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

NAME
       socket - Linux socket interface

SYNOPSIS
       #include <sys/socket.h>

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

DESCRIPTION
       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  ar‐
       riving  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 select(2).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚                                                                          I/O events                                                                            β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Event      β”‚ Poll flag β”‚ Occurrence                                                                                                                             β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read       β”‚ POLLIN    β”‚ New data arrived.                                                                                                                      β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read       β”‚ POLLIN    β”‚ A connection setup has been completed (for connection-oriented sockets)                                                                β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read       β”‚ POLLHUP   β”‚ A disconnection request has been initiated by the other end.                                                                           β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read       β”‚ POLLHUP   β”‚ A connection is broken (only for connection-oriented protocols).  When the socket is written SIGPIPE is also sent.                     β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Write      β”‚ POLLOUT   β”‚ Socket has enough send buffer space for writing new data.                                                                              β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read/Write β”‚ POLLIN |  β”‚ An outgoing connect(2) finished.                                                                                                       β”‚
       β”‚           β”‚ POLLOUT   β”‚                                                                                                                                        β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read/Write β”‚ POLLERR   β”‚ An asynchronous error occurred.                                                                                                        β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Read/Write β”‚ POLLHUP   β”‚ The other end has shut down one direction.                                                                                             β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚Exception  β”‚ POLLPRI   β”‚ Urgent data arrived.  SIGURG is sent then.                                                                                             β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜
       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), get‐
       sockname(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.

       SO_ACCEPTCONN
              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 lis‐
              tening 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 BPF_PROG_TYPE_SOCKET_FILTER.

              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

       SO_ATTACH_REUSEPORT_CBPF, SO_ATTACH_REUSEPORT_EBPF
              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_REUSE‐
              PORT_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 position.

              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.

       SO_BINDTODEVICE
              Bind  this  socket  to a particular device like β€œeth0”, 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  IFNAM‐
              SIZ.   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 argument.

       SO_BROADCAST
              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.

       SO_BSDCOMPAT
              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 2.2.

       SO_DEBUG
              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.

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

       SO_DONTROUTE
              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 flag.

       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 flag.

                  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 han‐
              dling the RX queue.  This provides optimal NUMA behavior and keeps CPU caches hot.

       SO_INCOMING_NAPI_ID (gettable since Linux 4.12)
              Returns a system-level unique ID called NAPI ID that is associated with a RX queue on which the last packet associated with that socket is received.

              This can be used by an application to split the incoming flows among worker threads based on the RX queue on which the packets associated with  the  flows
              are  received.   It  allows  each  worker  thread to be associated with a NIC HW receive queue and service all the connection requests received on that RX
              queue.  This mapping between a app thread and a HW NIC queue streamlines the flow of data from the NIC to the application.

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

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

                  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.

       SO_LOCK_FILTER
              When set, this option will prevent changing the filters associated with the socket.  These filters include any set using the socket options SO_ATTACH_FIL‐
              TER, SO_ATTACH_BPF, SO_ATTACH_REUSEPORT_CBPF, and SO_ATTACH_REUSEPORT_EBPF.

              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 fil‐
              ter, 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.

       SO_OOBINLINE
              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.

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

       SO_PASSSEC
              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 flag.

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

                  aabbccddeeff

              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

       SO_PEERCRED
              Return the credentials of the peer process connected to this socket.  For further details, see unix(7).

       SO_PEERSEC (since Linux 2.6.2)
              Return the security context of the peer socket connected to this socket.  For further details, see unix(7) and ip(7).

       SO_PRIORITY
              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 capability.

       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.

       SO_RCVBUF
              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.

       SO_RCVLOWAT and SO_SNDLOWAT
              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 sin‐
              gle byte of data was available.  A subsequent read from the socket would then block until SO_RCVLOWAT bytes are available.  Since Linux 2.6.28, select(2),
              poll(2), and epoll(7) indicate a socket as readable only if at least SO_RCVLOWAT bytes are available.

       SO_RCVTIMEO and SO_SNDTIMEO
              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 pe‐
              riod  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 trans‐
              ferred 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.

       SO_REUSEADDR
              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  connec‐
              tions, or having multiple threads that compete to accept(2) from the same socket.

              For UDP sockets, the use of this option can provide better distribution of incoming datagrams to multiple processes (or threads) as compared to the tradi‐
              tional technique of having multiple 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.

       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.

       SO_SNDBUF
              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 set‐
              sockopt(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 al‐
              lowed 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.

       SO_TIMESTAMP
              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.

       SO_TYPE
              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 found.

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

              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 us‐
              age.

   Signals
       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  re‐
       turned.  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 sig‐
       nal 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 later.

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

       rmem_default
              contains the default setting in bytes of the socket receive buffer.

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

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

       wmem_max
              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.

       netdev_max_backlog
              Maximum number of packets in the global input queue.

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

   Ioctls
       These operations can be accessed using ioctl(2):

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

       SIOCGSTAMP
              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).

       SIOCSPGRP
              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).

       FIOASYNC
              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.)

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

       Valid fcntl(2) operations:

       FIOGETOWN
              The same as the SIOCGPGRP ioctl(2).

       FIOSETOWN
              The same as the SIOCSPGRP ioctl(2).

VERSIONS
       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_SND‐
       TIMEO are supported since Linux 2.3.41.  Earlier, timeouts were fixed to a protocol-specific setting, and could not be read or written.

NOTES
       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  op‐
       tion.  Typically this difference is invisible, since, for example, a server program is designed to always set this option.

SEE ALSO
       wireshark(1),  bpf(2),  connect(2),  getsockopt(2),  setsockopt(2),  socket(2), pcap(3), address_families(7), capabilities(7), ddp(7), ip(7), ipv6(7), packet(7),
       tcp(7), udp(7), unix(7), tcpdump(8)

Linux                                                                          2021-03-22                                                                      SOCKET(7)