intro(4n)intro(4n)Name
networking - introduction to networking facilities
Syntax
#include <sys/socket.h>
#include <net/route.h>
#include <net/if.h>
Description
This section briefly describes the networking facilities available in
the system. Documentation in this part of Section 4 is broken up into
three areas: protocol families, protocols, and ``network interfaces''.
Entries describing a protocol family are marked ``4f'', while entries
describing protocol use are marked ``4p''. Hardware support for net‐
work interfaces is found among the standard ``4'' entries.
All network protocols are associated with a specific protocol family.
A protocol family provides basic services to the protocol implementa‐
tion to allow it to function within a specific network environment.
These services can include packet fragmentation and reassembly, rout‐
ing, addressing, and basic transport. A protocol family can support
multiple methods of addressing, though the current protocol implementa‐
tions do not. A protocol family normally comprises a number of proto‐
cols, one per socket type. It is not required that a protocol family
support all socket types. A protocol family can contain multiple pro‐
tocols supporting the same socket abstraction.
A protocol supports one of the socket abstractions detailed in A spe‐
cific protocol can 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-spe‐
cific. All protocols are expected to support the basic model for their
particular socket type, but may, in addition, provide nonstandard
facilities or extensions to a mechanism. For example, a protocol sup‐
porting 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 inter‐
faces make up the lowest layer of the networking subsystem, interacting
with the actual transport hardware. An interface may support one or
more protocol families or address formats. The SYNTAX section of each
network interface entry gives a sample specification of the related
drivers for use in providing a system description to The DIAGNOSTICS
section lists messages that may appear on the console and in the system
error log file due to errors in device operation.
Addressing
Associated with each protocol family is an address format. The follow‐
ing address formats are used by the system:
#define AF_UNIX 1 /* local to host (pipes, portals) */
#define AF_INET 2 /* internetwork: UDP, TCP, etc. */
#define AF_IMPLINK 3 /* arpanet imp addresses */
Routing
The network facilities provide limited packet routing. A simple set of
data structures make up a ``routing table'' used in selecting the
appropriate network interface when transmitting packets. This table
contains a single entry for each route to a specific network or host.
A user process, the routing daemon, maintains this data base with the
aid of two socket-specific commands, SIOCADDRT and SIOCDELRT. The com‐
mands allow the addition and deletion of a single routing table entry.
Routing table manipulations can only be carried out by superuser.
A routing table entry has the following form, as defined in
<net/route.h>:
struct rtentry {
u_long rt_hash;
struct sockaddr rt_dst;
struct sockaddr rt_gateway;
short rt_flags;
short rt_refcnt;
u_long rt_use;
struct rtentry *rt_next;
struct ifnet *rt_ifp;
};
with rt_flags defined from,
#define RTF_UP 0x1 /* route usable */
#define RTF_GATEWAY 0x2 /* destination is a gateway */
#define RTF_HOST 0x4 /* host entry (net otherwise) */
Routing table entries come in three types: for a specific host, for all
hosts on a specific network, and for any destination not matched by
entries of the first two types (a wildcard route). When the system is
booted, each network interface autoconfigured installs a routing table
entry when it wishes to have packets sent through it. Normally, the
interface specifies the route through it is a ``direct'' connection to
the destination host or network. If the route is direct, the transport
layer of a protocol family usually requests the packet be sent to the
same host specified in the packet. Otherwise, the interface may be
requested to address the packet to an entity different from the even‐
tual recipient (that is, the packet is forwarded).
Routing table entries installed by a user process cannot specify the
hash, reference count, use, or interface fields; these are filled in by
the routing routines. If a route is in use when it is deleted
(rt_refcnt is nonzero), the resources associated with it are not
reclaimed until further references to it are released.
The routing code returns EEXIST if requested to duplicate an existing
entry, ESRCH if requested to delete a nonexistent entry, or ENOBUFS if
insufficient resources were available to install a new route.
User processes read the routing tables through the device.
The rt_use field contains the number of packets sent along the route.
This value is used to select among multiple routes to the same destina‐
tion. When multiple routes to the same destination exist, the least
used route is selected.
A wildcard routing entry is specified with a zero destination address
value. Wildcard routes are used only when the system fails to find a
route to the destination host and network. The combination of wildcard
routes and routing redirects can provide an economical mechanism for
routing traffic.
Interfaces
Each network interface in a system corresponds to a path through which
messages can be sent and received. A network interface usually has a
hardware device associated with it, though certain interfaces such as
the loopback interface, do not.
At boot time, each interface that has underlying hardware support makes
itself known to the system during the autoconfiguration process. Once
the interface has acquired its address, it is expected to install a
routing table entry so that messages can be routed through it. Most
interfaces require some part of their address specified with an SIOCSI‐
FADDR ioctl before they allow traffic to flow through them. On inter‐
faces where the network-link layer address mapping is static, only the
network number is taken from the ioctl; the remainder is found in a
hardware-specific manner. On interfaces that provide dynamic network-
link layer address mapping facilities (for example, 10Mb/s Ethernets),
the entire address specified in the ioctl is used.
The following calls may be used to manipulate network interfaces.
Unless specified otherwise, the request takes an ifrequest structure as
its parameter. This structure has the form:
struct ifreq {
char ifr_name[16]; /* name of interface (e.g. "ec0") */
union {
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
short ifru_flags;
} ifr_ifru;
#define ifr_addr ifr_ifru.ifru_addr /* address */
#define ifr_dstaddr ifr_ifru.ifru_dstaddr /* end of p-to-p link */
#define ifr_flags ifr_ifru.ifru_flags /* flags */
};
SIOCSIFADDR
Set interface address. Following the address assignment, the
``initialization'' routine for the interface is called.
SIOCGIFADDR
Get interface address.
SIOCSIFDSTADDR
Set point-to-point address for interface.
SIOCGIFDSTADDR
Get point-to-point address for interface.
SIOCSTATE
Read or set ownership and state of a device.
SIOCSIFFLAGS
Set interface flags field. If the interface is marked down, any
processes currently routing packets through the interface are
notified.
SIOCGIFFLAGS
Get interface flags.
SIOCGIFCONF
Get interface configuration list. This request takes an ifconf
structure (see SIOCSIFBRDADDR) 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.
SIOCGIFNETMASK
Get network address mask.
SIOCSIFNETMASK
Set network address mask.
SIOCGIFBRDADDR
Get broadcast address associated with network interface.
SIOCSIFBRDADDR
Set broadcast address associated with network interface.
/*
* 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 */
};
The following is the structure used in an SIOCSTATE request to
set device state and ownership.
struct ifstate {
char ifr_name[IFNAMSIZ]; /* if name, e.g. "dmv0" */
u_short if_family; /* current family ownership */
u_short if_next_family; /* next family ownership */
u_short if_mode:3, /* mode of device */
if_ustate:1, /* user requested state */
if_nomuxhdr:1, /* if set, omit mux header */
if_dstate:4, /* current state of device */
if_xferctl:1, /* xfer control to nxt family */
if_rdstate:1, /* read current state */
if_wrstate:1 /* set current state */
if_reserved:4;
};
See Alsosocket(2), ioctl(2), intro(4), config(8), routed(8c)intro(4n)