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.

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>

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

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.

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.

Set or get the broadcast flag. When enabled, datagram sockets are allowed to send packets to a broadcast address. This option has no effect on stream-oriented sockets.

Enable BSD bug-to-bug compatibility. This is used by the UDP protocol module in Linux 2.0 and 2.2. If enabled, ICMP errors received for a UDP socket will not be passed to the user program. In later kernel versions, support for this option has been phased out: Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning (printk()) if a program uses this option. Linux 2.0 also enabled BSD bug-to-bug compatibility options (random header changing, skipping of the broadcast flag) for raw sockets with this option, but that was removed in Linux 2.2.

Enable socket debugging. Only allowed for processes with the CAP_NET_ADMIN capability or an effective user ID of 0.

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.

Retrieves the socket domain as an integer, returning a value such as AF_INET6. See socket(2) for details. This socket option is read-only.

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

Don't send via a gateway, send only to directly connected hosts. The same effect can be achieved by setting the MSG_DONTROUTE flag on a socket send(2) operation. Expects an integer boolean flag.

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

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

When enabled, a close(2) or shutdown(2) will not return until all queued messages for the socket have been successfully sent or the linger timeout has been reached. Otherwise, the call returns immediately and the closing is done in the background. When the socket is closed as part of exit(2), it always lingers in the background.

When set, this option will prevent changing the filters associated with the socket. These filters include any set using the socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, 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.

Set the mark for each packet sent through this socket (similar to the netfilter MARK target but socket-based). Changing the mark can be used for mark-based routing without netfilter or for packet filtering. Setting this option requires the CAP_NET_ADMIN capability.

If this option is enabled, out-of-band data is directly placed into the receive data stream. Otherwise, out-of-band data is passed only when the MSG_OOB flag is set during receiving.

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

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:

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.

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.

Retrieves the socket protocol as an integer, returning a value such as IPPROTO_SCTP. See socket(2) for details. This socket option is read-only.

Sets or gets the maximum socket receive buffer in bytes. The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(2), and this doubled value is returned by getsockopt(2). The default value is set by the /proc/sys/net/core/rmem_default file, and the maximum allowed value is set by the /proc/sys/net/core/rmem_max file. The minimum (doubled) value for this option is 256.

Using this socket option, a privileged (CAP_NET_ADMIN) process can perform the same task as SO_RCVBUF, but the rmem_max limit can be overridden.

Specify the minimum number of bytes in the buffer until the socket layer will pass the data to the protocol (SO_SNDLOWAT) or the user on receiving (SO_RCVLOWAT). These two values are initialized to 1. SO_SNDLOWAT is not changeable on Linux (setsockopt(2) fails with the error ENOPROTOOPT). SO_RCVLOWAT is changeable only since Linux 2.4. 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.

Specify the receiving or sending timeouts until reporting an error. The argument is a struct timeval. If an input or output function blocks for this period of time, and data has been sent or received, the return value of that function will be the amount of data transferred; if no data has been transferred and the timeout has been reached, then −1 is returned with errno set to EAGAIN or EWOULDBLOCK, or EINPROGRESS (for connect(2)) just as if the socket was specified to be nonblocking. If the timeout is set to zero (the default), then the operation will never timeout. Timeouts only have effect for system calls that perform socket I/O (e.g., read(2), recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for select(2), poll(2), epoll_wait(2), and so on.

Indicates that the rules used in validating addresses supplied in a bind(2) call should allow reuse of local addresses. For AF_INET sockets this means that a socket may bind, except when there is an active listening socket bound to the address. When the listening socket is bound to INADDR_ANY with a specific port then it is not possible to bind to this port for any local address. Argument is an integer boolean flag.

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.

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.

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.

Using this socket option, a privileged (CAP_NET_ADMIN) process can perform the same task as SO_SNDBUF, but the wmem_max limit can be overridden.

Enable or disable the receiving of the SO_TIMESTAMP control message. The timestamp control message is sent with level SOL_SOCKET and 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.

Gets the socket type as an integer (e.g., SOCK_STREAM). This socket option is read-only.

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.

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

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

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

contains the maximum socket send buffer size in bytes which a user may set by using the SO_SNDBUF socket option.

configure the token bucket filter used to load limit warning messages caused by external network events.

Maximum number of packets in the global input queue.

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

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).

Set the process or process group to send SIGIO or SIGURG signals to when an asynchronous I/O operation has finished or urgent data is available. The argument is a pointer to a pid_t. If the argument is positive, send the signals to that process. If the argument is negative, send the signals to the process group with the ID of the absolute value of the argument. The process may only choose itself or its own process group to receive signals unless it has the CAP_KILL capability or an effective UID of 0.

Change the O_ASYNC flag to enable or disable asynchronous I/O mode of the socket. Asynchronous I/O mode means that the SIGIO signal or the signal set with F_SETSIG is raised when a new I/O event occurs.

Argument is an integer boolean flag. (This operation is synonymous with the use of fcntl(2) to set the O_ASYNC flag.)

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

The same as the SIOCGPGRP ioctl(2).

The same as the SIOCSPGRP ioctl(2).