netintro, networking - Introduction to socket networking
facilities
#include <sys/socket.h> #include <net/route.h> #include
<net/if.h>
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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'' routine for the interface is
called. Sets point to point address for protocol 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 broadcast
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-specific 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 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. 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 */ };
Functions: socket(2), ioctl(2)
Files: config(8), routed(8)
Network Programmer's Guide
netintro(7)
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