Command Section
IFNET(9)               FreeBSD Kernel Developer's Manual              IFNET(9)

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

     #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);

     if_attach(struct ifnet *ifp);

     if_detach(struct ifnet *ifp);

     if_free(struct ifnet *ifp);

     if_free_type(struct ifnet *ifp, u_char type);

     if_down(struct ifnet *ifp);

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

     ifpromisc(struct ifnet *ifp, int pswitch);

     if_allmulti(struct ifnet *ifp, int amswitch);

     struct ifnet *
     ifunit(const char *name);

     struct ifnet *
     ifunit_ref(const char *name);

     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);

     ifa_ref(struct ifaddr *ifa);

     ifa_free(struct ifaddr *ifa);

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

     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
     (*if_input)(struct ifnet *ifp, struct mbuf *m);)(struct ifnet *ifp, struct mbuf *m);

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

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

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

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

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

     (*if_init)(void *if_softc);)(void *if_softc);

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

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

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

     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 functions 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 multicast group memberships, described by ifmultiaddr
     structures.  These memberships are reference-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_vnet              (struct vnet *) A pointer to the virtual
                                network stack instance.  (Initialized by

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

           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 (usually via

           if_dunit             (int) A unique number assigned to each
                                interface managed by a particular 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_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)).  (Initialized 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

           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 reference-count the
                                IFF_ALLMULTI flag.

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

           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

           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

                                (int) Used to track the current state of
                                address family initialization.

           if_afdata_lock       (struct rwlock) An rwlock(9) lock used to
                                protect if_afdata internals.

           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 network 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

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

     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 complete its interface with the generic interface layer:

           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 emulate 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.

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

           Transmit a packet on an interface or queue it if the interface is
           in use.  This function 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 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 suggested internal software
           queuing mechanism is buf_ring.

           Free mbufs in internally managed queues when the interface is
           marked down.  This function 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 suggested internal software queuing mechanism is

           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.

           Process interface-related ioctl(2) requests (defined in
           <sys/sockio.h>).  Preliminary processing is done by the generic
           routine ifioctl() to check for appropriate privileges, 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.

           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).

           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 manipulated 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
           IFF_LOOPBACK         <S> The interface is a loopback device.
           IFF_POINTOPOINT      <S*> The interface is point-to-point;
                                ``broadcast'' address is actually 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
           IFF_NOARP            <D> Disable network address resolution on this
           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
           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.  Primarily useful for IFT_USB
                                interfaces registered at the interface list.
           IFF_MONITOR          <D> This interface blocks transmission of
                                packets and discards incoming packets after
                                BPF processing.  Used to monitor network
                                traffic but not interact with the network in
           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_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) interfaces below 1500 bytes.
                                      This implies the ability of this
                                      interface to cope with frames somewhat
                                      longer than permitted by the Ethernet

           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_TOE4                 This Ethernet interface supports TCP

           IFCAP_TOE6                 This Ethernet interface supports TCP6

           IFCAP_TOE                  A shorthand for (IFCAP_TOE4 |

           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_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.

     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
     interface.  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(4) handler returns the number of packets

   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

           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 without 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 computational 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 statistics 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

           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 complete 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 protocol 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-

     The members of struct ifaddr are as follows:

           ifa_addr           (struct sockaddr *) The local address of the

           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

           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 representing 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 manipulated.

     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

     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 represents.  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 membership 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
           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.

           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 is accessed by the
           ifaddr_byindex() function.)  The ifp must have been allocated by

           Shut down and unlink the specified ifp from the interface list.

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

           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 interface type.

           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.

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

           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.

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

           Return an ifnet pointer for the interface named name.

           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.

           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
                                     interface name will update its interface
                                     list.  Caller must have appropriate
                                     privilege.  (No call-down to driver.)
                 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

                 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.

                 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.

                 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.

                 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

   Interface Multicast Address Functions
     The if_addmulti(), if_delmulti(), and if_findmulti() functions provide
     support for requesting and relinquishing multicast group memberships, and
     for querying an interface's membership 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

           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 argument 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.

     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.

     This manual page was written by Garrett A. Wollman.

FreeBSD 11.1-RELEASE-p4         March 22, 2017         FreeBSD 11.1-RELEASE-p4
Command Section