NAME
netintro —
introduction to networking
facilities
SYNOPSIS
#include <sys/types.h>
#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 4 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 normally comprises a number of protocols, one per
socket(2) 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
socket(2). 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/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
non-standard 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 and/or address formats. 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(1) program.
The
DIAGNOSTICS section lists messages which may appear on the
console and/or in the system error log,
/var/log/messages
(see
syslogd(8)), due to errors
in device operation.
PROTOCOLS
The system currently supports the Internet protocols and some of the ISO OSI
protocols. Raw socket interfaces are provided to the IP protocol layer of the
Internet, and to the IDP protocol of Xerox NS. Consult the appropriate manual
pages in this section for more information regarding the support for each
protocol family.
ADDRESSING
Associated with each protocol family is an address format. All network address
adhere to a general structure, called a sockaddr, described below. However,
each protocol imposes finer and more specific structure, generally renaming
the variant, which is discussed in the protocol family manual page alluded to
above.
struct sockaddr {
u_char sa_len;
u_char sa_family;
char sa_data[14];
};
The field
sa_len contains the total length of the of the
structure, which 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_LOCAL 1 /* local to host */
#define AF_INET 2 /* internetwork: UDP, TCP, etc. */
#define AF_NS 6 /* Xerox NS protocols */
#define AF_CCITT 10 /* CCITT protocols, X.25 etc */
#define AF_HYLINK 15 /* NSC Hyperchannel */
#define AF_INET6 24 /* internetwork, v6: UDP, TCP, etc. */
ROUTING
UNIX provides some 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 co-operating processes) maintains this
database by sending messages over a special kind of socket. This supplants
fixed size
ioctl(2) used in
earlier releases.
This facility is described in
route(4).
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(4), do not.
The following
ioctl(2) calls may be
used to manipulate network interfaces. The
ioctl(2) 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
form
struct ifreq {
#define IFNAMSIZ 16
char ifr_name[IFNAMSIZ]; /* if name, e.g. "en0" */
union {
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
struct sockaddr ifru_broadaddr;
short ifru_flags;
int ifru_metric;
void *ifru_data;
} ifr_ifru;
#define ifr_addr ifr_ifru.ifru_addr /* address */
#define ifr_dstaddr ifr_ifru.ifru_dstaddr /* other end of p-to-p link */
#define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */
#define ifr_space ifr_ifru.ifru_space /* sockaddr_storage */
#define ifr_flags ifr_ifru.ifru_flags /* flags */
#define ifr_metric ifr_ifru.ifru_metric /* metric */
#define ifr_mtu ifr_ifru.ifru_mtu /* mtu */
#define ifr_dlt ifr_ifru.ifru_dlt /* data link type (DLT_*) */
#define ifr_value ifr_ifru.ifru_value /* generic value */
#define ifr_media ifr_ifru.ifru_metric /* media options (overload) */
#define ifr_data ifr_ifru.ifru_data /* for use by interface */
#define ifr_buf ifr_ifru.ifru_b.b_buf /* new interface ioctls */
#define ifr_buflen ifr_ifru.ifru_b.b_buflen
#define ifr_index ifr_ifru.ifru_value /* interface index */
};
Calls which are now deprecated are:
-
-
SIOCSIFADDR
- Set interface address for protocol family. Following the
address assignment, the ``initialization'' routine for the interface is
called.
-
-
SIOCSIFDSTADDR
- Set point to point address for protocol family and
interface.
-
-
SIOCSIFBRDADDR
- Set broadcast address for protocol family and
interface.
ioctl(2) requests to obtain
addresses and requests both to set and retrieve other data are still fully
supported and use the
ifreq structure:
-
-
SIOCGIFADDR
- Get interface address for protocol family.
-
-
SIOCGIFDSTADDR
- Get point to point address for protocol family and
interface.
-
-
SIOCGIFBRDADDR
- Get broadcast address for protocol family and
interface.
-
-
SIOCSIFFLAGS
- Set 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
- Get interface flags.
-
-
SIOCSIFMETRIC
- Set interface routing metric. The metric is used only by
user-level routers.
-
-
SIOCGIFMETRIC
- Get interface metric.
-
-
SIOCGIFINDEX
- Get the interface index and populate ifr_index.
There are two requests that make use of a new structure:
-
-
SIOCAIFADDR
- 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 or broadcast addresses, or network masks (now an
integral feature of multiple protocols) a separate structure,
ifaliasreq, is used to specify all three facets
simultaneously (see below). One would use a slightly tailored version of
this struct specific to each family (replacing each sockaddr by one of the
family-specific type). Where the sockaddr itself is larger than the
default size, one needs to modify the
ioctl(2) identifier itself to
include the total size, as described in
ioctl(2).
-
-
SIOCDIFADDR
- This requests deletes the specified address from the list
associated with an interface. It also uses the
ifaliasreq structure to allow for the possibility of
protocols allowing multiple masks or destination addresses, and also
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.
-
-
SIOCGIFALIAS
- This request provides means to get additional addresses
together with netmask and broadcast/destination from an interface. It also
uses the ifaliasreq structure.
Request making use of the
ifconf structure:
-
-
SIOCGIFCONF
- 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 will contain the length, in bytes, of the configuration
list.
/*
* Structure used in SIOC[AD]IFADDR request.
*/
struct ifaliasreq {
char ifra_name[IFNAMSIZ]; /* if name, e.g. "en0" */
struct sockaddr ifra_addr;
struct sockaddr ifra_dstaddr;
#define ifra_broadaddr ifra_dstaddr
struct sockaddr ifra_mask;
};
/*
* 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 {
void *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 */
};
SEE ALSO
config(1),
ioctl(2),
socket(2),
intro(4),
routed(8)
HISTORY
The
netintro manual appeared in
4.3BSD-Tahoe.