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),
       sendfile(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_NONBLOCK flag on a socket file descriptor using fcntl(2).  Then all
       operations 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

       │                            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           │
       │           │           │ connection-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),
       getpeername(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
       mismatches in calls to the sockets API.

       In addition, the sockets API provides the data type struct
       sockaddr_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
              indicates that this is not a listening socket, the value 1
              indicates that this is a listening socket.  This socket option
              is read-only.

       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
              program 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
              previous 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
              representing 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
              specified 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-
              terminated 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

              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
              program.  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;
                  setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, sizeof(cpu));

              Because all of the packets for a single stream (i.e., all
              packets 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
              handling 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
              associated with the socket.  These filters include any set using
              the socket options SO_ATTACH_FILTER, SO_ATTACH_BPF,

              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
              descriptor 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).

              Enable or disable the receiving of the SCM_SECURITY 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)
              sockets, 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
              offset" 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 peer process connected to this
              socket.  For further details, see unix(7).

              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
              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/rmem_default file, and the maximum
              allowed value is set by the /proc/sys/net/core/rmem_max file.
              The minimum (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
              initialized to 1.  SO_SNDLOWAT is not changeable on Linux
              (setsockopt(2) fails with the error ENOPROTOOPT).  SO_RCVLOWAT
              is changeable only since Linux 2.4.

              Before Linux 2.6.28 select(2), poll(2), and epoll(7) did not
              respect the SO_RCVLOWAT setting on Linux, and indicated a socket
              as readable when even a single byte of data was available.  A
              subsequent read from the socket would then 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
              timeout is set to zero (the default), then the operation will
              never timeout.  Timeouts only have effect for system calls that
              perform 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
              having multiple threads that compete to accept(2) from the same

              For UDP sockets, the use of this option can provide better
              distribution of incoming datagrams to multiple processes (or
              threads) as compared to the traditional technique of having
              multiple processes compete to receive datagrams on the same

       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.

       SO_SELECT_ERR_QUEUE (since Linux 3.10)
              When this option is set on a socket, an error condition on a
              socket causes notification not only via the exceptfds set of
              select(2).  Similarly, poll(2) also returns a POLLPRI whenever
              an POLLERR event is returned.

              Background: this option was added when waking up on an error
              condition occurred only via the readfds and writefds sets of
              select(2).  The option was added to allow monitoring for error
              conditions via the exceptfds argument without simultaneously
              having to receive notifications (via readfds) for regular data
              that can be read from the socket.  After changes in Linux 4.16,
              the use of this flag to achieve the desired notifications is no
              longer necessary.  This option is nevertheless retained for
              backwards compatibility.

              Sets or gets the maximum socket send buffer in bytes.  The
              kernel 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
              minimum (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
              message.  The timestamp control message is sent with level
              SOL_SOCKET and a cmsg_type of SCM_TIMESTAMP.  The cmsg_data
              field is a struct timeval indicating the reception time of the
              last packet passed to the user in this call.  See cmsg(3) for
              details on control messages.

       SO_TIMESTAMPNS (since Linux 2.6.22)
              Enable or disable the receiving of the SO_TIMESTAMPNS control
              message.  The timestamp control message is sent with level
              SOL_SOCKET and a cmsg_type of SCM_TIMESTAMPNS.  The cmsg_data
              field is a struct timespec indicating the reception time of the
              last packet passed to the user in this call.  The clock used for
              the timestamp is CLOCK_REALTIME.  See cmsg(3) for details on
              control messages.

              A socket cannot mix SO_TIMESTAMP and SO_TIMESTAMPNS: the two
              modes are mutually exclusive.

              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
              controlled 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
       signal 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
       siginfo_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
       disappeared when the process reacts to the signal.  If this happens,
       the process should wait again because Linux will resend the signal

   /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

              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
              options SO_TIMESTAMP and SO_TIMESTAMPNS are not set on the
              socket.  Otherwise, it returns the timestamp of the last packet
              that was received while SO_TIMESTAMP and SO_TIMESTAMPNS were 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
              synonymous 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_RCVTIMEO and SO_SNDTIMEO are supported since Linux 2.3.41.  Earlier,
       timeouts 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), address_families(7), capabilities(7), ddp(7),
       ip(7), packet(7), tcp(7), udp(7), unix(7), tcpdump(8)

       This page is part of release 5.06 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

Linux                             2020-04-11                         SOCKET(7)