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NETINTRO(4)	       FreeBSD Kernel Interfaces Manual		   NETINTRO(4)

     netintro -- introduction to networking facilities

     #include <sys/socket.h>
     #include <net/route.h>
     #include <net/if.h>

     This section is a general introduction to the networking facilities
     available in the system.  Documentation in	this part of section 4 is bro-
     ken 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	ser-
     vices may include packet fragmentation and	reassembly, routing, address-
     ing, 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 support-
     ing 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 ex-
     plicitly when creating a socket.  Protocols normally accept only one type
     of	address	format,	usually	determined by the addressing structure inher-
     ent 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 ex-
     tensions 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 pro-
     tocol families and/or address formats.  The SYNOPSIS section of each net-
     work 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 er-
     rors in device operation.

     Network interfaces	may be collected together into interface groups.  An
     interface group is	a container that can be	used generically when refer-
     ring to any interface related by some criteria.  When an action is	per-
     formed on an interface group, such	as packet filtering by the pf(4) sub-
     system, the operation will	be applied to each member interface in the
     group, if supported by the	subsystem.  The	ifconfig(8) utility can	be
     used to view and assign membership	of an interface	to an interface	group
     with the group modifier.

     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

     Associated	with each protocol family is an	address	format.	 All network
     addresses adhere to a general structure, called a sockaddr, described be-
     low.  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.

	   struct sockaddr {
		   u_int8_t	   sa_len;	   /* total length */
		   sa_family_t	   sa_family;	   /* address family */
		   char		   sa_data[14];	   /* actually longer */

     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	imple-

     #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 */

     The sa_data field contains	the actual address value.  Note	that it	may be
     longer than 14 bytes.

     OpenBSD 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)s used in earlier releases.

     This facility is described	in route(4).

     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 inter-
     faces.  The ioctl(2) is made on a socket (typically of type SOCK_DGRAM)
     in	the desired domain.  Most of the requests take an ifreq	structure
     pointer as	their parameter.  This structure is as follows:

     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;
		     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_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 */

     The supported ioctl(2) requests are:

     SIOCSIFADDR struct	ifreq *
	     Set the interface address for a protocol family.  Following the
	     address assignment, the "initialization" routine for the inter-
	     face is called.

	     This call has been	deprecated and superseded by the SIOCAIFADDR
	     call, described below.

     SIOCSIFDSTADDR struct ifreq *
	     Set the point-to-point address for	a protocol family and inter-

	     This call has been	deprecated and superseded by the SIOCAIFADDR
	     call, described below.

     SIOCSIFBRDADDR struct ifreq *
	     Set the broadcast address for a protocol family and interface.

	     This call has been	deprecated and superseded by the SIOCAIFADDR
	     call, described below.

     SIOCGIFADDR struct	ifreq *
	     Get the interface address for a protocol family.

     SIOCGIFDSTADDR struct ifreq *
	     Get the point-to-point address for	a protocol family and inter-

     SIOCGIFBRDADDR struct ifreq *
	     Get the broadcast address for a protocol family and interface.

     SIOCGIFDESCR struct ifreq *
	     Get the interface description, returned in	the ifru_data field.

     SIOCSIFDESCR struct ifreq *
	     Set the interface description to the value	of the ifru_data
	     field, limited to the size	of IFDESCRSIZE.

     SIOCSIFFLAGS struct ifreq *
	     Set the interface flags.  If the interface	is marked down,	any
	     processes currently routing packets through the interface are no-
	     tified; some interfaces may be reset so that incoming packets are
	     no	longer received.  When marked up again,	the interface is

     SIOCGIFFLAGS struct ifreq *
	     Get the interface flags.

     SIOCGIFXFLAGS struct ifreq	*
	     Get the extended interface	flags.

     SIOCGIFMTU	struct ifreq *
	     Get the current MTU of the	interface.

     SIOCGIFHARDMTU struct ifreq *
	     Get the maximum hardware MTU of the interface.

     SIOCSIFMEDIA struct ifreq *
	     Set the interface media settings.	See ifmedia(4) for possible

     SIOCGIFMEDIA struct ifmediareq *
	     Get the interface media settings.	The ifmediareq structure is as

	     struct ifmediareq {
		     char	     ifm_name[IFNAMSIZ];     /*	if name, e.g. "en0" */
		     uint64_t	     ifm_current;    /*	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 */

	     See ifmedia(4) for	interpreting this value.

     SIOCSIFMETRIC struct ifreq	*
	     Set the interface routing metric.	The metric is used only	by
	     user-level	routers.

     SIOCGIFMETRIC struct ifreq	*
	     Get the interface metric.

     SIOCSIFPRIORITY struct ifreq *
	     Set the interface routing priority.  The interface	routing	prior-
	     ity 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 bet-

     SIOCGIFPRIORITY struct ifreq *
	     Get the interface priority.

     SIOCGIFRDOMAIN struct ifreq *
	     Get the interface routing domain.	This identifies	which routing
	     table is used for the interface.

     SIOCAIFADDR 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).

	     Rather than making	separate calls to set destination or broadcast
	     addresses,	or network masks (now an integral feature of multiple
	     protocols), a separate structure, ifaliasreq, is used to specify
	     all three facets simultaneously (see below).  One would use a
	     slightly tailored version of this structure specific to each fam-
	     ily (replacing each sockaddr by one of the	family-specific	type).
	     One should	always set the length of a sockaddr, as	described in

	     The ifaliasreq structure is as follows:

	     struct ifaliasreq {
		     char    ifra_name[IFNAMSIZ];    /*	if name, e.g. "en0" */
		     struct  sockaddr ifra_addr;
		     struct  sockaddr ifra_dstaddr;
	     #define ifra_broadaddr ifra_dstaddr
		     struct  sockaddr ifra_mask;

     SIOCDIFADDR struct	ifreq *
	     This request deletes the specified	address	from the list associ-
	     ated 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	speci-
	     fication 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 struct	ifconf *
	     Get the 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.

	     Alternately, if the ifc_len passed	in is set to 0,	SIOCGIFCONF
	     will set ifc_len to the size that ifc_buf needs to	be to fit the
	     entire configuration list and will	not fill in the	other parame-
	     ters.  This is useful for determining the exact size that ifc_buf
	     needs to be in advance.  Note, however, that this is an extension
	     that not all operating systems support.

	     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 ret'd */

     SIOCIFCREATE struct ifreq *
	     Attempt to	create the specified interface.

     SIOCIFDESTROY struct ifreq	*
	     Attempt to	destroy	the specified interface.

     SIOCIFGCLONERS struct if_clonereq *
	     Get the list of clonable interfaces.  This	request	takes an
	     if_clonereq structure pointer (see	below) as a value-result pa-
	     rameter.  The ifcr_count field should be set to the number	of
	     IFNAMSIZ-sized strings that can 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.

	     The if_clonereq structure is as follows:

	     struct if_clonereq	{
		     int   ifcr_total;	/* total cloners (out) */
		     int   ifcr_count;	/* room	for this many in user buf */
		     char *ifcr_buffer;	/* buffer for cloner names */

     SIOCAIFGROUP struct ifgroupreq *
	     Associate the interface named by ifgr_name	with the interface
	     group named by ifgr_group.	 The ifgroupreq	structure is as	fol-

	     struct ifg_req {
		     char		      ifgrq_group[IFNAMSIZ];

	     struct ifgroupreq {
		     char    ifgr_name[IFNAMSIZ];
		     u_int   ifgr_len;
		     union {
			     char    ifgru_group[IFNAMSIZ];
			     struct  ifg_req *ifgru_groups;
		     } ifgr_ifgru;
	     #define ifgr_group	     ifgr_ifgru.ifgru_group
	     #define ifgr_groups     ifgr_ifgru.ifgru_groups

     SIOCGIFGROUP struct ifgroupreq *
	     Retrieve the list of groups for which an interface	is a member.
	     The interface is named by ifgr_name.  On enter, the amount	of
	     memory in which the group names will be written is	stored in
	     ifgr_len, and the group names themselves will be written to the
	     memory pointed to by ifgr_groups.	On return, the amount of mem-
	     ory actually written is returned in ifgr_len.

	     Alternately, if the ifgr_len passed in is set to 0, SIOCGIFGROUP
	     will set ifgr_len to the size that	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	ifgr_groups
	     needs to be in advance.

     SIOCDIFGROUP struct ifgroupreq *
	     Remove the	membership of the interface named by ifgr_name from
	     the group ifgr_group.

     netstat(1), ioctl(2), socket(2), arp(4), bridge(4), ifmedia(4), inet(4),
     intro(4), ip(4), ip6(4), lo(4), mpe(4), pf(4), tcp(4), udp(4), unix(4),
     hosts(5), bgpd(8),	config(8), ifconfig(8),	mrouted(8), netstart(8),
     ospfd(8), ripd(8),	route(8)

     The netintro manual appeared in 4.3BSD-Tahoe.

FreeBSD	13.0			 July 9, 2018			  FreeBSD 13.0


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