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    Command:

    ifnet

    
         #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);
    
         void
         if_up(struct ifnet *ifp);
    
       Interface Address Functions
         struct ifaddr *
         ifa_ifwithaddr(struct sockaddr *addr);
    
         struct ifaddr *
         ifa_ifwithdstaddr(struct sockaddr *addr);
    
         struct ifaddr *
         ifa_ifwithnet(struct sockaddr *addr);
    
         struct ifaddr *
    
         struct ifmultiaddr *
         ifmaof_ifpforaddr(struct sockaddr *addr, struct ifnet *ifp);
    
       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, struct sockaddr *dst,
                 struct rtentry *rt);
    
         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, int cmd, caddr_t data);
    
         void
         (*if_watchdog)(struct ifnet *ifp);
    
         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 sockaddr *dst);
    
       Global Variables
         extern struct ifnethead ifnet;
         extern struct ifaddr **ifnet_addrs;
         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 #include
         <net/if.h>
         and #include <net/if_var.h>
         nel routines which manipulate interface as such accept or return pointers
         to these structures.  Each interface structure contains an if_data struc-
         ture, which contains statistics and identifying information used by man-
         agement programs, and which is exported to user programs by way of the
         ifmib(4) branch of the sysctl(3) MIB.  Each interface also has a TAILQ of
         interface addresses, described by ifaddr structures; the head of the
         queue is always an AF_LINK address (see link_addr(3)) describing the link
         layer implemented by the interface (if any).  (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_alloc().)
    
               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.  (Initial-
                                ized by driver (usually via if_initname()).)
    
               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 num-
                                ber is not associated with the device.  (Initial-
                                ized by driver (usually via if_initname()).)
    
               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_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 inter-
                                face 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_snd           (struct ifqueue) The output queue.  (Manipulated
                                by driver.)
    
         There are in addition a number of function pointers which the driver must
         initialize to complete 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 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 fram-
               ing, 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 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
               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_done()
               Not used.  We are not even sure what it was ever for.  The proto-
               type is faked.
    
               if_ioctl()
               Process interface-related ioctl(2) requests (defined in 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 flush-
               ing queues.  See the description of ifioctl() below for more infor-
               mation.
    
               if_watchdog()
               Routine called by the generic code when the watchdog timer,
               if_timer, expires.  Usually this will reset the interface.
    
               if_init()
               Initialize and bring up the hardware, e.g., reset the chip and the
               watchdog timer 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 capabili-
         ties; 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 code.
               IFF_LOOPBACK     <S> The interface is a loopback device.
               IFF_POINTOPOINT  <S*> The interface is point-to-point; "broadcast"
                                sions.
               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_POLLING      <D*> The interface is in polling(4) mode.  See
                                Interface Capabilities Flags for details.
    
       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 par-
         ticular 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_NETCONS         This interface can be a network console.
    
               IFCAP_POLLING         This interface supports polling(4).  See
                                     below for details.
    
               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 hard-
                                     ware 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_VLAN_HWTAGGING  This interface can do VLAN tagging on output
                                     and demultiplex frames by their VLAN tag on
                                     input.
    
         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 pack-
                              ets 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.  First, there is a system-wide sysctl(8) master switch named
         kern.polling.enable, which can toggle polling(4) globally.  If that vari-
         able is set to non-zero, polling(4) will be used on those devices where
         it is enabled individually.  Otherwise, polling(4) will not be used in
         the system.  Second, 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, thus initiating switch of the interface
         to polling(4) mode or interrupt mode, respectively.  The actual mode
         change will occur at an implementation-specific moment in the future,
         e.g., during the next interrupt or polling(4) cycle.  And finally, if the
         mode transition has been successful, the flag IFF_POLLING is marked or
         cleared in the interface's if_flags to indicate the current mode of the
         interface.
    
       The if_data Structure
         In 4.4BSD, a subset of the interface information believed to be of inter-
         est to management stations was segregated from the ifnet structure and
         moved into its own if_data structure to facilitate its use by user pro-
         grams.  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:
    
                               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 capabili-
                               ties 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 inter-
                               face 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 inter-
                               faces.  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 break-
                               downs 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 colli-
                               layer multicast.
    
               ifi_omcasts     Number of packets sent by link-layer multicast.
    
               ifi_iqdrops     Number of packets dropped on input.  Rarely imple-
                               mented.
    
               ifi_noproto     Number of packets received for unknown network-
                               layer protocol.
    
               ifi_lastchange  (struct timeval) The time of the last administra-
                               tive change to the interface (as required for
                               SNMP).
    
       Interface Types
         The header file #include <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
    
       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 func-
         tionality across all protocols.  Interface addresses are reference-
                              point-to-point interfaces.
    
               ifa_ifp        (struct ifnet *) A link back to the interface struc-
                              ture.
    
               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 ta-
                              ble.
    
               ifa_refcnt     (short) The reference count.
    
               ifa_metric     (int) A metric associated with this interface
                              address, for the use of some external routing proto-
                              col.
    
         References to ifaddr structures are gained manually, by incrementing the
         ifa_refcnt member.  References are released by calling either the
         ifafree() function or the IFAFREE() macro.
    
         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, resolve, or delete routes.  The cmd argument indi-
         cates the request in question: RTM_ADD, RTM_RESOLVE, or RTM_DELETE.  The
         rt argument is the route in question; the dst argument is the specific
         destination being manipulated for RTM_RESOLVE, or a null pointer other-
         wise.
    
    
    

    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 imple-
         menting a specific link layer over different hardware; see the documenta-
         tion 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 multi-
         cast 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 represents.  The memberships for various
    
               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 inter-
               faces.  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
               global array ifnet_addrs.)  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 interface 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 message 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 promiscuous flag.
    
               ifunit()
               Return an ifnet pointer for the interface named name.
    
               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 ordi-
               narily only called from the socket-layer ioctl(2) handler, and only
                     SIOCGIFCAP
                     SIOCGIFFLAGS
                     SIOCGIFMETRIC
                     SIOCGIFMTU
                     SIOCGIFPHYS      Get interface capabilities, 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 inter-
                                      face, 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.
    
                     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 struc-
                                      ture 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 check-
                                      ing and for actually modifying the MTU in
                                      the interface structure.
    
                     SIOCADDMULTI
                     SIOCDELMULTI     Add or delete permanent multicast group mem-
                                      berships on the interface.  Caller must have
                                      appropriate privilege.  The if_addmulti() or
                                      if_delmulti() function is called to perform
                                      the operation; qq.v.
    
                     SIOCSIFDSTADDR
                     SIOCSIFADDR
    
       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.
    
         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.
    
         ifaof_ifpforaddr() returns the most specific address configured on inter-
         face 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.
    
         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 ifmaof_ifpforaddr() functions pro-
         vide support for requesting and relinquishing multicast group member-
         ships, 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 fol-
         lowing four-step process:
    
               1.   Call the interface's if_resolvemulti() entry point to deter-
                    mine the link-layer address, if any, corresponding to this
                    membership request, and also to give the link layer an oppor-
                    tunity 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 inter-
                    face'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 stan-
         dard error number on failure.
    
    
    

    BSD March 14, 2007 BSD

    
    
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