INTRO(4N) INTRO(4N)
NAME
networking - introduction to networking facilities
SYNOPSIS
#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 (domains), protocols, and network interfaces. Entries describing a protocol family are marked ``4F,''
while entries describing protocol use are marked ``4P.'' Hardware support for network interfaces are 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 implemen-
tation 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 imple-
mentations 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 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, interact-
ing with the actual transport hardware. An interface may support one or more protocol families and/or address formats. The SYNOPSIS sec-
tion of each network interface entry gives a sample specification of the related drivers for use in providing a system description to the
/sys/conf/config script. The DIAGNOSTICS section lists messages which may appear on the console and/or in the system error log,
/usr/adm/messages (see syslogd(8)), due to errors in device operation.
PROTOCOLS
The system currently supports the DARPA Internet protocols and the Xerox Network Systems(tm) protocols. Raw socket interfaces are provided
to the IP protocol layer of the DARPA Internet, to the IMP link layer(1822), 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. The following address formats are used by the system (and additional formats
are defined for possible future implementation):
#define AF_UNIX 1 /* local to host (pipes, portals) */
#define AF_INET 2 /* internetwork: UDP, TCP, etc. */
#define AF_IMPLINK 3 /* arpanet imp addresses */
#define AF_PUP 4 /* pup protocols: e.g. BSP */
#define AF_NS 6 /* Xerox NS protocols */
#define AF_HYLINK 15 /* NSC Hyperchannel */
ROUTING
The network facilities provided limited packet routing. A simple set of data structures comprise 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 ioctl(2) commands, SIOCADDRT and
SIOCDELRT. The commands allow the addition and deletion of a single routing table entry, respectively. Routing table manipulations may
only be carried out by super-user.
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 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) */
#define RTF_DYNAMIC 0x10 /* created dynamically (by redirect) */
Routing table entries come in three flavors: for a specific host, for all hosts on a specific network, for any destination not matched by
entries of the first two types (a wildcard route). When the system is booted and addresses are assigned to the network interfaces, each
protocol family installs a routing table entry for each interface when it is ready for traffic. Normally the protocol specifies the route
through each interface as 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 is requested to
address the packet to the gateway listed in the routing entry (i.e. the packet is forwarded).
Routing table entries installed by a user process may not 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 non-zero), the routing entry will be marked down and
removed from the routing table, but the resources associated with it will not be reclaimed until all references to it are released. The
routing code returns EEXIST if requested to duplicate an existing entry, ESRCH if requested to delete a non-existent entry, or ENOBUFS if
insufficient resources were available to install a new route. User processes read the routing tables through the /dev/kmem device. The
rt_use field contains the number of packets sent along the route.
When routing a packet, the kernel will first attempt to find a route to the destination host. Failing that, a search is made for a route
to the network of the destination. Finally, any route to a default (``wildcard'') gateway is chosen. If multiple routes are present in
the table, the first route found will be used. If no entry is found, the destination is declared to be unreachable.
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 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 calls may be used to manipulate network interfaces. The ioctl is made on a socket (typically of type SOCK_DGRAM) in
the desired domain. Unless specified otherwise, the request takes an ifrequest 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;
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_data ifr_ifru.ifru_data /* for use by interface */
};
SIOCSIFADDR
Set interface address for protocol family. Following the address assignment, the ``initialization'' routine for the interface is
called.
SIOCGIFADDR
Get interface address for protocol family.
SIOCSIFDSTADDR
Set point to point address for protocol family and interface.
SIOCGIFDSTADDR
Get point to point address for protocol family and interface.
SIOCSIFBRDADDR
Set broadcast address for protocol family and interface.
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 noti-
fied; some interfaces may be reset so that incoming packets are no longer received. When marked up again, the interface is reini-
tialized.
SIOCGIFFLAGS
Get interface flags.
SIOCSIFMETRIC
Set interface routing metric. The metric is used only by user-level routers.
SIOCGIFMETRIC
Get interface metric.
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 contain the length, in bytes, of
the configuration list.
/*
* 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 returned */
};
SEE ALSO
socket(2), ioctl(2), intro(4), config(8), routed(8C)
4.2 Berkeley Distribution August 1, 1987 INTRO(4N)