man/man3socket/sockaddr.3socket

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.Dd April 9, 2016
.Dt SOCKADDR 3SOCKET
.Os
.Sh NAME
.Nm sockaddr ,
.Nm sockaddr_dl ,
.Nm sockaddr_in ,
.Nm sockaddr_in6 ,
.Nm sockaddr_ll ,
.Nm sockaddr_storage ,
.Nm sockaddr_un
.Nd Socket Address Structures
.Sh SYNOPSIS
.In sys/socket.h
.Lp
.Sy struct sockaddr
.Em sock ;
.Lp
.In sys/socket.h
.In net/if_dl.h
.Lp
.Sy struct sockaddr_dl
.Em dl_sock ;
.Lp
.In sys/socket.h
.In netinet/in.h
.Lp
.Sy struct sockaddr_in
.Em in_sock ;
.Lp
.In sys/socket.h
.In netinet/in.h
.Lp
.Sy struct sockaddr_in6
.Em in6_sock ;
.Lp
.In sys/socket.h
.Lp
.Sy struct sockaddr_ll
.Em ll_sock ;
.Lp
.In sys/socket.h
.Lp
.Sy struct sockaddr_storage
.Em storage_sock ;
.Lp
.In sys/un.h
.Lp
.Sy struct sockaddr_un
.Em un_sock ;
.Sh DESCRIPTION
The
.Nm
family of structures are designed to represent network addresses for
different networking protocols.
The structure
.Sy struct sockaddr
is a generic structure that is used across calls to various socket
library routines
.Po
.Xr libsocket 3LIB
.Pc
such as
.Xr accept 3SOCKET
and
.Xr bind 3SOCKET .
Applications do not use the
.Sy struct sockaddr
directly, but instead cast the appropriate networking family specific
.Nm
structure to a
.Sy struct sockaddr * .
.Lp
Every structure in the
.Nm
family begins with a member of the same type, the
.Sy sa_family_t ,
though the different structures all have different names for the member.
For example, the structure
.Sy struct sockaddr
has the following members defined:
.Bd -literal -offset indent
sa_family_t	sa_family	/* address family */
char		sa_data[]	/* socket address (variable-length data) */
.Ed
.Lp
The member
.Em 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
.Sy struct sockaddr
and the
.Sy struct sockaddr_storage
are not included, because these are both generic structures.
More on the
.Sy struct sockaddr_storage
can be found in the next section.
.Bl -column -offset indent ".Sy Socket Structure" ".Sy Address Family"
.It Sy Socket Structure Ta Sy Address Family
.It struct sockaddr_dl Ta AF_LINK
.It struct sockaddr_in Ta AF_INET
.It struct sockaddr_in6 Ta AF_INET6
.It struct sockaddr_ll Ta AF_PACKET
.It struct sockaddr_un Ta AF_UNIX
.El
.Ss struct sockaddr_storage
The
.Sy sockaddr_storage
structure is a
.Nm
that is not associated with an address family.
Instead, it is large enough to hold the contents of any of the other
.Nm
structures.
It can be used to embed sufficient storage for a
.Sy sockaddr
of any type within a larger structure.
.Lp
The structure only has a single member defined.
While there are other members that are used to pad out the size of the
.Sy struct sockaddr_storage ,
they are not defined and must not be consumed.
The only valid member is:
.Bd -literal -offset indent
sa_family_t	ss_family	/* address family */
.Ed
.Lp
For example,
.Sy struct sockaddr_storage
is useful when running a service that accepts traffic over both
.Sy IPv4
and
.Sy IPv6
where it is common to use a single socket for both address families.
In that case, rather than guessing whether a
.Sy struct sockaddr_in
or a
.Sy struct sockaddr_in6
is more appropriate, one can simply use a
.Sy struct sockaddr_storage
and cast to the appropriate family-specific structure type based on the
value of the member
.Em ss_family .
.Ss struct sockaddr_in
The
.Sy sockaddr_in
is the socket type which is used for for the Internet Protocol version
four (IPv4).
It has the following members defined:
.Bd -literal -offset indent
sa_family_t	sin_family	/* address family */
in_port_t	sin_port	/* IP port */
struct in_addr	sin_addr	/* IP address */
.Ed
.Lp
The member
.Em sin_family
must always have the value
.Sy AF_INET
for
.Sy IPv4 .
The members
.Em sin_port
and
.Em sin_addr
describe the IP address and IP port to use.
In the case of a call to
.Xr connect 3SOCKET
these represent the remote IP address and port to which the connection
is being made.
In the case of
.Xr 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
.Xr accept 3SOCKET
these represent the remote IP address and port of the machine whose
connection was accepted.
.Lp
The member
.Em sin_port
is always stored in
.Sy Network Byte Order .
On many systems, this differs from the native host byte order.
Applications should read from the member with the function
.Xr ntohs 3C
and write to the member with the function
.Xr htons 3C .
The member
.Em 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
.Xr inet_pton 3C
which converts between a human readable IP address such as "10.1.2.3"
and the corresponding representation.
.Lp
Example 1 shows how to prepare an IPv4 socket and deal with
network byte-order.
See
.Xr inet 4P
and
.Xr ip 4P
for more information on IPv4, socket options, etc.
.Ss struct sockaddr_in6
The
.Sy sockaddr_in6
structure is the
.Nm
for the Internet Protocol version six (IPv6).
Unlike the
.Sy struct sockaddr_in ,
the
.Sy struct sockaddr_in6
has additional members beyond those shown here which are required to be
initialized to zero through a function such as
.Xr bzero 3C
or
.Xr memset 3C .
If the entire
.Sy struct sockaddr_in6
is not zeroed before use, applications will experience undefined behavior.
The
.Sy struct sockaddr_in6
has the following public members:
.Bd -literal -offset indent
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 */
.Ed
.Lp
The member
.Em sin6_family
must always have the value
.Sy AF_INET6 .
The members
.Em sin6_port
and
.Em sin6_addr
are the IPv6 equivalents of the
.Sy struct sockaddr_in
.Em sin_port
and
.Em sin_addr .
Like their IPv4 counterparts, both of these members must be in network
byte order.
The member
.Em sin6_port
describes the IPv6 port and should be manipulated with the functions
.Xr ntohs 3C
and
.Xr htons 3C .
The member
.Em sin6_addr
describes the 16-byte IPv6 address.
In addition to the function
.Xr inet_pton 3C ,
the header file
.In netinet/in.h
defines many macros for manipulating and testing IPv6 addresses.
.Lp
The member
.Em sin6_flowinfo
contains the traffic class and flow label associated with the IPv6
header.
The member
.Em sin6_scope_id
may contain an identifier which varies based on the scope of the address
in
.Em sin6_addr .
Applications do not need to initialize
.Em sin6_scope_id ;
it will be populated by the operating system as a result of various library
calls.
.Lp
Example 2 shows how to prepare an IPv6 socket.
For more information on
IPv6, please see
.Xr inet6 4P
and
.Xr ip6 4P .
.Ss struct sockaddr_un
The
.Sy sockaddr_un
structure specifies the address of a socket used to communicate between
processes running on a single system, commonly known as a
.Em UNIX domain socket .
Sockets of this type are identified by a path in the file system.
The
.Sy struct sockaddr_un
has the following members:
.Bd -literal -offset indent
sa_family_t	sun_family	/* address family */
char		sun_path[108]	/* path name */
.Ed
.Lp
The member
.Em sun_family
must always have the value
.Sy AF_UNIX .
The member
.Em sun_path
is populated with a
.Sy NUL
terminated array of characters that specify a file system path.
The maximum length of any such path, including the
.Sy NUL
terminator, is 108 bytes.
.Ss struct sockaddr_dl
The
.Sy 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
.Xr arp 4P .
The structure has the following members:
.Bd -literal -offset indent
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
.Ed
.Lp
The member
.Em sdl_family
must always have the value
.Sy AF_LINK .
When the member
.Em sdl_index
is non-zero this refers to the interface identifier that corresponds to
the
.Sy struct sockaddr_dl .
This identifier is the same identifier that's shown by tools like
.Xr ifconfig 8
and used in the SIOC* set of socket ioctls.
The member
.Em 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
.Sy IFT_ETHER .
The full set of types is derived from RFC1573 and recorded in the file
.In net/if_types.h .
The member
.Em sdl_slen
describes the length of a selector, if it exists, for the specified
medium.
This is used in protocols such as Trill.
.Lp
The
.Em sdl_data ,
.Em sdl_nlen
and
.Em 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
.Em sdl_data ,
i.e. at
.Em sdl_data[0] .
The name of the interface occupies the next
.Em sdl_nlen
bytes and is not
.Sy NUL
terminated.
The link-layer network address begins immediately after the interface name,
and is
.Em sdl_alen
bytes long.
The macro
.Sy 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
.Em sdl_type .
For example, if the type is
.Sy IFT_ETHER
then the address is expressed as a 6-byte MAC address.
.Ss struct sockaddr_ll
The
.Sy sockaddr_ll
is used as part of a socket type which is responsible for packet
capture:
.Sy AF_PACKET
sockets.
It is generally designed for use with Ethernet networks.
The members of the
.Sy struct sockaddr_ll
are:
.Bd -literal -offset indent
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 */
.Ed
.Lp
The member
.Em sll_family
must be
.Sy AF_PACKET .
The member
.Em sll_protocol
refers to a link-layer protocol.
For example, when capturing Ethernet frames the value of
.Em sll_protocol
is the Ethertype.
This member is written in network byte order and applications should use
.Xr htons 3C
and
.Xr ntohs 3C
to read and write the member.
.Lp
The member
.Em sll_ifindex
is the interface's index.
It is used as an identifier in various ioctls and included in the output of
.Xr ifconfig 8 .
When calling
.Xr bind 3SOCKET
it should be filled in with the index that corresponds to the interface
for which packets should be captured on.
.Lp
The member
.Em sll_pkttype
describes the type of the packet based on a list of types in the header
file
.In netpacket/packet.h .
These types include:
.Sy PACKET_OUTGOING ,
a packet that was leaving the host and has been looped back for packet capture;
.Sy PACKET_HOST ,
a packet that was destined for this host;
.Sy PACKET_BROADCAST ,
a packet that was broadcast across the link-layer;
.Sy PACKET_MULTICAST ,
a packet that was sent to a link-layer multicast address; and
.Sy PACKET_OTHERHOST ,
a packet that was captured only because the device in question was in
promiscuous mode.
.Lp
The member
.Em sll_hatype
contains the hardware type as defined by
.Xr arp 4P .
The list of types can be found in
.In net/if_arp.h .
The member
.Em sll_halen
contains the length, in bytes, of the hardware address, while the member
.Em sll_addr
contains the actual address in network byte order.
.Sh EXAMPLES
.Sy Example 1
Preparing an IPv4
.Sy sockaddr_in
to connect to a remote host
.Lp
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 127.0.0.1
on port 80.
This program should be compiled with the C compiler
.Sy cc
and linked against the libraries libsocket and libnsl.
If this example was named ip4.c, then the full link line would be
.Ic cc ip4.c -lsocket -lnsl .
.Bd -literal
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <netinet/in.h>
#include <inttypes.h>
#include <strings.h>

int
main(void)
{
	int sock;
	struct sockaddr_in in;

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

	bzero(&in, sizeof (struct sockaddr_in));
	in.sin_family = AF_INET;
	in.sin_port = htons(80);
	if (inet_pton(AF_INET, "127.0.0.1", &in.sin_addr) != 1) {
		perror("inet_pton");
		return (1);
	}

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

	/* use socket */

	return (0);
}
.Ed
.Lp
.Sy Example 2
Preparing an IPv6
.Sy sockaddr_in6
to bind to a local address
.Lp
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
.Sy cc
and linked against the libraries libsocket and libnsl.
If this example was named ip6.c, then the full compiler line would be
.Ic cc ip6.c -lsocket -lnsl .
.Bd -literal
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <netinet/in.h>
#include <inttypes.h>
#include <strings.h>

int
main(void)
{
	int sock6;
	struct sockaddr_in6 in6;

	if ((sock6 = socket(AF_INET6, SOCK_STREAM, 0)) < 0) {
		perror("socket");
		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) {
		perror("inet_pton");
		return (1);
	}

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

	/* use server socket */

	return (0);
}
.Ed
.Sh SEE ALSO
.Xr socket 3HEAD ,
.Xr un.h 3HEAD ,
.Xr accept 3SOCKET ,
.Xr bind 3SOCKET ,
.Xr connect 3SOCKET ,
.Xr socket 3SOCKET ,
.Xr arp 4P ,
.Xr inet 4P ,
.Xr inet6 4P ,
.Xr ip 4P ,
.Xr ip6 4P