intro(7N) —

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 7 is divided 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(3I) 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(3I). 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, 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. The DIAGNOSTICS section lists messages which may appear on the console and/or in the system error log, /usr/adm/messages (see syslogd(1M)), due to errors in device operation.

All network-related ioctls are in the stream I_STR format (see streamio(7)).

Protocols

The system currently supports the DARPA Internet protocols. Raw socket interfaces are provided to the IP protocol layer of the DARPA Internet. Consult the appropriate manual entries 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):

#defineAF_INET2/* internetwork: UDP, TCP, etc. */

Routing

The network facilities provide 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 database with the aid of two socket-specific ioctl(2) commands, SIOCADDRT and SIOCDELRT. These commands allow the addition and deletion, respectively, of a single routing table entry. Routing table manipulations may only be carried out by superuser.

A routing table entry has the following form, as defined in <net/route.h>;

struct rtentry {
u_longrt_hash;
structsockaddr rt_dst;
structsockaddr rt_gateway;
shortrt_flags;
shortrt_refcnt;
u_longrt_use;
structifnet *rt_ifp;
};

with rt_flags defined from,

#defineRTF_UP0x1/* route usable */
#defineRTF_GATEWAY0x2 /*destination is a gateway */
#defineRTF_HOST0x4/* host entry (net otherwise) */
#defineRTF_DYNAMIC0x10/* created dynamically */
/* (by redirect) */

There are three types of routing table entries: 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 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 nonzero), 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(7), 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 ifreq structure as its parameter. This structure has the following form:

structifreq{
#define IFNAMSIZ16
char ifr_name[IFNAMSIZ]; /* if name, e.g. "ec0" */
union{
structsockaddr ifru_addr;
structsockaddr ifru_dstaddr;
structsockaddr ifru_broadaddr;
shortifru_flags;
intifru_metric;
intifru_lindex;
caddr_tifru_data;
structifstats ifru_stats;
structhwaddr ifru_hwaddr;
}ifr_ifru;

#define ifr_addrifr_ifru.ifru_addr/* address */
#define ifr_dstaddrifr_ifru.ifru_dstaddr /* other end of */
/* p-to-p link */
#define ifr_broadaddrifr_ifru.ifru_broadaddr /* broadcast */
/* address */
#define ifr_flagsifr_ifru.ifru_flags/* flags */
#define ifr_metricifr_ifru.ifru_metric/* routing metric */
#define ifr_lindexifr_ifru.ifru_lindex/* link index for if */
/* name set */
#define ifr_dataifr_ifru.ifru_data/* for use by */
/* interface */
#define ifr_statsifr_ifru.ifru_stats/* interface */
/* statistics */
#define ifr_hwaddrifr_ifru.ifru_hwaddr /* interface physical */
/* address */
};

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.

SIOCGPHYSADDR
Retrieve an interface physical address.

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.

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.

This ioctl is not 100 percent compatible with the BSD implementation. In all cases, ioctl calls to the TCP/IP modules must conform to the I_STR format described in streamio(7).

/*
* Structure used in SIOCGIFCONF request.
* Used to retrieve interface configuration
* for machine (useful for programs which
* must know all accessible networks).
*/
structifconf{
intifc_len;/* size of associated buffer */
union{
struct ifreq ifcu_req[1];
}ifc_ifcu;

#define ifc_bufifc_ifcu.ifcu_buf/* buffer address */
#define ifc_reqifc_ifcu.ifcu_req/* array of structures */
/* returned */
};

Rather than using a pointer to a buffer, the ifconf structure must be allocated with sufficient space to hold all of the data desired. The ifc_len member must contain the amount of space allocated for the data. The ifcu_req member is an array that indexes all of the data.

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