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.