NETINTRO(4) BSD Programmer's Manual NETINTRO(4)NAME
networking - introduction to 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 4 is bro-
ken 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 ser-
vices may include packet fragmentation and reassembly, routing, address-
ing, and basic transport. A protocol family may support multiple methods
of addressing, though the current protocol implementations do not. A
protocol family is normally composed of 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 support-
ing 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 ex-
plicitly when creating a socket. Protocols normally accept only one type
of address format, usually determined by the addressing structure inher-
ent 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 ex-
tensions 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 pro-
tocol families and/or address formats. The SYNOPSIS section of each net-
work interface entry gives a sample specification of the related 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 (see syslog.conf(5) and syslog(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 in-
terfaces 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 be-
low. 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. Additional formats are defined for possible future
implementation:
#define AF_LOCAL 1 /* local to host (pipes, portals) */
#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
The operating system provides packet forwarding facilities by maintaining
a ``forwarding information database'', or ``forwarding table''. This
database is used in selecting the appropriate network interface when
transmitting packets.
One or more co-operating user processes (see gated(8)and routed(8))
maintain this database by sending messages over a special type of socket,
the ``routing socket'' as described in route(4). This supplants the
fixed size ioctl(2) calls (SIOCADDRT and SIOCDELRT) used in earlier re-
leases.
The term ``routing'' technically refers to the use of routing protocols
to discover ``routes'' to remote networks and hosts. The term
``forwarding'' refers to the process of sending a packet on towards its
eventual destination. An entry in the operating system's forwarding
table can be referred to as a ``forwarding table entry'' or a ``route''.
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 inter-
faces. The ioctl(2) is made on a socket (typically of type SOCK_DGRAM) in
the desired domain. Descriptions of the ioctl(2) calls are grouped by
the structure used as an argument. All the listed ioctl(2) calls identi-
fy interfaces by name (e.g. ``ef0'').
struct ifreq
The struct ifreq is used to identify an interface by name (e.g. ``we0'')
and pass one attribute of the configuration of an interface. The layout
of the struct ifreq is:
struct ifreq {
#define IFNAMSIZ 16
char ifr_name[IFNAMSIZ]; /* if name, e.g. "we0" */
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 /* other 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_mtu ifr_ifru.ifru_metric /* MTU */
#define ifr_data ifr_ifru.ifru_data /* for use by interface */
};
SIOCDIFADDR Delete interface address. The specified address from the
list associated with the interface is deleted. Specifica-
tion of a default address for a protocol family is inter-
preted to mean that the primary (first) address in this
protocol family for the specified interface should be
deleted. Uses the ifr_addr field of the struct ifreq, or
a protocol family specific structure.
SIOCGIFFLAGS Get interface flags. Uses the ifr_flags field of the
struct ifreq.
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. Uses the
ifr_flags field of the struct ifreq.
SIOCGIFMETRIC Get interface metric. Uses the ifr_metric field of the
struct ifreq.
SIOCSIFMETRIC Set interface routing metric. The metric is used only by
user-level routers. Uses the ifr_metric field of the
struct ifreq.
SIOCSIFADDR Set interface address. The use of SIOCSIFADDR is depre-
cated for all address families except AF_LINK where it is
used to set the interface type using a sockaddr_dl struc-
ture as defined in <net/if_dl.h> Uses the ifr_addr field
of the struct ifreq.
SIOCGIFMTU Get the interface MTU. Uses the ifr_mtu field of the
struct ifreq.
SIOCSIFMTU Set the interface MTU. Uses the ifr_mtu field of the
struct ifreq. Not all interfaces support SIOCSIFMTU.
The following calls use the ifreq structure, but are deprecated. They
are not capable of distinguishing multiple addresses in the same protocol
assigned to a given interface. Their functionality is provided by the
sysctl(3) and getifaddrs(3) subroutines.
SIOCGIFADDR Get interface address for protocol family. Uses the
ifr_addr field of the struct ifreq.
SIOCGIFBRDADDR Get broadcast address for protocol family and interface.
Uses the ifr_broadaddr field of the struct ifreq.
SIOCGIFDSTADDR Get point to point destination address for protocol fami-
ly and interface. Uses the ifr_dstaddr field of the
struct ifreq.
SIOCGIFNETMASK Get the network mask for the specified interface. Uses
the ifr_addr field of the struct ifreq.
The following calls use the ifreq structure, but are deprecated. They
are not capable of distinguishing multiple addresses in the same protocol
assigned to a given interface. Their functionality is provided by the
SIOCAIFADDR and SIOCDIFADDR ioctl(2) system calls.
SIOCSIFBRDADDR Set broadcast address for protocol family and interface.
Uses the ifr_broadaddr field of the struct ifreq.
SIOCSIFDSTADDR Set point to point destination address for protocol fami-
ly and interface. Uses the ifr_dstaddr field of the
struct ifreq.
SIOCGIFNETMASK Set the network mask for the specified interface. Uses
the ifr_addr field of the struct ifreq.
struct ifaliasreq
The ifaliasreq structure is a model of the structure used to set the ad-
dress configuration of an interface. The actual structure used will con-
tain sockaddr structures appropriate to the protocol family being config-
ured. Address families with sockaddr structures smaller or larger than
the default will have alternate version of the ioctl(2) identifier encod-
ed with different lengths (e.g. SIOCAIFADDR_ISO).
struct ifaliasreq {
char ifra_name[IFNAMSIZ]; /* if name, e.g. "en0" */
struct sockaddr ifra_addr;
struct sockaddr ifra_broadaddr;
struct sockaddr ifra_mask;
};
The ioctl(2) call that makes use of this structure is:
SIOCAIFADDR Add or change the address of an interface. Protocol fami-
lies that support multiple protocol addresses per interface
allow an additional addresses (referred to as ``aliases'')
to be assigned to and interface when the specified address
does not match an existing address assigned to the inter-
face. When the specified address matches an existing ad-
dress the characteristics of that address are changed. If
the default address for the protocol family is specified,
the characteristics of the primary address are changed.
struct ifconf
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 */
};
The following call uses the struct ifconf.
SIOCGIFCONF This interface is deprecated, the interface list is avail-
able via sysctl(3) and getifaddrs(3).
Get interface configuration list. This request takes an
ifconf structure 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. If the buffer is not
large enough to hold the complete interface list, the errno
variable will be set to EFAULT.
SEE ALSOsocket(2), ioctl(2), getifaddrs(3), sysctl(3), inet(4), intro(4),
iso(4), localdomain(4), ns(4), route(4), config(8), gated(8),
ifconfig(8), routed(8)HISTORY
The netintro manual entry appeared in 4.3BSD-Tahoe.
4.2 Berkeley Distribution November 30, 1993 5
