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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 com-
patible sockets are the uniform interface between the user process and the network proto-
col 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 con-
nection.
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) re-
turn with EAGAIN (operation should be retried later); connect(2) will return EINPROGRESS
error. The user can then wait for various events via poll(2) or select(2).
+--------------------------------------------------------------------+
| I/O events |
+-----------+-----------+--------------------------------------------+
|Event | Poll flag | Occurrence |
+-----------+-----------+--------------------------------------------+
|Read | POLLIN | New data arrived. |
+-----------+-----------+--------------------------------------------+
|Read | POLLIN | A connection setup has been completed (for |
| | | connection-oriented sockets) |
+-----------+-----------+--------------------------------------------+
|Read | POLLHUP | A disconnection request has been initiated |
| | | by the other end. |
+-----------+-----------+--------------------------------------------+
|Read | POLLHUP | A connection is broken (only for connec- |
| | | tion-oriented protocols). When the socket |
| | | is written SIGPIPE is also sent. |
+-----------+-----------+--------------------------------------------+
|Write | POLLOUT | Socket has enough send buffer space for |
| | | writing new data. |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLIN | | An outgoing connect(2) finished. |
| | POLLOUT | |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLERR | An asynchronous error occurred. |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLHUP | The other end has shut down one direction. |
+-----------+-----------+--------------------------------------------+
|Exception | POLLPRI | Urgent data arrived. SIGURG is sent then. |
+-----------+-----------+--------------------------------------------+
An alternative to poll(2) and select(2) is to let the kernel inform the application about
events via a SIGIO signal. For that the O_ASYNC flag must be set on a socket file de-
scriptor via fcntl(2) and a valid signal handler for SIGIO must be installed via sigac-
tion(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 iden-
tify 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 getsock-
opt(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 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/fil-
ter.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 Documenta-
tion/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) pro-
gram 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 ad-
dress 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 re-
place 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 in-
terface 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-ter-
minated 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, particu-
larly 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 con-
tain the buffer size available to receive the device name and is recommended to be
IFNAMSIZ bytes. The real device name length is reported back in the optlen argu-
ment.
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 sock-
ets.
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 capa-
bility 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 ex-
tended BPF program attached to a socket with either SO_ATTACH_FILTER or SO_AT-
TACH_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 in-
coming 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_AT-
TACH_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 opera-
tion 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 rout-
ing 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 in-
formation 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 com-
ments:
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_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 IP-
PROTO_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 single
byte of data was available. A subsequent read from the socket would then block un-
til 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 EIN-
PROGRESS (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 ad-
dress. 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 in-
teger boolean flag.
SO_REUSEPORT (since Linux 3.9)
Permits multiple AF_INET or AF_INET6 sockets to be bound to an identical socket ad-
dress. 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 us-
ing 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 tech-
nique 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_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 condi-
tion 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 sig-
nal 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 ex-
ternal 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).
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_SNDTIMEO 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 option. 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), ad-
dress_families(7), capabilities(7), ddp(7), ip(7), packet(7), tcp(7), udp(7), unix(7),
tcpdump(8)
COLOPHON
This page is part of release 5.05 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 2019-08-02 SOCKET(7)
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