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.\" @(#)netintro.4 8.2 (Berkeley) 11/30/93
.\"
.Dd August 2, 2018
.Dt NETINTRO 4
.Os
.Sh NAME
.Nm netintro
.Nd introduction to networking facilities
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In net/route.h
.In net/if.h
.Sh 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:
.Em protocol families
(domains),
.Em protocols ,
and
.Em network interfaces .
.Pp
All network protocols are associated with a specific
.Em 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
.Xr 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.
.Pp
A protocol supports one of the socket abstractions detailed in
.Xr 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
.Dv SOCK_STREAM
abstraction may allow more than one byte of out-of-band
data to be transmitted per out-of-band message.
.Pp
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
.Em SYNOPSIS
section of each network interface entry gives a sample specification
of the related drivers for use in providing a system description to the
.Xr config 1
program.
.Pp
The
.Em DIAGNOSTICS
section lists messages which may appear on the console
and/or in the system error log,
.Pa /var/log/messages
(see
.Xr syslogd 8 ) ,
due to errors in device operation.
.Sh PROTOCOLS
The system currently supports the Internet protocols.
Raw socket interfaces are provided to the
.Tn IP
protocol layer of the Internet.
Consult the appropriate manual pages in this section for more
information regarding the support for a protocol.
.Sh 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.
.Bd -literal -offset indent
struct sockaddr {
u_char sa_len;
u_char sa_family;
char sa_data[14];
};
.Ed
.Pp
The field
.Ar sa_len
contains the total length of the of the structure, which may exceed 16 bytes.
The following address values for
.Ar sa_family
are known to the system
(and additional formats are defined for possible future implementation):
.Bd -literal
#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. */
.Ed
.Sh ROUTING
.Ux
provides some packet routing facilities.
The kernel maintains a routing information database, which
is used in selecting the appropriate network interface when
transmitting packets.
.Pp
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
.Xr ioctl 2
used in earlier releases.
.Pp
This facility is described in
.Xr route 4 .
.Sh 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,
.Xr lo 4 ,
do not.
.Pp
The following
.Xr ioctl 2
calls may be used to manipulate network interfaces.
The
.Xr ioctl 2
is made on a socket (typically of type
.Dv SOCK_DGRAM )
in the desired domain.
Most of the requests supported in earlier releases
take an
.Ar ifreq
structure as its parameter.
This structure has the form
.Bd -literal
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 */
};
.Ed
.Pp
Calls which are now deprecated are:
.Bl -tag -width SIOCGIFBRDADDR
.It Dv SIOCSIFADDR
Set interface address for protocol family.
Following the address assignment, the ``initialization'' routine for
the interface is called.
.It Dv SIOCSIFDSTADDR
Set point to point address for protocol family and interface.
.It Dv SIOCSIFBRDADDR
Set broadcast address for protocol family and interface.
.El
.Pp
.Xr ioctl 2
requests to obtain addresses and requests both to set and
retrieve other data are still fully supported
and use the
.Ar ifreq
structure:
.Bl -tag -width SIOCGIFBRDADDR
.It Dv SIOCGIFADDR
Get interface address for protocol family.
.It Dv SIOCGIFDSTADDR
Get point to point address for protocol family and interface.
.It Dv SIOCGIFBRDADDR
Get broadcast address for protocol family and interface.
.It Dv 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.
.It Dv SIOCGIFFLAGS
Get interface flags.
.It Dv SIOCSIFMETRIC
Set interface routing metric.
The metric is used only by user-level routers.
.It Dv SIOCGIFMETRIC
Get interface metric.
.It Dv SIOCGIFINDEX
Get the interface index and populate ifr_index.
.El
.Pp
There are two requests that make use of a new structure:
.Bl -tag -width SIOCGIFBRDADDR
.It Dv 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,
.Ar 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
.Xr ioctl 2
identifier itself to include the total size, as described in
.Xr ioctl 2 .
.It Dv SIOCDIFADDR
This requests deletes the specified address from the list
associated with an interface.
It also uses the
.Ar 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.
.It Dv SIOCGIFALIAS
This request provides means to get additional addresses together
with netmask and broadcast/destination from an interface.
It also uses the
.Ar ifaliasreq
structure.
.El
.Pp
Request making use of the
.Ar ifconf
structure:
.Bl -tag -width SIOCGIFBRDADDR
.It Dv SIOCGIFCONF
Get interface configuration list.
This request takes an
.Ar ifconf
structure (see below) as a value-result parameter.
The
.Ar ifc_len
field should be initially set to the size of the buffer
pointed to by
.Ar ifc_buf .
On return it will contain the length, in bytes, of the
configuration list.
.El
.Bd -literal
/*
* 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;
};
.Ed
.Pp
.Bd -literal
/*
* 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 */
};
.Ed
.Sh SEE ALSO
.Xr config 1 ,
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr intro 4 ,
.Xr routed 8
.Sh HISTORY
The
.Nm netintro
manual appeared in
.Bx 4.3 Tahoe .