man/man4/netintro.4

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.\"     @(#)netintro.4	8.2 (Berkeley) 11/30/93
.\" $FreeBSD$
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.Dd October 14, 2020
.Dt NETINTRO 4
.Os
.Sh NAME
.Nm networking
.Nd introduction to networking facilities
.Sh SYNOPSIS
.In sys/types.h
.In sys/time.h
.In sys/socket.h
.In net/if.h
.In net/route.h
.Sh 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:
.Em protocol families
(domains),
.Em protocols ,
and
.Em network interfaces .
.Pp
All network protocols are associated with a specific
.Em 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
.Xr 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.
.Pp
A protocol supports one of the socket abstractions detailed in
.Xr 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
.Dv SOCK_STREAM
abstraction may allow more than one byte of out-of-band
data to be transmitted per out-of-band message.
.Pp
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
.Xr config 8
program.
The DIAGNOSTICS section lists messages which may appear on the console
and/or in the system error log,
.Pa /var/log/messages
(see
.Xr syslogd 8 ) ,
due to errors in device operation.
.Sh PROTOCOLS
The system currently supports the
Internet
protocols, the Xerox Network Systems(tm) protocols,
and some of the
.Tn ISO OSI
protocols.
Raw socket interfaces are provided to the
.Tn IP
protocol
layer of the
Internet, and to the
.Tn IDP
protocol of Xerox
.Tn NS .
Consult the appropriate manual pages in this section for more
information regarding the support for each protocol family.
.Sh 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.
.Bd -literal -offset indent
struct sockaddr {
    u_char	sa_len;
    u_char	sa_family;
    char	sa_data[14];
};
.Ed
.Pp
The field
.Va sa_len
contains the total length of the structure,
which may exceed 16 bytes.
The following address values for
.Va sa_family
are known to the system
(and additional formats are defined for possible future implementation):
.Bd -literal
#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 */
.Ed
.Sh ROUTING
.Fx
provides some packet routing facilities.
The kernel maintains a routing information database, which
is used in selecting the appropriate network interface when
transmitting packets.
.Pp
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
.Xr ioctl 2
used in earlier releases.
.Pp
This facility is described in
.Xr route 4 .
.Sh 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,
.Xr lo 4 ,
do not.
.Pp
The following
.Xr ioctl 2
calls may be used to manipulate network interfaces.
The
.Fn ioctl
is made on a socket (typically of type
.Dv SOCK_DGRAM )
in the desired domain.
Most of the requests supported in earlier releases
take an
.Vt ifreq
structure as its parameter.
This structure has the form
.Bd -literal
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 */
};
.Ed
.Pp
.Fn Ioctl
requests to obtain addresses and requests both to set and
retrieve other data are still fully supported
and use the
.Vt ifreq
structure:
.Bl -tag -width SIOCGIFBRDADDR
.It Dv SIOCGIFADDR
Get interface address for protocol family.
.It Dv SIOCGIFDSTADDR
Get point to point address for protocol family and interface.
.It Dv SIOCGIFBRDADDR
Get broadcast address for protocol family and interface.
.It Dv SIOCSIFCAP
Attempt to set the enabled capabilities field for the interface
to the value of the
.Va ifr_reqcap
field of the
.Vt 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
.Va ifr_curcap
field is unused by this call.
.It Dv SIOCGIFCAP
Get the interface capabilities fields.
The values for supported and enabled capabilities will be returned in the
.Va ifr_reqcap
and
.Va ifr_curcap
fields of the
.Vt ifreq
structure, respectively.
.It Dv SIOCGIFDESCR
Get the interface description, returned in the
.Va buffer
field of
.Va ifru_buffer
struct.
The user supplied buffer length should be defined in the
.Va length
field of
.Va 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
.Va buffer
field of
.Va ifru_buffer
would be set to NULL.
The kernel will store the buffer length in the
.Va length
field upon return, regardless whether the buffer itself is
sufficient to hold the data.
.It Dv SIOCSIFDESCR
Set the interface description to the value of the
.Va buffer
field of
.Va ifru_buffer
struct, with
.Va length
field specifying its length (counting the terminating nul).
.It Dv 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.
.It Dv SIOCGIFFLAGS
Get interface flags.
.It Dv SIOCSIFMETRIC
Set interface routing metric.
The metric is used only by user-level routers.
.It Dv SIOCGIFMETRIC
Get interface metric.
.It Dv 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
.Va ifr_name
field will contain the new interface name.
.It Dv SIOCIFDESTROY
Attempt to destroy the specified interface.
.El
.Pp
There are two requests that make use of a new structure:
.Bl -tag -width SIOCGIFBRDADDR
.It Dv 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
.Fn ioctl
identifier itself to include the total size, as described in
.Fn ioctl .
.It Dv SIOCDIFADDR
This requests deletes the specified address from the list
associated with an interface.
It also uses the
.Vt 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.
.It Dv SIOCGIFALIAS
This request provides means to get additional addresses
together with netmask and broadcast/destination from an
interface.
It also uses the
.Vt ifaliasreq
structure.
.It Dv SIOCGIFCONF
Get interface configuration list.
This request takes an
.Vt ifconf
structure (see below) as a value-result parameter.
The
.Va ifc_len
field should be initially set to the size of the buffer
pointed to by
.Va ifc_buf .
On return it will contain the length, in bytes, of the
configuration list.
.It Dv SIOCIFGCLONERS
Get list of clonable interfaces.
This request takes an
.Vt if_clonereq
structure (see below) as a value-result parameter.
The
.Va ifcr_count
field should be set to the number of
.Dv IFNAMSIZ
sized strings that can be fit in the buffer pointed to by
.Va ifcr_buffer .
On return,
.Va ifcr_total
will be set to the number of clonable interfaces and the buffer pointed
to by
.Va ifcr_buffer
will be filled with the names of clonable interfaces aligned on
.Dv IFNAMSIZ
boundaries.
.El
.Bd -literal
/*
* 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;
};
.Ed
.Bd -literal
/*
* 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 */
};
.Ed
.Bd -literal
/* 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 */
};
.Ed
.Bd -literal
/* Structure used in SIOCGIFDESCR and SIOCSIFDESCR requests */
struct ifreq_buffer {
        size_t  length;         /* length of the buffer */
        void   *buffer;         /* pointer to userland space buffer */
};
.Ed
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr intro 4 ,
.Xr config 8 ,
.Xr routed 8 ,
.Xr ifnet 9
.Sh HISTORY
The
.Nm netintro
manual appeared in
.Bx 4.3 tahoe .