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 .\"
  21 .\" FIXME
  22 .\" The following are not yet documented:
  23 .\"
  24 .\" SO_PEERNAME (2.4?)
  25 .\"     get only
  26 .\"     Seems to do something similar to getpeername(), but then
  27 .\"     why is it necessary / how does it differ?
  28 .\"
  29 .\" SO_TIMESTAMPNS (2.6.22)
  30 .\"     Documentation/networking/timestamping.txt
  31 .\"     commit 92f37fd2ee805aa77925c1e64fd56088b46094fc
  32 .\"     Author: Eric Dumazet <dada1@cosmosbay.com>
  33 .\"
  34 .\" SO_TIMESTAMPING (2.6.30)
  35 .\"     Documentation/networking/timestamping.txt
  36 .\"     commit cb9eff097831007afb30d64373f29d99825d0068
  37 .\"     Author: Patrick Ohly <patrick.ohly@intel.com>
  38 .\"
  39 .\"  SO_WIFI_STATUS (3.3)
  40 .\"     commit 6e3e939f3b1bf8534b32ad09ff199d88800835a0
  41 .\"     Author: Johannes Berg <johannes.berg@intel.com>
  42 .\"     Also: SCM_WIFI_STATUS
  43 .\"
  44 .\" SO_NOFCS (3.4)
  45 .\"     commit 3bdc0eba0b8b47797f4a76e377dd8360f317450f
  46 .\"     Author: Ben Greear <greearb@candelatech.com>
  47 .\"
  48 .\"  SO_GET_FILTER (3.8)
  49 .\"     commit a8fc92778080c845eaadc369a0ecf5699a03bef0
  50 .\"     Author: Pavel Emelyanov <xemul@parallels.com>
  51 .\"
  52 .\" SO_SELECT_ERR_QUEUE (3.10)
  53 .\"     commit 7d4c04fc170087119727119074e72445f2bb192b
  54 .\"     Author: Keller, Jacob E <jacob.e.keller@intel.com>
  55 .\"
  56 .\" SO_MAX_PACING_RATE (3.13)
  57 .\"     commit 62748f32d501f5d3712a7c372bbb92abc7c62bc7
  58 .\"     Author: Eric Dumazet <edumazet@google.com>
  59 .\"
  60 .\" SO_BPF_EXTENSIONS (3.14)
  61 .\"     commit ea02f9411d9faa3553ed09ce0ec9f00ceae9885e
  62 .\"     Author: Michal Sekletar <msekleta@redhat.com>
  63 .\"
  64 .TH SOCKET 7 2016-10-08 Linux "Linux Programmer's Manual"
  65 .SH NAME
  66 socket \- Linux socket interface
  67 .SH SYNOPSIS
  68 .B #include <sys/socket.h>
  69 .sp
  70 .IB sockfd " = socket(int " socket_family ", int " socket_type ", int " protocol );
  71 .SH DESCRIPTION
  72 This manual page describes the Linux networking socket layer user
  73 interface.
  74 The BSD compatible sockets
  75 are the uniform interface
  76 between the user process and the network protocol stacks in the kernel.
  77 The protocol modules are grouped into
  78 .I protocol families
  79 such as
  80 .BR AF_INET ", " AF_IPX ", and " AF_PACKET ,
  81 and
  82 .I socket types
  83 such as
  84 .B SOCK_STREAM
  85 or
  86 .BR SOCK_DGRAM .
  87 See
  88 .BR socket (2)
  89 for more information on families and types.
  90 .SS Socket-layer functions
  91 These functions are used by the user process to send or receive packets
  92 and to do other socket operations.
  93 For more information see their respective manual pages.
  94
  95 .BR socket (2)
  96 creates a socket,
  97 .BR connect (2)
  98 connects a socket to a remote socket address,
  99 the
 100 .BR bind (2)
 101 function binds a socket to a local socket address,
 102 .BR listen (2)
 103 tells the socket that new connections shall be accepted, and
 104 .BR accept (2)
 105 is used to get a new socket with a new incoming connection.
 106 .BR socketpair (2)
 107 returns two connected anonymous sockets (implemented only for a few
 108 local families like
 109 .BR AF_UNIX )
 110 .PP
 111 .BR send (2),
 112 .BR sendto (2),
 113 and
 114 .BR sendmsg (2)
 115 send data over a socket, and
 116 .BR recv (2),
 117 .BR recvfrom (2),
 118 .BR recvmsg (2)
 119 receive data from a socket.
 120 .BR poll (2)
 121 and
 122 .BR select (2)
 123 wait for arriving data or a readiness to send data.
 124 In addition, the standard I/O operations like
 125 .BR write (2),
 126 .BR writev (2),
 127 .BR sendfile (2),
 128 .BR read (2),
 129 and
 130 .BR readv (2)
 131 can be used to read and write data.
 132 .PP
 133 .BR getsockname (2)
 134 returns the local socket address and
 135 .BR getpeername (2)
 136 returns the remote socket address.
 137 .BR getsockopt (2)
 138 and
 139 .BR setsockopt (2)
 140 are used to set or get socket layer or protocol options.
 141 .BR ioctl (2)
 142 can be used to set or read some other options.
 143 .PP
 144 .BR close (2)
 145 is used to close a socket.
 146 .BR shutdown (2)
 147 closes parts of a full-duplex socket connection.
 148 .PP
 149 Seeking, or calling
 150 .BR pread (2)
 151 or
 152 .BR pwrite (2)
 153 with a nonzero position is not supported on sockets.
 154 .PP
 155 It is possible to do nonblocking I/O on sockets by setting the
 156 .B O_NONBLOCK
 157 flag on a socket file descriptor using
 158 .BR fcntl (2).
 159 Then all operations that would block will (usually)
 160 return with
 161 .B EAGAIN
 162 (operation should be retried later);
 163 .BR connect (2)
 164 will return
 165 .B EINPROGRESS
 166 error.
 167 The user can then wait for various events via
 168 .BR poll (2)
 169 or
 170 .BR select (2).
 171 .TS
 172 tab(:) allbox;
 173 c s s
 174 l l l.
 175 I/O events
 176 Event:Poll flag:Occurrence
 177 Read:POLLIN:T{
 178 New data arrived.
 179 T}
 180 Read:POLLIN:T{
 181 A connection setup has been completed
 182 (for connection-oriented sockets)
 183 T}
 184 Read:POLLHUP:T{
 185 A disconnection request has been initiated by the other end.
 186 T}
 187 Read:POLLHUP:T{
 188 A connection is broken (only for connection-oriented protocols).
 189 When the socket is written
 190 .B SIGPIPE
 191 is also sent.
 192 T}
 193 Write:POLLOUT:T{
 194 Socket has enough send buffer space for writing new data.
 195 T}
 196 Read/Write:T{
 197 POLLIN |
 198 .br
 199 POLLOUT
 200 T}:T{
 201 An outgoing
 202 .BR connect (2)
 203 finished.
 204 T}
 205 Read/Write:POLLERR:An asynchronous error occurred.
 206 Read/Write:POLLHUP:The other end has shut down one direction.
 207 Exception:POLLPRI:T{
 208 Urgent data arrived.
 209 .B SIGURG
 210 is sent then.
 211 T}
 212 .\" FIXME . The following is not true currently:
 213 .\" It is no I/O event when the connection
 214 .\" is broken from the local end using
 215 .\" .BR shutdown (2)
 216 .\" or
 217 .\" .BR close (2).
 218 .TE
 219 .PP
 220 An alternative to
 221 .BR poll (2)
 222 and
 223 .BR select (2)
 224 is to let the kernel inform the application about events
 225 via a
 226 .B SIGIO
 227 signal.
 228 For that the
 229 .B O_ASYNC
 230 flag must be set on a socket file descriptor via
 231 .BR fcntl (2)
 232 and a valid signal handler for
 233 .B SIGIO
 234 must be installed via
 235 .BR sigaction (2).
 236 See the
 237 .I Signals
 238 discussion below.
 239 .SS Socket address structures
 240 Each socket domain has its own format for socket addresses,
 241 with a domain-specific address structure.
 242 Each of these structures begins with an
 243 integer "family" field (typed as
 244 .IR sa_family_t )
 245 that indicates the type of the address structure.
 246 This allows
 247 the various system calls (e.g.,
 248 .BR connect (2),
 249 .BR bind (2),
 250 .BR accept (2),
 251 .BR getsockname (2),
 252 .BR getpeername (2)),
 253 which are generic to all socket domains,
 254 to determine the domain of a particular socket address.
 255
 256 To allow any type of socket address to be passed to
 257 interfaces in the sockets API,
 258 the type
 259 .IR "struct sockaddr"
 260 is defined.
 261 The purpose of this type is purely to allow casting of
 262 domain-specific socket address types to a "generic" type,
 263 so as to avoid compiler warnings about type mismatches in
 264 calls to the sockets API.
 265
 266 In addition, the sockets API provides the data type
 267 .IR "struct sockaddr_storage".
 268 This type
 269 is suitable to accommodate all supported domain-specific socket
 270 address structures; it is large enough and is aligned properly.
 271 (In particular, it is large enough to hold
 272 IPv6 socket addresses.)
 273 The structure includes the following field, which can be used to identify
 274 the type of socket address actually stored in the structure:
 275
 276 .in +4n
 277 .nf
 278     sa_family_t ss_family;
 279 .fi
 280 .in
 281
 282 The
 283 .I sockaddr_storage
 284 structure is useful in programs that must handle socket addresses
 285 in a generic way
 286 (e.g., programs that must deal with both IPv4 and IPv6 socket addresses).
 287 .SS Socket options
 288 The socket options listed below can be set by using
 289 .BR setsockopt (2)
 290 and read with
 291 .BR getsockopt (2)
 292 with the socket level set to
 293 .B SOL_SOCKET
 294 for all sockets.
 295 Unless otherwise noted,
 296 .I optval
 297 is a pointer to an
 298 .IR int .
 299 .\" FIXME .
 300 .\" In the list below, the text used to describe argument types
 301 .\" for each socket option should be more consistent
 302 .\"
 303 .\" SO_ACCEPTCONN is in POSIX.1-2001, and its origin is explained in
 304 .\" W R Stevens, UNPv1
 305 .TP
 306 .B SO_ACCEPTCONN
 307 Returns a value indicating whether or not this socket has been marked
 308 to accept connections with
 309 .BR listen (2).
 310 The value 0 indicates that this is not a listening socket,
 311 the value 1 indicates that this is a listening socket.
 312 This socket option is read-only.
 313 .TP
 314 .BR SO_ATTACH_FILTER " (since Linux 2.2), " SO_ATTACH_BPF " (since Linux 3.19)"
 315 Attach a classic BPF
 316 .RB ( SO_ATTACH_FILTER )
 317 or an extended BPF
 318 .RB ( SO_ATTACH_BPF )
 319 program to the socket for use as a filter of incoming packets.
 320 A packet will be dropped if the filter program returns zero.
 321 If the filter program returns a
 322 non-zero value which is less than the packet's data length,
 323 the packet will be truncated to the length returned.
 324 If the value returned by the filter is greater than or equal to the
 325 packet's data length, the packet is allowed to proceed unmodified.
 326
 327 The argument for
 328 .BR SO_ATTACH_FILTER
 329 is a
 330 .I sock_fprog
 331 structure, defined in
 332 .IR <linux/filter.h> :
 333 .sp
 334 .in +4n
 335 .nf
 336 struct sock_fprog {
 337     unsigned short      len;
 338     struct sock_filter *filter;
 339 };
 340 .fi
 341 .in
 342 .IP
 343 The argument for
 344 .BR SO_ATTACH_BPF
 345 is a file descriptor returned by the
 346 .BR bpf (2)
 347 system call and must refer to a program of type
 348 .BR BPF_PROG_TYPE_SOCKET_FILTER.
 349
 350 These options may be set multiple times for a given socket,
 351 each time replacing the previous filter program.
 352 The classic and extended versions may be called on the same socket,
 353 but the previous filter will always be replaced such that a socket
 354 never has more than one filter defined.
 355
 356 Both classic and extended BPF are explained in the kernel source file
 357 .I Documentation/networking/filter.txt
 358 .TP
 359 .BR SO_ATTACH_REUSEPORT_CBPF ", " SO_ATTACH_REUSEPORT_EBPF
 360 For use with the
 361 .BR SO_REUSEPORT
 362 option, these options allow the user to set a classic BPF
 363 .RB ( SO_ATTACH_REUSEPORT_CBPF )
 364 or an extended BPF
 365 .RB ( SO_ATTACH_REUSEPORT_EBPF )
 366 program which defines how packets are assigned to
 367 the sockets in the reuseport group (that is, all sockets which have
 368 .BR SO_REUSEPORT
 369 set and are using the same local address to receive packets).
 370
 371 The BPF program must return an index between 0 and N\-1 representing
 372 the socket which should receive the packet
 373 (where N is the number of sockets in the group).
 374 If the BPF program returns an invalid index,
 375 socket selection will fall back to the plain
 376 .BR SO_REUSEPORT
 377 mechanism.
 378
 379 Sockets are numbered in the order in which they are added to the group
 380 (that is, the order of
 381 .BR bind (2)
 382 calls for UDP sockets or the order of
 383 .BR listen (2)
 384 calls for TCP sockets).
 385 New sockets added to a reuseport group will inherit the BPF program.
 386 When a socket is removed from a reuseport group (via
 387 .BR close (2)),
 388 the last socket in the group will be moved into the closed socket's
 389 position.
 390
 391 These options may be set repeatedly at any time on any socket in the group
 392 to replace the current BPF program used by all sockets in the group.
 393
 394 .BR SO_ATTACH_REUSEPORT_CBPF
 395 takes the same argument type as
 396 .BR SO_ATTACH_FILTER
 397 and
 398 .BR SO_ATTACH_REUSEPORT_EBPF
 399 takes the same argument type as
 400 .BR SO_ATTACH_BPF.
 401
 402 UDP support for this feature is available since Linux 4.5;
 403 TCP support is available since Linux 4.6.
 404 .TP
 405 .B SO_BINDTODEVICE
 406 Bind this socket to a particular device like \(lqeth0\(rq,
 407 as specified in the passed interface name.
 408 If the
 409 name is an empty string or the option length is zero, the socket device
 410 binding is removed.
 411 The passed option is a variable-length null-terminated
 412 interface name string with the maximum size of
 413 .BR IFNAMSIZ .
 414 If a socket is bound to an interface,
 415 only packets received from that particular interface are processed by the
 416 socket.
 417 Note that this works only for some socket types, particularly
 418 .B AF_INET
 419 sockets.
 420 It is not supported for packet sockets (use normal
 421 .BR bind (2)
 422 there).
 423
 424 Before Linux 3.8,
 425 this socket option could be set, but could not retrieved with
 426 .BR getsockopt (2).
 427 Since Linux 3.8, it is readable.
 428 The
 429 .I optlen
 430 argument should contain the buffer size available
 431 to receive the device name and is recommended to be
 432 .BR IFNAMSZ
 433 bytes.
 434 The real device name length is reported back in the
 435 .I optlen
 436 argument.
 437 .TP
 438 .B SO_BROADCAST
 439 Set or get the broadcast flag.
 440 When enabled, datagram sockets are allowed to send
 441 packets to a broadcast address.
 442 This option has no effect on stream-oriented sockets.
 443 .TP
 444 .B SO_BSDCOMPAT
 445 Enable BSD bug-to-bug compatibility.
 446 This is used by the UDP protocol module in Linux 2.0 and 2.2.
 447 If enabled, ICMP errors received for a UDP socket will not be passed
 448 to the user program.
 449 In later kernel versions, support for this option has been phased out:
 450 Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning
 451 (printk()) if a program uses this option.
 452 Linux 2.0 also enabled BSD bug-to-bug compatibility
 453 options (random header changing, skipping of the broadcast flag) for raw
 454 sockets with this option, but that was removed in Linux 2.2.
 455 .TP
 456 .B SO_DEBUG
 457 Enable socket debugging.
 458 Allowed only for processes with the
 459 .B CAP_NET_ADMIN
 460 capability or an effective user ID of 0.
 461 .TP
 462 .BR SO_DETACH_FILTER " (since Linux 2.2), " SO_DETACH_BPF " (since Linux 3.19)"
 463 These two options, which are synonyms,
 464 may be used to remove the classic or extended BPF
 465 program attached to a socket with either
 466 .BR SO_ATTACH_FILTER
 467 or
 468 .BR SO_ATTACH_BPF .
 469 The option value is ignored.
 470 .TP
 471 .BR SO_DOMAIN " (since Linux 2.6.32)"
 472 Retrieves the socket domain as an integer, returning a value such as
 473 .BR AF_INET6 .
 474 See
 475 .BR socket (2)
 476 for details.
 477 This socket option is read-only.
 478 .TP
 479 .B SO_ERROR
 480 Get and clear the pending socket error.
 481 This socket option is read-only.
 482 Expects an integer.
 483 .TP
 484 .B SO_DONTROUTE
 485 Don't send via a gateway, send only to directly connected hosts.
 486 The same effect can be achieved by setting the
 487 .B MSG_DONTROUTE
 488 flag on a socket
 489 .BR send (2)
 490 operation.
 491 Expects an integer boolean flag.
 492 .TP
 493 .B SO_KEEPALIVE
 494 Enable sending of keep-alive messages on connection-oriented sockets.
 495 Expects an integer boolean flag.
 496 .TP
 497 .B SO_LINGER
 498 Sets or gets the
 499 .B SO_LINGER
 500 option.
 501 The argument is a
 502 .I linger
 503 structure.
 504 .sp
 505 .in +4n
 506 .nf
 507 struct linger {
 508     int l_onoff;    /* linger active */
 509     int l_linger;   /* how many seconds to linger for */
 510 };
 511 .fi
 512 .in
 513 .IP
 514 When enabled, a
 515 .BR close (2)
 516 or
 517 .BR shutdown (2)
 518 will not return until all queued messages for the socket have been
 519 successfully sent or the linger timeout has been reached.
 520 Otherwise,
 521 the call returns immediately and the closing is done in the background.
 522 When the socket is closed as part of
 523 .BR exit (2),
 524 it always lingers in the background.
 525 .TP
 526 .B SO_LOCK_FILTER
 527 .\" commit d59577b6ffd313d0ab3be39cb1ab47e29bdc9182
 528 When set, this option will prevent
 529 changing the filters associated with the socket.
 530 These filters include any set using the socket options
 531 .BR SO_ATTACH_FILTER,
 532 .BR SO_ATTACH_BPF,
 533 .BR SO_ATTACH_REUSEPORT_CBPF
 534 and
 535 .BR SO_ATTACH_REUSEPORT_EPBF .
 536
 537 The typical use case is for a privileged process to set up a raw socket
 538 (an operation that requires the
 539 .BR CAP_NET_RAW
 540 capability), apply a restrictive filter, set the
 541 .BR SO_LOCK_FILTER
 542 option,
 543 and then either drop its privileges or pass the socket file descriptor
 544 to an unprivileged process via a UNIX domain socket.
 545
 546 Once the
 547 .BR SO_LOCK_FILTER
 548 option has been enabled, attempts to change or remove the filter
 549 attached to a socket, or to disable the
 550 .BR SO_LOCK_FILTER
 551 option will fail with the error
 552 .BR EPERM .
 553 .TP
 554 .BR SO_MARK " (since Linux 2.6.25)"
 555 .\" commit 4a19ec5800fc3bb64e2d87c4d9fdd9e636086fe0
 556 .\" and    914a9ab386a288d0f22252fc268ecbc048cdcbd5
 557 Set the mark for each packet sent through this socket
 558 (similar to the netfilter MARK target but socket-based).
 559 Changing the mark can be used for mark-based
 560 routing without netfilter or for packet filtering.
 561 Setting this option requires the
 562 .B CAP_NET_ADMIN
 563 capability.
 564 .TP
 565 .B SO_OOBINLINE
 566 If this option is enabled,
 567 out-of-band data is directly placed into the receive data stream.
 568 Otherwise, out-of-band data is passed only when the
 569 .B MSG_OOB
 570 flag is set during receiving.
 571 .\" don't document it because it can do too much harm.
 572 .\".B SO_NO_CHECK
 573 .\"     The kernel has support for the SO_NO_CHECK socket
 574 .\"     option (boolean: 0 == default, calculate checksum on xmit,
 575 .\"     1 == do not calculate checksum on xmit).
 576 .\" Additional note from Andi Kleen on SO_NO_CHECK (2010-08-30)
 577 .\"     On Linux UDP checksums are essentially free and there's no reason
 578 .\"     to turn them off and it would disable another safety line.
 579 .\"     That is why I didn't document the option.
 580 .TP
 581 .B SO_PASSCRED
 582 Enable or disable the receiving of the
 583 .B SCM_CREDENTIALS
 584 control message.
 585 For more information see
 586 .BR unix (7).
 587 .\" FIXME Document SO_PASSSEC, added in 2.6.18; there is some info
 588 .\" in the 2.6.18 ChangeLog
 589 .TP
 590 .BR SO_PEEK_OFF " (since Linux 3.4)"
 591 .\" commit ef64a54f6e558155b4f149bb10666b9e914b6c54
 592 This option, which is currently supported only for
 593 .BR unix (7)
 594 sockets, sets the value of the "peek offset" for the
 595 .BR recv (2)
 596 system call when used with
 597 .BR MSG_PEEK
 598 flag.
 599
 600 When this option is set to a negative value
 601 (it is set to \-1 for all new sockets),
 602 traditional behavior is provided:
 603 .BR recv (2)
 604 with the
 605 .BR MSG_PEEK
 606 flag will peek data from the front of the queue.
 607
 608 When the option is set to a value greater than or equal to zero,
 609 then the next peek at data queued in the socket will occur at
 610 the byte offset specified by the option value.
 611 At the same time, the "peek offset" will be
 612 incremented by the number of bytes that were peeked from the queue,
 613 so that a subsequent peek will return the next data in the queue.
 614
 615 If data is removed from the front of the queue via a call to
 616 .BR recv (2)
 617 (or similar) without the
 618 .BR MSG_PEEK
 619 flag, the "peek offset" will be decreased by the number of bytes removed.
 620 In other words, receiving data without the
 621 .B MSG_PEEK
 622 flag will cause the "peek offset" to be adjusted to maintain
 623 the correct relative position in the queued data,
 624 so that a subsequent peek will retrieve the data that would have been
 625 retrieved had the data not been removed.
 626
 627 For datagram sockets, if the "peek offset" points to the middle of a packet,
 628 the data returned will be marked with the
 629 .BR MSG_TRUNC
 630 flag.
 631
 632 The following example serves to illustrate the use of
 633 .BR SO_PEEK_OFF .
 634 Suppose a stream socket has the following queued input data:
 635
 636     aabbccddeeff
 637 .IP
 638 The following sequence of
 639 .BR recv (2)
 640 calls would have the effect noted in the comments:
 641
 642 .in +4n
 643 .nf
 644 int ov = 4;                  // Set peek offset to 4
 645 setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
 646
 647 recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
 648 recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
 649 recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
 650 recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8
 651 .fi
 652 .in
 653 .TP
 654 .B SO_PEERCRED
 655 Return the credentials of the foreign process connected to this socket.
 656 This is possible only for connected
 657 .B AF_UNIX
 658 stream sockets and
 659 .B AF_UNIX
 660 stream and datagram socket pairs created using
 661 .BR socketpair (2);
 662 see
 663 .BR unix (7).
 664 The returned credentials are those that were in effect at the time
 665 of the call to
 666 .BR connect (2)
 667 or
 668 .BR socketpair (2).
 669 The argument is a
 670 .I ucred
 671 structure; define the
 672 .B _GNU_SOURCE
 673 feature test macro to obtain the definition of that structure from
 674 .IR <sys/socket.h> .
 675 This socket option is read-only.
 676 .TP
 677 .B SO_PRIORITY
 678 Set the protocol-defined priority for all packets to be sent on
 679 this socket.
 680 Linux uses this value to order the networking queues:
 681 packets with a higher priority may be processed first depending
 682 on the selected device queueing discipline.
 683 .\" For
 684 .\" .BR ip (7),
 685 .\" this also sets the IP type-of-service (TOS) field for outgoing packets.
 686 Setting a priority outside the range 0 to 6 requires the
 687 .B CAP_NET_ADMIN
 688 capability.
 689 .TP
 690 .BR SO_PROTOCOL " (since Linux 2.6.32)"
 691 Retrieves the socket protocol as an integer, returning a value such as
 692 .BR IPPROTO_SCTP .
 693 See
 694 .BR socket (2)
 695 for details.
 696 This socket option is read-only.
 697 .TP
 698 .B SO_RCVBUF
 699 Sets or gets the maximum socket receive buffer in bytes.
 700 The kernel doubles this value (to allow space for bookkeeping overhead)
 701 when it is set using
 702 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 703 .BR setsockopt (2),
 704 and this doubled value is returned by
 705 .BR getsockopt (2).
 706 .\" The following thread on LMKL is quite informative:
 707 .\" getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard" behavior
 708 .\" 17 July 2012
 709 .\" http://thread.gmane.org/gmane.linux.kernel/1328935
 710 The default value is set by the
 711 .I /proc/sys/net/core/rmem_default
 712 file, and the maximum allowed value is set by the
 713 .I /proc/sys/net/core/rmem_max
 714 file.
 715 The minimum (doubled) value for this option is 256.
 716 .TP
 717 .BR SO_RCVBUFFORCE " (since Linux 2.6.14)"
 718 Using this socket option, a privileged
 719 .RB ( CAP_NET_ADMIN )
 720 process can perform the same task as
 721 .BR SO_RCVBUF ,
 722 but the
 723 .I rmem_max
 724 limit can be overridden.
 725 .TP
 726 .BR SO_RCVLOWAT " and " SO_SNDLOWAT
 727 Specify the minimum number of bytes in the buffer until the socket layer
 728 will pass the data to the protocol
 729 .RB ( SO_SNDLOWAT )
 730 or the user on receiving
 731 .RB ( SO_RCVLOWAT ).
 732 These two values are initialized to 1.
 733 .B SO_SNDLOWAT
 734 is not changeable on Linux
 735 .RB ( setsockopt (2)
 736 fails with the error
 737 .BR ENOPROTOOPT ).
 738 .B SO_RCVLOWAT
 739 is changeable
 740 only since Linux 2.4.
 741 The
 742 .BR select (2)
 743 and
 744 .BR poll (2)
 745 system calls currently do not respect the
 746 .B SO_RCVLOWAT
 747 setting on Linux,
 748 and mark a socket readable when even a single byte of data is available.
 749 A subsequent read from the socket will block until
 750 .B SO_RCVLOWAT
 751 bytes are available.
 752 .\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=111049368106984&w=2
 753 .\" Tested on kernel 2.6.14 -- mtk, 30 Nov 05
 754 .TP
 755 .BR SO_RCVTIMEO " and " SO_SNDTIMEO
 756 .\" Not implemented in 2.0.
 757 .\" Implemented in 2.1.11 for getsockopt: always return a zero struct.
 758 .\" Implemented in 2.3.41 for setsockopt, and actually used.
 759 Specify the receiving or sending timeouts until reporting an error.
 760 The argument is a
 761 .IR "struct timeval" .
 762 If an input or output function blocks for this period of time, and
 763 data has been sent or received, the return value of that function
 764 will be the amount of data transferred; if no data has been transferred
 765 and the timeout has been reached, then \-1 is returned with
 766 .I errno
 767 set to
 768 .BR EAGAIN
 769 or
 770 .BR EWOULDBLOCK ,
 771 .\" in fact to EAGAIN
 772 or
 773 .B EINPROGRESS
 774 (for
 775 .BR connect (2))
 776 just as if the socket was specified to be nonblocking.
 777 If the timeout is set to zero (the default),
 778 then the operation will never timeout.
 779 Timeouts only have effect for system calls that perform socket I/O (e.g.,
 780 .BR read (2),
 781 .BR recvmsg (2),
 782 .BR send (2),
 783 .BR sendmsg (2));
 784 timeouts have no effect for
 785 .BR select (2),
 786 .BR poll (2),
 787 .BR epoll_wait (2),
 788 and so on.
 789 .TP
 790 .B SO_REUSEADDR
 791 .\"     commit c617f398edd4db2b8567a28e899a88f8f574798d
 792 .\"     https://lwn.net/Articles/542629/
 793 Indicates that the rules used in validating addresses supplied in a
 794 .BR bind (2)
 795 call should allow reuse of local addresses.
 796 For
 797 .B AF_INET
 798 sockets this
 799 means that a socket may bind, except when there
 800 is an active listening socket bound to the address.
 801 When the listening socket is bound to
 802 .B INADDR_ANY
 803 with a specific port then it is not possible
 804 to bind to this port for any local address.
 805 Argument is an integer boolean flag.
 806 .TP
 807 .BR SO_REUSEPORT " (since Linux 3.9)"
 808 Permits multiple
 809 .B AF_INET
 810 or
 811 .B AF_INET6
 812 sockets to be bound to an identical socket address.
 813 This option must be set on each socket (including the first socket)
 814 prior to calling
 815 .BR bind (2)
 816 on the socket.
 817 To prevent port hijacking,
 818 all of the processes binding to the same address must have the same
 819 effective UID.
 820 This option can be employed with both TCP and UDP sockets.
 821
 822 For TCP sockets, this option allows
 823 .BR accept (2)
 824 load distribution in a multi-threaded server to be improved by
 825 using a distinct listener socket for each thread.
 826 This provides improved load distribution as compared
 827 to traditional techniques such using a single
 828 .BR accept (2)ing
 829 thread that distributes connections,
 830 or having multiple threads that compete to
 831 .BR accept (2)
 832 from the same socket.
 833
 834 For UDP sockets,
 835 the use of this option can provide better distribution
 836 of incoming datagrams to multiple processes (or threads) as compared
 837 to the traditional technique of having multiple processes
 838 compete to receive datagrams on the same socket.
 839 .TP
 840 .BR SO_RXQ_OVFL " (since Linux 2.6.33)"
 841 .\" commit 3b885787ea4112eaa80945999ea0901bf742707f
 842 Indicates that an unsigned 32-bit value ancillary message (cmsg)
 843 should be attached to received skbs indicating
 844 the number of packets dropped by the socket between
 845 the last received packet and this received packet.
 846 .TP
 847 .B SO_SNDBUF
 848 Sets or gets the maximum socket send buffer in bytes.
 849 The kernel doubles this value (to allow space for bookkeeping overhead)
 850 when it is set using
 851 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 852 .\" See also the comment to SO_RCVBUF (17 Jul 2012 LKML mail)
 853 .BR setsockopt (2),
 854 and this doubled value is returned by
 855 .BR getsockopt (2).
 856 The default value is set by the
 857 .I /proc/sys/net/core/wmem_default
 858 file and the maximum allowed value is set by the
 859 .I /proc/sys/net/core/wmem_max
 860 file.
 861 The minimum (doubled) value for this option is 2048.
 862 .TP
 863 .BR SO_SNDBUFFORCE " (since Linux 2.6.14)"
 864 Using this socket option, a privileged
 865 .RB ( CAP_NET_ADMIN )
 866 process can perform the same task as
 867 .BR SO_SNDBUF ,
 868 but the
 869 .I wmem_max
 870 limit can be overridden.
 871 .TP
 872 .B SO_TIMESTAMP
 873 Enable or disable the receiving of the
 874 .B SO_TIMESTAMP
 875 control message.
 876 The timestamp control message is sent with level
 877 .B SOL_SOCKET
 878 and the
 879 .I cmsg_data
 880 field is a
 881 .I "struct timeval"
 882 indicating the
 883 reception time of the last packet passed to the user in this call.
 884 See
 885 .BR cmsg (3)
 886 for details on control messages.
 887 .TP
 888 .B SO_TYPE
 889 Gets the socket type as an integer (e.g.,
 890 .BR SOCK_STREAM ).
 891 This socket option is read-only.
 892 .TP
 893 .BR SO_BUSY_POLL " (since Linux 3.11)"
 894 Sets the approximate time in microseconds to busy poll on a blocking receive
 895 when there is no data.
 896 Increasing this value requires
 897 .BR CAP_NET_ADMIN .
 898 The default for this option is controlled by the
 899 .I /proc/sys/net/core/busy_read
 900 file.
 901
 902 The value in the
 903 .I /proc/sys/net/core/busy_poll
 904 file determines how long
 905 .BR select (2)
 906 and
 907 .BR poll (2)
 908 will busy poll when they operate on sockets with
 909 .BR SO_BUSY_POLL
 910 set and no events to report are found.
 911
 912 In both cases,
 913 busy polling will only be done when the socket last received data
 914 from a network device that supports this option.
 915
 916 While busy polling may improve latency of some applications,
 917 care must be taken when using it since this will increase
 918 both CPU utilization and power usage.
 919 .SS Signals
 920 When writing onto a connection-oriented socket that has been shut down
 921 (by the local or the remote end)
 922 .B SIGPIPE
 923 is sent to the writing process and
 924 .B EPIPE
 925 is returned.
 926 The signal is not sent when the write call
 927 specified the
 928 .B MSG_NOSIGNAL
 929 flag.
 930 .PP
 931 When requested with the
 932 .B FIOSETOWN
 933 .BR fcntl (2)
 934 or
 935 .B SIOCSPGRP
 936 .BR ioctl (2),
 937 .B SIGIO
 938 is sent when an I/O event occurs.
 939 It is possible to use
 940 .BR poll (2)
 941 or
 942 .BR select (2)
 943 in the signal handler to find out which socket the event occurred on.
 944 An alternative (in Linux 2.2) is to set a real-time signal using the
 945 .B F_SETSIG
 946 .BR fcntl (2);
 947 the handler of the real time signal will be called with
 948 the file descriptor in the
 949 .I si_fd
 950 field of its
 951 .IR siginfo_t .
 952 See
 953 .BR fcntl (2)
 954 for more information.
 955 .PP
 956 Under some circumstances (e.g., multiple processes accessing a
 957 single socket), the condition that caused the
 958 .B SIGIO
 959 may have already disappeared when the process reacts to the signal.
 960 If this happens, the process should wait again because Linux
 961 will resend the signal later.
 962 .\" .SS Ancillary messages
 963 .SS /proc interfaces
 964 The core socket networking parameters can be accessed
 965 via files in the directory
 966 .IR /proc/sys/net/core/ .
 967 .TP
 968 .I rmem_default
 969 contains the default setting in bytes of the socket receive buffer.
 970 .TP
 971 .I rmem_max
 972 contains the maximum socket receive buffer size in bytes which a user may
 973 set by using the
 974 .B SO_RCVBUF
 975 socket option.
 976 .TP
 977 .I wmem_default
 978 contains the default setting in bytes of the socket send buffer.
 979 .TP
 980 .I wmem_max
 981 contains the maximum socket send buffer size in bytes which a user may
 982 set by using the
 983 .B SO_SNDBUF
 984 socket option.
 985 .TP
 986 .IR message_cost " and " message_burst
 987 configure the token bucket filter used to load limit warning messages
 988 caused by external network events.
 989 .TP
 990 .I netdev_max_backlog
 991 Maximum number of packets in the global input queue.
 992 .TP
 993 .I optmem_max
 994 Maximum length of ancillary data and user control data like the iovecs
 995 per socket.
 996 .\" netdev_fastroute is not documented because it is experimental
 997 .SS Ioctls
 998 These operations can be accessed using
 999 .BR ioctl (2):
1000
1001 .in +4n
1002 .nf
1003 .IB error " = ioctl(" ip_socket ", " ioctl_type ", " &value_result ");"
1004 .fi
1005 .in
1006 .TP
1007 .B SIOCGSTAMP
1008 Return a
1009 .I struct timeval
1010 with the receive timestamp of the last packet passed to the user.
1011 This is useful for accurate round trip time measurements.
1012 See
1013 .BR setitimer (2)
1014 for a description of
1015 .IR "struct timeval" .
1016 .\"
1017 This ioctl should be used only if the socket option
1018 .B SO_TIMESTAMP
1019 is not set on the socket.
1020 Otherwise, it returns the timestamp of the
1021 last packet that was received while
1022 .B SO_TIMESTAMP
1023 was not set, or it fails if no such packet has been received,
1024 (i.e.,
1025 .BR ioctl (2)
1026 returns \-1 with
1027 .I errno
1028 set to
1029 .BR ENOENT ).
1030 .TP
1031 .B SIOCSPGRP
1032 Set the process or process group that is to receive
1033 .B SIGIO
1034 or
1035 .B SIGURG
1036 signals when I/O becomes possible or urgent data is available.
1037 The argument is a pointer to a
1038 .IR pid_t .
1039 For further details, see the description of
1040 .BR F_SETOWN
1041 in
1042 .BR fcntl (2).
1043 .TP
1044 .B FIOASYNC
1045 Change the
1046 .B O_ASYNC
1047 flag to enable or disable asynchronous I/O mode of the socket.
1048 Asynchronous I/O mode means that the
1049 .B SIGIO
1050 signal or the signal set with
1051 .B F_SETSIG
1052 is raised when a new I/O event occurs.
1053 .IP
1054 Argument is an integer boolean flag.
1055 (This operation is synonymous with the use of
1056 .BR fcntl (2)
1057 to set the
1058 .B O_ASYNC
1059 flag.)
1060 .\"
1061 .TP
1062 .B SIOCGPGRP
1063 Get the current process or process group that receives
1064 .B SIGIO
1065 or
1066 .B SIGURG
1067 signals,
1068 or 0
1069 when none is set.
1070 .PP
1071 Valid
1072 .BR fcntl (2)
1073 operations:
1074 .TP
1075 .B FIOGETOWN
1076 The same as the
1077 .B SIOCGPGRP
1078 .BR ioctl (2).
1079 .TP
1080 .B FIOSETOWN
1081 The same as the
1082 .B SIOCSPGRP
1083 .BR ioctl (2).
1084 .SH VERSIONS
1085 .B SO_BINDTODEVICE
1086 was introduced in Linux 2.0.30.
1087 .B SO_PASSCRED
1088 is new in Linux 2.2.
1089 The
1090 .I /proc
1091 interfaces were introduced in Linux 2.2.
1092 .B SO_RCVTIMEO
1093 and
1094 .B SO_SNDTIMEO
1095 are supported since Linux 2.3.41.
1096 Earlier, timeouts were fixed to
1097 a protocol-specific setting, and could not be read or written.
1098 .SH NOTES
1099 Linux assumes that half of the send/receive buffer is used for internal
1100 kernel structures; thus the values in the corresponding
1101 .I /proc
1102 files are twice what can be observed on the wire.
1103
1104 Linux will allow port reuse only with the
1105 .B SO_REUSEADDR
1106 option
1107 when this option was set both in the previous program that performed a
1108 .BR bind (2)
1109 to the port and in the program that wants to reuse the port.
1110 This differs from some implementations (e.g., FreeBSD)
1111 where only the later program needs to set the
1112 .B SO_REUSEADDR
1113 option.
1114 Typically this difference is invisible, since, for example, a server
1115 program is designed to always set this option.
1116 .\" .SH AUTHORS
1117 .\" This man page was written by Andi Kleen.
1118 .SH SEE ALSO
1119 .BR wireshark (1),
1120 .BR bpf (2),
1121 .BR connect (2),
1122 .BR getsockopt (2),
1123 .BR setsockopt (2),
1124 .BR socket (2),
1125 .BR pcap (3),
1126 .BR capabilities (7),
1127 .BR ddp (7),
1128 .BR ip (7),
1129 .BR packet (7),
1130 .BR tcp (7),
1131 .BR udp (7),
1132 .BR unix (7),
1133 .BR tcpdump (8)