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 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 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 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(.). 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 nonstandard 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 SYNTAX section of each network interface entry gives a sample specification of the related drivers for use in providing a system description to config() The DIAGNOSTICS section lists messages that may appear on the console and in the system error log file /usr/adm/syserr/syserr.<hostname> due to errors in device operation.
ADDRESSING
Associated with each protocol family is an address format. The following 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 */
#define AF_PUP 4 /* pup protocols: e.g. BSP */
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 data base with the aid of two socket-specific ioctl() 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 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 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, 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 eventual recipient (that is, 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 resources associated with it will not be 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 /dev/kmem 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 destination. 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 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, 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 may be routed through it. Most interfaces require some part of their address specified with an SIOCSIFADDR ioctl before they will allow traffic to flow through them. On interfaces 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 which provide dynamic network-link layer address mapping facilities (for example, 10Mb/s Ethernets), the entire address specified in the ioctl is used.
The following ioctl 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 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.
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;
};