.\"	$OpenBSD: netintro.4,v 1.55 2024/09/23 20:38:49 kn Exp $
.\"	$NetBSD: netintro.4,v 1.4 1995/10/19 08:03:40 jtc Exp $
.\"
.\" Copyright (c) 1983, 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)netintro.4	8.2 (Berkeley) 11/30/93
.\"
.Dd $Mdocdate: September 23 2024 $
.Dt NETINTRO 4
.Os
.Sh NAME
.Nm netintro
.Nd introduction to networking facilities
.Sh SYNOPSIS
.In sys/socket.h
.In net/route.h
.In net/if.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.
.Pp
Network interfaces may be collected together into interface groups.
An interface group is a container that can be used generically when
referring to any interface related by some criteria.
When an action is performed on an interface group, such as packet
filtering by the
.Xr pf 4
subsystem, the operation will be applied to each member interface in the
group, if supported by the subsystem.
The
.Xr ifconfig 8
utility can be used to view and assign membership of an interface to an
interface group with the
.Cm group
modifier.
.Sh PROTOCOLS
The system currently supports the
Internet protocols (IPv4 and IPv6),
MPLS,
and a few others.
Raw socket interfaces are provided to the IP protocol
layer of the
Internet.
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
.Vt sockaddr ,
described below.
However, each protocol imposes a finer, 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_int8_t	sa_len;		/* total length */
	sa_family_t	sa_family;	/* address family */
	char		sa_data[14];	/* actually longer */
};
.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 */
#define AF_INET		2	/* internetwork: UDP, TCP, etc. */
#define AF_INET6	24	/* IPv6 */
#define AF_MPLS		33	/* Multiprotocol Label Switching */
.Ed
.Pp
The
.Va sa_data
field contains the actual address value.
Note that it may be longer than 14 bytes.
.Sh ROUTING
.Ox
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 Ns s
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
.Xr ioctl 2
is made on a socket (typically of type
.Dv SOCK_DGRAM )
in the desired domain.
Most of the requests
take an
.Vt ifreq
structure pointer as their parameter.
This structure is as follows:
.Bd -literal
struct	ifreq {
	char	ifr_name[IFNAMSIZ];		/* if name, e.g. "en0" */
	union {
		struct	sockaddr	ifru_addr;
		struct	sockaddr	ifru_dstaddr;
		struct	sockaddr	ifru_broadaddr;
		short			ifru_flags;
		int			ifru_metric;
		int64_t			ifru_vnetid;
		uint64_t		ifru_media;
		caddr_t			ifru_data;
		unsigned int		ifru_index;
	} 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_flags	ifr_ifru.ifru_flags	/* flags */
#define	ifr_metric	ifr_ifru.ifru_metric	/* metric */
#define	ifr_mtu		ifr_ifru.ifru_metric	/* mtu (overload) */
#define	ifr_hardmtu	ifr_ifru.ifru_metric	/* hardmtu (overload) */
#define	ifr_media	ifr_ifru.ifru_media	/* media options */
#define	ifr_rdomainid	ifr_ifru.ifru_metric	/* VRF instance (overload) */
#define ifr_vnetid	ifr_ifru.ifru_vnetid	/* Virtual Net Id */
#define ifr_ttl		ifr_ifru.ifru_metric	/* tunnel TTL (overload) */
#define ifr_df		ifr_ifru.ifru_metric	/* tunnel DF (overload) */
#define	ifr_data	ifr_ifru.ifru_data	/* for use by interface */
#define ifr_index	ifr_ifru.ifru_index	/* interface index */
#define ifr_llprio	ifr_ifru.ifru_metric	/* link layer priority */
#define ifr_hdrprio	ifr_ifru.ifru_metric	/* header prio field config */
#define ifr_pwe3	ifr_ifru.ifru_metric	/* PWE3 type */
};
.Ed
.Pp
The supported
.Xr ioctl 2
requests are:
.Bl -tag -width Ds
.It Dv SIOCSIFADDR Fa "struct ifreq *"
Set the interface address for a protocol family.
Following the address assignment, the
.Dq initialization
routine for the
interface is called.
.Pp
This call has been deprecated and superseded by the
.Dv SIOCAIFADDR
call, described below.
.It Dv SIOCSIFDSTADDR Fa "struct ifreq *"
Set the point-to-point address for a protocol family and interface.
.Pp
This call has been deprecated and superseded by the
.Dv SIOCAIFADDR
call, described below.
.It Dv SIOCSIFBRDADDR Fa "struct ifreq *"
Set the broadcast address for a protocol family and interface.
.Pp
This call has been deprecated and superseded by the
.Dv SIOCAIFADDR
call, described below.
.It Dv SIOCGIFADDR Fa "struct ifreq *"
Get the interface address for a protocol family.
.It Dv SIOCGIFDSTADDR Fa "struct ifreq *"
Get the point-to-point address for a protocol family and interface.
.It Dv SIOCGIFBRDADDR Fa "struct ifreq *"
Get the broadcast address for a protocol family and interface.
.It Dv SIOCGIFDESCR Fa "struct ifreq *"
Get the interface description, returned in the
.Va ifru_data
field.
.It Dv SIOCSIFDESCR Fa "struct ifreq *"
Set the interface description to the value of the
.Va ifru_data
field, limited to the size of
.Dv IFDESCRSIZE .
.It Dv SIOCSIFFLAGS Fa "struct ifreq *"
Set the interface flags.
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 Fa "struct ifreq *"
Get the interface flags.
.It Dv SIOCGIFXFLAGS Fa "struct ifreq *"
Get the extended interface flags.
.It Dv SIOCSIFMTU Fa "struct ifreq *"
Set the MTU of the interface.
.It Dv SIOCGIFMTU Fa "struct ifreq *"
Get the current MTU of the interface.
.It Dv SIOCGIFHARDMTU Fa "struct ifreq *"
Get the maximum hardware MTU of the interface.
.It Dv SIOCSIFMEDIA Fa "struct ifreq *"
Set the interface media settings.
See
.Xr ifmedia 4
for possible values.
.It Dv SIOCGIFMEDIA Fa "struct ifmediareq *"
Get the interface media settings.
The
.Vt ifmediareq
structure is as follows:
.Bd -literal
struct ifmediareq {
	char		ifm_name[IFNAMSIZ];	/* if name, e.g. "en0" */
	uint64_t	ifm_current;		/* get/set current media options */
	uint64_t	ifm_mask;		/* don't care mask */
	uint64_t	ifm_status;		/* media status */
	uint64_t	ifm_active;		/* active options */
	int		ifm_count;		/* # entries in ifm_ulist array */
	uint64_t	*ifm_ulist;		/* media words */
};
.Ed
.Pp
See
.Xr ifmedia 4
for interpreting this value.
.It Dv SIOCSIFMETRIC Fa "struct ifreq *"
Set the interface routing metric.
The metric is used only by user-level routers.
.It Dv SIOCGIFMETRIC Fa "struct ifreq *"
Get the interface metric.
.It Dv SIOCSIFPRIORITY Fa "struct ifreq *"
Set the interface routing priority.
The interface routing priority influences the resulting routing priority of
new static routes added to the kernel using the specified interface.
The value is in the range of 0 to 16 with smaller numbers being better.
.It Dv SIOCGIFPRIORITY Fa "struct ifreq *"
Get the interface priority.
.It Dv SIOCGIFRDOMAIN Fa "struct ifreq *"
Get the interface routing domain.
This identifies which routing table is used for the interface.
.It Dv SIOCAIFADDR Fa "struct ifaliasreq *"
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).
.Pp
Rather than making separate calls to set destination or broadcast addresses,
or network masks (now an integral feature of multiple protocols), a separate
structure,
.Vt ifaliasreq ,
is used to specify all three facets simultaneously (see below).
One would use a slightly tailored version of this structure specific
to each family (replacing each
.Vt sockaddr
by one
of the family-specific type).
One should always set the length of a
.Vt sockaddr ,
as described in
.Xr ioctl 2 .
.Pp
The
.Vt ifaliasreq
structure is as follows:
.Bd -literal
struct ifaliasreq {
	char	ifra_name[IFNAMSIZ];		/* if name, e.g. "en0" */
	union {
		struct	sockaddr ifrau_addr;
		int	ifrau_align;
	 } ifra_ifrau;
#ifndef ifra_addr
#define ifra_addr	ifra_ifrau.ifrau_addr
#endif
	struct	sockaddr ifra_dstaddr;
#define	ifra_broadaddr	ifra_dstaddr
	struct	sockaddr ifra_mask;
};
.Ed
.It Dv SIOCDIFADDR Fa "struct ifreq *"
This request 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 SIOCGIFCONF Fa "struct ifconf *"
Get the 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.
.Pp
Alternately, if the
.Va ifc_len
passed in is set to 0,
.Dv SIOCGIFCONF
will set
.Va ifc_len
to the size that
.Va ifc_buf
needs to be to fit the entire configuration list and will not
fill in the other parameters.
This is useful for determining the exact size that
.Va ifc_buf
needs to be in advance.
Note, however, that this is an extension
that not all operating systems support.
.Bd -literal
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
.It Dv SIOCIFCREATE Fa "struct ifreq *"
Attempt to create the specified interface.
.It Dv SIOCIFDESTROY Fa "struct ifreq *"
Attempt to destroy the specified interface.
.It Dv SIOCIFGCLONERS Fa "struct if_clonereq *"
Get the list of clonable interfaces.
This request takes an
.Vt if_clonereq
structure pointer (see below) as a value-result parameter.
The
.Va ifcr_count
field should be set to the number of
.Dv IFNAMSIZ Ns -sized
strings that can 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.
.Pp
The
.Vt if_clonereq
structure is as follows:
.Bd -literal
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
.It Dv SIOCAIFGROUP Fa "struct ifgroupreq *"
Associate the interface named by
.Va ifgr_name
with the interface group named by
.Va ifgr_group .
The
.Vt ifgroupreq
structure is as follows:
.Bd -literal
struct ifg_req {
	union {
		char			 ifgrqu_group[IFNAMSIZ];
		char			 ifgrqu_member[IFNAMSIZ];
	} ifgrq_ifgrqu;
#define	ifgrq_group	ifgrq_ifgrqu.ifgrqu_group
#define	ifgrq_member	ifgrq_ifgrqu.ifgrqu_member
};

struct ifgroupreq {
	char	ifgr_name[IFNAMSIZ];
	u_int	ifgr_len;
	union {
		char			 ifgru_group[IFNAMSIZ];
		struct	ifg_req		*ifgru_groups;
		struct	ifg_attrib	 ifgru_attrib;
	} ifgr_ifgru;
#define ifgr_group	ifgr_ifgru.ifgru_group
#define ifgr_groups	ifgr_ifgru.ifgru_groups
#define ifgr_attrib	ifgr_ifgru.ifgru_attrib
};
.Ed
.It Dv SIOCGIFGROUP Fa "struct ifgroupreq *"
Retrieve the list of groups for which an interface is a member.
The interface is named by
.Va ifgr_name .
On enter, the amount of memory in which the group names will
be written is stored in
.Va ifgr_len ,
and the group names themselves will be written to the memory
pointed to by
.Va ifgr_groups .
On return, the amount of memory actually written is returned in
.Va ifgr_len .
.Pp
Alternately, if the
.Va ifgr_len
passed in is set to 0,
.Dv SIOCGIFGROUP
will set
.Va ifgr_len
to the size that
.Va ifgr_groups
needs to be to fit the entire group list and will not
fill in the other parameters.
This is useful for determining the exact size that
.Va ifgr_groups
needs to be in advance.
.It Dv SIOCDIFGROUP Fa "struct ifgroupreq *"
Remove the membership of the interface named by
.Va ifgr_name
from the group
.Va ifgr_group .
.El
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr arp 4 ,
.Xr bridge 4 ,
.Xr ifmedia 4 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr lo 4 ,
.Xr mpe 4 ,
.Xr pf 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr unix 4 ,
.Xr hosts 5 ,
.Xr bgpd 8 ,
.Xr config 8 ,
.Xr ifconfig 8 ,
.Xr mrouted 8 ,
.Xr netstart 8 ,
.Xr ospfd 8 ,
.Xr ripd 8 ,
.Xr route 8
.Sh HISTORY
The
.Nm
manual appeared in
.Bx 4.3 Tahoe .