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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 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 │
│ │ │ 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.
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 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 non-
zero 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_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 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 “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
IFNAMSZ 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;
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
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.
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_FILTER, SO_ATTACH_BPF,
SO_ATTACH_REUSEPORT_CBPF and SO_ATTACH_REUSEPORT_EPBF.
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.
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_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 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.
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. The select(2) and poll(2)
system 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.
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 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.
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 connections, 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 traditional 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 between the last
received packet and this received packet.
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 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.
SO_TIMESTAMP
Enable or disable the receiving of the SO_TIMESTAMP control
message. The timestamp control message is sent with level
SOL_SOCKET and 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_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 usage.
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 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
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
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).
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).
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), capabilities(7), ddp(7), ip(7), packet(7),
tcp(7), udp(7), unix(7), tcpdump(8)
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 SOCKET(7)
Pages that refer to this page: accept(2), bind(2), bpf(2), getpeername(2), getsockname(2), getsockopt(2), intro(2), listen(2), recv(2), recvmmsg(2), seccomp(2), send(2), sendmmsg(2), shutdown(2), socket(2), socketpair(2), cmsg(3), sd_is_fifo(3), sd_journal_print(3), systemd.exec(5), systemd.socket(5), ddp(7), ip(7), ipv6(7), packet(7), raw(7), sctp(7), tcp(7), udp(7), udplite(7), unix(7), x25(7), tc-fq(8), tc-mqprio(8), tc-prio(8)