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NAME
networking — introduction to networking facilities
SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
DESCRIPTION
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 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 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.
PROTOCOLS
The system currently supports the Internet protocols, the Xerox Network Systems(tm) protocols, and some
of the ISO OSI protocols. 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.
ADDRESSING
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 finer and more specific
structure, generally renaming the variant, which is discussed in the protocol family manual page alluded
to above.
struct sockaddr {
u_char sa_len;
u_char sa_family;
char sa_data[14];
};
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_UNIX 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_ISO 18 /* ISO protocols */
ROUTING
FreeBSD 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) used in earlier releases.
This facility is described in route(4).
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(2) calls may be used to manipulate network interfaces. The ioctl() is made on a
socket (typically of type SOCK_DGRAM) in the desired domain. Most of the requests supported in earlier
releases take an ifreq 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;
struct ifreq_buffer ifru_buffer;
short ifru_flags[2];
short ifru_index;
int ifru_metric;
int ifru_mtu;
int ifru_phys;
int ifru_media;
caddr_t ifru_data;
int ifru_cap[2];
} 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_buffer ifr_ifru.ifru_buffer /* user supplied buffer with its length */
#define ifr_flags ifr_ifru.ifru_flags[0] /* flags (low 16 bits) */
#define ifr_flagshigh ifr_ifru.ifru_flags[1] /* flags (high 16 bits) */
#define ifr_metric ifr_ifru.ifru_metric /* metric */
#define ifr_mtu ifr_ifru.ifru_mtu /* mtu */
#define ifr_phys ifr_ifru.ifru_phys /* physical wire */
#define ifr_media ifr_ifru.ifru_media /* physical media */
#define ifr_data ifr_ifru.ifru_data /* for use by interface */
#define ifr_reqcap ifr_ifru.ifru_cap[0] /* requested capabilities */
#define ifr_curcap ifr_ifru.ifru_cap[1] /* current capabilities */
#define ifr_index ifr_ifru.ifru_index /* interface index */
};
Ioctl() requests to obtain addresses and requests both to set and retrieve other data are still fully
supported and use the ifreq structure:
SIOCGIFADDR Get interface address for protocol family.
SIOCGIFDSTADDR Get point to point address for protocol family and interface.
SIOCGIFBRDADDR Get broadcast address for protocol family and interface.
SIOCSIFCAP Attempt to set the enabled capabilities field for the interface to the value of the
ifr_reqcap field of the ifreq structure. Note that, depending on the particular
interface features, some capabilities may appear hard-coded to enabled, or toggling a
capability may affect the status of other ones. The supported capabilities field is
read-only, and the ifr_curcap field is unused by this call.
SIOCGIFCAP Get the interface capabilities fields. The values for supported and enabled capabilities
will be returned in the ifr_reqcap and ifr_curcap fields of the ifreq structure,
respectively.
SIOCGIFDESCR Get the interface description, returned in the buffer field of ifru_buffer struct. The
user supplied buffer length should be defined in the length field of ifru_buffer struct
passed in as parameter, and the length would include the terminating nul character. If
there is not enough space to hold the interface length, no copy would be done and the
buffer field of ifru_buffer would be set to NULL. The kernel will store the buffer
length in the length field upon return, regardless whether the buffer itself is
sufficient to hold the data.
SIOCSIFDESCR Set the interface description to the value of the buffer field of ifru_buffer struct,
with length field specifying its length (counting the terminating nul).
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.
SIOCIFCREATE Attempt to create the specified interface. If the interface name is given without a unit
number the system will attempt to create a new interface with an arbitrary unit number.
On successful return the ifr_name field will contain the new interface name.
SIOCIFDESTROY Attempt to destroy the specified interface.
There are two requests that make use of a new structure:
SIOCAIFADDR 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 is used to specify all three
facets simultaneously (see below). One would use a slightly tailored version of this
struct specific to each family (replacing each sockaddr by one of the family-specific
type). Where the sockaddr itself is larger than the default size, one needs to modify
the ioctl() identifier itself to include the total size, as described in ioctl().
SIOCDIFADDR This requests 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 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.
SIOCIFGCLONERS Get list of clonable interfaces. This request takes an if_clonereq structure (see below)
as a value-result parameter. The ifcr_count field should be set to the number of
IFNAMSIZ sized strings that can be 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.
/*
* Structure used in SIOCAIFADDR request.
*/
struct ifaliasreq {
char ifra_name[IFNAMSIZ]; /* if name, e.g. "en0" */
struct sockaddr ifra_addr;
struct sockaddr ifra_broadaddr;
struct sockaddr ifra_mask;
};
/*
* 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 */
};
/* Structure used in SIOCIFGCLONERS request. */
struct if_clonereq {
int ifcr_total; /* total cloners (out) */
int ifcr_count; /* room for this many in user buffer */
char *ifcr_buffer; /* buffer for cloner names */
};
/* Structure used in SIOCGIFDESCR and SIOCSIFDESCR requests */
struct ifreq_buffer {
size_t length; /* length of the buffer */
void *buffer; /* pointer to userland space buffer */
};
SEE ALSO
ioctl(2), socket(2), intro(4), config(8), routed(8), ifnet(9)
HISTORY
The netintro manual appeared in 4.3BSD-Tahoe.
Debian January 26, 2012 NETINTRO(4)