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 2016-03-15 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 " (since Linux 2.2), " SO_ATTACH_BPF " (since Linux 3.19)"
 305 Attach a classic BPF
 306 .RB ( SO_ATTACH_FILTER )
 307 or an extended BPF
 308 .RB ( SO_ATTACH_BPF )
 309 program to the socket for use as a filter of incoming packets.
 310 A packet will be dropped if the filter program returns zero.
 311 If the filter program returns a
 312 non-zero value which is less than the packet's data length,
 313 the packet will be truncated to the length returned.
 314 If the value returned by the filter is greater than or equal to the
 315 packet's data length, the packet is allowed to proceed unmodified.
 316
 317 The argument for
 318 .BR SO_ATTACH_FILTER
 319 is a
 320 .I sock_fprog
 321 structure, defined in
 322 .IR <linux/filter.h> :
 323 .sp
 324 .in +4n
 325 .nf
 326 struct sock_fprog {
 327     unsigned short      len;
 328     struct sock_filter *filter;
 329 };
 330 .fi
 331 .in
 332 .IP
 333 The argument for
 334 .BR SO_ATTACH_BPF
 335 is a file descriptor returned by the
 336 .BR bpf (2)
 337 system call and must refer to a program of type
 338 .BR BPF_PROG_TYPE_SOCKET_FILTER.
 339
 340 These options may be set multiple times for a given socket,
 341 each time replacing the previous filter program.
 342 The classic and extended versions may be called on the same socket,
 343 but the previous filter will always be replaced such that a socket
 344 never has more than one filter defined.
 345
 346 Both classic and extended BPF are explained in the kernel source file
 347 .I Documentation/networking/filter.txt
 348 .TP
 349 .BR SO_ATTACH_REUSEPORT_CBPF ", " SO_ATTACH_REUSEPORT_EBPF
 350 For use with the
 351 .BR SO_REUSEPORT
 352 option, these options allow the user to set a classic BPF
 353 .RB ( SO_ATTACH_REUSEPORT_CBPF )
 354 or an extended BPF
 355 .RB ( SO_ATTACH_REUSEPORT_EBPF )
 356 program which defines how packets are assigned to
 357 the sockets in the reuseport group (that is, all sockets which have
 358 .BR SO_REUSEPORT
 359 set and are using the same local address to receive packets).
 360
 361 The BPF program must return an index between 0 and N\-1 representing
 362 the socket which should receive the packet
 363 (where N is the number of sockets in the group).
 364 If the BPF program returns an invalid index,
 365 socket selection will fall back to the plain
 366 .BR SO_REUSEPORT
 367 mechanism.
 368
 369 Sockets are numbered in the order in which they are added to the group
 370 (that is, the order of
 371 .BR bind (2)
 372 calls for UDP sockets or the order of
 373 .BR listen (2)
 374 calls for TCP sockets).
 375 New sockets added to a reuseport group will inherit the BPF program.
 376 When a socket is removed from a reuseport group (via
 377 .BR close (2)),
 378 the last socket in the group will be moved into the closed socket's
 379 position.
 380
 381 These options may be set repeatedly at any time on any socket in the group
 382 to replace the current BPF program used by all sockets in the group.
 383
 384 .BR SO_ATTACH_REUSEPORT_CBPF
 385 takes the same argument type as
 386 .BR SO_ATTACH_FILTER
 387 and
 388 .BR SO_ATTACH_REUSEPORT_EBPF
 389 takes the same argument type as
 390 .BR SO_ATTACH_BPF.
 391
 392 UDP support for this feature is available since Linux 4.5;
 393 TCP support is available since Linux 4.6.
 394 .TP
 395 .B SO_BINDTODEVICE
 396 Bind this socket to a particular device like \(lqeth0\(rq,
 397 as specified in the passed interface name.
 398 If the
 399 name is an empty string or the option length is zero, the socket device
 400 binding is removed.
 401 The passed option is a variable-length null-terminated
 402 interface name string with the maximum size of
 403 .BR IFNAMSIZ .
 404 If a socket is bound to an interface,
 405 only packets received from that particular interface are processed by the
 406 socket.
 407 Note that this works only for some socket types, particularly
 408 .B AF_INET
 409 sockets.
 410 It is not supported for packet sockets (use normal
 411 .BR bind (2)
 412 there).
 413
 414 Before Linux 3.8,
 415 this socket option could be set, but could not retrieved with
 416 .BR getsockopt (2).
 417 Since Linux 3.8, it is readable.
 418 The
 419 .I optlen
 420 argument should contain the buffer size available
 421 to receive the device name and is recommended to be
 422 .BR IFNAMSZ
 423 bytes.
 424 The real device name length is reported back in the
 425 .I optlen
 426 argument.
 427 .TP
 428 .B SO_BROADCAST
 429 Set or get the broadcast flag.
 430 When enabled, datagram sockets are allowed to send
 431 packets to a broadcast address.
 432 This option has no effect on stream-oriented sockets.
 433 .TP
 434 .B SO_BSDCOMPAT
 435 Enable BSD bug-to-bug compatibility.
 436 This is used by the UDP protocol module in Linux 2.0 and 2.2.
 437 If enabled, ICMP errors received for a UDP socket will not be passed
 438 to the user program.
 439 In later kernel versions, support for this option has been phased out:
 440 Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning
 441 (printk()) if a program uses this option.
 442 Linux 2.0 also enabled BSD bug-to-bug compatibility
 443 options (random header changing, skipping of the broadcast flag) for raw
 444 sockets with this option, but that was removed in Linux 2.2.
 445 .TP
 446 .B SO_DEBUG
 447 Enable socket debugging.
 448 Only allowed for processes with the
 449 .B CAP_NET_ADMIN
 450 capability or an effective user ID of 0.
 451 .TP
 452 .BR SO_DETACH_FILTER " (since Linux 2.2), " SO_DETACH_BPF " (since Linux 3.19)"
 453 These two options, which are synonyms,
 454 may be used to remove the classic or extended BPF
 455 program attached to a socket with either
 456 .BR SO_ATTACH_FILTER
 457 or
 458 .BR SO_ATTACH_BPF .
 459 The option value is ignored.
 460 .TP
 461 .BR SO_DOMAIN " (since Linux 2.6.32)"
 462 Retrieves the socket domain as an integer, returning a value such as
 463 .BR AF_INET6 .
 464 See
 465 .BR socket (2)
 466 for details.
 467 This socket option is read-only.
 468 .TP
 469 .B SO_ERROR
 470 Get and clear the pending socket error.
 471 This socket option is read-only.
 472 Expects an integer.
 473 .TP
 474 .B SO_DONTROUTE
 475 Don't send via a gateway, send only to directly connected hosts.
 476 The same effect can be achieved by setting the
 477 .B MSG_DONTROUTE
 478 flag on a socket
 479 .BR send (2)
 480 operation.
 481 Expects an integer boolean flag.
 482 .TP
 483 .B SO_KEEPALIVE
 484 Enable sending of keep-alive messages on connection-oriented sockets.
 485 Expects an integer boolean flag.
 486 .TP
 487 .B SO_LINGER
 488 Sets or gets the
 489 .B SO_LINGER
 490 option.
 491 The argument is a
 492 .I linger
 493 structure.
 494 .sp
 495 .in +4n
 496 .nf
 497 struct linger {
 498     int l_onoff;    /* linger active */
 499     int l_linger;   /* how many seconds to linger for */
 500 };
 501 .fi
 502 .in
 503 .IP
 504 When enabled, a
 505 .BR close (2)
 506 or
 507 .BR shutdown (2)
 508 will not return until all queued messages for the socket have been
 509 successfully sent or the linger timeout has been reached.
 510 Otherwise,
 511 the call returns immediately and the closing is done in the background.
 512 When the socket is closed as part of
 513 .BR exit (2),
 514 it always lingers in the background.
 515 .TP
 516 .B SO_LOCK_FILTER
 517 .\" commit d59577b6ffd313d0ab3be39cb1ab47e29bdc9182
 518 When set, this option will prevent
 519 changing the filters associated with the socket.
 520 These filters include any set using the socket options
 521 .BR SO_ATTACH_FILTER,
 522 .BR SO_ATTACH_BPF,
 523 .BR SO_ATTACH_REUSEPORT_CBPF
 524 and
 525 .BR SO_ATTACH_REUSEPORT_EPBF .
 526
 527 The typical use case is for a privileged process to set up a raw socket
 528 (an operation that requires the
 529 .BR CAP_NET_RAW
 530 capability), apply a restrictive filter, set the
 531 .BR SO_LOCK_FILTER
 532 option,
 533 and then either drop its privileges or pass the socket file descriptor
 534 to an unprivileged process via a UNIX domain socket.
 535
 536 Once the
 537 .BR SO_LOCK_FILTER
 538 option has been enabled, attempts to change or remove the filter
 539 attached to a socket, or to disable the
 540 .BR SO_LOCK_FILTER
 541 option will fail with the error
 542 .BR EPERM .
 543 .TP
 544 .BR SO_MARK " (since Linux 2.6.25)"
 545 .\" commit 4a19ec5800fc3bb64e2d87c4d9fdd9e636086fe0
 546 .\" and    914a9ab386a288d0f22252fc268ecbc048cdcbd5
 547 Set the mark for each packet sent through this socket
 548 (similar to the netfilter MARK target but socket-based).
 549 Changing the mark can be used for mark-based
 550 routing without netfilter or for packet filtering.
 551 Setting this option requires the
 552 .B CAP_NET_ADMIN
 553 capability.
 554 .TP
 555 .B SO_OOBINLINE
 556 If this option is enabled,
 557 out-of-band data is directly placed into the receive data stream.
 558 Otherwise, out-of-band data is passed only when the
 559 .B MSG_OOB
 560 flag is set during receiving.
 561 .\" don't document it because it can do too much harm.
 562 .\".B SO_NO_CHECK
 563 .\"     The kernel has support for the SO_NO_CHECK socket
 564 .\"     option (boolean: 0 == default, calculate checksum on xmit,
 565 .\"     1 == do not calculate checksum on xmit).
 566 .\" Additional note from Andi Kleen on SO_NO_CHECK (2010-08-30)
 567 .\"     On Linux UDP checksums are essentially free and there's no reason
 568 .\"     to turn them off and it would disable another safety line.
 569 .\"     That is why I didn't document the option.
 570 .TP
 571 .B SO_PASSCRED
 572 Enable or disable the receiving of the
 573 .B SCM_CREDENTIALS
 574 control message.
 575 For more information see
 576 .BR unix (7).
 577 .\" FIXME Document SO_PASSSEC, added in 2.6.18; there is some info
 578 .\" in the 2.6.18 ChangeLog
 579 .TP
 580 .BR SO_PEEK_OFF " (since Linux 3.4)"
 581 .\" commit ef64a54f6e558155b4f149bb10666b9e914b6c54
 582 This option, which is currently supported only for
 583 .BR unix (7)
 584 sockets, sets the value of the "peek offset" for the
 585 .BR recv (2)
 586 system call when used with
 587 .BR MSG_PEEK
 588 flag.
 589
 590 When this option is set to a negative value
 591 (it is set to \-1 for all new sockets),
 592 traditional behavior is provided:
 593 .BR recv (2)
 594 with the
 595 .BR MSG_PEEK
 596 flag will peek data from the front of the queue.
 597
 598 When the option is set to a value greater than or equal to zero,
 599 then the next peek at data queued in the socket will occur at
 600 the byte offset specified by the option value.
 601 At the same time, the "peek offset" will be
 602 incremented by the number of bytes that were peeked from the queue,
 603 so that a subsequent peek will return the next data in the queue.
 604
 605 If data is removed from the front of the queue via a call to
 606 .BR recv (2)
 607 (or similar) without the
 608 .BR MSG_PEEK
 609 flag, the "peek offset" will be decreased by the number of bytes removed.
 610 In other words, receiving data without the
 611 .B MSG_PEEK
 612 flag will cause the "peek offset" to be adjusted to maintain
 613 the correct relative position in the queued data,
 614 so that a subsequent peek will retrieve the data that would have been
 615 retrieved had the data not been removed.
 616
 617 For datagram sockets, if the "peek offset" points to the middle of a packet,
 618 the data returned will be marked with the
 619 .BR MSG_TRUNC
 620 flag.
 621
 622 The following example serves to illustrate the use of
 623 .BR SO_PEEK_OFF .
 624 Suppose a stream socket has the following queued input data:
 625
 626     aabbccddeeff
 627 .IP
 628 The following sequence of
 629 .BR recv (2)
 630 calls would have the effect noted in the comments:
 631
 632 .in +4n
 633 .nf
 634 int ov = 4;                  // Set peek offset to 4
 635 setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
 636
 637 recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
 638 recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
 639 recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
 640 recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8
 641 .fi
 642 .in
 643 .TP
 644 .B SO_PEERCRED
 645 Return the credentials of the foreign process connected to this socket.
 646 This is possible only for connected
 647 .B AF_UNIX
 648 stream sockets and
 649 .B AF_UNIX
 650 stream and datagram socket pairs created using
 651 .BR socketpair (2);
 652 see
 653 .BR unix (7).
 654 The returned credentials are those that were in effect at the time
 655 of the call to
 656 .BR connect (2)
 657 or
 658 .BR socketpair (2).
 659 The argument is a
 660 .I ucred
 661 structure; define the
 662 .B _GNU_SOURCE
 663 feature test macro to obtain the definition of that structure from
 664 .IR <sys/socket.h> .
 665 This socket option is read-only.
 666 .TP
 667 .B SO_PRIORITY
 668 Set the protocol-defined priority for all packets to be sent on
 669 this socket.
 670 Linux uses this value to order the networking queues:
 671 packets with a higher priority may be processed first depending
 672 on the selected device queueing discipline.
 673 .\" For
 674 .\" .BR ip (7),
 675 .\" this also sets the IP type-of-service (TOS) field for outgoing packets.
 676 Setting a priority outside the range 0 to 6 requires the
 677 .B CAP_NET_ADMIN
 678 capability.
 679 .TP
 680 .BR SO_PROTOCOL " (since Linux 2.6.32)"
 681 Retrieves the socket protocol as an integer, returning a value such as
 682 .BR IPPROTO_SCTP .
 683 See
 684 .BR socket (2)
 685 for details.
 686 This socket option is read-only.
 687 .TP
 688 .B SO_RCVBUF
 689 Sets or gets the maximum socket receive buffer in bytes.
 690 The kernel doubles this value (to allow space for bookkeeping overhead)
 691 when it is set using
 692 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 693 .BR setsockopt (2),
 694 and this doubled value is returned by
 695 .BR getsockopt (2).
 696 .\" The following thread on LMKL is quite informative:
 697 .\" getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard" behavior
 698 .\" 17 July 2012
 699 .\" http://thread.gmane.org/gmane.linux.kernel/1328935
 700 The default value is set by the
 701 .I /proc/sys/net/core/rmem_default
 702 file, and the maximum allowed value is set by the
 703 .I /proc/sys/net/core/rmem_max
 704 file.
 705 The minimum (doubled) value for this option is 256.
 706 .TP
 707 .BR SO_RCVBUFFORCE " (since Linux 2.6.14)"
 708 Using this socket option, a privileged
 709 .RB ( CAP_NET_ADMIN )
 710 process can perform the same task as
 711 .BR SO_RCVBUF ,
 712 but the
 713 .I rmem_max
 714 limit can be overridden.
 715 .TP
 716 .BR SO_RCVLOWAT " and " SO_SNDLOWAT
 717 Specify the minimum number of bytes in the buffer until the socket layer
 718 will pass the data to the protocol
 719 .RB ( SO_SNDLOWAT )
 720 or the user on receiving
 721 .RB ( SO_RCVLOWAT ).
 722 These two values are initialized to 1.
 723 .B SO_SNDLOWAT
 724 is not changeable on Linux
 725 .RB ( setsockopt (2)
 726 fails with the error
 727 .BR ENOPROTOOPT ).
 728 .B SO_RCVLOWAT
 729 is changeable
 730 only since Linux 2.4.
 731 The
 732 .BR select (2)
 733 and
 734 .BR poll (2)
 735 system calls currently do not respect the
 736 .B SO_RCVLOWAT
 737 setting on Linux,
 738 and mark a socket readable when even a single byte of data is available.
 739 A subsequent read from the socket will block until
 740 .B SO_RCVLOWAT
 741 bytes are available.
 742 .\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=111049368106984&w=2
 743 .\" Tested on kernel 2.6.14 -- mtk, 30 Nov 05
 744 .TP
 745 .BR SO_RCVTIMEO " and " SO_SNDTIMEO
 746 .\" Not implemented in 2.0.
 747 .\" Implemented in 2.1.11 for getsockopt: always return a zero struct.
 748 .\" Implemented in 2.3.41 for setsockopt, and actually used.
 749 Specify the receiving or sending timeouts until reporting an error.
 750 The argument is a
 751 .IR "struct timeval" .
 752 If an input or output function blocks for this period of time, and
 753 data has been sent or received, the return value of that function
 754 will be the amount of data transferred; if no data has been transferred
 755 and the timeout has been reached, then \-1 is returned with
 756 .I errno
 757 set to
 758 .BR EAGAIN
 759 or
 760 .BR EWOULDBLOCK ,
 761 .\" in fact to EAGAIN
 762 or
 763 .B EINPROGRESS
 764 (for
 765 .BR connect (2))
 766 just as if the socket was specified to be nonblocking.
 767 If the timeout is set to zero (the default),
 768 then the operation will never timeout.
 769 Timeouts only have effect for system calls that perform socket I/O (e.g.,
 770 .BR read (2),
 771 .BR recvmsg (2),
 772 .BR send (2),
 773 .BR sendmsg (2));
 774 timeouts have no effect for
 775 .BR select (2),
 776 .BR poll (2),
 777 .BR epoll_wait (2),
 778 and so on.
 779 .TP
 780 .B SO_REUSEADDR
 781 .\"     commit c617f398edd4db2b8567a28e899a88f8f574798d
 782 .\"     https://lwn.net/Articles/542629/
 783 Indicates that the rules used in validating addresses supplied in a
 784 .BR bind (2)
 785 call should allow reuse of local addresses.
 786 For
 787 .B AF_INET
 788 sockets this
 789 means that a socket may bind, except when there
 790 is an active listening socket bound to the address.
 791 When the listening socket is bound to
 792 .B INADDR_ANY
 793 with a specific port then it is not possible
 794 to bind to this port for any local address.
 795 Argument is an integer boolean flag.
 796 .TP
 797 .BR SO_REUSEPORT " (since Linux 3.9)"
 798 Permits multiple
 799 .B AF_INET
 800 or
 801 .B AF_INET6
 802 sockets to be bound to an identical socket address.
 803 This option must be set on each socket (including the first socket)
 804 prior to calling
 805 .BR bind (2)
 806 on the socket.
 807 To prevent port hijacking,
 808 all of the processes binding to the same address must have the same
 809 effective UID.
 810 This option can be employed with both TCP and UDP sockets.
 811
 812 For TCP sockets, this option allows
 813 .BR accept (2)
 814 load distribution in a multi-threaded server to be improved by
 815 using a distinct listener socket for each thread.
 816 This provides improved load distribution as compared
 817 to traditional techniques such using a single
 818 .BR accept (2)ing
 819 thread that distributes connections,
 820 or having multiple threads that compete to
 821 .BR accept (2)
 822 from the same socket.
 823
 824 For UDP sockets,
 825 the use of this option can provide better distribution
 826 of incoming datagrams to multiple processes (or threads) as compared
 827 to the traditional technique of having multiple processes
 828 compete to receive datagrams on the same socket.
 829 .TP
 830 .BR SO_RXQ_OVFL " (since Linux 2.6.33)"
 831 .\" commit 3b885787ea4112eaa80945999ea0901bf742707f
 832 Indicates that an unsigned 32-bit value ancillary message (cmsg)
 833 should be attached to received skbs indicating
 834 the number of packets dropped by the socket between
 835 the last received packet and this received packet.
 836 .TP
 837 .B SO_SNDBUF
 838 Sets or gets the maximum socket send buffer in bytes.
 839 The kernel doubles this value (to allow space for bookkeeping overhead)
 840 when it is set using
 841 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 842 .\" See also the comment to SO_RCVBUF (17 Jul 2012 LKML mail)
 843 .BR setsockopt (2),
 844 and this doubled value is returned by
 845 .BR getsockopt (2).
 846 The default value is set by the
 847 .I /proc/sys/net/core/wmem_default
 848 file and the maximum allowed value is set by the
 849 .I /proc/sys/net/core/wmem_max
 850 file.
 851 The minimum (doubled) value for this option is 2048.
 852 .TP
 853 .BR SO_SNDBUFFORCE " (since Linux 2.6.14)"
 854 Using this socket option, a privileged
 855 .RB ( CAP_NET_ADMIN )
 856 process can perform the same task as
 857 .BR SO_SNDBUF ,
 858 but the
 859 .I wmem_max
 860 limit can be overridden.
 861 .TP
 862 .B SO_TIMESTAMP
 863 Enable or disable the receiving of the
 864 .B SO_TIMESTAMP
 865 control message.
 866 The timestamp control message is sent with level
 867 .B SOL_SOCKET
 868 and the
 869 .I cmsg_data
 870 field is a
 871 .I "struct timeval"
 872 indicating the
 873 reception time of the last packet passed to the user in this call.
 874 See
 875 .BR cmsg (3)
 876 for details on control messages.
 877 .TP
 878 .B SO_TYPE
 879 Gets the socket type as an integer (e.g.,
 880 .BR SOCK_STREAM ).
 881 This socket option is read-only.
 882 .TP
 883 .BR SO_BUSY_POLL " (since Linux 3.11)"
 884 Sets the approximate time in microseconds to busy poll on a blocking receive
 885 when there is no data.
 886 Increasing this value requires
 887 .BR CAP_NET_ADMIN .
 888 The default for this option is controlled by the
 889 .I /proc/sys/net/core/busy_read
 890 file.
 891
 892 The value in the
 893 .I /proc/sys/net/core/busy_poll
 894 file determines how long
 895 .BR select (2)
 896 and
 897 .BR poll (2)
 898 will busy poll when they operate on sockets with
 899 .BR SO_BUSY_POLL
 900 set and no events to report are found.
 901
 902 In both cases,
 903 busy polling will only be done when the socket last received data
 904 from a network device that supports this option.
 905
 906 While busy polling may improve latency of some applications,
 907 care must be taken when using it since this will increase
 908 both CPU utilization and power usage.
 909 .SS Signals
 910 When writing onto a connection-oriented socket that has been shut down
 911 (by the local or the remote end)
 912 .B SIGPIPE
 913 is sent to the writing process and
 914 .B EPIPE
 915 is returned.
 916 The signal is not sent when the write call
 917 specified the
 918 .B MSG_NOSIGNAL
 919 flag.
 920 .PP
 921 When requested with the
 922 .B FIOSETOWN
 923 .BR fcntl (2)
 924 or
 925 .B SIOCSPGRP
 926 .BR ioctl (2),
 927 .B SIGIO
 928 is sent when an I/O event occurs.
 929 It is possible to use
 930 .BR poll (2)
 931 or
 932 .BR select (2)
 933 in the signal handler to find out which socket the event occurred on.
 934 An alternative (in Linux 2.2) is to set a real-time signal using the
 935 .B F_SETSIG
 936 .BR fcntl (2);
 937 the handler of the real time signal will be called with
 938 the file descriptor in the
 939 .I si_fd
 940 field of its
 941 .IR siginfo_t .
 942 See
 943 .BR fcntl (2)
 944 for more information.
 945 .PP
 946 Under some circumstances (e.g., multiple processes accessing a
 947 single socket), the condition that caused the
 948 .B SIGIO
 949 may have already disappeared when the process reacts to the signal.
 950 If this happens, the process should wait again because Linux
 951 will resend the signal later.
 952 .\" .SS Ancillary messages
 953 .SS /proc interfaces
 954 The core socket networking parameters can be accessed
 955 via files in the directory
 956 .IR /proc/sys/net/core/ .
 957 .TP
 958 .I rmem_default
 959 contains the default setting in bytes of the socket receive buffer.
 960 .TP
 961 .I rmem_max
 962 contains the maximum socket receive buffer size in bytes which a user may
 963 set by using the
 964 .B SO_RCVBUF
 965 socket option.
 966 .TP
 967 .I wmem_default
 968 contains the default setting in bytes of the socket send buffer.
 969 .TP
 970 .I wmem_max
 971 contains the maximum socket send buffer size in bytes which a user may
 972 set by using the
 973 .B SO_SNDBUF
 974 socket option.
 975 .TP
 976 .IR message_cost " and " message_burst
 977 configure the token bucket filter used to load limit warning messages
 978 caused by external network events.
 979 .TP
 980 .I netdev_max_backlog
 981 Maximum number of packets in the global input queue.
 982 .TP
 983 .I optmem_max
 984 Maximum length of ancillary data and user control data like the iovecs
 985 per socket.
 986 .\" netdev_fastroute is not documented because it is experimental
 987 .SS Ioctls
 988 These operations can be accessed using
 989 .BR ioctl (2):
 990
 991 .in +4n
 992 .nf
 993 .IB error " = ioctl(" ip_socket ", " ioctl_type ", " &value_result ");"
 994 .fi
 995 .in
 996 .TP
 997 .B SIOCGSTAMP
 998 Return a
 999 .I struct timeval
1000 with the receive timestamp of the last packet passed to the user.
1001 This is useful for accurate round trip time measurements.
1002 See
1003 .BR setitimer (2)
1004 for a description of
1005 .IR "struct timeval" .
1006 .\"
1007 This ioctl should be used only if the socket option
1008 .B SO_TIMESTAMP
1009 is not set on the socket.
1010 Otherwise, it returns the timestamp of the
1011 last packet that was received while
1012 .B SO_TIMESTAMP
1013 was not set, or it fails if no such packet has been received,
1014 (i.e.,
1015 .BR ioctl (2)
1016 returns \-1 with
1017 .I errno
1018 set to
1019 .BR ENOENT ).
1020 .TP
1021 .B SIOCSPGRP
1022 Set the process or process group that is to receive
1023 .B SIGIO
1024 or
1025 .B SIGURG
1026 signals when I/O becomes possible or urgent data is available.
1027 The argument is a pointer to a
1028 .IR pid_t .
1029 For further details, see the description of
1030 .BR F_SETOWN
1031 in
1032 .BR fcntl (2).
1033 .TP
1034 .B FIOASYNC
1035 Change the
1036 .B O_ASYNC
1037 flag to enable or disable asynchronous I/O mode of the socket.
1038 Asynchronous I/O mode means that the
1039 .B SIGIO
1040 signal or the signal set with
1041 .B F_SETSIG
1042 is raised when a new I/O event occurs.
1043 .IP
1044 Argument is an integer boolean flag.
1045 (This operation is synonymous with the use of
1046 .BR fcntl (2)
1047 to set the
1048 .B O_ASYNC
1049 flag.)
1050 .\"
1051 .TP
1052 .B SIOCGPGRP
1053 Get the current process or process group that receives
1054 .B SIGIO
1055 or
1056 .B SIGURG
1057 signals,
1058 or 0
1059 when none is set.
1060 .PP
1061 Valid
1062 .BR fcntl (2)
1063 operations:
1064 .TP
1065 .B FIOGETOWN
1066 The same as the
1067 .B SIOCGPGRP
1068 .BR ioctl (2).
1069 .TP
1070 .B FIOSETOWN
1071 The same as the
1072 .B SIOCSPGRP
1073 .BR ioctl (2).
1074 .SH VERSIONS
1075 .B SO_BINDTODEVICE
1076 was introduced in Linux 2.0.30.
1077 .B SO_PASSCRED
1078 is new in Linux 2.2.
1079 The
1080 .I /proc
1081 interfaces were introduced in Linux 2.2.
1082 .B SO_RCVTIMEO
1083 and
1084 .B SO_SNDTIMEO
1085 are supported since Linux 2.3.41.
1086 Earlier, timeouts were fixed to
1087 a protocol-specific setting, and could not be read or written.
1088 .SH NOTES
1089 Linux assumes that half of the send/receive buffer is used for internal
1090 kernel structures; thus the values in the corresponding
1091 .I /proc
1092 files are twice what can be observed on the wire.
1093
1094 Linux will allow port reuse only with the
1095 .B SO_REUSEADDR
1096 option
1097 when this option was set both in the previous program that performed a
1098 .BR bind (2)
1099 to the port and in the program that wants to reuse the port.
1100 This differs from some implementations (e.g., FreeBSD)
1101 where only the later program needs to set the
1102 .B SO_REUSEADDR
1103 option.
1104 Typically this difference is invisible, since, for example, a server
1105 program is designed to always set this option.
1106 .\" .SH AUTHORS
1107 .\" This man page was written by Andi Kleen.
1108 .SH SEE ALSO
1109 .BR wireshark (1),
1110 .BR bpf (2),
1111 .BR connect (2),
1112 .BR getsockopt (2),
1113 .BR setsockopt (2),
1114 .BR socket (2),
1115 .BR pcap (3),
1116 .BR capabilities (7),
1117 .BR ddp (7),
1118 .BR ip (7),
1119 .BR packet (7),
1120 .BR tcp (7),
1121 .BR udp (7),
1122 .BR unix (7),
1123 .BR tcpdump (8)