SOCKET(7)                  Linux Programmer's Manual                 SOCKET(7)

       socket - Linux socket interface

       #include <sys/socket.h>

       sockfd = socket(int socket_family, int socket_type, int protocol);

       This  manual  page  describes  the  Linux  networking socket layer user
       interface.  The  BSD  compatible  sockets  are  the  uniform  interface
       between the user process and the network protocol stacks in the kernel.
       The protocol  modules  are  grouped  into  protocol  families  such  as
       AF_INET, AF_IPX, and AF_PACKET, and socket types such as SOCK_STREAM or
       SOCK_DGRAM.  See socket(2) for more information on families and types.

   Socket-layer functions
       These functions are used by the user process to send or receive packets
       and  to  do  other  socket  operations.  For more information see their
       respective manual pages.

       socket(2) creates a socket, connect(2) connects a socket  to  a  remote
       socket  address,  the bind(2) function binds a socket to a local socket
       address, listen(2) tells the  socket  that  new  connections  shall  be
       accepted, and accept(2) is used to get a new socket with a new incoming
       connection.  socketpair(2)  returns  two  connected  anonymous  sockets
       (implemented only for a few local families like AF_UNIX)

       send(2),  sendto(2),  and  sendmsg(2)  send  data  over  a  socket, and
       recv(2), recvfrom(2), recvmsg(2) receive data from a  socket.   poll(2)
       and  select(2)  wait for arriving data or a readiness to send data.  In
       addition, the standard I/O operations like write(2),  writev(2),  send-
       file(2), read(2), and readv(2) can be used to read and write data.

       getsockname(2)  returns  the  local  socket  address and getpeername(2)
       returns the remote socket address.  getsockopt(2) and setsockopt(2) are
       used  to  set or get socket layer or protocol options.  ioctl(2) can be
       used to set or read some other options.

       close(2) is used to close a socket.   shutdown(2)  closes  parts  of  a
       full-duplex socket connection.

       Seeking,  or  calling  pread(2) or pwrite(2) with a nonzero position is
       not supported on sockets.

       It is possible to do nonblocking I/O on sockets by setting  the  O_NON-
       BLOCK flag on a socket file descriptor using fcntl(2).  Then all opera-
       tions that would block will (usually)  return  with  EAGAIN  (operation
       should  be  retried  later);  connect(2) will return EINPROGRESS error.
       The user can then wait for various events via poll(2) or select(2).

       |                            I/O events                              |
       |Event      | Poll flag | Occurrence                                 |
       |Read       | POLLIN    | New data arrived.                          |
       |Read       | POLLIN    | A connection setup has been completed (for |
       |           |           | connection-oriented sockets)               |
       |Read       | POLLHUP   | A disconnection request has been initiated |
       |           |           | by the other end.                          |
       |Read       | POLLHUP   | A connection is broken (only  for  connec- |
       |           |           | tion-oriented protocols).  When the socket |
       |           |           | is written SIGPIPE is also sent.           |
       |Write      | POLLOUT   | Socket has enough send  buffer  space  for |
       |           |           | writing new data.                          |
       |Read/Write | POLLIN |  | An outgoing connect(2) finished.           |
       |           | POLLOUT   |                                            |
       |Read/Write | POLLERR   | An asynchronous error occurred.            |
       |Read/Write | POLLHUP   | The other end has shut down one direction. |
       |Exception  | POLLPRI   | Urgent data arrived.  SIGURG is sent then. |
       An alternative to poll(2) and select(2) is to let the kernel inform the
       application about events via a SIGIO signal.  For that the O_ASYNC flag
       must be set on a socket file descriptor via fcntl(2) and a valid signal
       handler for SIGIO must be installed via sigaction(2).  See the  Signals
       discussion below.

   Socket address structures
       Each  socket  domain  has  its  own format for socket addresses, with a
       domain-specific address structure.  Each  of  these  structures  begins
       with  an  integer  "family" field (typed as sa_family_t) that indicates
       the type of the address structure.   This  allows  the  various  system
       calls  (e.g.,  connect(2), bind(2), accept(2), getsockname(2), getpeer-
       name(2)), which are generic to all socket  domains,  to  determine  the
       domain of a particular socket address.

       To  allow  any type of socket address to be passed to interfaces in the
       sockets API, the type struct sockaddr is defined.  The purpose of  this
       type is purely to allow casting of domain-specific socket address types
       to a "generic" type, so as to avoid compiler warnings about  type  mis-
       matches in calls to the sockets API.

       In  addition,  the  sockets  API  provides  the  data type struct sock-
       addr_storage.  This type  is  suitable  to  accommodate  all  supported
       domain-specific  socket  address  structures; it is large enough and is
       aligned properly.  (In particular, it is  large  enough  to  hold  IPv6
       socket  addresses.)   The structure includes the following field, which
       can be used to identify the type of socket address actually  stored  in
       the structure:

               sa_family_t ss_family;

       The  sockaddr_storage  structure is useful in programs that must handle
       socket addresses in a generic way (e.g., programs that must  deal  with
       both IPv4 and IPv6 socket addresses).

   Socket options
       The  socket  options listed below can be set by using setsockopt(2) and
       read with getsockopt(2) with the socket level set to SOL_SOCKET for all
       sockets.  Unless otherwise noted, optval is a pointer to an int.

              Returns  a  value indicating whether or not this socket has been
              marked to accept connections with listen(2).  The value 0  indi-
              cates that this is not a listening socket, the value 1 indicates
              that this is a listening socket.  This socket  option  is  read-

       SO_ATTACH_FILTER (since Linux 2.2), SO_ATTACH_BPF (since Linux 3.19)
              Attach  a  classic  BPF  (SO_ATTACH_FILTER)  or  an extended BPF
              (SO_ATTACH_BPF) program to the socket for use  as  a  filter  of
              incoming  packets.   A packet will be dropped if the filter pro-
              gram returns zero.  If the  filter  program  returns  a  nonzero
              value  which  is  less than the packet's data length, the packet
              will be truncated to the length returned.  If the value returned
              by  the  filter  is  greater  than or equal to the packet's data
              length, the packet is allowed to proceed unmodified.

              The argument for SO_ATTACH_FILTER  is  a  sock_fprog  structure,
              defined in <linux/filter.h>:

                  struct sock_fprog {
                      unsigned short      len;
                      struct sock_filter *filter;

              The  argument for SO_ATTACH_BPF is a file descriptor returned by
              the bpf(2) system call and must  refer  to  a  program  of  type

              These options may be set multiple times for a given socket, each
              time replacing the previous filter  program.   The  classic  and
              extended versions may be called on the same socket, but the pre-
              vious filter will always be replaced such that  a  socket  never
              has more than one filter defined.

              Both classic and extended BPF are explained in the kernel source
              file Documentation/networking/filter.txt

              For use with the SO_REUSEPORT option, these  options  allow  the
              user  to  set  a  classic  BPF  (SO_ATTACH_REUSEPORT_CBPF) or an
              extended BPF (SO_ATTACH_REUSEPORT_EBPF)  program  which  defines
              how  packets  are assigned to the sockets in the reuseport group
              (that is, all sockets which have SO_REUSEPORT set and are  using
              the same local address to receive packets).

              The  BPF  program  must return an index between 0 and N-1 repre-
              senting the socket which should receive the packet (where  N  is
              the number of sockets in the group).  If the BPF program returns
              an invalid index, socket selection will fall back to  the  plain
              SO_REUSEPORT mechanism.

              Sockets are numbered in the order in which they are added to the
              group (that is, the order of bind(2) calls for  UDP  sockets  or
              the  order  of  listen(2)  calls  for TCP sockets).  New sockets
              added to a reuseport group will inherit the BPF program.  When a
              socket  is  removed  from  a reuseport group (via close(2)), the
              last socket in the group will be moved into the closed  socket's

              These options may be set repeatedly at any time on any socket in
              the group to replace the current BPF program used by all sockets
              in the group.

              SO_ATTACH_REUSEPORT_CBPF   takes   the  same  argument  type  as
              SO_ATTACH_FILTER and  SO_ATTACH_REUSEPORT_EBPF  takes  the  same
              argument type as SO_ATTACH_BPF.

              UDP  support  for this feature is available since Linux 4.5; TCP
              support is available since Linux 4.6.

              Bind this socket to a particular device like "eth0",  as  speci-
              fied  in  the  passed  interface  name.  If the name is an empty
              string or the option length is zero, the socket  device  binding
              is  removed.  The passed option is a variable-length null-termi-
              nated interface name string with the maximum size  of  IFNAMSIZ.
              If a socket is bound to an interface, only packets received from
              that particular interface are processed  by  the  socket.   Note
              that this works only for some socket types, particularly AF_INET
              sockets.  It is not supported for  packet  sockets  (use  normal
              bind(2) there).

              Before Linux 3.8, this socket option could be set, but could not
              retrieved with getsockopt(2).  Since Linux 3.8, it is  readable.
              The  optlen argument should contain the buffer size available to
              receive the device name and is recommended to be IFNAMSIZ bytes.
              The real device name length is reported back in the optlen argu-

              Set or get the broadcast flag.  When enabled,  datagram  sockets
              are allowed to send packets to a broadcast address.  This option
              has no effect on stream-oriented sockets.

              Enable BSD bug-to-bug compatibility.  This is used  by  the  UDP
              protocol  module  in Linux 2.0 and 2.2.  If enabled, ICMP errors
              received for a UDP socket will not be passed to  the  user  pro-
              gram.   In  later  kernel  versions, support for this option has
              been phased out: Linux 2.4 silently ignores it,  and  Linux  2.6
              generates  a  kernel  warning  (printk()) if a program uses this
              option.  Linux 2.0 also  enabled  BSD  bug-to-bug  compatibility
              options (random header changing, skipping of the broadcast flag)
              for raw sockets with this option, but that was removed in  Linux

              Enable  socket  debugging.   Allowed only for processes with the
              CAP_NET_ADMIN capability or an effective user ID of 0.

       SO_DETACH_FILTER (since Linux 2.2), SO_DETACH_BPF (since Linux 3.19)
              These two options, which are synonyms, may be used to remove the
              classic or extended BPF program attached to a socket with either
              SO_ATTACH_FILTER or SO_ATTACH_BPF.  The option value is ignored.

       SO_DOMAIN (since Linux 2.6.32)
              Retrieves the socket domain as an  integer,  returning  a  value
              such  as  AF_INET6.   See  socket(2)  for  details.  This socket
              option is read-only.

              Get and clear the pending socket error.  This socket  option  is
              read-only.  Expects an integer.

              Don't send via a gateway, send only to directly connected hosts.
              The same effect can be achieved  by  setting  the  MSG_DONTROUTE
              flag  on a socket send(2) operation.  Expects an integer boolean

       SO_INCOMING_CPU (gettable since Linux 3.19, settable since Linux 4.4)
              Sets or gets the CPU affinity of a socket.  Expects  an  integer

                  int cpu = 1;
                  socklen_t len = sizeof(cpu);
                  setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, &len);

              Because  all of the packets for a single stream (i.e., all pack-
              ets for the same 4-tuple) arrive on the single RX queue that  is
              associated  with  a  particular  CPU, the typical use case is to
              employ one listening process per RX  queue,  with  the  incoming
              flow  being  handled  by a listener on the same CPU that is han-
              dling the RX queue.  This provides  optimal  NUMA  behavior  and
              keeps CPU caches hot.

              Enable  sending  of  keep-alive  messages on connection-oriented
              sockets.  Expects an integer boolean flag.

              Sets or gets the SO_LINGER option.  The  argument  is  a  linger

                  struct linger {
                      int l_onoff;    /* linger active */
                      int l_linger;   /* how many seconds to linger for */

              When  enabled,  a  close(2) or shutdown(2) will not return until
              all queued messages for the socket have been  successfully  sent
              or  the  linger  timeout  has been reached.  Otherwise, the call
              returns immediately and the closing is done in  the  background.
              When  the socket is closed as part of exit(2), it always lingers
              in the background.

              When set, this option will prevent changing the filters  associ-
              ated  with  the socket.  These filters include any set using the
              socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE-

              The typical use case is for a privileged process to set up a raw
              socket (an operation that requires the CAP_NET_RAW  capability),
              apply  a  restrictive filter, set the SO_LOCK_FILTER option, and
              then either drop its privileges or pass the socket file descrip-
              tor to an unprivileged process via a UNIX domain socket.

              Once  the  SO_LOCK_FILTER  option  has been enabled, attempts to
              change or remove the filter attached to a socket, or to  disable
              the SO_LOCK_FILTER option will fail with the error EPERM.

       SO_MARK (since Linux 2.6.25)
              Set  the  mark for each packet sent through this socket (similar
              to the netfilter MARK target but  socket-based).   Changing  the
              mark can be used for mark-based routing without netfilter or for
              packet   filtering.    Setting   this   option   requires    the
              CAP_NET_ADMIN capability.

              If  this  option is enabled, out-of-band data is directly placed
              into the receive data stream.  Otherwise,  out-of-band  data  is
              passed only when the MSG_OOB flag is set during receiving.

              Enable  or  disable the receiving of the SCM_CREDENTIALS control
              message.  For more information see unix(7).

       SO_PEEK_OFF (since Linux 3.4)
              This option, which is currently supported only for unix(7) sock-
              ets,  sets the value of the "peek offset" for the recv(2) system
              call when used with MSG_PEEK flag.

              When this option is set to a negative value (it is set to -1 for
              all new sockets), traditional behavior is provided: recv(2) with
              the MSG_PEEK flag will peek data from the front of the queue.

              When the option is set to a value greater than or equal to zero,
              then  the  next  peek at data queued in the socket will occur at
              the byte offset specified by the  option  value.   At  the  same
              time,  the  "peek  offset"  will be incremented by the number of
              bytes that were peeked from the queue, so that a subsequent peek
              will return the next data in the queue.

              If  data  is  removed  from the front of the queue via a call to
              recv(2) (or similar) without the MSG_PEEK flag, the  "peek  off-
              set" will be decreased by the number of bytes removed.  In other
              words, receiving data without the MSG_PEEK flag will  cause  the
              "peek  offset"  to  be adjusted to maintain the correct relative
              position in the queued data, so  that  a  subsequent  peek  will
              retrieve  the  data  that would have been retrieved had the data
              not been removed.

              For datagram sockets, if the "peek offset" points to the  middle
              of a packet, the data returned will be marked with the MSG_TRUNC

              The  following  example  serves  to  illustrate   the   use   of
              SO_PEEK_OFF.   Suppose  a stream socket has the following queued
              input data:


              The following sequence of recv(2) calls would  have  the  effect
              noted in the comments:

                  int ov = 4;                  // Set peek offset to 4
                  setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));

                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
                  recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8

              Return  the credentials of the foreign process connected to this
              socket.  This is possible  only  for  connected  AF_UNIX  stream
              sockets  and  AF_UNIX  stream  and datagram socket pairs created
              using socketpair(2); see unix(7).  The returned credentials  are
              those  that were in effect at the time of the call to connect(2)
              or socketpair(2).  The argument is a ucred structure; define the
              _GNU_SOURCE  feature test macro to obtain the definition of that
              structure from <sys/socket.h>.  This socket option is read-only.

              Set the protocol-defined priority for all packets to be sent  on
              this  socket.   Linux  uses  this  value to order the networking
              queues: packets with a higher priority may  be  processed  first
              depending on the selected device queueing discipline.  Setting a
              priority outside the range 0 to  6  requires  the  CAP_NET_ADMIN

       SO_PROTOCOL (since Linux 2.6.32)
              Retrieves  the  socket protocol as an integer, returning a value
              such as IPPROTO_SCTP.  See socket(2) for details.   This  socket
              option is read-only.

              Sets  or  gets  the maximum socket receive buffer in bytes.  The
              kernel doubles this value (to allow space for bookkeeping  over-
              head) when it is set using setsockopt(2), and this doubled value
              is returned by getsockopt(2).  The default value is set  by  the
              /proc/sys/net/core/rmem_default  file,  and  the maximum allowed
              value is set by the /proc/sys/net/core/rmem_max file.  The mini-
              mum (doubled) value for this option is 256.

       SO_RCVBUFFORCE (since Linux 2.6.14)
              Using  this  socket option, a privileged (CAP_NET_ADMIN) process
              can perform the same task as SO_RCVBUF, but the  rmem_max  limit
              can be overridden.

              Specify  the  minimum  number  of  bytes in the buffer until the
              socket layer will pass the data to the protocol (SO_SNDLOWAT) or
              the  user on receiving (SO_RCVLOWAT).  These two values are ini-
              tialized to 1.  SO_SNDLOWAT is not changeable on Linux (setsock-
              opt(2)  fails  with  the  error  ENOPROTOOPT).   SO_RCVLOWAT  is
              changeable only since Linux 2.4.  The select(2) and poll(2) sys-
              tem  calls  currently  do not respect the SO_RCVLOWAT setting on
              Linux, and mark a socket readable when even  a  single  byte  of
              data is available.  A subsequent read from the socket will block
              until SO_RCVLOWAT bytes are available.

              Specify the receiving or sending  timeouts  until  reporting  an
              error.  The argument is a struct timeval.  If an input or output
              function blocks for this period of time, and data has been  sent
              or  received,  the  return  value  of  that function will be the
              amount of data transferred; if no data has been transferred  and
              the timeout has been reached, then -1 is returned with errno set
              to EAGAIN or EWOULDBLOCK, or EINPROGRESS (for  connect(2))  just
              as  if the socket was specified to be nonblocking.  If the time-
              out is set to zero (the default), then the operation will  never
              timeout.   Timeouts  only have effect for system calls that per-
              form   socket   I/O   (e.g.,   read(2),   recvmsg(2),   send(2),
              sendmsg(2));  timeouts  have  no  effect for select(2), poll(2),
              epoll_wait(2), and so on.

              Indicates that the rules used in validating  addresses  supplied
              in  a  bind(2)  call should allow reuse of local addresses.  For
              AF_INET sockets this means that a socket may bind,  except  when
              there  is an active listening socket bound to the address.  When
              the listening socket is bound to INADDR_ANY with a specific port
              then  it  is  not  possible  to  bind to this port for any local
              address.  Argument is an integer boolean flag.

       SO_REUSEPORT (since Linux 3.9)
              Permits multiple AF_INET or AF_INET6 sockets to be bound  to  an
              identical  socket  address.   This  option  must  be set on each
              socket (including the first socket) prior to calling bind(2)  on
              the  socket.   To  prevent  port hijacking, all of the processes
              binding to the same address must have the  same  effective  UID.
              This option can be employed with both TCP and UDP sockets.

              For  TCP sockets, this option allows accept(2) load distribution
              in a multi-threaded server to be improved by  using  a  distinct
              listener  socket  for  each thread.  This provides improved load
              distribution as compared to traditional techniques such using  a
              single accept(2)ing thread that distributes connections, or hav-
              ing multiple threads that compete to  accept(2)  from  the  same

              For  UDP sockets, the use of this option can provide better dis-
              tribution  of  incoming  datagrams  to  multiple  processes  (or
              threads) as compared to the traditional technique of having mul-
              tiple processes compete to receive datagrams on the same socket.

       SO_RXQ_OVFL (since Linux 2.6.33)
              Indicates that an unsigned 32-bit value ancillary message (cmsg)
              should  be  attached  to  received skbs indicating the number of
              packets dropped by the socket since its creation.

              Sets or gets the maximum socket send buffer in bytes.  The  ker-
              nel doubles this value (to allow space for bookkeeping overhead)
              when it is set using setsockopt(2), and this  doubled  value  is
              returned  by  getsockopt(2).   The  default  value is set by the
              /proc/sys/net/core/wmem_default file  and  the  maximum  allowed
              value is set by the /proc/sys/net/core/wmem_max file.  The mini-
              mum (doubled) value for this option is 2048.

       SO_SNDBUFFORCE (since Linux 2.6.14)
              Using this socket option, a privileged  (CAP_NET_ADMIN)  process
              can  perform  the same task as SO_SNDBUF, but the wmem_max limit
              can be overridden.

              Enable or disable the receiving of the SO_TIMESTAMP control mes-
              sage.    The  timestamp  control  message  is  sent  with  level
              SOL_SOCKET and the cmsg_data field is a struct timeval  indicat-
              ing  the reception time of the last packet passed to the user in
              this call.  See cmsg(3) for details on control messages.

              Gets the socket type as an integer  (e.g.,  SOCK_STREAM).   This
              socket option is read-only.

       SO_BUSY_POLL (since Linux 3.11)
              Sets  the  approximate  time  in  microseconds to busy poll on a
              blocking receive when there is no data.  Increasing  this  value
              requires  CAP_NET_ADMIN.   The  default  for this option is con-
              trolled by the /proc/sys/net/core/busy_read file.

              The value in the  /proc/sys/net/core/busy_poll  file  determines
              how  long select(2) and poll(2) will busy poll when they operate
              on sockets with SO_BUSY_POLL set and no  events  to  report  are

              In  both  cases,  busy polling will only be done when the socket
              last received data from a  network  device  that  supports  this

              While  busy  polling  may  improve latency of some applications,
              care must be taken when using it since this will  increase  both
              CPU utilization and power usage.

       When  writing onto a connection-oriented socket that has been shut down
       (by the local or the remote end) SIGPIPE is sent to the writing process
       and  EPIPE  is  returned.   The  signal is not sent when the write call
       specified the MSG_NOSIGNAL flag.

       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO
       is  sent  when  an  I/O event occurs.  It is possible to use poll(2) or
       select(2) in the signal handler to find  out  which  socket  the  event
       occurred  on.  An alternative (in Linux 2.2) is to set a real-time sig-
       nal using the F_SETSIG fcntl(2); the handler of the  real  time  signal
       will  be called with the file descriptor in the si_fd field of its sig-
       info_t.  See fcntl(2) for more information.

       Under some circumstances (e.g., multiple processes accessing  a  single
       socket),  the  condition  that caused the SIGIO may have already disap-
       peared when the process reacts to the signal.   If  this  happens,  the
       process should wait again because Linux will resend the signal later.

   /proc interfaces
       The  core socket networking parameters can be accessed via files in the
       directory /proc/sys/net/core/.

              contains the default setting in bytes of the socket receive buf-

              contains the maximum socket receive buffer size in bytes which a
              user may set by using the SO_RCVBUF socket option.

              contains the default setting in bytes of the socket send buffer.

              contains the maximum socket send buffer size in  bytes  which  a
              user may set by using the SO_SNDBUF socket option.

       message_cost and message_burst
              configure  the  token  bucket  filter used to load limit warning
              messages caused by external network events.

              Maximum number of packets in the global input queue.

              Maximum length of ancillary data and user control data like  the
              iovecs per socket.

       These operations can be accessed using ioctl(2):

           error = ioctl(ip_socket, ioctl_type, &value_result);

              Return  a  struct timeval with the receive timestamp of the last
              packet passed to the user.  This is useful  for  accurate  round
              trip  time  measurements.  See setitimer(2) for a description of
              struct timeval.  This ioctl should be used only  if  the  socket
              option  SO_TIMESTAMP  is  not  set on the socket.  Otherwise, it
              returns the timestamp of the last packet that was received while
              SO_TIMESTAMP was not set, or it fails if no such packet has been
              received, (i.e., ioctl(2) returns -1 with errno set to ENOENT).

              Set the process or process group that is  to  receive  SIGIO  or
              SIGURG  signals  when  I/O  becomes  possible  or urgent data is
              available.  The argument is a pointer to a pid_t.   For  further
              details, see the description of F_SETOWN in fcntl(2).

              Change  the  O_ASYNC  flag to enable or disable asynchronous I/O
              mode of the socket.  Asynchronous I/O mode means that the  SIGIO
              signal  or the signal set with F_SETSIG is raised when a new I/O
              event occurs.

              Argument is an integer boolean flag.  (This operation is synony-
              mous with the use of fcntl(2) to set the O_ASYNC flag.)

              Get  the current process or process group that receives SIGIO or
              SIGURG signals, or 0 when none is set.

       Valid fcntl(2) operations:

              The same as the SIOCGPGRP ioctl(2).

              The same as the SIOCSPGRP ioctl(2).

       SO_BINDTODEVICE was introduced in Linux 2.0.30.  SO_PASSCRED is new  in
       Linux 2.2.  The /proc interfaces were introduced in Linux 2.2.  SO_RCV-
       TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41.  Earlier, time-
       outs  were  fixed to a protocol-specific setting, and could not be read
       or written.

       Linux assumes that half of the send/receive buffer is used for internal
       kernel structures; thus the values in the corresponding /proc files are
       twice what can be observed on the wire.

       Linux will allow port reuse only with the SO_REUSEADDR option when this
       option was set both in the previous program that performed a bind(2) to
       the port and in the program that wants to reuse the port.  This differs
       from  some implementations (e.g., FreeBSD) where only the later program
       needs to set the SO_REUSEADDR option.   Typically  this  difference  is
       invisible,  since,  for example, a server program is designed to always
       set this option.

       wireshark(1),   bpf(2),   connect(2),   getsockopt(2),   setsockopt(2),
       socket(2),  pcap(3), capabilities(7), ddp(7), ip(7), packet(7), tcp(7),
       udp(7), unix(7), tcpdump(8)

       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

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