netintro - introduction to networking facilities
#include <sys/socket.h>
#include <net/route.h>
#include <net/if.h>
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 is normally comprised 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 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 outof-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(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.
Network interfaces may be collected together into interface
groups. An
interface group is a container that can be used generically
when referring
to any interface related by some criteria. Interfaces
may be a member
of any number of interface groups. All interfaces are
members of
their interface family group by default. For example, a PPP
interface
such as ppp0 is a member of the PPP interface family group,
ppp. When an
action is performed on an interface group, such as packet
filtering by
the pf(4) subsystem, the operation will be applied to each
member interface
in the group, if supported by the subsystem. The ifconfig(8) utility
can be used to view and assign membership of an interface
to an interface
group with the group modifier.
The system currently supports the Internet protocols (IPv4
and IPv6), the
Xerox Network Systems(tm) protocols, CCITT, Appletalk, Novell's IPX protocols,
and a few others. 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.
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 a finer, more specific
structure,
generally renaming the variant, which is discussed in the
protocol family
manual page alluded to above.
struct sockaddr {
u_int8_t sa_len; /* total
length */
sa_family_t sa_family; /* address
family */
char sa_data[14]; /* actually
longer */
};
The field sa_len contains the total length 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 (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_APPLETALK 16 /* AppleTalk */
#define AF_IPX 23 /* Novell Internet Protocol
*/
#define AF_INET6 24 /* IPv6 */
#define AF_NATM 27 /* native ATM access */
The sa_data field contains the actual address value. Note
that it may be
longer than 14 bytes.
OpenBSD 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)'s used in earlier releases.
This facility is described in route(4).
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 take an ifreq
structure
pointer as their parameter. This structure is as follows:
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;
caddr_t ifru_data;
} ifr_ifru;
#define ifr_addr ifr_ifru.ifru_addr /* address
*/
#define ifr_dstaddr ifr_ifru.ifru_dstaddr /* p-to-p
peer */
#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
(overload) */
#define ifr_media ifr_ifru.ifru_metric /* media options */
#define ifr_data ifr_ifru.ifru_data /* used by
interface */
};
The supported ioctl(2) requests are:
SIOCSIFADDR struct ifreq *
Set the interface address for a protocol family.
Following the
address assignment, the ``initialization'' routine
for the interface
is called.
This call has been deprecated and superceded by the
SIOCAIFADDR
call, described below.
SIOCSIFDSTADDR struct ifreq *
Set the point-to-point address for a protocol family
and interface.
This call has been deprecated and superceded by the
SIOCAIFADDR
call, described below.
SIOCSIFBRDADDR struct ifreq *
Set the broadcast address for a protocol family and
interface.
This call has been deprecated and superceded by the
SIOCAIFADDR
call, described below.
SIOCGIFADDR struct ifreq *
Get the interface address for a protocol family.
SIOCGIFDSTADDR struct ifreq *
Get the point-to-point address for a protocol family
and interface.
SIOCGIFBRDADDR struct ifreq *
Get the broadcast address for a protocol family and
interface.
SIOCGIFDESCR struct ifreq *
Get the interface description, returned in the
ifru_data field.
SIOCSIFDESCR struct ifreq *
Set the interface description to the value of the
ifru_data
field, limited to the size of IFDESCRSIZE.
SIOCSIFFLAGS struct ifreq *
Set the interface flags. 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 struct ifreq *
Get the interface flags.
SIOCSIFMEDIA struct ifreq *
Set the interface media settings. See ifmedia(4)
for possible
values.
SIOCGIFMEDIA struct ifmediareq *
Get the interface media settings. The ifmediareq
structure is as
follows:
struct ifmediareq {
char ifm_name[IFNAMSIZ]; /* if name,
e.g. "en0" */
int ifm_current; /* current media options */
int ifm_mask; /* don't care mask
*/
int ifm_status; /* media status */
int ifm_active; /* active options */
int ifm_count; /* #entries in
ifm_ulist array */
int *ifm_ulist; /* media words */
};
See ifmedia(4) for interpreting this value.
SIOCSIFMETRIC struct ifreq *
Set the interface routing metric. The metric is
used only by user-level
routers.
SIOCGIFMETRIC struct ifreq *
Get the interface metric.
SIOCAIFADDR struct ifaliasreq *
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 structure specific
to each family
(replacing each sockaddr by one of the familyspecific type).
One should always set the length of a sockaddr, as
described in
ioctl(2).
The ifaliasreq structure is as follows:
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;
};
SIOCDIFADDR struct ifreq *
This request 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.
SIOCGIFCONF struct ifconf *
Get the 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.
Alternately, if the ifc_len passed in is set to 0,
SIOCGIFCONF
will set ifc_len to the size that ifc_buf needs to
be to fit the
entire configuration list and will not fill in the
other parameters.
This is useful for determining the exact size
that ifc_buf
needs to be in advance. Note, however, that this is
an extension
that not all operating systems support.
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 ret'd */
};
SIOCIFCREATE struct ifreq *
Attempt to create the specified interface.
SIOCIFDESTROY struct ifreq *
Attempt to destroy the specified interface.
SIOCIFGCLONERS struct if_clonereq *
Get the list of clonable interfaces. This request
takes an
if_clonereq structure pointer (see below) as a value-result parameter.
The ifcr_count field should be set to the
number of
IFNAMSIZ-sized strings that can fit in the buffer
pointed to by
ifcr_buffer. On return, ifcr_total will be set to
the number of
clonable interfaces, and the buffer pointed to by
ifcr_buffer
will be filled with the names of clonable interfaces
aligned on
IFNAMSIZ boundaries.
The if_clonereq structure is as follows:
struct if_clonereq {
int ifcr_total; /* total cloners (out) */
int ifcr_count; /* room for this many in
user buf */
char *ifcr_buffer; /* buffer for cloner
names */
};
SIOCAIFGROUP struct ifgroupreq *
Associate the interface named by ifgr_name with the
interface
group named by ifgr_group. The ifgroupreq structure
is as follows:
struct ifgroupreq {
char ifgr_name[IFNAMSIZ];
u_int ifgr_len;
union {
char ifgru_group[IFNAMSIZ];
struct ifgroup *ifgru_groups;
} ifgr_ifgru;
#define ifgr_group ifgr_ifgru.ifgru_group
#define ifgr_groups ifgr_ifgru.ifgru_groups
};
SIOCGIFGROUP struct ifgroupreq *
Retrieve the list of groups for which an interface
is a member.
The interface is named by ifgr_name. On enter, the
amount of
memory in which the group names will be written is
stored in
ifgr_len, and the group names themselves will be
written to the
memory pointed to by ifgr_groups. On return, the
amount of memory
actually written is returned in ifgr_len.
Alternately, if the ifgr_len passed in is set to 0,
SIOCGIFGROUP
will set ifgr_len to the size that ifgr_groups needs
to be to fit
the entire group list and will not fill in the other
parameters.
This is useful for determining the exact size that
ifgr_groups
needs to be in advance.
SIOCDIFGROUP struct ifgroupreq *
Remove the membership of the interface named by
ifgr_name from
the group ifgr_group.
netstat(1), ioctl(2), socket(2), inet(3), ipx(3), arp(4),
bridge(4),
ifmedia(4), inet(4), intro(4), ip(4), ip6(4), lo(4), pf(4),
tcp(4),
udp(4), hosts(5), networks(5), config(8), ifconfig(8), netstart(8),
route(8), routed(8)
The netintro manual appeared in 4.3BSD-Tahoe.
OpenBSD 3.6 September 3, 1994
[ Back ] |
