CONTENTS

SYNOPSIS

The programmer’s interface is BSD sockets compatible. For more information on sockets, see socket(7).

An IP socket is created by calling the socket(2) function as "socket(PF_INET, socket_type, protocol)". Valid socket types are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM to open a udp(7) socket, or SOCK_RAW to open a raw(7) socket to access the IP protocol directly. protocol is the IP protocol in the IP header to be received or sent. The only valid values for protocol are 0 and IPPROTO_TCP for TCP sockets and 0 and IPPROTO_UDP for UDP sockets. For SOCK_RAW you may specify a valid IANA IP protocol defined in RFC1700 assigned numbers.

When a process wants to receive new incoming packets or connections, it should bind a socket to a local interface address using bind(2). Only one IP socket may be bound to any given local (address, port) pair. When INADDR_ANY is specified in the bind call the socket will be bound to all local interfaces. When listen(2) or connect(2) are called on a unbound socket the socket is automatically bound to a random free port with the local address set to INADDR_ANY.

A TCP local socket address that has been bound is unavailable for some time after closing, unless the SO_REUSEADDR flag has been set. Care should be taken when using this flag as it makes TCP less reliable.

struct sockaddr_in {
sa_family_t sin_family; /* address family: AF_INET */
u_int16_t sin_port; /* port in network byte order */
struct in_addr sin_addr; /* internet address */
};

/* Internet address. */
struct in_addr {
u_int32_t s_addr;/* address in network byte order */
};

sin_family is always set to AF_INET. This is required; in Linux 2.2 most networking functions return EINVAL when this setting is missing. sin_port contains the port in network byte order. The port numbers below 1024 are called "reserved ports". Only processes with effective user id 0 or the CAP_NET_BIND_SERVICE capability may bind(2) to these sockets. Note that the raw IPv4 protocol as such has no concept of a port, they are only implemented by higher protocols like tcp(7) and udp(7).

sin_addr is the IP host address. The addr member of struct in_addr contains the host interface address in network order. in_addr should be only accessed using the inet_aton(3), inet_addr(3), inet_makeaddr(3) library functions or directly with the name resolver (see gethostbyname(3)). IPv4 addresses are divided into unicast, broadcast and multicast addresses. Unicast addresses specify a single interface of a host, broadcast addresses specify all hosts on a network and multicast addresses address all hosts in a multicast group. Datagrams to broadcast addresses can be only sent or received when the SO_BROADCAST socket flag is set. In the current implementation connection oriented sockets are only allowed to use unicast addresses.

Note that the address and the port are always stored in network order. In particular, this means that you need to call htons(3) on the number that is assigned to a port. All address/port manipulation functions in the standard library work in network order.

There are several special addresses: INADDR_LOOPBACK (127.0.0.1) always refers to the local host via the loopback device; INADDR_ANY (0.0.0.0) means any address for binding; INADDR_BROADCAST (255.255.255.255) means any host and has the same effect on bind as INADDR_ANY for historical reasons.

IP supports some protocol specific socket options that can be set with setsockopt(2) and read with getsockopt(2). The socket option level for IP is SOL_IP. A boolean integer flag is zero when it is false, otherwise true.