NETINTRO(4) BSD Kernel Interfaces Manual NETINTRO(4)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 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 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 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
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. Cer‐
tain semantics of the basic socket abstractions are protocol specific.
All protocols are expected to support the basic model for their particu‐
lar 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 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(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_ISO 18 /* ISO protocols */
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 inter‐
faces. 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_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:
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 inter‐
face.
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 struc‐
ture:
SIOCGIFADDR Get interface address for protocol family.
SIOCGIFDSTADDR Get point to point address for protocol family and inter‐
face.
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.
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 sepa‐
rate calls to set destination or broadcast addresses, or
network masks (now an integral feature of multiple proto‐
cols) a separate structure, ifaliasreq, is used to spec‐
ify all three facets simultaneously (see below). One
would use a slightly tailored version of this struct spe‐
cific 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 pro‐
tocols 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.
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 con‐
tain 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 ALSOconfig(1), ioctl(2), socket(2), intro(4), routed(8)HISTORY
The netintro manual appeared in 4.3BSD-Tahoe.
BSD September 7, 2006 BSD
