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IFNET(9)			  BSD Kernel Developer's Manual 			 IFNET(9)

NAME
     ifnet, ifaddr, ifqueue, if_data -- kernel interfaces for manipulating network interfaces

SYNOPSIS
     #include <sys/param.h>
     #include <sys/time.h>
     #include <sys/socket.h>
     #include <net/if.h>
     #include <net/if_var.h>
     #include <net/if_types.h>

   Interface Manipulation Functions
     struct ifnet *
     if_alloc(u_char type);

     void
     if_attach(struct ifnet *ifp);

     void
     if_detach(struct ifnet *ifp);

     void
     if_free(struct ifnet *ifp);

     void
     if_free_type(struct ifnet *ifp, u_char type);

     void
     if_down(struct ifnet *ifp);

     int
     ifioctl(struct socket *so, u_long cmd, caddr_t data, struct thread *td);

     int
     ifpromisc(struct ifnet *ifp, int pswitch);

     int
     if_allmulti(struct ifnet *ifp, int amswitch);

     struct ifnet *
     ifunit(const char *name);

     struct ifnet *
     ifunit_ref(const char *name);

     void
     if_up(struct ifnet *ifp);

   Interface Address Functions
     struct ifaddr *
     ifaddr_byindex(u_short idx);

     struct ifaddr *
     ifa_ifwithaddr(struct sockaddr *addr);

     struct ifaddr *
     ifa_ifwithdstaddr(struct sockaddr *addr, int fib);

     struct ifaddr *
     ifa_ifwithnet(struct sockaddr *addr, int ignore_ptp, int fib);

     struct ifaddr *
     ifaof_ifpforaddr(struct sockaddr *addr, struct ifnet *ifp);

     void
     ifa_ref(struct ifaddr *ifa);

     void
     ifa_free(struct ifaddr *ifa);

   Interface Multicast Address Functions
     int
     if_addmulti(struct ifnet *ifp, struct sockaddr *sa, struct ifmultiaddr **ifmap);

     int
     if_delmulti(struct ifnet *ifp, struct sockaddr *sa);

     struct ifmultiaddr *
     if_findmulti(struct ifnet *ifp, struct sockaddr *sa);

   Output queue macros
     IF_DEQUEUE(struct ifqueue *ifq, struct mbuf *m);

   struct ifnet Member Functions
     void
     (*if_input)(struct ifnet *ifp, struct mbuf *m);

     int
     (*if_output)(struct ifnet *ifp, struct mbuf *m, const struct sockaddr *dst,
	 struct route *ro);

     void
     (*if_start)(struct ifnet *ifp);

     int
     (*if_transmit)(struct ifnet *ifp, struct mbuf *m);

     void
     (*if_qflush)(struct ifnet *ifp);

     int
     (*if_ioctl)(struct ifnet *ifp, u_long cmd, caddr_t data);

     void
     (*if_init)(void *if_softc);

     int
     (*if_resolvemulti)(struct ifnet *ifp, struct sockaddr **retsa, struct sockaddr *addr);

   struct ifaddr member function
     void
     (*ifa_rtrequest)(int cmd, struct rtentry *rt, struct rt_addrinfo *info);

   Global Variables
     extern struct ifnethead ifnet;
     extern int if_index;
     extern int ifqmaxlen;

DATA STRUCTURES
     The kernel mechanisms for handling network interfaces reside primarily in the ifnet,
     if_data, ifaddr, and ifmultiaddr structures in <net/if.h> and <net/if_var.h> and the func-
     tions named above and defined in /sys/net/if.c.  Those interfaces which are intended to be
     used by user programs are defined in <net/if.h>; these include the interface flags, the
     if_data structure, and the structures defining the appearance of interface-related messages
     on the route(4) routing socket and in sysctl(3).  The header file <net/if_var.h> defines the
     kernel-internal interfaces, including the ifnet, ifaddr, and ifmultiaddr structures and the
     functions which manipulate them.  (A few user programs will need <net/if_var.h> because it
     is the prerequisite of some other header file like <netinet/if_ether.h>.  Most references to
     those two files in particular can be replaced by <net/ethernet.h>.)

     The system keeps a linked list of interfaces using the TAILQ macros defined in queue(3);
     this list is headed by a struct ifnethead called ifnet.  The elements of this list are of
     type struct ifnet, and most kernel routines which manipulate interface as such accept or
     return pointers to these structures.  Each interface structure contains an if_data structure
     used for statistics and information.  Each interface also has a TAILQ of interface
     addresses, described by ifaddr structures.  An AF_LINK address (see link_addr(3)) describing
     the link layer implemented by the interface (if any) is accessed by the ifaddr_byindex()
     function or if_addr structure.  (Some trivial interfaces do not provide any link layer
     addresses; this structure, while still present, serves only to identify the interface name
     and index.)

     Finally, those interfaces supporting reception of multicast datagrams have a TAILQ of multi-
     cast group memberships, described by ifmultiaddr structures.  These memberships are refer-
     ence-counted.

     Interfaces are also associated with an output queue, defined as a struct ifqueue; this
     structure is used to hold packets while the interface is in the process of sending another.

   The ifnet structure
     The fields of struct ifnet are as follows:

	   if_softc	    (void *) A pointer to the driver's private state block.  (Initialized
			    by driver.)

	   if_l2com	    (void *) A pointer to the common data for the interface's layer 2
			    protocol.  (Initialized by if_alloc().)

	   if_vnet	    (struct vnet *) A pointer to the virtual network stack instance.
			    (Initialized by if_attach().)

	   if_home_vnet     (struct vnet *) A pointer to the parent virtual network stack, where
			    this struct ifnet originates from.	(Initialized by if_attach().)

	   if_link	    (TAILQ_ENTRY(ifnet)) queue(3) macro glue.

	   if_xname	    (char *) The name of the interface, (e.g., ``fxp0'' or ``lo0'').
			    (Initialized by driver (usually via if_initname()).)

	   if_dname	    (const char *) The name of the driver.  (Initialized by driver (usu-
			    ally via if_initname()).)

	   if_dunit	    (int) A unique number assigned to each interface managed by a partic-
			    ular driver.  Drivers may choose to set this to IF_DUNIT_NONE if a
			    unit number is not associated with the device.  (Initialized by
			    driver (usually via if_initname()).)

	   if_refcount	    (u_int) The reference count.  (Initialized by if_alloc().)

	   if_addrhead	    (struct ifaddrhead) The head of the queue(3) TAILQ containing the
			    list of addresses assigned to this interface.

	   if_pcount	    (int) A count of promiscuous listeners on this interface, used to
			    reference-count the IFF_PROMISC flag.

	   if_carp	    (struct carp_if *) A pointer to the CARP interface structure,
			    carp(4).  (Initialized by the driver-specific if_ioctl() routine.)

	   if_bpf	    (struct bpf_if *) Opaque per-interface data for the packet filter,
			    bpf(4).  (Initialized by bpf_attach().)

	   if_index	    (u_short) A unique number assigned to each interface in sequence as
			    it is attached.  This number can be used in a struct sockaddr_dl to
			    refer to a particular interface by index (see link_addr(3)).  (Ini-
			    tialized by if_alloc().)

	   if_vlantrunk     (struct ifvlantrunk *) A pointer to 802.1Q trunk structure, vlan(4).
			    (Initialized by the driver-specific if_ioctl() routine.)

	   if_flags	    (int) Flags describing operational parameters of this interface (see
			    below).  (Manipulated by generic code.)

	   if_drv_flags     (int) Flags describing operational status of this interface (see
			    below).  (Manipulated by driver.)

	   if_capabilities  (int) Flags describing the capabilities the interface supports (see
			    below).

	   if_capenable     (int) Flags describing the enabled capabilities of the interface (see
			    below).

	   if_linkmib	    (void *) A pointer to an interface-specific MIB structure exported by
			    ifmib(4).  (Initialized by driver.)

	   if_linkmiblen    (size_t) The size of said structure.  (Initialized by driver.)

	   if_data	    (struct if_data) More statistics and information; see The if_data
			    structure, below.  (Initialized by driver, manipulated by both driver
			    and generic code.)

	   if_multiaddrs    (struct ifmultihead) The head of the queue(3) TAILQ containing the
			    list of multicast addresses assigned to this interface.

	   if_amcount	    (int) A number of multicast requests on this interface, used to ref-
			    erence-count the IFF_ALLMULTI flag.

	   if_addr	    (struct ifaddr *) A pointer to the link-level interface address.
			    (Initialized by if_alloc().)

	   if_snd	    (struct ifaltq) The output queue.  (Manipulated by driver.)

	   if_broadcastaddr
			    (const u_int8_t *) A link-level broadcast bytestring for protocols
			    with variable address length.

	   if_bridge	    (void *) A pointer to the bridge interface structure, if_bridge(4).
			    (Initialized by the driver-specific if_ioctl() routine.)

	   if_label	    (struct label *) A pointer to the MAC Framework label structure,
			    mac(4).  (Initialized by if_alloc().)

	   if_afdata	    (void *) An address family dependent data region.

	   if_afdata_initialized
			    (int) Used to track the current state of address family initializa-
			    tion.

	   if_afdata_lock   (struct rwlock) An rwlock(9) lock used to protect if_afdata inter-
			    nals.

	   if_linktask	    (struct task) A taskqueue(9) task scheduled for link state change
			    events of the interface.

	   if_addr_lock     (struct rwlock) An rwlock(9) lock used to protect interface-related
			    address lists.

	   if_clones	    (LIST_ENTRY(ifnet)) queue(3) macro glue for the list of clonable net-
			    work interfaces.

	   if_groups	    (TAILQ_HEAD(, ifg_list)) The head of the queue(3) TAILQ containing
			    the list of groups per interface.

	   if_pf_kif	    (void *) A pointer to the structure used for interface abstraction by
			    pf(4).

	   if_lagg	    (void *) A pointer to the lagg(4) interface structure.

	   if_alloctype     (u_char) The type of the interface as it was at the time of its allo-
			    cation.  It is used to cache the type passed to if_alloc(), but
			    unlike if_type, it would not be changed by drivers.

     References to ifnet structures are gained by calling the if_ref() function and released by
     calling the if_rele() function.  They are used to allow kernel code walking global interface
     lists to release the ifnet lock yet keep the ifnet structure stable.

     There are in addition a number of function pointers which the driver must initialize to com-
     plete its interface with the generic interface layer:

	   if_input()
	   Pass a packet to an appropriate upper layer as determined from the link-layer header
	   of the packet.  This routine is to be called from an interrupt handler or used to emu-
	   late reception of a packet on this interface.  A single function implementing
	   if_input() can be shared among multiple drivers utilizing the same link-layer framing,
	   e.g., Ethernet.

	   if_output()
	   Output a packet on interface ifp, or queue it on the output queue if the interface is
	   already active.

	   if_transmit()
	   Transmit a packet on an interface or queue it if the interface is in use.  This func-
	   tion will return ENOBUFS if the devices software and hardware queues are both full.
	   This function must be installed after if_attach() to override the default implementa-
	   tion.  This function is exposed in order to allow drivers to manage their own queues
	   and to reduce the latency caused by a frequently gratuitous enqueue / dequeue pair to
	   ifq.  The suggested internal software queueing mechanism is buf_ring.

	   if_qflush()
	   Free mbufs in internally managed queues when the interface is marked down.  This func-
	   tion must be installed after if_attach() to override the default implementation.  This
	   function is exposed in order to allow drivers to manage their own queues and to reduce
	   the latency caused by a frequently gratuitous enqueue / dequeue pair to ifq.  The sug-
	   gested internal software queueing mechanism is buf_ring.

	   if_start()
	   Start queued output on an interface.  This function is exposed in order to provide for
	   some interface classes to share a if_output() among all drivers.  if_start() may only
	   be called when the IFF_DRV_OACTIVE flag is not set.	(Thus, IFF_DRV_OACTIVE does not
	   literally mean that output is active, but rather that the device's internal output
	   queue is full.) Please note that this function will soon be deprecated.

	   if_ioctl()
	   Process interface-related ioctl(2) requests (defined in <sys/sockio.h>).  Preliminary
	   processing is done by the generic routine ifioctl() to check for appropriate privi-
	   leges, locate the interface being manipulated, and perform certain generic operations
	   like twiddling flags and flushing queues.  See the description of ifioctl() below for
	   more information.

	   if_init()
	   Initialize and bring up the hardware, e.g., reset the chip and enable the receiver
	   unit.  Should mark the interface running, but not active (IFF_DRV_RUNNING,
	   ~IIF_DRV_OACTIVE).

	   if_resolvemulti()
	   Check the requested multicast group membership, addr, for validity, and if necessary
	   compute a link-layer group which corresponds to that address which is returned in
	   *retsa.  Returns zero on success, or an error code on failure.

   Interface Flags
     Interface flags are used for a number of different purposes.  Some flags simply indicate
     information about the type of interface and its capabilities; others are dynamically manipu-
     lated to reflect the current state of the interface.  Flags of the former kind are marked
     <S> in this table; the latter are marked <D>.  Flags which begin with ``IFF_DRV_'' are
     stored in if_drv_flags; all other flags are stored in if_flags.

     The macro IFF_CANTCHANGE defines the bits which cannot be set by a user program using the
     SIOCSIFFLAGS command to ioctl(2); these are indicated by an asterisk ('*') in the following
     listing.

	   IFF_UP	    <D> The interface has been configured up by the user-level code.
	   IFF_BROADCAST    <S*> The interface supports broadcast.
	   IFF_DEBUG	    <D> Used to enable/disable driver debugging code.
	   IFF_LOOPBACK     <S> The interface is a loopback device.
	   IFF_POINTOPOINT  <S*> The interface is point-to-point; ``broadcast'' address is actu-
			    ally the address of the other end.
	   IFF_DRV_RUNNING  <D*> The interface has been configured and dynamic resources were
			    successfully allocated.  Probably only useful internal to the inter-
			    face.
	   IFF_NOARP	    <D> Disable network address resolution on this interface.
	   IFF_PROMISC	    <D*> This interface is in promiscuous mode.
	   IFF_PPROMISC     <D> This interface is in the permanently promiscuous mode (implies
			    IFF_PROMISC).
	   IFF_ALLMULTI     <D*> This interface is in all-multicasts mode (used by multicast
			    routers).
	   IFF_DRV_OACTIVE  <D*> The interface's hardware output queue (if any) is full; output
			    packets are to be queued.
	   IFF_SIMPLEX	    <S*> The interface cannot hear its own transmissions.
	   IFF_LINK0
	   IFF_LINK1
	   IFF_LINK2	    <D> Control flags for the link layer.  (Currently abused to select
			    among multiple physical layers on some devices.)
	   IFF_MULTICAST    <S*> This interface supports multicast.
	   IFF_CANTCONFIG   <S*> The interface is not configurable in a meaningful way.  Primar-
			    ily useful for IFT_USB interfaces registered at the interface list.
	   IFF_MONITOR	    <D> This interface blocks transmission of packets and discards incom-
			    ing packets after BPF processing.  Used to monitor network traffic
			    but not interact with the network in question.
	   IFF_STATICARP    <D> Used to enable/disable ARP requests on this interface.
	   IFF_DYING	    <D*> Set when the ifnet structure of this interface is being released
			    and still has if_refcount references.
	   IFF_RENAMING     <D*> Set when this interface is being renamed.

   Interface Capabilities Flags
     Interface capabilities are specialized features an interface may or may not support.  These
     capabilities are very hardware-specific and allow, when enabled, to offload specific network
     processing to the interface or to offer a particular feature for use by other kernel parts.

     It should be stressed that a capability can be completely uncontrolled (i.e., stay always
     enabled with no way to disable it) or allow limited control over itself (e.g., depend on
     another capability's state.)  Such peculiarities are determined solely by the hardware and
     driver of a particular interface.	Only the driver possesses the knowledge on whether and
     how the interface capabilities can be controlled.	Consequently, capabilities flags in
     if_capenable should never be modified directly by kernel code other than the interface
     driver.  The command SIOCSIFCAP to ifioctl() is the dedicated means to attempt altering
     if_capenable on an interface.  Userland code shall use ioctl(2).

     The following capabilities are currently supported by the system:

	   IFCAP_RXCSUM 	  This interface can do checksum validation on receiving data.
				  Some interfaces do not have sufficient buffer storage to store
				  frames above a certain MTU-size completely.  The driver for the
				  interface might disable hardware checksum validation if the MTU
				  is set above the hardcoded limit.

	   IFCAP_TXCSUM 	  This interface can do checksum calculation on transmitting
				  data.

	   IFCAP_HWCSUM 	  A shorthand for (IFCAP_RXCSUM | IFCAP_TXCSUM).

	   IFCAP_NETCONS	  This interface can be a network console.

	   IFCAP_VLAN_MTU	  The vlan(4) driver can operate over this interface in software
				  tagging mode without having to decrease MTU on vlan(4) inter-
				  faces below 1500 bytes.  This implies the ability of this
				  interface to cope with frames somewhat longer than permitted by
				  the Ethernet specification.

	   IFCAP_VLAN_HWTAGGING   This interface can do VLAN tagging on output and demultiplex
				  frames by their VLAN tag on input.

	   IFCAP_JUMBO_MTU	  This Ethernet interface can transmit and receive frames up to
				  9000 bytes long.

	   IFCAP_POLLING	  This interface supports polling(4).  See below for details.

	   IFCAP_VLAN_HWCSUM	  This interface can do checksum calculation on both transmitting
				  and receiving data on vlan(4) interfaces (implies
				  IFCAP_HWCSUM).

	   IFCAP_TSO4		  This Ethernet interface supports TCP4 Segmentation offloading.

	   IFCAP_TSO6		  This Ethernet interface supports TCP6 Segmentation offloading.

	   IFCAP_TSO		  A shorthand for (IFCAP_TSO4 | IFCAP_TSO6).

	   IFCAP_TOE4		  This Ethernet interface supports TCP offloading.

	   IFCAP_TOE6		  This Ethernet interface supports TCP6 offloading.

	   IFCAP_TOE		  A shorthand for (IFCAP_TOE4 | IFCAP_TOE6).

	   IFCAP_WOL_UCAST	  This Ethernet interface supports waking up on any Unicast
				  packet.

	   IFCAP_WOL_MCAST	  This Ethernet interface supports waking up on any Multicast
				  packet.

	   IFCAP_WOL_MAGIC	  This Ethernet interface supports waking up on any Magic packet
				  such as those sent by wake(8).

	   IFCAP_WOL		  A shorthand for (IFCAP_WOL_UCAST | IFCAP_WOL_MCAST |
				  IFCAP_WOL_MAGIC).

	   IFCAP_TOE4		  This Ethernet interface supports TCP4 Offload Engine.

	   IFCAP_TOE6		  This Ethernet interface supports TCP6 Offload Engine.

	   IFCAP_TOE		  A shorthand for (IFCAP_TOE4 | IFCAP_TOE6).

	   IFCAP_VLAN_HWFILTER	  This interface supports frame filtering in hardware on vlan(4)
				  interfaces.

	   IFCAP_POLLING_NOCOUNT  The return value for the number of processed packets should be
				  skipped for this interface.

	   IFCAP_VLAN_HWTSO	  This interface supports TCP Segmentation offloading on vlan(4)
				  interfaces (implies IFCAP_TSO).

	   IFCAP_LINKSTATE	  This Ethernet interface supports dynamic link state changes.

     The ability of advanced network interfaces to offload certain computational tasks from the
     host CPU to the board is limited mostly to TCP/IP.  Therefore a separate field associated
     with an interface (see ifnet.if_data.ifi_hwassist below) keeps a detailed description of its
     enabled capabilities specific to TCP/IP processing.  The TCP/IP module consults the field to
     see which tasks can be done on an outgoing packet by the interface.  The flags defined for
     that field are a superset of those for mbuf.m_pkthdr.csum_flags, namely:

	   CSUM_IP	  The interface will compute IP checksums.

	   CSUM_TCP	  The interface will compute TCP checksums.

	   CSUM_UDP	  The interface will compute UDP checksums.

	   CSUM_IP_FRAGS  The interface can compute a TCP or UDP checksum for a packet fragmented
			  by the host CPU.  Makes sense only along with CSUM_TCP or CSUM_UDP.

	   CSUM_FRAGMENT  The interface will do the fragmentation of IP packets if necessary.
			  The host CPU does not need to care about MTU on this interface as long
			  as a packet to transmit through it is an IP one and it does not exceed
			  the size of the hardware buffer.

     An interface notifies the TCP/IP module about the tasks the former has performed on an
     incoming packet by setting the corresponding flags in the field mbuf.m_pkthdr.csum_flags of
     the mbuf chain containing the packet.  See mbuf(9) for details.

     The capability of a network interface to operate in polling(4) mode involves several flags
     in different global variables and per-interface fields.  The capability flag IFCAP_POLLING
     set in interface's if_capabilities indicates support for polling(4) on the particular inter-
     face.  If set in if_capabilities, the same flag can be marked or cleared in the interface's
     if_capenable within ifioctl(), thus initiating switch of the interface to polling(4) mode or
     interrupt mode, respectively.  The actual mode change is managed by the driver-specific
     if_ioctl() routine.  The polling handler returns the number of packets processed.

   The if_data Structure
     The if_data structure contains statistics and identifying information used by management
     programs, and which is exported to user programs by way of the ifmib(4) branch of the
     sysctl(3) MIB.  The following elements of the if_data structure are initialized by the
     interface and are not expected to change significantly over the course of normal operation:

	   ifi_type	   (u_char) The type of the interface, as defined in <net/if_types.h> and
			   described below in the Interface Types section.

	   ifi_physical    (u_char) Intended to represent a selection of physical layers on
			   devices which support more than one; never implemented.

	   ifi_addrlen	   (u_char) Length of a link-layer address on this device, or zero if
			   there are none.  Used to initialized the address length field in
			   sockaddr_dl structures referring to this interface.

	   ifi_hdrlen	   (u_char) Maximum length of any link-layer header which might be
			   prepended by the driver to a packet before transmission.  The generic
			   code computes the maximum over all interfaces and uses that value to
			   influence the placement of data in mbufs to attempt to ensure that
			   there is always sufficient space to prepend a link-layer header with-
			   out allocating an additional mbuf.

	   ifi_datalen	   (u_char) Length of the if_data structure.  Allows some stabilization
			   of the routing socket ABI in the face of increases in the length of
			   struct ifdata.

	   ifi_mtu	   (u_long) The maximum transmission unit of the medium, exclusive of any
			   link-layer overhead.

	   ifi_metric	   (u_long) A dimensionless metric interpreted by a user-mode routing
			   process.

	   ifi_baudrate    (u_long) The line rate of the interface, in bits per second.

	   ifi_hwassist    (u_long) A detailed interpretation of the capabilities to offload com-
			   putational tasks for outgoing packets.  The interface driver must keep
			   this field in accord with the current value of if_capenable.

	   ifi_epoch	   (time_t) The system uptime when interface was attached or the statis-
			   tics below were reset.  This is intended to be used to set the SNMP
			   variable ifCounterDiscontinuityTime.  It may also be used to determine
			   if two successive queries for an interface of the same index have
			   returned results for the same interface.

     The structure additionally contains generic statistics applicable to a variety of different
     interface types (except as noted, all members are of type u_long):

	   ifi_link_state  (u_char) The current link state of Ethernet interfaces.  See the
			   Interface Link States section for possible values.

	   ifi_ipackets    Number of packets received.

	   ifi_ierrors	   Number of receive errors detected (e.g., FCS errors, DMA overruns,
			   etc.).  More detailed breakdowns can often be had by way of a link-
			   specific MIB.

	   ifi_opackets    Number of packets transmitted.

	   ifi_oerrors	   Number of output errors detected (e.g., late collisions, DMA overruns,
			   etc.).  More detailed breakdowns can often be had by way of a link-
			   specific MIB.

	   ifi_collisions  Total number of collisions detected on output for CSMA interfaces.
			   (This member is sometimes [ab]used by other types of interfaces for
			   other output error counts.)

	   ifi_ibytes	   Total traffic received, in bytes.

	   ifi_obytes	   Total traffic transmitted, in bytes.

	   ifi_imcasts	   Number of packets received which were sent by link-layer multicast.

	   ifi_omcasts	   Number of packets sent by link-layer multicast.

	   ifi_iqdrops	   Number of packets dropped on input.	Rarely implemented.

	   ifi_noproto	   Number of packets received for unknown network-layer protocol.

	   ifi_lastchange  (struct timeval) The time of the last administrative change to the
			   interface (as required for SNMP).

   Interface Types
     The header file <net/if_types.h> defines symbolic constants for a number of different types
     of interfaces.  The most common are:

	   IFT_OTHER	    none of the following
	   IFT_ETHER	    Ethernet
	   IFT_ISO88023     ISO 8802-3 CSMA/CD
	   IFT_ISO88024     ISO 8802-4 Token Bus
	   IFT_ISO88025     ISO 8802-5 Token Ring
	   IFT_ISO88026     ISO 8802-6 DQDB MAN
	   IFT_FDDI	    FDDI
	   IFT_PPP	    Internet Point-to-Point Protocol (ppp(8))
	   IFT_LOOP	    The loopback (lo(4)) interface
	   IFT_SLIP	    Serial Line IP
	   IFT_PARA	    Parallel-port IP (``PLIP'')
	   IFT_ATM	    Asynchronous Transfer Mode
	   IFT_USB	    USB Interface

   Interface Link States
     The following link states are currently defined:

	   LINK_STATE_UNKNOWN  The link is in an invalid or unknown state.
	   LINK_STATE_DOWN     The link is down.
	   LINK_STATE_UP       The link is up.

   The ifaddr Structure
     Every interface is associated with a list (or, rather, a TAILQ) of addresses, rooted at the
     interface structure's if_addrlist member.	The first element in this list is always an
     AF_LINK address representing the interface itself; multi-access network drivers should com-
     plete this structure by filling in their link-layer addresses after calling if_attach().
     Other members of the structure represent network-layer addresses which have been configured
     by means of the SIOCAIFADDR command to ioctl(2), called on a socket of the appropriate pro-
     tocol family.  The elements of this list consist of ifaddr structures.  Most protocols will
     declare their own protocol-specific interface address structures, but all begin with a
     struct ifaddr which provides the most-commonly-needed functionality across all protocols.
     Interface addresses are reference-counted.

     The members of struct ifaddr are as follows:

	   ifa_addr	  (struct sockaddr *) The local address of the interface.

	   ifa_dstaddr	  (struct sockaddr *) The remote address of point-to-point interfaces,
			  and the broadcast address of broadcast interfaces.  (ifa_broadaddr is a
			  macro for ifa_dstaddr.)

	   ifa_netmask	  (struct sockaddr *) The network mask for multi-access interfaces, and
			  the confusion generator for point-to-point interfaces.

	   ifa_ifp	  (struct ifnet *) A link back to the interface structure.

	   ifa_link	  (TAILQ_ENTRY(ifaddr)) queue(3) glue for list of addresses on each
			  interface.

	   ifa_rtrequest  See below.

	   ifa_flags	  (u_short) Some of the flags which would be used for a route represent-
			  ing this address in the route table.

	   ifa_refcnt	  (short) The reference count.

     References to ifaddr structures are gained by calling the ifa_ref() function and released by
     calling the ifa_free() function.

     ifa_rtrequest() is a pointer to a function which receives callouts from the routing code
     (rtrequest()) to perform link-layer-specific actions upon requests to add, or delete routes.
     The cmd argument indicates the request in question: RTM_ADD, or RTM_DELETE.  The rt argument
     is the route in question; the info argument contains the specific destination being manipu-
     lated.

FUNCTIONS
     The functions provided by the generic interface code can be divided into two groups: those
     which manipulate interfaces, and those which manipulate interface addresses.  In addition to
     these functions, there may also be link-layer support routines which are used by a number of
     drivers implementing a specific link layer over different hardware; see the documentation
     for that link layer for more details.

   The ifmultiaddr Structure
     Every multicast-capable interface is associated with a list of multicast group memberships,
     which indicate at a low level which link-layer multicast addresses (if any) should be
     accepted, and at a high level, in which network-layer multicast groups a user process has
     expressed interest.

     The elements of the structure are as follows:

	   ifma_link	  (LIST_ENTRY(ifmultiaddr)) queue(3) macro glue.

	   ifma_addr	  (struct sockaddr *) A pointer to the address which this record repre-
			  sents.  The memberships for various address families are stored in
			  arbitrary order.

	   ifma_lladdr	  (struct sockaddr *) A pointer to the link-layer multicast address, if
			  any, to which the network-layer multicast address in ifma_addr is
			  mapped, else a null pointer.	If this element is non-nil, this member-
			  ship also holds an invisible reference to another membership for that
			  link-layer address.

	   ifma_refcount  (u_int) A reference count of requests for this particular membership.

   Interface Manipulation Functions
	   if_alloc()
	   Allocate and initialize struct ifnet.  Initialization includes the allocation of an
	   interface index and may include the allocation of a type specific structure in
	   if_l2com.

	   if_attach()
	   Link the specified interface ifp into the list of network interfaces.  Also initialize
	   the list of addresses on that interface, and create a link-layer ifaddr structure to
	   be the first element in that list.  (A pointer to this address structure is saved in
	   the ifnet structure and shall be accessed by the ifaddr_byindex() function.)  The ifp
	   must have been allocated by if_alloc().

	   if_detach()
	   Shut down and unlink the specified ifp from the interface list.

	   if_free()
	   Free the given ifp back to the system.  The interface must have been previously
	   detached if it was ever attached.

	   if_free_type()
	   Identical to if_free() except that the given type is used to free if_l2com instead of
	   the type in if_type.  This is intended for use with drivers that change their inter-
	   face type.

	   if_down()
	   Mark the interface ifp as down (i.e., IFF_UP is not set), flush its output queue,
	   notify protocols of the transition, and generate a message from the route(4) routing
	   socket.

	   if_up()
	   Mark the interface ifp as up, notify protocols of the transition, and generate a mes-
	   sage from the route(4) routing socket.

	   ifpromisc()
	   Add or remove a promiscuous reference to ifp.  If pswitch is true, add a reference; if
	   it is false, remove a reference.  On reference count transitions from zero to one and
	   one to zero, set the IFF_PROMISC flag appropriately and call if_ioctl() to set up the
	   interface in the desired mode.

	   if_allmulti()
	   As ifpromisc(), but for the all-multicasts (IFF_ALLMULTI) flag instead of the promis-
	   cuous flag.

	   ifunit()
	   Return an ifnet pointer for the interface named name.

	   ifunit_ref()
	   Return a reference-counted (via ifa_ref()) ifnet pointer for the interface named name.
	   This is the preferred function over ifunit().  The caller is responsible for releasing
	   the reference with if_rele() when it is finished with the ifnet.

	   ifioctl()
	   Process the ioctl request cmd, issued on socket so by thread td, with data parameter
	   data.  This is the main routine for handling all interface configuration requests from
	   user mode.  It is ordinarily only called from the socket-layer ioctl(2) handler, and
	   only for commands with class 'i'.  Any unrecognized commands will be passed down to
	   socket so's protocol for further interpretation.  The following commands are handled
	   by ifioctl():

		 SIOCGIFCONF	 Get interface configuration.  (No call-down to driver.)

		 SIOCSIFNAME	 Set the interface name.  RTM_IFANNOUNCE departure and arrival
				 messages are sent so that routing code that relies on the inter-
				 face name will update its interface list.  Caller must have
				 appropriate privilege.  (No call-down to driver.)
		 SIOCGIFCAP
		 SIOCGIFFIB
		 SIOCGIFFLAGS
		 SIOCGIFMETRIC
		 SIOCGIFMTU
		 SIOCGIFPHYS	 Get interface capabilities, FIB, flags, metric, MTU, medium
				 selection.  (No call-down to driver.)

		 SIOCSIFCAP	 Enable or disable interface capabilities.  Caller must have
				 appropriate privilege.  Before a call to the driver-specific
				 if_ioctl() routine, the requested mask for enabled capabilities
				 is checked against the mask of capabilities supported by the
				 interface, if_capabilities.  Requesting to enable an unsupported
				 capability is invalid.  The rest is supposed to be done by the
				 driver, which includes updating if_capenable and
				 if_data.ifi_hwassist appropriately.

		 SIOCSIFFIB	 Sets interface FIB.  Caller must have appropriate privilege.
				 FIB values start at 0 and values greater or equals than net.fibs
				 are considered invalid.
		 SIOCSIFFLAGS	 Change interface flags.  Caller must have appropriate privilege.
				 If a change to the IFF_UP flag is requested, if_up() or
				 if_down() is called as appropriate.  Flags listed in
				 IFF_CANTCHANGE are masked off, and the field if_flags in the
				 interface structure is updated.  Finally, the driver if_ioctl()
				 routine is called to perform any setup requested.

		 SIOCSIFMETRIC
		 SIOCSIFPHYS	 Change interface metric or medium.  Caller must have appropriate
				 privilege.

		 SIOCSIFMTU	 Change interface MTU.	Caller must have appropriate privilege.
				 MTU values less than 72 or greater than 65535 are considered
				 invalid.  The driver if_ioctl() routine is called to implement
				 the change; it is responsible for any additional sanity checking
				 and for actually modifying the MTU in the interface structure.

		 SIOCADDMULTI
		 SIOCDELMULTI	 Add or delete permanent multicast group memberships on the
				 interface.  Caller must have appropriate privilege.  The
				 if_addmulti() or if_delmulti() function is called to perform the
				 operation; qq.v.

		 SIOCAIFADDR
		 SIOCDIFADDR	 The socket's protocol control routine is called to implement the
				 requested action.

     if_down(), ifioctl(), ifpromisc(), and if_up() must be called at splnet() or higher.

   Interface Address Functions
     Several functions exist to look up an interface address structure given an address.
     ifa_ifwithaddr() returns an interface address with either a local address or a broadcast
     address precisely matching the parameter addr.  ifa_ifwithdstaddr() returns an interface
     address for a point-to-point interface whose remote (``destination'') address is addr and a
     fib is fib.  If fib is RT_ALL_FIBS, then the first interface address matching addr will be
     returned.

     ifa_ifwithnet() returns the most specific interface address which matches the specified
     address, addr, subject to its configured netmask, or a point-to-point interface address
     whose remote address is addr if one is found.  If ignore_ptp is true, skip point-to-point
     interface addresses.  The fib parameter is handled the same way as by ifa_ifwithdstaddr().

     ifaof_ifpforaddr() returns the most specific address configured on interface ifp which
     matches address addr, subject to its configured netmask.  If the interface is point-to-
     point, only an interface address whose remote address is precisely addr will be returned.

     ifaddr_byindex() returns the link-level address of the interface with the given index idx.

     All of these functions return a null pointer if no such address can be found.

   Interface Multicast Address Functions
     The if_addmulti(), if_delmulti(), and if_findmulti() functions provide support for request-
     ing and relinquishing multicast group memberships, and for querying an interface's member-
     ship list, respectively.  The if_addmulti() function takes a pointer to an interface, ifp,
     and a generic address, sa.  It also takes a pointer to a struct ifmultiaddr * which is
     filled in on successful return with the address of the group membership control block.  The
     if_addmulti() function performs the following four-step process:

	   1.	Call the interface's if_resolvemulti() entry point to determine the link-layer
		address, if any, corresponding to this membership request, and also to give the
		link layer an opportunity to veto this membership request should it so desire.

	   2.	Check the interface's group membership list for a pre-existing membership for
		this group.  If one is not found, allocate a new one; if one is, increment its
		reference count.

	   3.	If the if_resolvemulti() routine returned a link-layer address corresponding to
		the group, repeat the previous step for that address as well.

	   4.	If the interface's multicast address filter needs to be changed because a new
		membership was added, call the interface's if_ioctl() routine (with a cmd argu-
		ment of SIOCADDMULTI) to request that it do so.

     The if_delmulti() function, given an interface ifp and an address, sa, reverses this
     process.  Both functions return zero on success, or a standard error number on failure.

     The if_findmulti() function examines the membership list of interface ifp for an address
     matching sa, and returns a pointer to that struct ifmultiaddr if one is found, else it
     returns a null pointer.

SEE ALSO
     ioctl(2), link_addr(3), queue(3), sysctl(3), bpf(4), ifmib(4), lo(4), netintro(4),
     polling(4), config(8), ppp(8), mbuf(9), rtentry(9)

     Gary R. Wright and W. Richard Stevens, TCP/IP Illustrated, Vol. 2, Addison-Wesley, ISBN
     0-201-63354-X.

AUTHORS
     This manual page was written by Garrett A. Wollman.

BSD					  July 29, 2014 				      BSD
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