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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 in- terface. 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 re- spective 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 ac- cepted, and accept(2) is used to get a new socket with a new incoming connection. socketpair(2) returns two connected anonymous sockets (im- plemented only for a few local families like AF_UNIX) send(2), sendto(2), and sendmsg(2) send data over a socket, and recv(2), recvfrom(2), recvmsg(2) receive data from a socket. poll(2) and select(2) wait for arriving data or a readiness to send data. In addition, the standard I/O operations like write(2), writev(2), send- file(2), read(2), and readv(2) can be used to read and write data. getsockname(2) returns the local socket address and getpeername(2) re- turns the remote socket address. getsockopt(2) and setsockopt(2) are used to set or get socket layer or protocol options. ioctl(2) can be used to set or read some other options. close(2) is used to close a socket. shutdown(2) closes parts of a full-duplex socket connection. Seeking, or calling pread(2) or pwrite(2) with a nonzero position is not supported on sockets. It is possible to do nonblocking I/O on sockets by setting the O_NON- BLOCK flag on a socket file descriptor using fcntl(2). Then all opera- tions that would block will (usually) return with EAGAIN (operation should be retried later); connect(2) will return EINPROGRESS error. The user can then wait for various events via poll(2) or select(2). +--------------------------------------------------------------------+ | I/O events | +-----------+-----------+--------------------------------------------+ |Event | Poll flag | Occurrence | +-----------+-----------+--------------------------------------------+ |Read | POLLIN | New data arrived. | +-----------+-----------+--------------------------------------------+ |Read | POLLIN | A connection setup has been completed (for | | | | connection-oriented sockets) | +-----------+-----------+--------------------------------------------+ |Read | POLLHUP | A disconnection request has been initiated | | | | by the other end. | +-----------+-----------+--------------------------------------------+ |Read | POLLHUP | A connection is broken (only for connec- | | | | tion-oriented protocols). When the socket | | | | is written SIGPIPE is also sent. | +-----------+-----------+--------------------------------------------+ |Write | POLLOUT | Socket has enough send buffer space for | | | | writing new data. | +-----------+-----------+--------------------------------------------+ |Read/Write | POLLIN | | An outgoing connect(2) finished. | | | POLLOUT | | +-----------+-----------+--------------------------------------------+ |Read/Write | POLLERR | An asynchronous error occurred. | +-----------+-----------+--------------------------------------------+ |Read/Write | POLLHUP | The other end has shut down one direction. | +-----------+-----------+--------------------------------------------+ |Exception | POLLPRI | Urgent data arrived. SIGURG is sent then. | +-----------+-----------+--------------------------------------------+ An alternative to poll(2) and select(2) is to let the kernel inform the application about events via a SIGIO signal. For that the O_ASYNC flag must be set on a socket file descriptor via fcntl(2) and a valid signal handler for SIGIO must be installed via sigaction(2). See the Signals discussion below. Socket address structures Each socket domain has its own format for socket addresses, with a do- main-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 mis- matches in calls to the sockets API. In addition, the sockets API provides the data type struct sock- addr_storage. This type is suitable to accommodate all supported do- main-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 indi- cates 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 in- coming 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, de- fined 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 ex- tended versions may be called on the same socket, but the previ- ous 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 ex- tended BPF (SO_ATTACH_REUSEPORT_EBPF) program which defines how packets are assigned to the sockets in the reuseport group (that is, all sockets which have SO_REUSEPORT set and are using the same local address to receive packets). The BPF program must return an index between 0 and N-1 repre- senting the socket which should receive the packet (where N is the number of sockets in the group). If the BPF program returns an invalid index, socket selection will fall back to the plain SO_REUSEPORT mechanism. Sockets are numbered in the order in which they are added to the group (that is, the order of bind(2) calls for UDP sockets or the order of listen(2) calls for TCP sockets). New sockets added to a reuseport group will inherit the BPF program. When a socket is removed from a reuseport group (via close(2)), the last socket in the group will be moved into the closed socket's 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_AT- TACH_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 speci- fied in the passed interface name. If the name is an empty string or the option length is zero, the socket device binding is removed. The passed option is a variable-length null-termi- nated interface name string with the maximum size of IFNAMSIZ. If a socket is bound to an interface, only packets received from that particular interface are processed by the socket. Note that this works only for some socket types, particularly AF_INET sockets. It is not supported for packet sockets (use normal bind(2) there). Before Linux 3.8, this socket option could be set, but could not retrieved with getsockopt(2). Since Linux 3.8, it is readable. The optlen argument should contain the buffer size available to receive the device name and is recommended to be IFNAMSIZ bytes. The real device name length is reported back in the optlen argu- 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 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 pro- gram. In later kernel versions, support for this option has been phased out: Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning (printk()) if a program uses this op- tion. Linux 2.0 also enabled BSD bug-to-bug compatibility op- tions (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 op- tion 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 pack- ets for the same 4-tuple) arrive on the single RX queue that is associated with a particular CPU, the typical use case is to em- ploy 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_INCOMING_NAPI_ID (gettable since Linux 4.12) Returns a system-level unique ID called NAPI ID that is associ- ated 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 re- turns 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 associ- ated with the socket. These filters include any set using the socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE- PORT_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 filter, set the SO_LOCK_FILTER option, and then either drop its privileges or pass the socket file descrip- tor to an unprivileged process via a UNIX domain socket. Once the SO_LOCK_FILTER option has been enabled, attempts to change or remove the filter attached to a socket, or to disable the SO_LOCK_FILTER option will fail with the error EPERM. SO_MARK (since Linux 2.6.25) Set the mark for each packet sent through this socket (similar to the netfilter MARK target but socket-based). Changing the mark can be used for mark-based routing without netfilter or for packet filtering. Setting this option requires the CAP_NET_AD- MIN 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 mes- sage. For more information see unix(7). SO_PEEK_OFF (since Linux 3.4) This option, which is currently supported only for unix(7) sock- ets, sets the value of the "peek offset" for the recv(2) system call when used with MSG_PEEK flag. When this option is set to a negative value (it is set to -1 for all new sockets), traditional behavior is provided: recv(2) with the MSG_PEEK flag will peek data from the front of the queue. When the option is set to a value greater than or equal to zero, then the next peek at data queued in the socket will occur at the byte offset specified by the option value. At the same time, the "peek offset" will be incremented by the number of bytes that were peeked from the queue, so that a subsequent peek will return the next data in the queue. If data is removed from the front of the queue via a call to recv(2) (or similar) without the MSG_PEEK flag, the "peek off- set" will be decreased by the number of bytes removed. In other words, receiving data without the MSG_PEEK flag will cause the "peek offset" to be adjusted to maintain the correct relative position in the queued data, so that a subsequent peek will re- trieve 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 ca- pability. 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 over- head) when it is set using setsockopt(2), and this doubled value is returned by getsockopt(2). The default value is set by the /proc/sys/net/core/rmem_default file, and the maximum allowed value is set by the /proc/sys/net/core/rmem_max file. The mini- mum (doubled) value for this option is 256. SO_RCVBUFFORCE (since Linux 2.6.14) Using this socket option, a privileged (CAP_NET_ADMIN) process can perform the same task as SO_RCVBUF, but the rmem_max limit can be overridden. 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 ini- tialized to 1. SO_SNDLOWAT is not changeable on Linux (setsock- opt(2) fails with the error ENOPROTOOPT). SO_RCVLOWAT is changeable only since Linux 2.4. Before Linux 2.6.28 select(2), poll(2), and epoll(7) did not re- spect the SO_RCVLOWAT setting on Linux, and indicated a socket as readable when even a single byte of data was available. A subsequent read from the socket would then block until SO_RCVLOWAT bytes are available. 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 er- ror. The argument is a struct timeval. If an input or output function blocks for this period of time, and data has been sent or received, the return value of that function will be the amount of data transferred; if no data has been transferred and the timeout has been reached, then -1 is returned with errno set to EAGAIN or EWOULDBLOCK, or EINPROGRESS (for connect(2)) just as if the socket was specified to be nonblocking. If the time- out is set to zero (the default), then the operation will never timeout. Timeouts only have effect for system calls that per- form socket I/O (e.g., read(2), recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for select(2), poll(2), epoll_wait(2), and so on. 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 ad- dress. Argument is an integer boolean flag. SO_REUSEPORT (since Linux 3.9) Permits multiple AF_INET or AF_INET6 sockets to be bound to an identical socket address. This option must be set on each socket (including the first socket) prior to calling bind(2) on the socket. To prevent port hijacking, all of the processes binding to the same address must have the same effective UID. This option can be employed with both TCP and UDP sockets. For TCP sockets, this option allows accept(2) load distribution in a multi-threaded server to be improved by using a distinct listener socket for each thread. This provides improved load distribution as compared to traditional techniques such using a single accept(2)ing thread that distributes connections, or hav- ing multiple threads that compete to accept(2) from the same socket. For UDP sockets, the use of this option can provide better dis- tribution of incoming datagrams to multiple processes (or threads) as compared to the traditional technique of having mul- tiple processes compete to receive datagrams on the same socket. SO_RXQ_OVFL (since Linux 2.6.33) Indicates that an unsigned 32-bit value ancillary message (cmsg) should be attached to received skbs indicating the number of packets dropped by the socket since its creation. 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 se- lect(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 se- lect(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 ker- nel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(2), and this doubled value is returned by getsockopt(2). The default value is set by the /proc/sys/net/core/wmem_default file and the maximum allowed value is set by the /proc/sys/net/core/wmem_max file. The mini- mum (doubled) value for this option is 2048. SO_SNDBUFFORCE (since Linux 2.6.14) Using this socket option, a privileged (CAP_NET_ADMIN) process can perform the same task as SO_SNDBUF, but the wmem_max limit can be overridden. SO_TIMESTAMP Enable or disable the receiving of the SO_TIMESTAMP control mes- sage. 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 con- trolled by the /proc/sys/net/core/busy_read file. The value in the /proc/sys/net/core/busy_poll file determines how long select(2) and poll(2) will busy poll when they operate on sockets with SO_BUSY_POLL set and no events to report are found. In both cases, busy polling will only be done when the socket last received data from a network device that supports this op- tion. 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 se- lect(2) in the signal handler to find out which socket the event oc- curred 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 disap- peared when the process reacts to the signal. If this happens, the process should wait again because Linux will resend the signal later. /proc interfaces The core socket networking parameters can be accessed via files in the directory /proc/sys/net/core/. rmem_default contains the default setting in bytes of the socket receive buf- fer. 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 synony- mous 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_RCV- TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41. Earlier, time- outs were fixed to a protocol-specific setting, and could not be read or written. 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 in- visible, 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) COLOPHON This page is part of release 5.10 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 2020-08-13 SOCKET(7) Socket(3perl) Perl Programmers Reference Guide Socket(3perl) NAME "Socket" - networking constants and support functions SYNOPSIS "Socket" a low-level module used by, among other things, the IO::Socket family of modules. The following examples demonstrate some low-level uses but a practical program would likely use the higher-level API provided by "IO::Socket" or similar instead. use Socket qw(PF_INET SOCK_STREAM pack_sockaddr_in inet_aton); socket(my $socket, PF_INET, SOCK_STREAM, 0) or die "socket: $!"; my $port = getservbyname "echo", "tcp"; connect($socket, pack_sockaddr_in($port, inet_aton("localhost"))) or die "connect: $!"; print $socket "Hello, world!\n"; print <$socket>; See also the "EXAMPLES" section. DESCRIPTION This module provides a variety of constants, structure manipulators and other functions related to socket-based networking. The values and functions provided are useful when used in conjunction with Perl core functions such as socket(), setsockopt() and bind(). It also provides several other support functions, mostly for dealing with conversions of network addresses between human-readable and native binary forms, and for hostname resolver operations. Some constants and functions are exported by default by this module; but for backward-compatibility any recently-added symbols are not exported by default and must be requested explicitly. When an import list is provided to the "use Socket" line, the default exports are not automatically imported. It is therefore best practice to always to explicitly list all the symbols required. Also, some common socket "newline" constants are provided: the constants "CR", "LF", and "CRLF", as well as $CR, $LF, and $CRLF, which map to "\015", "\012", and "\015\012". If you do not want to use the literal characters in your programs, then use the constants provided here. They are not exported by default, but can be imported individually, and with the ":crlf" export tag: use Socket qw(:DEFAULT :crlf); $sock->print("GET / HTTP/1.0$CRLF"); The entire getaddrinfo() subsystem can be exported using the tag ":addrinfo"; this exports the getaddrinfo() and getnameinfo() functions, and all the "AI_*", "NI_*", "NIx_*" and "EAI_*" constants. CONSTANTS In each of the following groups, there may be many more constants provided than just the ones given as examples in the section heading. If the heading ends "..." then this means there are likely more; the exact constants provided will depend on the OS and headers found at compile-time. PF_INET, PF_INET6, PF_UNIX, ... Protocol family constants to use as the first argument to socket() or the value of the "SO_DOMAIN" or "SO_FAMILY" socket option. AF_INET, AF_INET6, AF_UNIX, ... Address family constants used by the socket address structures, to pass to such functions as inet_pton() or getaddrinfo(), or are returned by such functions as sockaddr_family(). SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, ... Socket type constants to use as the second argument to socket(), or the value of the "SO_TYPE" socket option. SOCK_NONBLOCK. SOCK_CLOEXEC Linux-specific shortcuts to specify the "O_NONBLOCK" and "FD_CLOEXEC" flags during a socket(2) call. socket( my $sockh, PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, 0 ) SOL_SOCKET Socket option level constant for setsockopt() and getsockopt(). SO_ACCEPTCONN, SO_BROADCAST, SO_ERROR, ... Socket option name constants for setsockopt() and getsockopt() at the "SOL_SOCKET" level. IP_OPTIONS, IP_TOS, IP_TTL, ... Socket option name constants for IPv4 socket options at the "IPPROTO_IP" level. IP_PMTUDISC_WANT, IP_PMTUDISC_DONT, ... Socket option value constants for "IP_MTU_DISCOVER" socket option. IPTOS_LOWDELAY, IPTOS_THROUGHPUT, IPTOS_RELIABILITY, ... Socket option value constants for "IP_TOS" socket option. MSG_BCAST, MSG_OOB, MSG_TRUNC, ... Message flag constants for send() and recv(). SHUT_RD, SHUT_RDWR, SHUT_WR Direction constants for shutdown(). INADDR_ANY, INADDR_BROADCAST, INADDR_LOOPBACK, INADDR_NONE Constants giving the special "AF_INET" addresses for wildcard, broadcast, local loopback, and invalid addresses. Normally equivalent to inet_aton('0.0.0.0'), inet_aton('255.255.255.255'), inet_aton('localhost') and inet_aton('255.255.255.255') respectively. IPPROTO_IP, IPPROTO_IPV6, IPPROTO_TCP, ... IP protocol constants to use as the third argument to socket(), the level argument to getsockopt() or setsockopt(), or the value of the "SO_PROTOCOL" socket option. TCP_CORK, TCP_KEEPALIVE, TCP_NODELAY, ... Socket option name constants for TCP socket options at the "IPPROTO_TCP" level. IN6ADDR_ANY, IN6ADDR_LOOPBACK Constants giving the special "AF_INET6" addresses for wildcard and local loopback. Normally equivalent to inet_pton(AF_INET6, "::") and inet_pton(AF_INET6, "::1") respectively. IPV6_ADD_MEMBERSHIP, IPV6_MTU, IPV6_V6ONLY, ... Socket option name constants for IPv6 socket options at the "IPPROTO_IPV6" level. STRUCTURE MANIPULATORS The following functions convert between lists of Perl values and packed binary strings representing structures. $family = sockaddr_family $sockaddr Takes a packed socket address (as returned by pack_sockaddr_in(), pack_sockaddr_un() or the perl builtin functions getsockname() and getpeername()). Returns the address family tag. This will be one of the "AF_*" constants, such as "AF_INET" for a "sockaddr_in" addresses or "AF_UNIX" for a "sockaddr_un". It can be used to figure out what unpack to use for a sockaddr of unknown type. $sockaddr = pack_sockaddr_in $port, $ip_address Takes two arguments, a port number and an opaque string (as returned by inet_aton(), or a v-string). Returns the "sockaddr_in" structure with those arguments packed in and "AF_INET" filled in. For Internet domain sockets, this structure is normally what you need for the arguments in bind(), connect(), and send(). An undefined $port argument is taken as zero; an undefined $ip_address is considered a fatal error. ($port, $ip_address) = unpack_sockaddr_in $sockaddr Takes a "sockaddr_in" structure (as returned by pack_sockaddr_in(), getpeername() or recv()). Returns a list of two elements: the port and an opaque string representing the IP address (you can use inet_ntoa() to convert the address to the four-dotted numeric format). Will croak if the structure does not represent an "AF_INET" address. In scalar context will return just the IP address. $sockaddr = sockaddr_in $port, $ip_address ($port, $ip_address) = sockaddr_in $sockaddr A wrapper of pack_sockaddr_in() or unpack_sockaddr_in(). In list context, unpacks its argument and returns a list consisting of the port and IP address. In scalar context, packs its port and IP address arguments as a "sockaddr_in" and returns it. Provided largely for legacy compatibility; it is better to use pack_sockaddr_in() or unpack_sockaddr_in() explicitly. $sockaddr = pack_sockaddr_in6 $port, $ip6_address, [$scope_id, [$flowinfo]] Takes two to four arguments, a port number, an opaque string (as returned by inet_pton()), optionally a scope ID number, and optionally a flow label number. Returns the "sockaddr_in6" structure with those arguments packed in and "AF_INET6" filled in. IPv6 equivalent of pack_sockaddr_in(). An undefined $port argument is taken as zero; an undefined $ip6_address is considered a fatal error. ($port, $ip6_address, $scope_id, $flowinfo) = unpack_sockaddr_in6 $sockaddr Takes a "sockaddr_in6" structure. Returns a list of four elements: the port number, an opaque string representing the IPv6 address, the scope ID, and the flow label. (You can use inet_ntop() to convert the address to the usual string format). Will croak if the structure does not represent an "AF_INET6" address. In scalar context will return just the IP address. $sockaddr = sockaddr_in6 $port, $ip6_address, [$scope_id, [$flowinfo]] ($port, $ip6_address, $scope_id, $flowinfo) = sockaddr_in6 $sockaddr A wrapper of pack_sockaddr_in6() or unpack_sockaddr_in6(). In list context, unpacks its argument according to unpack_sockaddr_in6(). In scalar context, packs its arguments according to pack_sockaddr_in6(). Provided largely for legacy compatibility; it is better to use pack_sockaddr_in6() or unpack_sockaddr_in6() explicitly. $sockaddr = pack_sockaddr_un $path Takes one argument, a pathname. Returns the "sockaddr_un" structure with that path packed in with "AF_UNIX" filled in. For "PF_UNIX" sockets, this structure is normally what you need for the arguments in bind(), connect(), and send(). ($path) = unpack_sockaddr_un $sockaddr Takes a "sockaddr_un" structure (as returned by pack_sockaddr_un(), getpeername() or recv()). Returns a list of one element: the pathname. Will croak if the structure does not represent an "AF_UNIX" address. $sockaddr = sockaddr_un $path ($path) = sockaddr_un $sockaddr A wrapper of pack_sockaddr_un() or unpack_sockaddr_un(). In a list context, unpacks its argument and returns a list consisting of the pathname. In a scalar context, packs its pathname as a "sockaddr_un" and returns it. Provided largely for legacy compatibility; it is better to use pack_sockaddr_un() or unpack_sockaddr_un() explicitly. These are only supported if your system has <sys/un.h>. $ip_mreq = pack_ip_mreq $multiaddr, $interface Takes an IPv4 multicast address and optionally an interface address (or "INADDR_ANY"). Returns the "ip_mreq" structure with those arguments packed in. Suitable for use with the "IP_ADD_MEMBERSHIP" and "IP_DROP_MEMBERSHIP" sockopts. ($multiaddr, $interface) = unpack_ip_mreq $ip_mreq Takes an "ip_mreq" structure. Returns a list of two elements; the IPv4 multicast address and interface address. $ip_mreq_source = pack_ip_mreq_source $multiaddr, $source, $interface Takes an IPv4 multicast address, source address, and optionally an interface address (or "INADDR_ANY"). Returns the "ip_mreq_source" structure with those arguments packed in. Suitable for use with the "IP_ADD_SOURCE_MEMBERSHIP" and "IP_DROP_SOURCE_MEMBERSHIP" sockopts. ($multiaddr, $source, $interface) = unpack_ip_mreq_source $ip_mreq Takes an "ip_mreq_source" structure. Returns a list of three elements; the IPv4 multicast address, source address and interface address. $ipv6_mreq = pack_ipv6_mreq $multiaddr6, $ifindex Takes an IPv6 multicast address and an interface number. Returns the "ipv6_mreq" structure with those arguments packed in. Suitable for use with the "IPV6_ADD_MEMBERSHIP" and "IPV6_DROP_MEMBERSHIP" sockopts. ($multiaddr6, $ifindex) = unpack_ipv6_mreq $ipv6_mreq Takes an "ipv6_mreq" structure. Returns a list of two elements; the IPv6 address and an interface number. FUNCTIONS $ip_address = inet_aton $string Takes a string giving the name of a host, or a textual representation of an IP address and translates that to an packed binary address structure suitable to pass to pack_sockaddr_in(). If passed a hostname that cannot be resolved, returns "undef". For multi-homed hosts (hosts with more than one address), the first address found is returned. For portability do not assume that the result of inet_aton() is 32 bits wide, in other words, that it would contain only the IPv4 address in network order. This IPv4-only function is provided largely for legacy reasons. Newly- written code should use getaddrinfo() or inet_pton() instead for IPv6 support. $string = inet_ntoa $ip_address Takes a packed binary address structure such as returned by unpack_sockaddr_in() (or a v-string representing the four octets of the IPv4 address in network order) and translates it into a string of the form "d.d.d.d" where the "d"s are numbers less than 256 (the normal human-readable four dotted number notation for Internet addresses). This IPv4-only function is provided largely for legacy reasons. Newly- written code should use getnameinfo() or inet_ntop() instead for IPv6 support. $address = inet_pton $family, $string Takes an address family (such as "AF_INET" or "AF_INET6") and a string containing a textual representation of an address in that family and translates that to an packed binary address structure. See also getaddrinfo() for a more powerful and flexible function to look up socket addresses given hostnames or textual addresses. $string = inet_ntop $family, $address Takes an address family and a packed binary address structure and translates it into a human-readable textual representation of the address; typically in "d.d.d.d" form for "AF_INET" or "hhhh:hhhh::hhhh" form for "AF_INET6". See also getnameinfo() for a more powerful and flexible function to turn socket addresses into human-readable textual representations. ($err, @result) = getaddrinfo $host, $service, [$hints] Given both a hostname and service name, this function attempts to resolve the host name into a list of network addresses, and the service name into a protocol and port number, and then returns a list of address structures suitable to connect() to it. Given just a host name, this function attempts to resolve it to a list of network addresses, and then returns a list of address structures giving these addresses. Given just a service name, this function attempts to resolve it to a protocol and port number, and then returns a list of address structures that represent it suitable to bind() to. This use should be combined with the "AI_PASSIVE" flag; see below. Given neither name, it generates an error. If present, $hints should be a reference to a hash, where the following keys are recognised: flags => INT A bitfield containing "AI_*" constants; see below. family => INT Restrict to only generating addresses in this address family socktype => INT Restrict to only generating addresses of this socket type protocol => INT Restrict to only generating addresses for this protocol The return value will be a list; the first value being an error indication, followed by a list of address structures (if no error occurred). The error value will be a dualvar; comparable to the "EAI_*" error constants, or printable as a human-readable error message string. If no error occurred it will be zero numerically and an empty string. Each value in the results list will be a hash reference containing the following fields: family => INT The address family (e.g. "AF_INET") socktype => INT The socket type (e.g. "SOCK_STREAM") protocol => INT The protocol (e.g. "IPPROTO_TCP") addr => STRING The address in a packed string (such as would be returned by pack_sockaddr_in()) canonname => STRING The canonical name for the host if the "AI_CANONNAME" flag was provided, or "undef" otherwise. This field will only be present on the first returned address. The following flag constants are recognised in the $hints hash. Other flag constants may exist as provided by the OS. AI_PASSIVE Indicates that this resolution is for a local bind() for a passive (i.e. listening) socket, rather than an active (i.e. connecting) socket. AI_CANONNAME Indicates that the caller wishes the canonical hostname ("canonname") field of the result to be filled in. AI_NUMERICHOST Indicates that the caller will pass a numeric address, rather than a hostname, and that getaddrinfo() must not perform a resolve operation on this name. This flag will prevent a possibly-slow network lookup operation, and instead return an error if a hostname is passed. ($err, $hostname, $servicename) = getnameinfo $sockaddr, [$flags, [$xflags]] Given a packed socket address (such as from getsockname(), getpeername(), or returned by getaddrinfo() in a "addr" field), returns the hostname and symbolic service name it represents. $flags may be a bitmask of "NI_*" constants, or defaults to 0 if unspecified. The return value will be a list; the first value being an error condition, followed by the hostname and service name. The error value will be a dualvar; comparable to the "EAI_*" error constants, or printable as a human-readable error message string. The host and service names will be plain strings. The following flag constants are recognised as $flags. Other flag constants may exist as provided by the OS. NI_NUMERICHOST Requests that a human-readable string representation of the numeric address be returned directly, rather than performing a name resolve operation that may convert it into a hostname. This will also avoid potentially-blocking network IO. NI_NUMERICSERV Requests that the port number be returned directly as a number representation rather than performing a name resolve operation that may convert it into a service name. NI_NAMEREQD If a name resolve operation fails to provide a name, then this flag will cause getnameinfo() to indicate an error, rather than returning the numeric representation as a human-readable string. NI_DGRAM Indicates that the socket address relates to a "SOCK_DGRAM" socket, for the services whose name differs between TCP and UDP protocols. The following constants may be supplied as $xflags. NIx_NOHOST Indicates that the caller is not interested in the hostname of the result, so it does not have to be converted. "undef" will be returned as the hostname. NIx_NOSERV Indicates that the caller is not interested in the service name of the result, so it does not have to be converted. "undef" will be returned as the service name. getaddrinfo() / getnameinfo() ERROR CONSTANTS The following constants may be returned by getaddrinfo() or getnameinfo(). Others may be provided by the OS. EAI_AGAIN A temporary failure occurred during name resolution. The operation may be successful if it is retried later. EAI_BADFLAGS The value of the "flags" hint to getaddrinfo(), or the $flags parameter to getnameinfo() contains unrecognised flags. EAI_FAMILY The "family" hint to getaddrinfo(), or the family of the socket address passed to getnameinfo() is not supported. EAI_NODATA The host name supplied to getaddrinfo() did not provide any usable address data. EAI_NONAME The host name supplied to getaddrinfo() does not exist, or the address supplied to getnameinfo() is not associated with a host name and the "NI_NAMEREQD" flag was supplied. EAI_SERVICE The service name supplied to getaddrinfo() is not available for the socket type given in the $hints. EXAMPLES Lookup for connect() The getaddrinfo() function converts a hostname and a service name into a list of structures, each containing a potential way to connect() to the named service on the named host. use IO::Socket; use Socket qw(SOCK_STREAM getaddrinfo); my %hints = (socktype => SOCK_STREAM); my ($err, @res) = getaddrinfo("localhost", "echo", \%hints); die "Cannot getaddrinfo - $err" if $err; my $sock; foreach my $ai (@res) { my $candidate = IO::Socket->new(); $candidate->socket($ai->{family}, $ai->{socktype}, $ai->{protocol}) or next; $candidate->connect($ai->{addr}) or next; $sock = $candidate; last; } die "Cannot connect to localhost:echo" unless $sock; $sock->print("Hello, world!\n"); print <$sock>; Because a list of potential candidates is returned, the "while" loop tries each in turn until it finds one that succeeds both the socket() and connect() calls. This function performs the work of the legacy functions gethostbyname(), getservbyname(), inet_aton() and pack_sockaddr_in(). In practice this logic is better performed by IO::Socket::IP. Making a human-readable string out of an address The getnameinfo() function converts a socket address, such as returned by getsockname() or getpeername(), into a pair of human-readable strings representing the address and service name. use IO::Socket::IP; use Socket qw(getnameinfo); my $server = IO::Socket::IP->new(LocalPort => 12345, Listen => 1) or die "Cannot listen - $@"; my $socket = $server->accept or die "accept: $!"; my ($err, $hostname, $servicename) = getnameinfo($socket->peername); die "Cannot getnameinfo - $err" if $err; print "The peer is connected from $hostname\n"; Since in this example only the hostname was used, the redundant conversion of the port number into a service name may be omitted by passing the "NIx_NOSERV" flag. use Socket qw(getnameinfo NIx_NOSERV); my ($err, $hostname) = getnameinfo($socket->peername, 0, NIx_NOSERV); This function performs the work of the legacy functions unpack_sockaddr_in(), inet_ntoa(), gethostbyaddr() and getservbyport(). In practice this logic is better performed by IO::Socket::IP. Resolving hostnames into IP addresses To turn a hostname into a human-readable plain IP address use getaddrinfo() to turn the hostname into a list of socket structures, then getnameinfo() on each one to make it a readable IP address again. use Socket qw(:addrinfo SOCK_RAW); my ($err, @res) = getaddrinfo($hostname, "", {socktype => SOCK_RAW}); die "Cannot getaddrinfo - $err" if $err; while( my $ai = shift @res ) { my ($err, $ipaddr) = getnameinfo($ai->{addr}, NI_NUMERICHOST, NIx_NOSERV); die "Cannot getnameinfo - $err" if $err; print "$ipaddr\n"; } The "socktype" hint to getaddrinfo() filters the results to only include one socket type and protocol. Without this most OSes return three combinations, for "SOCK_STREAM", "SOCK_DGRAM" and "SOCK_RAW", resulting in triplicate output of addresses. The "NI_NUMERICHOST" flag to getnameinfo() causes it to return a string-formatted plain IP address, rather than reverse resolving it back into a hostname. This combination performs the work of the legacy functions gethostbyname() and inet_ntoa(). Accessing socket options The many "SO_*" and other constants provide the socket option names for getsockopt() and setsockopt(). use IO::Socket::INET; use Socket qw(SOL_SOCKET SO_RCVBUF IPPROTO_IP IP_TTL); my $socket = IO::Socket::INET->new(LocalPort => 0, Proto => 'udp') or die "Cannot create socket: $@"; $socket->setsockopt(SOL_SOCKET, SO_RCVBUF, 64*1024) or die "setsockopt: $!"; print "Receive buffer is ", $socket->getsockopt(SOL_SOCKET, SO_RCVBUF), " bytes\n"; print "IP TTL is ", $socket->getsockopt(IPPROTO_IP, IP_TTL), "\n"; As a convenience, IO::Socket's setsockopt() method will convert a number into a packed byte buffer, and getsockopt() will unpack a byte buffer of the correct size back into a number. AUTHOR This module was originally maintained in Perl core by the Perl 5 Porters. It was extracted to dual-life on CPAN at version 1.95 by Paul Evans <leonerd AT leonerd.uk> perl v5.34.0 2025-07-25 Socket(3perl)
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