netintro(4)

NAME

networking - introduction to networking facilities

SYNOPSIS

#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>

DESCRIPTION

This section is a general introduction to the networking fa

cilities
available in the system. Documentation in this part of sec

tion 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 im

plementation to
allow it to function within a specific network environment.

These services may include packet fragmentation and reassembly, rout

ing, 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 proto

cols, one per
socket(2) type. It is not required that a protocol family

support all
socket types. A protocol family may contain multiple proto

cols 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 ac

cept only one
type of address format, usually determined by the addressing

structure
inherent in the design of the protocol family/network archi

tecture. 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 support

ing 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. Net

work interfaces
comprise the lowest layer of the networking subsystem, in

teracting 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 re

lated drivers
for use in providing a system description to the config(8)

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, the

Xerox Network
Systems(tm) protocols, and some of the ISO OSI protocols.

Raw socket
interfaces are provided to the IP protocol layer of the In

ternet, and to
the IDP protocol of Xerox NS. Consult the appropriate manu

al 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
addresses adhere to a general structure, called a sockaddr,

described
below. However, each protocol imposes finer and more spe

cific 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 sal_en contains the total length of the structure,

which may
exceed 16 bytes. The following address values for saf_amily

are known to
the system (and additional formats are defined for possible

future implementation):

#define AF_UNIX 1 /* local to host (pipes, por

tals) */
#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_ISO 18 /* ISO protocols */

ROUTING

FreeBSD 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 usu

ally has a
hardware device associated with it, though certain inter

faces such as the
loopback interface, lo(4), do not.

The following ioctl(2) calls may be used to manipulate net

work interfaces. The ioctl() is made on a socket (typically of type

SOCK_DGRAM) in
the desired domain. Most of the requests supported in ear

lier 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[2];
short ifru_index;
int ifru_metric;
int ifru_mtu;
int ifru_phys;
int ifru_media;
caddr_t ifru_data;
int ifru_cap[2];

} 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_flags ifr_ifru.ifru_flags[0] /* flags (low

16 bits) */
#define ifr_flagshigh ifr_ifru.ifru_flags[1] /* flags (high

16 bits) */
#define ifr_metric ifr_ifru.ifru_metric /* metric */
#define ifr_mtu ifr_ifru.ifru_mtu /* mtu */
#define ifr_phys ifr_ifru.ifru_phys /* physical

wire */
#define ifr_media ifr_ifru.ifru_media /* physical

media */
#define ifr_data ifr_ifru.ifru_data /* for use by

interface */
#define ifr_reqcap ifr_ifru.ifru_cap[0] /* requested

capabilities */
#define ifr_curcap ifr_ifru.ifru_cap[1] /* current ca

pabilities */
#define ifr_index ifr_ifru.ifru_index /* 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 fam

ily and inter

face.

SIOCSIFBRDADDR Set broadcast address for protocol family

and interface.

Ioctl() 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 fam

ily and inter

face.

SIOCGIFBRDADDR Get broadcast address for protocol family

and interface.

SIOCSIFCAP Attempt to set the enabled capabilities

field for the

interface to the value of the ifrr_eqcap field

of the
ifreq structure. Note that, depending on

the particular
interface features, some capabilities may

appear hardcoded to enabled, or toggling a capability

may affect the
status of other ones. The supported capa

bilities field
is read-only, and the ifrc_urcap field is un

used by this
call.

SIOCGIFCAP Get the interface capabilities fields. The

values for

supported and enabled capabilities will be

returned in
the ifrr_eqcap and ifrc_urcap fields of the

ifreq structure, respectively.

SIOCSIFFLAGS Set interface flags field. If the interface

is marked

down, any processes currently routing pack

ets through the
interface are notified; some interfaces may

be reset so
that incoming packets are no longer re

ceived. When
marked up again, the interface is reinitial

ized.

SIOCGIFFLAGS Get interface flags.

SIOCSIFMETRIC Set interface routing metric. The metric is

used only by

user-level routers.

SIOCGIFMETRIC Get interface metric.

SIOCIFCREATE Attempt to create the specified interface.

If the inter

face name is given without a unit number the

system will
attempt to create a new interface with an

arbitrary unit
number. On successful return the ifrn_ame

field will
contain the new interface name.

SIOCIFDESTROY Attempt to destroy the specified interface.

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 char

acteristics 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 is used to speci

fy 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() identifier itself to include the total size, as

described in
ioctl().

SIOCDIFADDR This requests deletes the specified address

from the list

associated with an interface. It also uses

the
ifaliasreq structure to allow for the possi

bility of protocols allowing multiple masks or destina

tion addresses,
and also adopts the convention that specifi

cation 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 Get interface configuration list. This re

quest takes an

ifconf structure (see below) as a value-re

sult parameter.
The ifcl_en field should be initially set to

the size of
the buffer pointed to by ifcb_uf. On return

it will contain the length, in bytes, of the configura

tion list.

SIOCIFGCLONERS Get list of clonable interfaces. This re

quest takes an

ifc_lonereq structure (see below) as a value

result
parameter. The ifcrc_ount field should be set

to the
number of IFNAMSIZ sized strings that can be

fit in the
buffer pointed to by ifcrb_uffer. On return,

ifcrt_otal
will be set to the number of clonable inter

faces and the
buffer pointed to by ifcrb_uffer will be

filled with the
names of clonable interfaces aligned on IF

NAMSIZ boundaries.

/*
* Structure used in SIOCAIFCONF request.
*/
struct ifaliasreq {

char ifra_name[IFNAMSIZ]; /* if name, e.g.

"en0" */
struct sockaddr ifra_addr;
struct sockaddr ifra_broadaddr;
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 {

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 re

turned */
};

/* Structure used in SIOCIFGCLONERS request. */
struct if_clonereq {

int ifcr_total; /* total cloners (out) */
int ifcr_count; /* room for this many in us

er buffer */
char *ifcr_buffer; /* buffer for cloner names

*/

};

SEE ALSO

ioctl(2), socket(2), intro(4), config(8), routed(8),

ifnet(9)

HISTORY

The netintro manual appeared in 4.3BSD-Tahoe.

BSD June 18, 2004

BSD