sockaddr, sockaddr_dl, sockaddr_in, sockaddr_in6, sockaddr_ll,
sockaddr_storage, sockaddr_un - Socket Address Structures


#include <sys/socket.h>

struct sockaddr sock;

#include <sys/socket.h>
#include <net/if_dl.h>

struct sockaddr_dl dl_sock;

#include <sys/socket.h>
#include <netinet/in.h>

struct sockaddr_in in_sock;

#include <sys/socket.h>
#include <netinet/in.h>

struct sockaddr_in6 in6_sock;

#include <sys/socket.h>

struct sockaddr_ll ll_sock;

#include <sys/socket.h>

struct sockaddr_storage storage_sock;

#include <sys/un.h>

struct sockaddr_un un_sock;


The sockaddr family of structures are designed to represent network
addresses for different networking protocols. The structure struct
sockaddr is a generic structure that is used across calls to various socket
library routines (libsocket(3LIB)) such as accept(3SOCKET) and
bind(3SOCKET). Applications do not use the struct sockaddr directly, but
instead cast the appropriate networking family specific sockaddr structure
to a struct sockaddr *.

Every structure in the sockaddr family begins with a member of the same
type, the sa_family_t, though the different structures all have different
names for the member. For example, the structure struct sockaddr has the
following members defined:

sa_family_t sa_family /* address family */
char sa_data[] /* socket address (variable-length data) */

The member sa_family corresponds to the socket family that's actually in
use. The following table describes the mapping between the address family
and the corresponding socket structure that's used. Note that both the
generic struct sockaddr and the struct sockaddr_storage are not included,
because these are both generic structures. More on the struct
sockaddr_storage can be found in the next section.

Socket Structure Address Family
struct sockaddr_dl AF_LINK
struct sockaddr_in AF_INET
struct sockaddr_in6 AF_INET6
struct sockaddr_ll AF_PACKET
struct sockaddr_un AF_UNIX

struct sockaddr_storage
The sockaddr_storage structure is a sockaddr that is not associated with an
address family. Instead, it is large enough to hold the contents of any of
the other sockaddr structures. It can be used to embed sufficient storage
for a sockaddr of any type within a larger structure.

The structure only has a single member defined. While there are other
members that are used to pad out the size of the struct sockaddr_storage,
they are not defined and must not be consumed. The only valid member is:

sa_family_t ss_family /* address family */

For example, struct sockaddr_storage is useful when running a service that
accepts traffic over both IPv4 and IPv6 where it is common to use a single
socket for both address families. In that case, rather than guessing
whether a struct sockaddr_in or a struct sockaddr_in6 is more appropriate,
one can simply use a struct sockaddr_storage and cast to the appropriate
family-specific structure type based on the value of the member ss_family.

struct sockaddr_in
The sockaddr_in is the socket type which is used for for the Internet
Protocol version four (IPv4). It has the following members defined:

sa_family_t sin_family /* address family */
in_port_t sin_port /* IP port */
struct in_addr sin_addr /* IP address */

The member sin_family must always have the value AF_INET for IPv4. The
members sin_port and sin_addr describe the IP address and IP port to use.
In the case of a call to connect(3SOCKET) these represent the remote IP
address and port to which the connection is being made. In the case of
bind(3SOCKET) these represent the IP address and port on the local host to
which the socket is to be bound. In the case of accept(3SOCKET) these
represent the remote IP address and port of the machine whose connection
was accepted.

The member sin_port is always stored in Network Byte Order. On many
systems, this differs from the native host byte order. Applications should
read from the member with the function ntohs(3C) and write to the member
with the function htons(3C). The member sin_addr is the four byte IPv4
address. It is also stored in network byte order. The common way to write
out the address is to use the function inet_pton(3C) which converts between
a human readable IP address such as "" and the corresponding

Example 1 shows how to prepare an IPv4 socket and deal with network byte-
order. See inet(4P) and ip(4P) for more information on IPv4, socket
options, etc.

struct sockaddr_in6
The sockaddr_in6 structure is the sockaddr for the Internet Protocol
version six (IPv6). Unlike the struct sockaddr_in, the struct sockaddr_in6
has additional members beyond those shown here which are required to be
initialized to zero through a function such as bzero(3C) or memset(3C). If
the entire struct sockaddr_in6 is not zeroed before use, applications will
experience undefined behavior. The struct sockaddr_in6 has the following
public members:

sa_family_t sin6_family /* address family */
in_port_t sin6_port /* IPv6 port */
struct in6_addr sin6_addr /* IPv6 address */
uint32_t sin6_flowinfo; /* traffic class and flow info */
uint32_t sin6_scope_id; /* interface scope */

The member sin6_family must always have the value AF_INET6. The members
sin6_port and sin6_addr are the IPv6 equivalents of the struct sockaddr_in
sin_port and sin_addr. Like their IPv4 counterparts, both of these members
must be in network byte order. The member sin6_port describes the IPv6
port and should be manipulated with the functions ntohs(3C) and htons(3C).
The member sin6_addr describes the 16-byte IPv6 address. In addition to
the function inet_pton(3C), the header file <netinet/in.h> defines many
macros for manipulating and testing IPv6 addresses.

The member sin6_flowinfo contains the traffic class and flow label
associated with the IPv6 header. The member sin6_scope_id may contain an
identifier which varies based on the scope of the address in sin6_addr.
Applications do not need to initialize sin6_scope_id; it will be populated
by the operating system as a result of various library calls.

Example 2 shows how to prepare an IPv6 socket. For more information on
IPv6, please see inet6(4P) and ip6(4P).

struct sockaddr_un
The sockaddr_un structure specifies the address of a socket used to
communicate between processes running on a single system, commonly known as
a UNIX domain socket. Sockets of this type are identified by a path in the
file system. The struct sockaddr_un has the following members:

sa_family_t sun_family /* address family */
char sun_path[108] /* path name */

The member sun_family must always have the value AF_UNIX. The member
sun_path is populated with a NUL terminated array of characters that
specify a file system path. The maximum length of any such path, including
the NUL terminator, is 108 bytes.

struct sockaddr_dl
The sockaddr_dl structure is used to describe a layer 2 link-level address.
This is used as part of various socket ioctls, such as those for arp(4P).
The structure has the following members:

ushort_t sdl_family; /* address family */
ushort_t sdl_index; /* if != 0, system interface index */
uchar_t sdl_type; /* interface type */
uchar_t sdl_nlen; /* interface name length */
uchar_t sdl_alen; /* link level address length */
uchar_t sdl_slen; /* link layer selector length */
char sdl_data[244]; /* contains both if name and ll address

The member sdl_family must always have the value AF_LINK. When the member
sdl_index is non-zero this refers to the interface identifier that
corresponds to the struct sockaddr_dl. This identifier is the same
identifier that's shown by tools like ifconfig(8) and used in the SIOC* set
of socket ioctls. The member sdl_type refers to the media that is used for
the socket. The most common case is that the medium for the interface is
Ethernet which has the value IFT_ETHER. The full set of types is derived
from RFC1573 and recorded in the file <net/if_types.h>. The member
sdl_slen describes the length of a selector, if it exists, for the
specified medium. This is used in protocols such as Trill.

The sdl_data, sdl_nlen and sdl_alen members together describe a character
string containing the interface name and the link-layer network address.
The name starts at the beginning of sdl_data, i.e. at sdl_data[0]. The
name of the interface occupies the next sdl_nlen bytes and is not NUL
terminated. The link-layer network address begins immediately after the
interface name, and is sdl_alen bytes long. The macro LLADDR(struct
sockaddr_dl *) returns the start of the link-layer network address. The
interpretation of the link-layer address depends on the value of sdl_type.
For example, if the type is IFT_ETHER then the address is expressed as a
6-byte MAC address.

struct sockaddr_ll
The sockaddr_ll is used as part of a socket type which is responsible for
packet capture: AF_PACKET sockets. It is generally designed for use with
Ethernet networks. The members of the struct sockaddr_ll are:

uint16_t sll_family; /* address family */
uint16_t sll_protocol; /* link layer protocol */
int32_t sll_ifindex; /* interface index */
uint16_t sll_hatype; /* ARP hardware type */
uint8_t sll_pkttype; /* packet type */
uint8_t sll_halen; /* hardware address length */
uint8_t sll_addr[8]; /* hardware type */

The member sll_family must be AF_PACKET. The member sll_protocol refers to
a link-layer protocol. For example, when capturing Ethernet frames the
value of sll_protocol is the Ethertype. This member is written in network
byte order and applications should use htons(3C) and ntohs(3C) to read and
write the member.

The member sll_ifindex is the interface's index. It is used as an
identifier in various ioctls and included in the output of ifconfig(8).
When calling bind(3SOCKET) it should be filled in with the index that
corresponds to the interface for which packets should be captured on.

The member sll_pkttype describes the type of the packet based on a list of
types in the header file <netpacket/packet.h>. These types include:
PACKET_OUTGOING, a packet that was leaving the host and has been looped
back for packet capture; PACKET_HOST, a packet that was destined for this
host; PACKET_BROADCAST, a packet that was broadcast across the link-layer;
PACKET_MULTICAST, a packet that was sent to a link-layer multicast address;
and PACKET_OTHERHOST, a packet that was captured only because the device in
question was in promiscuous mode.

The member sll_hatype contains the hardware type as defined by arp(4P).
The list of types can be found in <net/if_arp.h>. The member sll_halen
contains the length, in bytes, of the hardware address, while the member
sll_addr contains the actual address in network byte order.


Example 1 Preparing an IPv4 sockaddr_in to connect to a remote host

The following example shows how one would open a socket and prepare it to
connect to the remote host whose address is the IP address on
port 80. This program should be compiled with the C compiler cc and linked
against the libraries libsocket and libnsl. If this example was named
ip4.c, then the full link line would be cc ip4.c -lsocket -lnsl.

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <netinet/in.h>
#include <inttypes.h>
#include <strings.h>

int sock;
struct sockaddr_in in;

if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
return (1);

bzero(&in, sizeof (struct sockaddr_in));
in.sin_family = AF_INET;
in.sin_port = htons(80);
if (inet_pton(AF_INET, "", &in.sin_addr) != 1) {
return (1);

if (connect(sock, (struct sockaddr *)&in,
sizeof (struct sockaddr_in)) != 0) {
return (1);

/* use socket */

return (0);

Example 2 Preparing an IPv6 sockaddr_in6 to bind to a local address

The following example shows how one would open a socket and prepare it to
bind to the local IPv6 address ::1 port on port 12345. This program should
be compiled with the C compiler cc and linked against the libraries
libsocket and libnsl. If this example was named ip6.c, then the full
compiler line would be cc ip6.c -lsocket -lnsl.

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <netinet/in.h>
#include <inttypes.h>
#include <strings.h>

int sock6;
struct sockaddr_in6 in6;

if ((sock6 = socket(AF_INET6, SOCK_STREAM, 0)) < 0) {
return (1);

bzero(&in6, sizeof (struct sockaddr_in6));
in6.sin6_family = AF_INET6;
in6.sin6_port = htons(12345);
if (inet_pton(AF_INET6, "::1", &in6.sin6_addr) != 1) {
return (1);

if (bind(sock6, (struct sockaddr *)&in6,
sizeof (struct sockaddr_in6)) != 0) {
return (1);

/* use server socket */

return (0);


socket(3HEAD), un.h(3HEAD), accept(3SOCKET), bind(3SOCKET),
connect(3SOCKET), socket(3SOCKET), arp(4P), inet(4P), inet6(4P), ip(4P),

illumos April 9, 2016 illumos