bde623ef378e547b73f91
 209 This file is a handle for the user namespace of the process.
 210 .TP
 211 .IR /proc/[pid]/ns/uts " (since Linux 3.0)"
 212 This file is a handle for the UTS namespace of the process.
 213 .PP
 214 Permission to dereference or read
 215 .RB ( readlink (2))
 216 these symbolic links is governed by a ptrace access mode
 217 .B PTRACE_MODE_READ_FSCREDS
 218 check; see
 219 .BR ptrace (2).
 220 .\"
 221 .\" ==================== The /proc/sys/user directory ====================
 222 .\"
 223 .SS The /proc/sys/user directory
 224 The files in the
 225 .I /proc/sys/user
 226 directory (which is present since Linux 4.9) expose limits
 227 on the number of namespaces of various types that can be created.
 228 The files are as follows:
 229 .TP
 230 .IR max_cgroup_namespaces
 231 The value in this file defines a per-user limit on the number of
 232 cgroup namespaces that may be created in the user namespace.
 233 .TP
 234 .IR max_ipc_namespaces
 235 The value in this file defines a per-user limit on the number of
 236 ipc namespaces that may be created in the user namespace.
 237 .TP
 238 .IR max_mnt_namespaces
 239 The value in this file defines a per-user limit on the number of
 240 mount namespaces that may be created in the user namespace.
 241 .TP
 242 .IR max_net_namespaces
 243 The value in this file defines a per-user limit on the number of
 244 network namespaces that may be created in the user namespace.
 245 .TP
 246 .IR max_pid_namespaces
 247 The value in this file defines a per-user limit on the number of
 248 pid namespaces that may be created in the user namespace.
 249 .TP
 250 .IR max_user_namespaces
 251 The value in this file defines a per-user limit on the number of
 252 user namespaces that may be created in the user namespace.
 253 .TP
 254 .IR max_uts_namespaces
 255 The value in this file defines a per-user limit on the number of
 256 user namespaces that may be created in the user namespace.
 257 .PP
 258 Note the following details about these files:
 259 .IP * 3
 260 The values in these files are modifiable by privileged processes.
 261 .IP *
 262 The values exposed by these files are the limits for the user namespace
 263 in which the opening process resides.
 264 .IP *
 265 The limits are per-user.
 266 Each user in the same user namespace
 267 can create namespaces up to the defined limit.
 268 .IP *
 269 The limits apply to all users, including UID 0.
 270 .IP *
 271 These limits apply in addition to any other per-namespace
 272 limits (such as those for PID and user namespaces) that may be enforced.
 273 .IP *
 274 Upon encountering these limits,
 275 .BR clone (2)
 276 and
 277 .BR unshare (2)
 278 fail with the error
 279 .BR ENOSPC .
 280 .IP *
 281 For the initial user namespace,
 282 the default value in each of these files is half the limit on the number
 283 of threads that may be created
 284 .RI ( /proc/sys/kernel/threads-max ).
 285 In all descendant user namespaces, the default value in each file is
 286 .BR MAXINT .
 287 .IP *
 288 When a namespace is created, the object is also accounted
 289 against ancestor namespaces.
 290 More precisely:
 291 .RS
 292 .IP + 3
 293 Each user namespace has a creator UID.
 294 .IP +
 295 When a namespace is created,
 296 it is accounted against the creator UIDs in each of the
 297 ancestor user namespaces,
 298 and the kernel ensures that the corresponding namespace limit
 299 for the creator UID in the ancestor namespace is not exceeded.
 300 .IP +
 301 The aforementioned point ensures that creating a new user namespace
 302 cannot be used as a means to escape the limits in force
 303 in the current user namespace.
 304 .RE
 305 .PP
 306 .\"
 307 .\" ==================== Cgroup namespaces ====================
 308 .\"
 309 .SS Cgroup namespaces (CLONE_NEWCGROUP)
 310 See
 311 .BR cgroup_namespaces (7).
 312 .\"
 313 .\" ==================== IPC namespaces ====================
 314 .\"
 315 .SS IPC namespaces (CLONE_NEWIPC)
 316 IPC namespaces isolate certain IPC resources,
 317 namely, System V IPC objects (see
 318 .BR svipc (7))
 319 and (since Linux 2.6.30)
 320 .\" commit 7eafd7c74c3f2e67c27621b987b28397110d643f
 321 .\" https://lwn.net/Articles/312232/
 322 POSIX message queues (see
 323 .BR mq_overview (7)).
 324 The common characteristic of these IPC mechanisms is that IPC
 325 objects are identified by mechanisms other than filesystem
 326 pathnames.
 327
 328 Each IPC namespace has its own set of System V IPC identifiers and
 329 its own POSIX message queue filesystem.
 330 Objects created in an IPC namespace are visible to all other processes
 331 that are members of that namespace,
 332 but are not visible to processes in other IPC namespaces.
 333
 334 The following
 335 .I /proc
 336 interfaces are distinct in each IPC namespace:
 337 .IP * 3
 338 The POSIX message queue interfaces in
 339 .IR /proc/sys/fs/mqueue .
 340 .IP *
 341 The System V IPC interfaces in
 342 .IR /proc/sys/kernel ,
 343 namely:
 344 .IR msgmax ,
 345 .IR msgmnb  ,
 346 .IR msgmni ,
 347 .IR sem ,
 348 .IR shmall ,
 349 .IR shmmax ,
 350 .IR shmmni ,
 351 and
 352 .IR shm_rmid_forced .
 353 .IP *
 354 The System V IPC interfaces in
 355 .IR /proc/sysvipc .
 356 .PP
 357 When an IPC namespace is destroyed
 358 (i.e., when the last process that is a member of the namespace terminates),
 359 all IPC objects in the namespace are automatically destroyed.
 360
 361 Use of IPC namespaces requires a kernel that is configured with the
 362 .B CONFIG_IPC_NS
 363 option.
 364 .\"
 365 .\" ==================== Network namespaces ====================
 366 .\"
 367 .SS Network namespaces (CLONE_NEWNET)
 368 Network namespaces provide isolation of the system resources associated
 369 with networking: network devices, IPv4 and IPv6 protocol stacks,
 370 IP routing tables, firewalls, the
 371 .I /proc/net
 372 directory, the
 373 .I /sys/class/net
 374 directory, port numbers (sockets), and so on.
 375 A physical network device can live in exactly one
 376 network namespace.
 377 A virtual network device ("veth") pair provides a pipe-like abstraction
 378 .\" FIXME . Add pointer to veth(4) page when it is eventually completed
 379 that can be used to create tunnels between network namespaces,
 380 and can be used to create a bridge to a physical network device
 381 in another namespace.
 382
 383 When a network namespace is freed
 384 (i.e., when the last process in the namespace terminates),
 385 its physical network devices are moved back to the
 386 initial network namespace (not to the parent of the process).
 387
 388 Use of network namespaces requires a kernel that is configured with the
 389 .B CONFIG_NET_NS
 390 option.
 391 .\"
 392 .\" ==================== Mount namespaces ====================
 393 .\"
 394 .SS Mount namespaces (CLONE_NEWNS)
 395 See
 396 .BR mount_namespaces (7).
 397 .\"
 398 .\" ==================== PID namespaces ====================
 399 .\"
 400 .SS PID namespaces (CLONE_NEWPID)
 401 See
 402 .BR pid_namespaces (7).
 403 .\"
 404 .\" ==================== User namespaces ====================
 405 .\"
 406 .SS User namespaces (CLONE_NEWUSER)
 407 See
 408 .BR user_namespaces (7).
 409 .\"
 410 .\" ==================== UTS namespaces ====================
 411 .\"
 412 .SS UTS namespaces (CLONE_NEWUTS)
 413 UTS namespaces provide isolation of two system identifiers:
 414 the hostname and the NIS domain name.
 415 These identifiers are set using
 416 .BR sethostname (2)
 417 and
 418 .BR setdomainname (2),
 419 and can be retrieved using
 420 .BR uname (2),
 421 .BR gethostname (2),
 422 and
 423 .BR getdomainname (2).
 424
 425 Use of UTS namespaces requires a kernel that is configured with the
 426 .B CONFIG_UTS_NS
 427 option.
 428 .fi
 429 .SH EXAMPLE
 430 See
 431 .BR user_namespaces (7).
 432 .SH SEE ALSO
 433 .BR nsenter (1),
 434 .BR readlink (1),
 435 .BR unshare (1),
 436 .BR clone (2),
 437 .BR ioctl_ns (2),
 438 .BR setns (2),
 439 .BR unshare (2),
 440 .BR proc (5),
 441 .BR capabilities (7),
 442 .BR cgroup_namespaces (7),
 443 .BR cgroups (7),
 444 .BR credentials (7),
 445 .BR pid_namespaces (7),
 446 .BR user_namespaces (7),
 447 .BR ip-netns (8),
 448 .BR lsns (8),
 449 .BR switch_root (8)