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netintro(7)
NAME
netintro, networking - Introduction to socket networking facilities
SYNOPSIS
#include <sys/socket.h>
#include <net/route.h>
#include <net/if.h>
DESCRIPTION
This section is a general introduction to the networking facilities
available in the system. Documentation in this part of Section 7 is broken
up into three areas: protocol families (domains), protocols, and network
interfaces.
All network protocols are associated with a specific protocol family. A
protocol family provides basic services to the protocol implementation to
allow it to function within a specific network environment. These services
may include packet fragmentation and reassembly, routing, addressing, and
basic transport. A protocol family may support multiple methods of
addressing, though the current protocol implementations do not. A protocol
family is normally comprised of a number of protocols, one per socket type.
It is not required that a protocol family support all socket types. A
protocol family may contain multiple protocols supporting the same socket
abstraction.
A protocol supports one of the socket abstractions detailed in the
reference page for the socket() function. A specific protocol may be
accessed either by creating a socket of the appropriate type and protocol
family, or by requesting the protocol explicitly when creating a socket.
Protocols normally accept only one type of address format, usually
determined by the addressing structure inherent in the design of the
protocol family and network architecture. Certain semantics of the basic
socket abstractions are protocol specific. All protocols are expected to
support the basic model for their particular socket type, but may, in
addition, provide nonstandard facilities or extensions to a mechanism. For
example, a protocol supporting the SOCK_STREAM abstraction may allow more
than one byte of out-of-band data to be transmitted per out-of-band
message.
A network interface is similar to a device interface. Network interfaces
comprise the lowest layer of the networking subsystem, interacting with the
actual transport hardware. An interface may support one or more protocol
families, address formats, or both. The SYNOPSIS section of each network
interface entry gives a sample specification of the related drivers for use
in providing a system description to the config program. The ERRORS section
lists messages which may appear on the console and/or in the system error
log, /var/log/messages (see the syslogd function), due to errors in device
operation.
The system currently supports the DARPA Internet protocols. Raw socket
interfaces are provided to the IP layer of the DARPA Internet. Consult the
appropriate manual pages in this section for more information regarding
this support.
Addressing
Associated with each protocol family is an address format. All network
address adhere to a general structure, called a sockaddr. However, each
protocol imposes finer and more specific structure, generally renaming the
variant.
Both the 4.3BSD and 4.4BSD sockaddr structures are supported by Tru64 UNIX.
The default sockaddr structure is the 4.3BSD structure, which is as
follows:
struct sockaddr(
u_short sa_family,
char sa_data[14] );
If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h
header file is included, however, the 4.4BSD sockaddr structure is defined,
which is as follows:
struct sockaddr(
u_char sa_len,
u_char sa_family,
char sa_data[14] );
The 4.4BSD sockaddr structure provides for a sa_len field, which contains
the total length of the structure. Unlike the 4.3BSD sockaddr structure,
this length may exceed 16 bytes.
The following address values for sa_family are known to the system (and
additional formats are defined for possible future implementation):
#define AF_UNIX 1 /* local to host (pipes, portals) */
#define AF_INET 2 /* internetwork: UDP, TCP (IPv4 address format) */
#define AF_INET6 26 /* internetwork: UDP, TCP (IPv6 address format) */
Internet domain addresses vary with the underlying protocol. The following
table lists the underlying protocol and their associated addresses:
_______________________
Protocol Address
_______________________
IPv4 sockaddr_in
IPv6 sockaddr_in6
_______________________
IPv4 Addressing
The sockaddr_in structure, defined in <netinet/in.h>, is the IPv4 sockaddr
variant, and is as follows:
struct sockaddr_in(
u_short sin_family,
u_short sin_port,
struct in_addr sin_addr,
char sin_zero[8] );
If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h
header file is included, however, the 4.4BSD sockaddr_in structure is
defined, which is as follows:
struct sockaddr_in(
u_char sin_len,
sa_family_t sin_family,
in_port_t sin_port,
struct in_addr sin_addr,
char sin_zero[8] );
IPv6 Addressing
The sockaddr_in6 structure, defined in <netinet/in6.h>, is the IPv6
sockaddr variant, and is as follows:
struct sockaddr_in6(
unsigned short sin6_family,
in_port_t sin_port,
uint32_t sin6_flowinfo,
struct in6_addr sin6_addr,
uint32_t sin6_scope_id );
If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h
header file is included, however, the 4.4BSD sockaddr_in6 structure is
defined, which is as follows:
struct sockaddr_in6(
uint8_t sin6_len,
sa_family_t sin6_family,
in_port_t sin_port,
uint32_t sin6_flowinfo,
struct in6_addr sin6_addr,
uint32_t sin6_scope_id );
The in6_addr structure is defined in <netinet/in6.h>.
Routing
The UNIX operating system provides packet routing facilities. The kernel
maintains a routing information database, which is used in selecting the
appropriate network interface when transmitting packets.
A user process (or possibly multiple cooperating processes) maintains this
database by sending messages over a special kind of socket. This supplants
fixed size ioctl's used in earlier releases.
This facility is described in the files reference page for the route()
function.
Interfaces
Each network interface in a system corresponds to a path through which
messages may be sent and received. A network interface usually has a
hardware device associated with it, though certain interfaces such as the
loopback interface, lo, do not.
The following ioctl calls may be used to manipulate network interfaces. The
ioctl is made on a socket (typically of type SOCK_DGRAM) in the desired
domain. Most of the requests supported in earlier releases take an ifreq
structure as its parameter. This structure has the following form:
struct ifreq {
#define IFNAMSIZ 16
char ifr_name[IFNAMSIZE]; /*interface name */
union {
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
struct sockaddr ifru_broadaddr;
short ifru_flags;
int ifru_metric;
caddr_t ifru_data;
} ifr_ifru;
#define ifr_addr ifr_ifru.ifru_addr
/* address */
#define ifr_dstaddr
ifr_ifru.ifru_dstaddr
/* end of p-to-p link */
#define ifr_broadaddr
ifr_ifru.ifru_broadaddr
/* broadcast address */
#define ifr_flags ifr_ifru.ifru_flags
/* flags */
#define ifr_metric
ifr_ifru.ifru_metric /* metric */
#define ifr_data ifr_ifru.ifru_data
/* for use by interface */
};
Calls which are now deprecated are:
SIOCSIFADDR (IPv4)
Sets interface address for protocol family. Following the address
assignment, the ``initialization'' routine for the interface is called.
SIOCSIFDSTADDR (IPv4)
Sets point to point address for protocol family and interface.
SIOCSIFBRDADDR (IPv4)
Sets broadcast address for protocol family and interface. All ioctl
requests to obtain addresses and requests both to set and retrieve
other data are still fully supported and use the ifreq structure:
SIOCGIFADDR (IPv4)
Gets interface address for protocol family.
SIOCGIFDSTADDR (IPv4)
Gets point-to-point address for protocol family and interface.
SIOCGIFBRDADDR (IPv4)
Gets broadcast address for protocol family and interface.
SIOCGIFMASK (IPv4)
Gets the network mask for protocol family and interface.
SIOCSIFFLAGS (IPv4)
Sets interface flags field. If the interface is marked down, any
processes currently routing packets through the interface are notified;
some interfaces may be reset so that incoming packets are no longer
received. When marked up again, the interface is reinitialized.
SIOCGIFFLAGS (IPv4)
Gets interface flags.
SIOCSIFMETRIC (IPv4)
Sets interface routing metric. The metric is used only by user-level
routers.
SIOCGIFMETRIC (IPv4)
Gets interface metric.
There are three requests that make use of a new structure:
SIOCAIFADDR (IPv4)
An interface may have more than one address associated with it in some
protocols. This request provides a means to add additional addresses
(or modify characteristics of the primary address if the default
address for the address family is specified). Rather than making
separate calls to set destination addresses, broadcast addresses, or
network masks (now an integral feature of multiple protocols) a
separate structure is used to specify all three facets simultaneously:
struct ifaliasreq(
char ifra_name[IFNAMSIZ],
struct sockaddr ifra_addr,
struct sockaddr ifra_broadaddr,
struct sockaddr ifra_mask );
You would use a slightly tailored version of this structure for each
family (replacing each sockaddr by one of the family-specific type).
When the sockaddr itself is larger than the default size, you must
modify the ioctl identifier itself to include the total size.
SIOCDIFADDR (IPv4)
This request deletes the specified address from the list associated
with an interface. It uses the if_aliasreq structure to permit
protocols to allow multiple masks or destination addresses, and it
adopts the convention that specification of the default address means
to delete the first address for the interface belonging to the address
family in which the original socket was opened.
SIOCGIFCONF (IPv4 and IPv6)
Get interface configuration list. This request takes an ifconf
structure (see below) as a value-result parameter. The ifc_len field
should be initially set to the size of the buffer pointed to by
ifc_buf. On return it contains the length, in bytes, of the
configuration list.
/*
* Structure used in SIOCGIFCONF request.
* Used to retrieve interface configuration
* for machine (useful for programs which
* must know all networks accessible).
*/
struct ifconf {
int ifc_len; /* size of associated buffer */
union {
caddr_t ifcu_buf;
struct ifreq *ifcu_req;
} ifc_ifcu;
#define ifc_buf ifc_ifcu.ifcu_buf
/* buffer address */
#define ifc_req ifc_ifcu.ifcu_req
/* array of structures returned */
#define ifc_req ifc_ifcu.ifcu_req
/* array of structures returned */
};
SEE ALSO
Functions: socket(2), ioctl(2)
Files: config(8), routed(8)
Network Programmer's Guide
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