man7/socket.7

   1 '\" t
   2 .\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
   3 .\" and copyright (c) 1999 Matthew Wilcox.
   4 .\"
   5 .\" %%%LICENSE_START(VERBATIM_ONE_PARA)
   6 .\" Permission is granted to distribute possibly modified copies
   7 .\" of this page provided the header is included verbatim,
   8 .\" and in case of nontrivial modification author and date
   9 .\" of the modification is added to the header.
  10 .\" %%%LICENSE_END
  11 .\"
  12 .\" 2002-10-30, Michael Kerrisk, <mtk.manpages@gmail.com>
  13 .\"     Added description of SO_ACCEPTCONN
  14 .\" 2004-05-20, aeb, added SO_RCVTIMEO/SO_SNDTIMEO text.
  15 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  16 .\"     Added notes on capability requirements
  17 .\"     A few small grammar fixes
  18 .\" 2010-06-13 Jan Engelhardt <jengelh@medozas.de>
  19 .\"     Documented SO_DOMAIN and SO_PROTOCOL.
  20 .\" FIXME
  21 .\" The following are not yet documented:
  22 .\"     SO_PEERNAME (2.4?)
  23 .\"             get only
  24 .\"             Seems to do something similar to getpeername(), but then
  25 .\"             why is it necessary / how does it differ?
  26 .\"     SO_TIMESTAMPNS (2.6.22)
  27 .\"             Documentation/networking/timestamping.txt
  28 .\"             commit 92f37fd2ee805aa77925c1e64fd56088b46094fc
  29 .\"             Author: Eric Dumazet <dada1@cosmosbay.com>
  30 .\"     SO_TIMESTAMPING (2.6.30)
  31 .\"             Documentation/networking/timestamping.txt
  32 .\"             commit cb9eff097831007afb30d64373f29d99825d0068
  33 .\"             Author: Patrick Ohly <patrick.ohly@intel.com>
  34 .\"     SO_WIFI_STATUS (3.3)
  35 .\"             commit 6e3e939f3b1bf8534b32ad09ff199d88800835a0
  36 .\"             Author: Johannes Berg <johannes.berg@intel.com>
  37 .\"             Also: SCM_WIFI_STATUS
  38 .\"     SO_NOFCS (3.4)
  39 .\"             commit 3bdc0eba0b8b47797f4a76e377dd8360f317450f
  40 .\"             Author: Ben Greear <greearb@candelatech.com>
  41 .\"     SO_GET_FILTER (3.8)
  42 .\"             commit a8fc92778080c845eaadc369a0ecf5699a03bef0
  43 .\"             Author: Pavel Emelyanov <xemul@parallels.com>
  44 .\"     SO_SELECT_ERR_QUEUE (3.10)
  45 .\"             commit 7d4c04fc170087119727119074e72445f2bb192b
  46 .\"             Author: Keller, Jacob E <jacob.e.keller@intel.com>
  47 .\"     SO_MAX_PACING_RATE (3.13)
  48 .\"             commit 62748f32d501f5d3712a7c372bbb92abc7c62bc7
  49 .\"             Author: Eric Dumazet <edumazet@google.com>
  50 .\"     SO_BPF_EXTENSIONS (3.14)
  51 .\"             commit ea02f9411d9faa3553ed09ce0ec9f00ceae9885e
  52 .\"             Author: Michal Sekletar <msekleta@redhat.com>
  53 .\"
  54 .TH SOCKET 7 2015-05-07 Linux "Linux Programmer's Manual"
  55 .SH NAME
  56 socket \- Linux socket interface
  57 .SH SYNOPSIS
  58 .B #include <sys/socket.h>
  59 .sp
  60 .IB sockfd " = socket(int " socket_family ", int " socket_type ", int " protocol );
  61 .SH DESCRIPTION
  62 This manual page describes the Linux networking socket layer user
  63 interface.
  64 The BSD compatible sockets
  65 are the uniform interface
  66 between the user process and the network protocol stacks in the kernel.
  67 The protocol modules are grouped into
  68 .I protocol families
  69 such as
  70 .BR AF_INET ", " AF_IPX ", and " AF_PACKET ,
  71 and
  72 .I socket types
  73 such as
  74 .B SOCK_STREAM
  75 or
  76 .BR SOCK_DGRAM .
  77 See
  78 .BR socket (2)
  79 for more information on families and types.
  80 .SS Socket-layer functions
  81 These functions are used by the user process to send or receive packets
  82 and to do other socket operations.
  83 For more information see their respective manual pages.
  84
  85 .BR socket (2)
  86 creates a socket,
  87 .BR connect (2)
  88 connects a socket to a remote socket address,
  89 the
  90 .BR bind (2)
  91 function binds a socket to a local socket address,
  92 .BR listen (2)
  93 tells the socket that new connections shall be accepted, and
  94 .BR accept (2)
  95 is used to get a new socket with a new incoming connection.
  96 .BR socketpair (2)
  97 returns two connected anonymous sockets (implemented only for a few
  98 local families like
  99 .BR AF_UNIX )
 100 .PP
 101 .BR send (2),
 102 .BR sendto (2),
 103 and
 104 .BR sendmsg (2)
 105 send data over a socket, and
 106 .BR recv (2),
 107 .BR recvfrom (2),
 108 .BR recvmsg (2)
 109 receive data from a socket.
 110 .BR poll (2)
 111 and
 112 .BR select (2)
 113 wait for arriving data or a readiness to send data.
 114 In addition, the standard I/O operations like
 115 .BR write (2),
 116 .BR writev (2),
 117 .BR sendfile (2),
 118 .BR read (2),
 119 and
 120 .BR readv (2)
 121 can be used to read and write data.
 122 .PP
 123 .BR getsockname (2)
 124 returns the local socket address and
 125 .BR getpeername (2)
 126 returns the remote socket address.
 127 .BR getsockopt (2)
 128 and
 129 .BR setsockopt (2)
 130 are used to set or get socket layer or protocol options.
 131 .BR ioctl (2)
 132 can be used to set or read some other options.
 133 .PP
 134 .BR close (2)
 135 is used to close a socket.
 136 .BR shutdown (2)
 137 closes parts of a full-duplex socket connection.
 138 .PP
 139 Seeking, or calling
 140 .BR pread (2)
 141 or
 142 .BR pwrite (2)
 143 with a nonzero position is not supported on sockets.
 144 .PP
 145 It is possible to do nonblocking I/O on sockets by setting the
 146 .B O_NONBLOCK
 147 flag on a socket file descriptor using
 148 .BR fcntl (2).
 149 Then all operations that would block will (usually)
 150 return with
 151 .B EAGAIN
 152 (operation should be retried later);
 153 .BR connect (2)
 154 will return
 155 .B EINPROGRESS
 156 error.
 157 The user can then wait for various events via
 158 .BR poll (2)
 159 or
 160 .BR select (2).
 161 .TS
 162 tab(:) allbox;
 163 c s s
 164 l l l.
 165 I/O events
 166 Event:Poll flag:Occurrence
 167 Read:POLLIN:T{
 168 New data arrived.
 169 T}
 170 Read:POLLIN:T{
 171 A connection setup has been completed
 172 (for connection-oriented sockets)
 173 T}
 174 Read:POLLHUP:T{
 175 A disconnection request has been initiated by the other end.
 176 T}
 177 Read:POLLHUP:T{
 178 A connection is broken (only for connection-oriented protocols).
 179 When the socket is written
 180 .B SIGPIPE
 181 is also sent.
 182 T}
 183 Write:POLLOUT:T{
 184 Socket has enough send buffer space for writing new data.
 185 T}
 186 Read/Write:T{
 187 POLLIN|
 188 .br
 189 POLLOUT
 190 T}:T{
 191 An outgoing
 192 .BR connect (2)
 193 finished.
 194 T}
 195 Read/Write:POLLERR:An asynchronous error occurred.
 196 Read/Write:POLLHUP:The other end has shut down one direction.
 197 Exception:POLLPRI:T{
 198 Urgent data arrived.
 199 .B SIGURG
 200 is sent then.
 201 T}
 202 .\" FIXME . The following is not true currently:
 203 .\" It is no I/O event when the connection
 204 .\" is broken from the local end using
 205 .\" .BR shutdown (2)
 206 .\" or
 207 .\" .BR close (2).
 208 .TE
 209 .PP
 210 An alternative to
 211 .BR poll (2)
 212 and
 213 .BR select (2)
 214 is to let the kernel inform the application about events
 215 via a
 216 .B SIGIO
 217 signal.
 218 For that the
 219 .B O_ASYNC
 220 flag must be set on a socket file descriptor via
 221 .BR fcntl (2)
 222 and a valid signal handler for
 223 .B SIGIO
 224 must be installed via
 225 .BR sigaction (2).
 226 See the
 227 .I Signals
 228 discussion below.
 229 .SS Socket address structures
 230 Each socket domain has its own format for socket addresses,
 231 with a domain-specific address structure.
 232 Each of these structures begins with an
 233 integer "family" field (typed as
 234 .IR sa_family_t )
 235 that indicates the type of the address structure.
 236 This allows
 237 the various system calls (e.g.,
 238 .BR connect (2),
 239 .BR bind (2),
 240 .BR accept (2),
 241 .BR getsockname (2),
 242 .BR getpeername (2)),
 243 which are generic to all socket domains,
 244 to determine the domain of a particular socket address.
 245
 246 To allow any type of socket address to be passed to
 247 interfaces in the sockets API,
 248 the type
 249 .IR "struct sockaddr"
 250 is defined.
 251 The purpose of this type is purely to allow casting of
 252 domain-specific socket address types to a "generic" type,
 253 so as to avoid compiler warnings about type mismatches in
 254 calls to the sockets API.
 255
 256 In addition, the sockets API provides the data type
 257 .IR "struct sockaddr_storage".
 258 This type
 259 is suitable to accommodate all supported domain-specific socket
 260 address structures; it is large enough and is aligned properly.
 261 (In particular, it is large enough to hold
 262 IPv6 socket addresses.)
 263 The structure includes the following field, which can be used to identify
 264 the type of socket address actually stored in the structure:
 265
 266 .in +4n
 267 .nf
 268     sa_family_t ss_family;
 269 .fi
 270 .in
 271
 272 The
 273 .I sockaddr_storage
 274 structure is useful in programs that must handle socket addresses
 275 in a generic way
 276 (e.g., programs that must deal with both IPv4 and IPv6 socket addresses).
 277 .SS Socket options
 278 The socket options listed below can be set by using
 279 .BR setsockopt (2)
 280 and read with
 281 .BR getsockopt (2)
 282 with the socket level set to
 283 .B SOL_SOCKET
 284 for all sockets.
 285 Unless otherwise noted,
 286 .I optval
 287 is a pointer to an
 288 .IR int .
 289 .\" FIXME .
 290 .\" In the list below, the text used to describe argument types
 291 .\" for each socket option should be more consistent
 292 .\"
 293 .\" SO_ACCEPTCONN is in POSIX.1-2001, and its origin is explained in
 294 .\" W R Stevens, UNPv1
 295 .TP
 296 .B SO_ACCEPTCONN
 297 Returns a value indicating whether or not this socket has been marked
 298 to accept connections with
 299 .BR listen (2).
 300 The value 0 indicates that this is not a listening socket,
 301 the value 1 indicates that this is a listening socket.
 302 This socket option is read-only.
 303 .TP
 304 .BR SO_ATTACH_FILTER " and " SO_ATTACH_BPF
 305 Attach a classic or extended BPF program (respectively) to the socket
 306 for use as a filter of incoming packets. A packet will be dropped if
 307 the filter program returns zero.  If the filter program returns a
 308 non-zero value which is less than the packet's data length, the packet
 309 will be truncated to the length returned.  If the value returned by
 310 the filter is greater than or equal to the packet's data length, the
 311 packet is allowed to proceed unmodified.
 312
 313 The argument for
 314 .BR SO_ATTACH_FILTER
 315 is a
 316 .I sock_fprog
 317 structure in
 318 .B <linux/filter.h>.
 319 .sp
 320 .in +4n
 321 .nf
 322 struct sock_fprog {
 323     unsigned short      len;
 324     struct sock_filter *filter;
 325 };
 326 .fi
 327 .in
 328 .IP
 329 The argument for
 330 .BR SO_ATTACH_BPF
 331 is a file descriptor returned by the
 332 .BR bpf (2)
 333 system call and must refer to a program of type
 334 .BR BPF_PROG_TYPE_SOCKET_FILTER.
 335 These options may be set multiple times for a given socket, each time
 336 replacing the previous filter program.  The classic and extended
 337 versions may be called on the same socket, but the previous filter
 338 will always be replaced such that a socket never has more than one
 339 filter defined.
 340
 341 .BR SO_ATTACH_FILTER
 342 is available since Linux 2.2.
 343 .BR SO_ATTACH_BPF
 344 is available since Linux 3.19.  Both classic and extended BPF are
 345 explained in the kernel source file
 346 .I Documentation/networking/filter.txt
 347 .TP
 348 .BR SO_ATTACH_REUSEPORT_CBPF ", " SO_ATTACH_REUSEPORT_EBPF " (since Linux 4.5)"
 349 For use with the
 350 .BR SO_REUSEPORT
 351 option, these options allow the user to set a classic or extended
 352 BPF program (respectively) which defines how packets are assigned to
 353 the sockets in the reuseport group (that is, all sockets which have
 354 .BR SO_REUSEPORT
 355 set and are using the same local address to receive packets).  The BPF
 356 program must return an index between 0 and N-1 representing the socket
 357 which should receive the packet (where N is the number of sockets in
 358 the group). If the BPF program returns an invalid index, socket
 359 selection will fall back to the plain
 360 .BR SO_REUSEPORT
 361 mechanism.
 362
 363 Sockets are numbered in the order in which they are added to the group
 364 (that is, the order of
 365 .BR bind (2)
 366 calls for UDP sockets or the order of
 367 .BR listen (2)
 368 calls for TCP sockets).  New sockets added to a reuseport group will
 369 inherit the BPF program.  When a socket is removed from a reuseport
 370 group (via
 371 .BR close (2))
 372 the last socket in the group will be moved into the closed socket's
 373 position.
 374
 375 These options may be set repeatedly at any time on any single socket
 376 in the group to replace the current BPF program used by all sockets in
 377 the group.
 378 .BR SO_ATTACH_REUSEPORT_CBPF
 379 takes the same socket argument type as
 380 .BR SO_ATTACH_FILTER
 381 and
 382 .BR SO_ATTACH_REUSEPORT_EBPF
 383 takes the same socket argument type as
 384 .BR SO_ATTACH_BPF.
 385 UDP support for this feature is available since Linux 4.5.
 386 TCP support for this feature is available since Linux 4.6.
 387 .TP
 388 .B SO_BINDTODEVICE
 389 Bind this socket to a particular device like \(lqeth0\(rq,
 390 as specified in the passed interface name.
 391 If the
 392 name is an empty string or the option length is zero, the socket device
 393 binding is removed.
 394 The passed option is a variable-length null-terminated
 395 interface name string with the maximum size of
 396 .BR IFNAMSIZ .
 397 If a socket is bound to an interface,
 398 only packets received from that particular interface are processed by the
 399 socket.
 400 Note that this works only for some socket types, particularly
 401 .B AF_INET
 402 sockets.
 403 It is not supported for packet sockets (use normal
 404 .BR bind (2)
 405 there).
 406
 407 Before Linux 3.8,
 408 this socket option could be set, but could not retrieved with
 409 .BR getsockopt (2).
 410 Since Linux 3.8, it is readable.
 411 The
 412 .I optlen
 413 argument should contain the buffer size available
 414 to receive the device name and is recommended to be
 415 .BR IFNAMSZ
 416 bytes.
 417 The real device name length is reported back in the
 418 .I optlen
 419 argument.
 420 .TP
 421 .B SO_BROADCAST
 422 Set or get the broadcast flag.
 423 When enabled, datagram sockets are allowed to send
 424 packets to a broadcast address.
 425 This option has no effect on stream-oriented sockets.
 426 .TP
 427 .B SO_BSDCOMPAT
 428 Enable BSD bug-to-bug compatibility.
 429 This is used by the UDP protocol module in Linux 2.0 and 2.2.
 430 If enabled, ICMP errors received for a UDP socket will not be passed
 431 to the user program.
 432 In later kernel versions, support for this option has been phased out:
 433 Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning
 434 (printk()) if a program uses this option.
 435 Linux 2.0 also enabled BSD bug-to-bug compatibility
 436 options (random header changing, skipping of the broadcast flag) for raw
 437 sockets with this option, but that was removed in Linux 2.2.
 438 .TP
 439 .B SO_DEBUG
 440 Enable socket debugging.
 441 Only allowed for processes with the
 442 .B CAP_NET_ADMIN
 443 capability or an effective user ID of 0.
 444 .TP
 445 .BR SO_DETACH_FILTER " and " SO_DETACH_BPF
 446 These options may be used to remove the BPF program attached to the
 447 socket with either
 448 .BR SO_ATTACH_FILTER
 449 or
 450 .BR SO_ATTACH_BPF.
 451 The option value is ignored.
 452 .BR SO_DETACH_FILTER
 453 is available since Linux 2.2.
 454 .BR SO_DETACH_BPF
 455 is available since Linux 3.19.
 456 .TP
 457 .BR SO_DOMAIN " (since Linux 2.6.32)"
 458 Retrieves the socket domain as an integer, returning a value such as
 459 .BR AF_INET6 .
 460 See
 461 .BR socket (2)
 462 for details.
 463 This socket option is read-only.
 464 .TP
 465 .B SO_ERROR
 466 Get and clear the pending socket error.
 467 This socket option is read-only.
 468 Expects an integer.
 469 .TP
 470 .B SO_DONTROUTE
 471 Don't send via a gateway, send only to directly connected hosts.
 472 The same effect can be achieved by setting the
 473 .B MSG_DONTROUTE
 474 flag on a socket
 475 .BR send (2)
 476 operation.
 477 Expects an integer boolean flag.
 478 .TP
 479 .B SO_KEEPALIVE
 480 Enable sending of keep-alive messages on connection-oriented sockets.
 481 Expects an integer boolean flag.
 482 .TP
 483 .B SO_LINGER
 484 Sets or gets the
 485 .B SO_LINGER
 486 option.
 487 The argument is a
 488 .I linger
 489 structure.
 490 .sp
 491 .in +4n
 492 .nf
 493 struct linger {
 494     int l_onoff;    /* linger active */
 495     int l_linger;   /* how many seconds to linger for */
 496 };
 497 .fi
 498 .in
 499 .IP
 500 When enabled, a
 501 .BR close (2)
 502 or
 503 .BR shutdown (2)
 504 will not return until all queued messages for the socket have been
 505 successfully sent or the linger timeout has been reached.
 506 Otherwise,
 507 the call returns immediately and the closing is done in the background.
 508 When the socket is closed as part of
 509 .BR exit (2),
 510 it always lingers in the background.
 511 .TP
 512 .B SO_LOCK_FILTER
 513 When set, this option will prevent an unprivileged process from
 514 changing the filters associated with the socket.  These filters
 515 include any set using the socket options
 516 .BR SO_ATTACH_FILTER,
 517 .BR SO_ATTACH_BPF,
 518 .BR SO_ATTACH_REUSEPORT_CBPF
 519 or
 520 .BR SO_ATTACH_REUSEPORT_EPBF.
 521 The typical use case is for a privileged process to setup a socket with
 522 restrictive filters, set
 523 .BR SO_LOCK_FILTER
 524 and then either drop its privileges or pass the socket file descriptor
 525 to an unprivileged process.  Attempts to change a filter by an
 526 unprivileged process while
 527 .BR SO_LOCK_FILTER
 528 is set will result in an error with value
 529 .BR EPERM.
 530 .TP
 531 .BR SO_MARK " (since Linux 2.6.25)"
 532 .\" commit 4a19ec5800fc3bb64e2d87c4d9fdd9e636086fe0
 533 .\" and    914a9ab386a288d0f22252fc268ecbc048cdcbd5
 534 Set the mark for each packet sent through this socket
 535 (similar to the netfilter MARK target but socket-based).
 536 Changing the mark can be used for mark-based
 537 routing without netfilter or for packet filtering.
 538 Setting this option requires the
 539 .B CAP_NET_ADMIN
 540 capability.
 541 .TP
 542 .B SO_OOBINLINE
 543 If this option is enabled,
 544 out-of-band data is directly placed into the receive data stream.
 545 Otherwise, out-of-band data is passed only when the
 546 .B MSG_OOB
 547 flag is set during receiving.
 548 .\" don't document it because it can do too much harm.
 549 .\".B SO_NO_CHECK
 550 .\"     The kernel has support for the SO_NO_CHECK socket
 551 .\"     option (boolean: 0 == default, calculate checksum on xmit,
 552 .\"     1 == do not calculate checksum on xmit).
 553 .\" Additional note from Andi Kleen on SO_NO_CHECK (2010-08-30)
 554 .\"     On Linux UDP checksums are essentially free and there's no reason
 555 .\"     to turn them off and it would disable another safety line.
 556 .\"     That is why I didn't document the option.
 557 .TP
 558 .B SO_PASSCRED
 559 Enable or disable the receiving of the
 560 .B SCM_CREDENTIALS
 561 control message.
 562 For more information see
 563 .BR unix (7).
 564 .\" FIXME Document SO_PASSSEC, added in 2.6.18; there is some info
 565 .\" in the 2.6.18 ChangeLog
 566 .TP
 567 .BR SO_PEEK_OFF " (since Linux 3.4)"
 568 .\" commit ef64a54f6e558155b4f149bb10666b9e914b6c54
 569 This option, which is currently supported only for
 570 .BR unix (7)
 571 sockets, sets the value of the "peek offset" for the
 572 .BR recv (2)
 573 system call when used with
 574 .BR MSG_PEEK
 575 flag.
 576
 577 When this option is set to a negative value
 578 (it is set to \-1 for all new sockets),
 579 traditional behavior is provided:
 580 .BR recv (2)
 581 with the
 582 .BR MSG_PEEK
 583 flag will peek data from the front of the queue.
 584
 585 When the option is set to a value greater than or equal to zero,
 586 then the next peek at data queued in the socket will occur at
 587 the byte offset specified by the option value.
 588 At the same time, the "peek offset" will be
 589 incremented by the number of bytes that were peeked from the queue,
 590 so that a subsequent peek will return the next data in the queue.
 591
 592 If data is removed from the front of the queue via a call to
 593 .BR recv (2)
 594 (or similar) without the
 595 .BR MSG_PEEK
 596 flag, the "peek offset" will be decreased by the number of bytes removed.
 597 In other words, receiving data without the
 598 .B MSG_PEEK
 599 flag will cause the "peek offset" to be adjusted to maintain
 600 the correct relative position in the queued data,
 601 so that a subsequent peek will retrieve the data that would have been
 602 retrieved had the data not been removed.
 603
 604 For datagram sockets, if the "peek offset" points to the middle of a packet,
 605 the data returned will be marked with the
 606 .BR MSG_TRUNC
 607 flag.
 608
 609 The following example serves to illustrate the use of
 610 .BR SO_PEEK_OFF .
 611 Suppose a stream socket has the following queued input data:
 612
 613     aabbccddeeff
 614 .IP
 615 The following sequence of
 616 .BR recv (2)
 617 calls would have the effect noted in the comments:
 618
 619 .in +4n
 620 .nf
 621 int ov = 4;                  // Set peek offset to 4
 622 setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
 623
 624 recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
 625 recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
 626 recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
 627 recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8
 628 .fi
 629 .in
 630 .TP
 631 .B SO_PEERCRED
 632 Return the credentials of the foreign process connected to this socket.
 633 This is possible only for connected
 634 .B AF_UNIX
 635 stream sockets and
 636 .B AF_UNIX
 637 stream and datagram socket pairs created using
 638 .BR socketpair (2);
 639 see
 640 .BR unix (7).
 641 The returned credentials are those that were in effect at the time
 642 of the call to
 643 .BR connect (2)
 644 or
 645 .BR socketpair (2).
 646 The argument is a
 647 .I ucred
 648 structure; define the
 649 .B _GNU_SOURCE
 650 feature test macro to obtain the definition of that structure from
 651 .IR <sys/socket.h> .
 652 This socket option is read-only.
 653 .TP
 654 .B SO_PRIORITY
 655 Set the protocol-defined priority for all packets to be sent on
 656 this socket.
 657 Linux uses this value to order the networking queues:
 658 packets with a higher priority may be processed first depending
 659 on the selected device queueing discipline.
 660 .\" For
 661 .\" .BR ip (7),
 662 .\" this also sets the IP type-of-service (TOS) field for outgoing packets.
 663 Setting a priority outside the range 0 to 6 requires the
 664 .B CAP_NET_ADMIN
 665 capability.
 666 .TP
 667 .BR SO_PROTOCOL " (since Linux 2.6.32)"
 668 Retrieves the socket protocol as an integer, returning a value such as
 669 .BR IPPROTO_SCTP .
 670 See
 671 .BR socket (2)
 672 for details.
 673 This socket option is read-only.
 674 .TP
 675 .B SO_RCVBUF
 676 Sets or gets the maximum socket receive buffer in bytes.
 677 The kernel doubles this value (to allow space for bookkeeping overhead)
 678 when it is set using
 679 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 680 .BR setsockopt (2),
 681 and this doubled value is returned by
 682 .BR getsockopt (2).
 683 .\" The following thread on LMKL is quite informative:
 684 .\" getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard" behavior
 685 .\" 17 July 2012
 686 .\" http://thread.gmane.org/gmane.linux.kernel/1328935
 687 The default value is set by the
 688 .I /proc/sys/net/core/rmem_default
 689 file, and the maximum allowed value is set by the
 690 .I /proc/sys/net/core/rmem_max
 691 file.
 692 The minimum (doubled) value for this option is 256.
 693 .TP
 694 .BR SO_RCVBUFFORCE " (since Linux 2.6.14)"
 695 Using this socket option, a privileged
 696 .RB ( CAP_NET_ADMIN )
 697 process can perform the same task as
 698 .BR SO_RCVBUF ,
 699 but the
 700 .I rmem_max
 701 limit can be overridden.
 702 .TP
 703 .BR SO_RCVLOWAT " and " SO_SNDLOWAT
 704 Specify the minimum number of bytes in the buffer until the socket layer
 705 will pass the data to the protocol
 706 .RB ( SO_SNDLOWAT )
 707 or the user on receiving
 708 .RB ( SO_RCVLOWAT ).
 709 These two values are initialized to 1.
 710 .B SO_SNDLOWAT
 711 is not changeable on Linux
 712 .RB ( setsockopt (2)
 713 fails with the error
 714 .BR ENOPROTOOPT ).
 715 .B SO_RCVLOWAT
 716 is changeable
 717 only since Linux 2.4.
 718 The
 719 .BR select (2)
 720 and
 721 .BR poll (2)
 722 system calls currently do not respect the
 723 .B SO_RCVLOWAT
 724 setting on Linux,
 725 and mark a socket readable when even a single byte of data is available.
 726 A subsequent read from the socket will block until
 727 .B SO_RCVLOWAT
 728 bytes are available.
 729 .\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=111049368106984&w=2
 730 .\" Tested on kernel 2.6.14 -- mtk, 30 Nov 05
 731 .TP
 732 .BR SO_RCVTIMEO " and " SO_SNDTIMEO
 733 .\" Not implemented in 2.0.
 734 .\" Implemented in 2.1.11 for getsockopt: always return a zero struct.
 735 .\" Implemented in 2.3.41 for setsockopt, and actually used.
 736 Specify the receiving or sending timeouts until reporting an error.
 737 The argument is a
 738 .IR "struct timeval" .
 739 If an input or output function blocks for this period of time, and
 740 data has been sent or received, the return value of that function
 741 will be the amount of data transferred; if no data has been transferred
 742 and the timeout has been reached, then \-1 is returned with
 743 .I errno
 744 set to
 745 .BR EAGAIN
 746 or
 747 .BR EWOULDBLOCK ,
 748 .\" in fact to EAGAIN
 749 or
 750 .B EINPROGRESS
 751 (for
 752 .BR connect (2))
 753 just as if the socket was specified to be nonblocking.
 754 If the timeout is set to zero (the default),
 755 then the operation will never timeout.
 756 Timeouts only have effect for system calls that perform socket I/O (e.g.,
 757 .BR read (2),
 758 .BR recvmsg (2),
 759 .BR send (2),
 760 .BR sendmsg (2));
 761 timeouts have no effect for
 762 .BR select (2),
 763 .BR poll (2),
 764 .BR epoll_wait (2),
 765 and so on.
 766 .TP
 767 .B SO_REUSEADDR
 768 .\"     commit c617f398edd4db2b8567a28e899a88f8f574798d
 769 .\"     https://lwn.net/Articles/542629/
 770 Indicates that the rules used in validating addresses supplied in a
 771 .BR bind (2)
 772 call should allow reuse of local addresses.
 773 For
 774 .B AF_INET
 775 sockets this
 776 means that a socket may bind, except when there
 777 is an active listening socket bound to the address.
 778 When the listening socket is bound to
 779 .B INADDR_ANY
 780 with a specific port then it is not possible
 781 to bind to this port for any local address.
 782 Argument is an integer boolean flag.
 783 .TP
 784 .BR SO_REUSEPORT " (since Linux 3.9)"
 785 Permits multiple
 786 .B AF_INET
 787 or
 788 .B AF_INET6
 789 sockets to be bound to an identical socket address.
 790 This option must be set on each socket (including the first socket)
 791 prior to calling
 792 .BR bind (2)
 793 on the socket.
 794 To prevent port hijacking,
 795 all of the processes binding to the same address must have the same
 796 effective UID.
 797 This option can be employed with both TCP and UDP sockets.
 798
 799 For TCP sockets, this option allows
 800 .BR accept (2)
 801 load distribution in a multi-threaded server to be improved by
 802 using a distinct listener socket for each thread.
 803 This provides improved load distribution as compared
 804 to traditional techniques such using a single
 805 .BR accept (2)ing
 806 thread that distributes connections,
 807 or having multiple threads that compete to
 808 .BR accept (2)
 809 from the same socket.
 810
 811 For UDP sockets,
 812 the use of this option can provide better distribution
 813 of incoming datagrams to multiple processes (or threads) as compared
 814 to the traditional technique of having multiple processes
 815 compete to receive datagrams on the same socket.
 816 .TP
 817 .BR SO_RXQ_OVFL " (since Linux 2.6.33)"
 818 .\" commit 3b885787ea4112eaa80945999ea0901bf742707f
 819 Indicates that an unsigned 32-bit value ancillary message (cmsg)
 820 should be attached to received skbs indicating
 821 the number of packets dropped by the socket between
 822 the last received packet and this received packet.
 823 .TP
 824 .B SO_SNDBUF
 825 Sets or gets the maximum socket send buffer in bytes.
 826 The kernel doubles this value (to allow space for bookkeeping overhead)
 827 when it is set using
 828 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 829 .\" See also the comment to SO_RCVBUF (17 Jul 2012 LKML mail)
 830 .BR setsockopt (2),
 831 and this doubled value is returned by
 832 .BR getsockopt (2).
 833 The default value is set by the
 834 .I /proc/sys/net/core/wmem_default
 835 file and the maximum allowed value is set by the
 836 .I /proc/sys/net/core/wmem_max
 837 file.
 838 The minimum (doubled) value for this option is 2048.
 839 .TP
 840 .BR SO_SNDBUFFORCE " (since Linux 2.6.14)"
 841 Using this socket option, a privileged
 842 .RB ( CAP_NET_ADMIN )
 843 process can perform the same task as
 844 .BR SO_SNDBUF ,
 845 but the
 846 .I wmem_max
 847 limit can be overridden.
 848 .TP
 849 .B SO_TIMESTAMP
 850 Enable or disable the receiving of the
 851 .B SO_TIMESTAMP
 852 control message.
 853 The timestamp control message is sent with level
 854 .B SOL_SOCKET
 855 and the
 856 .I cmsg_data
 857 field is a
 858 .I "struct timeval"
 859 indicating the
 860 reception time of the last packet passed to the user in this call.
 861 See
 862 .BR cmsg (3)
 863 for details on control messages.
 864 .TP
 865 .B SO_TYPE
 866 Gets the socket type as an integer (e.g.,
 867 .BR SOCK_STREAM ).
 868 This socket option is read-only.
 869 .TP
 870 .BR SO_BUSY_POLL " (since Linux 3.11)"
 871 Sets the approximate time in microseconds to busy poll on a blocking receive
 872 when there is no data.
 873 Increasing this value requires
 874 .BR CAP_NET_ADMIN .
 875 The default for this option is controlled by the
 876 .I /proc/sys/net/core/busy_read
 877 file.
 878
 879 The value in the
 880 .I /proc/sys/net/core/busy_poll
 881 file determines how long
 882 .BR select (2)
 883 and
 884 .BR poll (2)
 885 will busy poll when they operate on sockets with
 886 .BR SO_BUSY_POLL
 887 set and no events to report are found.
 888
 889 In both cases,
 890 busy polling will only be done when the socket last received data
 891 from a network device that supports this option.
 892
 893 While busy polling may improve latency of some applications,
 894 care must be taken when using it since this will increase
 895 both CPU utilization and power usage.
 896 .SS Signals
 897 When writing onto a connection-oriented socket that has been shut down
 898 (by the local or the remote end)
 899 .B SIGPIPE
 900 is sent to the writing process and
 901 .B EPIPE
 902 is returned.
 903 The signal is not sent when the write call
 904 specified the
 905 .B MSG_NOSIGNAL
 906 flag.
 907 .PP
 908 When requested with the
 909 .B FIOSETOWN
 910 .BR fcntl (2)
 911 or
 912 .B SIOCSPGRP
 913 .BR ioctl (2),
 914 .B SIGIO
 915 is sent when an I/O event occurs.
 916 It is possible to use
 917 .BR poll (2)
 918 or
 919 .BR select (2)
 920 in the signal handler to find out which socket the event occurred on.
 921 An alternative (in Linux 2.2) is to set a real-time signal using the
 922 .B F_SETSIG
 923 .BR fcntl (2);
 924 the handler of the real time signal will be called with
 925 the file descriptor in the
 926 .I si_fd
 927 field of its
 928 .IR siginfo_t .
 929 See
 930 .BR fcntl (2)
 931 for more information.
 932 .PP
 933 Under some circumstances (e.g., multiple processes accessing a
 934 single socket), the condition that caused the
 935 .B SIGIO
 936 may have already disappeared when the process reacts to the signal.
 937 If this happens, the process should wait again because Linux
 938 will resend the signal later.
 939 .\" .SS Ancillary messages
 940 .SS /proc interfaces
 941 The core socket networking parameters can be accessed
 942 via files in the directory
 943 .IR /proc/sys/net/core/ .
 944 .TP
 945 .I rmem_default
 946 contains the default setting in bytes of the socket receive buffer.
 947 .TP
 948 .I rmem_max
 949 contains the maximum socket receive buffer size in bytes which a user may
 950 set by using the
 951 .B SO_RCVBUF
 952 socket option.
 953 .TP
 954 .I wmem_default
 955 contains the default setting in bytes of the socket send buffer.
 956 .TP
 957 .I wmem_max
 958 contains the maximum socket send buffer size in bytes which a user may
 959 set by using the
 960 .B SO_SNDBUF
 961 socket option.
 962 .TP
 963 .IR message_cost " and " message_burst
 964 configure the token bucket filter used to load limit warning messages
 965 caused by external network events.
 966 .TP
 967 .I netdev_max_backlog
 968 Maximum number of packets in the global input queue.
 969 .TP
 970 .I optmem_max
 971 Maximum length of ancillary data and user control data like the iovecs
 972 per socket.
 973 .\" netdev_fastroute is not documented because it is experimental
 974 .SS Ioctls
 975 These operations can be accessed using
 976 .BR ioctl (2):
 977
 978 .in +4n
 979 .nf
 980 .IB error " = ioctl(" ip_socket ", " ioctl_type ", " &value_result ");"
 981 .fi
 982 .in
 983 .TP
 984 .B SIOCGSTAMP
 985 Return a
 986 .I struct timeval
 987 with the receive timestamp of the last packet passed to the user.
 988 This is useful for accurate round trip time measurements.
 989 See
 990 .BR setitimer (2)
 991 for a description of
 992 .IR "struct timeval" .
 993 .\"
 994 This ioctl should be used only if the socket option
 995 .B SO_TIMESTAMP
 996 is not set on the socket.
 997 Otherwise, it returns the timestamp of the
 998 last packet that was received while
 999 .B SO_TIMESTAMP
1000 was not set, or it fails if no such packet has been received,
1001 (i.e.,
1002 .BR ioctl (2)
1003 returns \-1 with
1004 .I errno
1005 set to
1006 .BR ENOENT ).
1007 .TP
1008 .B SIOCSPGRP
1009 Set the process or process group to send
1010 .B SIGIO
1011 or
1012 .B SIGURG
1013 signals
1014 to when an
1015 asynchronous I/O operation has finished or urgent data is available.
1016 The argument is a pointer to a
1017 .IR pid_t .
1018 If the argument is positive, send the signals to that process.
1019 If the
1020 argument is negative, send the signals to the process group with the ID
1021 of the absolute value of the argument.
1022 The process may only choose itself or its own process group to receive
1023 signals unless it has the
1024 .B CAP_KILL
1025 capability or an effective UID of 0.
1026 .TP
1027 .B FIOASYNC
1028 Change the
1029 .B O_ASYNC
1030 flag to enable or disable asynchronous I/O mode of the socket.
1031 Asynchronous I/O mode means that the
1032 .B SIGIO
1033 signal or the signal set with
1034 .B F_SETSIG
1035 is raised when a new I/O event occurs.
1036 .IP
1037 Argument is an integer boolean flag.
1038 (This operation is synonymous with the use of
1039 .BR fcntl (2)
1040 to set the
1041 .B O_ASYNC
1042 flag.)
1043 .\"
1044 .TP
1045 .B SIOCGPGRP
1046 Get the current process or process group that receives
1047 .B SIGIO
1048 or
1049 .B SIGURG
1050 signals,
1051 or 0
1052 when none is set.
1053 .PP
1054 Valid
1055 .BR fcntl (2)
1056 operations:
1057 .TP
1058 .B FIOGETOWN
1059 The same as the
1060 .B SIOCGPGRP
1061 .BR ioctl (2).
1062 .TP
1063 .B FIOSETOWN
1064 The same as the
1065 .B SIOCSPGRP
1066 .BR ioctl (2).
1067 .SH VERSIONS
1068 .B SO_BINDTODEVICE
1069 was introduced in Linux 2.0.30.
1070 .B SO_PASSCRED
1071 is new in Linux 2.2.
1072 The
1073 .I /proc
1074 interfaces were introduced in Linux 2.2.
1075 .B SO_RCVTIMEO
1076 and
1077 .B SO_SNDTIMEO
1078 are supported since Linux 2.3.41.
1079 Earlier, timeouts were fixed to
1080 a protocol-specific setting, and could not be read or written.
1081 .SH NOTES
1082 Linux assumes that half of the send/receive buffer is used for internal
1083 kernel structures; thus the values in the corresponding
1084 .I /proc
1085 files are twice what can be observed on the wire.
1086
1087 Linux will allow port reuse only with the
1088 .B SO_REUSEADDR
1089 option
1090 when this option was set both in the previous program that performed a
1091 .BR bind (2)
1092 to the port and in the program that wants to reuse the port.
1093 This differs from some implementations (e.g., FreeBSD)
1094 where only the later program needs to set the
1095 .B SO_REUSEADDR
1096 option.
1097 Typically this difference is invisible, since, for example, a server
1098 program is designed to always set this option.
1099 .\" .SH AUTHORS
1100 .\" This man page was written by Andi Kleen.
1101 .SH SEE ALSO
1102 .BR bpf (2),
1103 .BR connect (2),
1104 .BR getsockopt (2),
1105 .BR setsockopt (2),
1106 .BR socket (2),
1107 .BR pcap (3),
1108 .BR capabilities (7),
1109 .BR ddp (7),
1110 .BR ip (7),
1111 .BR packet (7),
1112 .BR tcp (7),
1113 .BR udp (7),
1114 .BR unix (7)