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ifnet (9)
  • >> ifnet (9) ( FreeBSD man: Ядро )

  • BSD mandoc


     - 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);
    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 * ifaof_ifpforaddr (struct sockaddr *addr struct ifnet *ifp);
    void ifafree (struct ifaddr *ifa);
    IFAFREE (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 * ifmaof_ifpforaddr (struct sockaddr *addr struct ifnet *ifp);

    Output queue macros

    IF_DEQUEUE (struct ifqueue *ifq struct mbuf *m);

    struct ifnet Member Functions

    void Fn *if_input struct ifnet *ifp struct mbuf *m int Fo *if_output Fa struct ifnet *ifp struct mbuf *m Fa struct sockaddr *dst struct rtentry *rt Fc Ft void Fn *if_start struct ifnet *ifp int Fn *if_ioctl struct ifnet *ifp int cmd caddr_t data void Fn *if_watchdog struct ifnet *ifp void Fn *if_init void *if_softc int Fo *if_resolvemulti Fa struct ifnet *ifp struct sockaddr **retsa struct sockaddr *addr Fc  

    struct ifaddr member function

    void Fo *ifa_rtrequest Fa int cmd struct rtentry *rt struct sockaddr *dst Fc  

    Global Variables

    Vt extern struct ifnethead ifnet ; Vt extern struct ifaddr **ifnet_addrs ; Vt extern int if_index ; Vt extern int ifqmaxlen ;  


    The kernel mechanisms for handling network interfaces reside primarily in the Vt ifnet , if_data , ifaddr , and Vt ifmultiaddr structures in    #include <net/if.h>
    and    #include <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 In net/if.h ; these include the interface flags, the Vt 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    #include <net/if_var.h>
    defines the kernel-internal interfaces, including the Vt ifnet , ifaddr , and Vt ifmultiaddr structures and the functions which manipulate them. (A few user programs will need    #include <net/if_var.h>
    because it is the prerequisite of some other header file like In netinet/if_ether.h . Most references to those two files in particular can be replaced by In 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 Vt struct ifnethead called ifnet The elements of this list are of type Vt struct ifnet , and most kernel routines which manipulate interface as such accept or return pointers to these structures. Each interface structure contains an Vt if_data structure, which 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. Each interface also has a TAILQ of interface addresses, described by Vt ifaddr structures; the head of the queue is always an AF_LINK address (see link_addr3) 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 Vt ifmultiaddr structures. These memberships are reference-counted.

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


    The Vt ifnet  


    The fields of Vt struct ifnet are as follows:

    (Vt void * ) A pointer to the driver's private state block. (Initialized by driver.)
    (Vt void * ) A pointer to the common data for the interface's layer 2 protocol. (Initialized by if_alloc (. ));
    (Fn TAILQ_ENTRY ifnet ) queue(3) macro glue.
    (Vt char * ) The name of the interface, (e.g., ``fxp0 '' or ``lo0 '' (Initialized by driver.)
    (Vt const char * ) The name of the driver. (Initialized by driver.)
    (Vt 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.)
    (Vt struct ifaddrhead ) The head of the queue(3) TAILQ containing the list of addresses assigned to this interface.
    (Vt int ) A count of promiscuous listeners on this interface, used to reference-count the IFF_PROMISC flag.
    (Vt struct bpf_if * ) Opaque per-interface data for the packet filter, bpf(4). (Initialized by bpf_attach (. ));
    (Vt u_short ) A unique number assigned to each interface in sequence as it is attached. This number can be used in a Vt struct sockaddr_dl to refer to a particular interface by index (see link_addr3). (Initialized by if_alloc (. ));
    (Vt short ) Number of seconds until the watchdog timer if_watchdog ();
    is called, or zero if the timer is disabled. (Set by driver, decremented by generic watchdog code.)
    (Vt int ) Flags describing operational parameters of this interface (see below). (Manipulated by both driver and generic code.)
    (Vt int ) Flags describing the capabilities the interface supports (see below).
    (Vt int ) Flags describing the enabled capabilities of the interface (see below).
    (Vt void * ) A pointer to an interface-specific MIB structure exported by ifmib(4). (Initialized by driver.)
    (Vt size_t ) The size of said structure. (Initialized by driver.)
    (Vt struct if_data ) More statistics and information; see Sx The if_data structure , below. (Initialized by driver, manipulated by both driver and generic code.)
    (Vt 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:

    Fn 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 framing, e.g., Ethernet.
    Fn if_output
    Output a packet on interface Fa ifp , or queue it on the output queue if the interface is already active.
    Fn 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_OACTIVE flag is not set. (Thus, IFF_OACTIVE does not literally mean that output is active, but rather that the device's internal output queue is full.)
    Fn if_done
    Not used. We are not even sure what it was ever for. The prototype is faked.
    Fn if_ioctl
    Process interface-related ioctl(2) requests (defined in 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.
    Fn if_watchdog
    Routine called by the generic code when the watchdog timer, if_timer expires. Usually this will reset the interface.
    Fn 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_RUNNING , ~IIF_OACTIVE )
    Fn if_resolvemulti
    Check the requested multicast group membership, Fa addr , for validity, and if necessary compute a link-layer group which corresponds to that address which is returned in Fa *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 Aq S in this table; the latter are marked Aq D .

    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.

    Aq D The interface has been configured up by the user-level code.
    Aq S* The interface supports broadcast.
    Aq D Used to enable/disable driver debugging code.
    Aq S The interface is a loopback device.
    Aq S* The interface is point-to-point; ``broadcast'' address is actually the address of the other end.
    Aq D* The interface has been configured and dynamic resources were successfully allocated. Probably only useful internal to the interface.
    Aq D Disable network address resolution on this interface.
    Aq D* This interface is in promiscuous mode.
    Aq D This interface is in the permanently promiscuous mode (implies IFF_PROMISC )
    Aq D* This interface is in all-multicasts mode (used by multicast routers).
    Aq D* The interface's hardware output queue (if any) is full; output packets are to be queued.
    Aq S* The interface cannot hear its own transmissions.
    Aq D Control flags for the link layer. (Currently abused to select among multiple physical layers on some devices.)
    Aq S* This interface supports multicast.
    Aq D* The interface is in polling(4) mode. See Sx 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 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:

    This interface can be a network console.
    This interface supports polling(4). See below for details.
    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.
    This interface can do checksum calculation on transmitting data.
    A shorthand for (IFCAP_RXCSUM | IFCAP_TXCSUM )
    This interface can do VLAN tagging on output and demultiplex frames by their VLAN tag on input.
    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 specification.
    This Ethernet interface can transmit and receive frames up to 9000 bytes long.

    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:

    The interface will compute IP checksums.
    The interface will compute TCP checksums.
    The interface will compute UDP checksums.
    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
    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 Vt 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 variable 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 interface. 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 Vt if_data  


    In BSD 4.4 a subset of the interface information believed to be of interest to management stations was segregated from the Vt ifnet structure and moved into its own Vt if_data structure to facilitate its use by user programs. The following elements of the Vt if_data structure are initialized by the interface and are not expected to change significantly over the course of normal operation:

    (Vt u_char ) The type of the interface, as defined in    #include <net/if_types.h>
    and described below in the Sx Interface Types section.
    (Vt u_char ) Intended to represent a selection of physical layers on devices which support more than one; never implemented.
    (Vt 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 Vt sockaddr_dl structures referring to this interface.
    (Vt 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 Vt mbuf Ns s to attempt to ensure that there is always sufficient space to prepend a link-layer header without allocating an additional Vt mbuf .
    (Vt u_char ) Length of the Vt if_data structure. Allows some stabilization of the routing socket ABI in the face of increases in the length of Vt struct ifdata .
    (Vt u_long ) The maximum transmission unit of the medium, exclusive of any link-layer overhead.
    (Vt u_long ) A dimensionless metric interpreted by a user-mode routing process.
    (Vt u_long ) The line rate of the interface, in bits per second.
    (Vt 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
    (Vt 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 Vt u_long ) :

    (Vt u_char ) The current link state of Ethernet interfaces. See the Sx Interface Link States section for possible values.
    Number of packets received.
    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.
    Number of packets transmitted.
    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.
    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.)
    Total traffic received, in bytes.
    Total traffic transmitted, in bytes.
    Number of packets received which were sent by link-layer multicast.
    Number of packets sent by link-layer multicast.
    Number of packets dropped on input. Rarely implemented.
    Number of packets received for unknown network-layer protocol.
    (Vt struct timeval ) The time of the last administrative 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:

    none of the following
    ISO 8802-3 CSMA/CD
    ISO 8802-4 Token Bus
    ISO 8802-5 Token Ring
    ISO 8802-6 DQDB MAN
    Internet Point-to-Point Protocol (ppp(8) )
    The loopback (lo(4) ) interface
    Serial Line IP
    Parallel-port IP (``PLIP '' )
    Asynchronous Transfer Mode


    Interface Link States

    The following link states are currently defined:

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


    The Vt ifaddr  


    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 Vt ifaddr structures. Most protocols will declare their own protocol-specific interface address structures, but all begin with a Vt struct ifaddr which provides the most-commonly-needed functionality across all protocols. Interface addresses are reference-counted.

    The members of Vt struct ifaddr are as follows:

    (Vt struct sockaddr * ) The local address of the interface.
    (Vt 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 .
    (Vt struct sockaddr * ) The network mask for multi-access interfaces, and the confusion generator for point-to-point interfaces.
    (Vt struct ifnet * ) A link back to the interface structure.
    (Fn TAILQ_ENTRY ifaddr ) queue(3) glue for list of addresses on each interface.
    See below.
    (Vt u_short ) Some of the flags which would be used for a route representing this address in the route table.
    (Vt short ) The reference count.
    (Vt int ) A metric associated with this interface address, for the use of some external routing protocol.

    References to Vt ifaddr structures are gained manually, by incrementing the ifa_refcnt member. References are released by calling either the ifafree ();
    function or the IFAFREE ();

    ifa_rtrequest ();
    is a pointer to a function which receives callouts from the routing code (Fn rtrequest ) to perform link-layer-specific actions upon requests to add, resolve, or delete routes. The Fa cmd argument indicates the request in question: RTM_ADD , RTM_RESOLVE or RTM_DELETE The Fa rt argument is the route in question; the Fa dst argument is the specific destination being manipulated for RTM_RESOLVE or a null pointer otherwise.  


    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 Vt ifmultiaddr  


    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:

    (Fn LIST_ENTRY ifmultiaddr ) queue(3) macro glue.
    (Vt struct sockaddr * ) A pointer to the address which this record represents. The memberships for various address families are stored in arbitrary order.
    (Vt 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.
    (Vt u_int ) A reference count of requests for this particular membership.


    Interface Manipulation Functions

    Fn if_alloc
    Allocate and initialize Vt struct ifnet . Initialization includes the allocation of an interface index and may include the allocation of a Fa type specific structure in if_l2com
    Fn if_attach
    Link the specified interface Fa ifp into the list of network interfaces. Also initialize the list of addresses on that interface, and create a link-layer Vt 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 Fa ifp must have been allocated by if_alloc (.);
    Fn if_detach
    Shut down and unlink the specified Fa ifp from the interface list.
    Fn if_free
    Free the given Fa ifp back to the system. The interface must have been previously detached if it was ever attached.
    Fn if_free_type
    Identical to if_free ();
    except that the given Fa 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.
    Fn if_down
    Mark the interface Fa 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.
    Fn if_up
    Mark the interface Fa ifp as up, notify protocols of the transition, and generate a message from the route(4) routing socket.
    Fn ifpromisc
    Add or remove a promiscuous reference to Fa ifp . If Fa 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.
    Fn if_allmulti
    As ifpromisc (,);
    but for the all-multicasts (IFF_ALLMULTI ) flag instead of the promiscuous flag.
    Fn ifunit
    Return an Vt ifnet pointer for the interface named Fa name .
    Fn ifioctl
    Process the ioctl request Fa cmd , issued on socket Fa so by thread Fa td , with data parameter Fa 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 Fa so Ns 's protocol for further interpretation. The following commands are handled by ifioctl (:);

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

    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.)
    Get interface capabilities, flags, metric, MTU, medium selection. (No call-down to driver.)

    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.

    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.

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

    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.

    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.

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

    The socket's protocol control routine is called to implement the requested action. On return, Vt sockaddr structures are converted into old-style (no sa_len member).

    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 Fa addr . ifa_ifwithdstaddr ();
    returns an interface address for a point-to-point interface whose remote (``destination'' ) address is Fa addr .

    ifa_ifwithnet ();
    returns the most specific interface address which matches the specified address, Fa addr , subject to its configured netmask, or a point-to-point interface address whose remote address is Fa addr if one is found.

    ifaof_ifpforaddr ();
    returns the most specific address configured on interface Fa ifp which matches address Fa addr , subject to its configured netmask. If the interface is point-to-point, only an interface address whose remote address is precisely Fa 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 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, Fa ifp , and a generic address, Fa sa . It also takes a pointer to a Vt 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 Fa cmd argument of SIOCADDMULTI to request that it do so.

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

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


    ioctl(2), link_addr3, 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 W. Richard Stevens TCP/IP Illustrated Vol. 2 Addison-Wesley, ISBN 0-201-63354-X


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



    Interface Manipulation Functions
    Interface Address Functions
    Interface Multicast Address Functions
    Output queue macros
    struct ifnet Member Functions
    struct ifaddr member function
    Global Variables
    The Vt ifnet Ss structure
    Interface Flags
    Interface Capabilities Flags
    The Vt if_data Ss Structure
    Interface Types
    Interface Link States
    The Vt ifaddr Ss Structure
    The Vt ifmultiaddr Ss Structure
    Interface Manipulation Functions
    Interface Address Functions
    Interface Multicast Address Functions

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