netintro(7)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 multi‐
ple methods of addressing, though the current protocol implementations
do not. A protocol family is normally comprised of a number of proto‐
cols, one per socket type. It is not required that a protocol family
support all socket types. A protocol family may contain multiple pro‐
tocols supporting the same socket abstraction.
A protocol supports one of the socket abstractions detailed in the ref‐
erence page for the socket() function. A specific protocol may be
accessed either by creating a socket of the appropriate type and proto‐
col family, or by requesting the protocol explicitly when creating a
socket. Protocols normally accept only one type of address format, usu‐
ally 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 abstrac‐
tion 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 inter‐
faces comprise the lowest layer of the networking subsystem, interact‐
ing with the actual transport hardware. An interface may support one
or more protocol families, address formats, or both. The SYNOPSIS sec‐
tion 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 con‐
tains 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 fol‐
lowing 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 sock‐
addr 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 ker‐
nel maintains a routing information database, which is used in select‐
ing 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: Sets interface address for protocol
family. Following the address assignment, the ``initialization'' rou‐
tine for the interface is called. Sets point to point address for pro‐
tocol family and interface. 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: Gets interface address for protocol family. Gets
point-to-point address for protocol family and interface. Gets broad‐
cast address for protocol family and interface. Gets the network mask
for protocol family and interface. 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. Gets interface flags. Sets interface
routing metric. The metric is used only by user-level routers. Gets
interface metric.
There are three requests that make use of a new structure: 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-spe‐
cific type). When the sockaddr itself is larger than the default
size, you must modify the ioctl identifier itself to include the
total size. 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 desti‐
nation addresses, and it adopts the convention that specifica‐
tion 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. 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 con‐
tains 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
netintro(7)