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NAME

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

SYNOPSIS

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

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

     void
     if_attach(struct ifnet *ifp);

     void
     if_detach(struct ifnet *ifp);

     void
     if_free(struct ifnet *ifp);

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

     void
     if_down(struct ifnet *ifp);

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

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

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

     struct ifnet *
     ifunit(const char *name);

     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
     (*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_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>
     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
     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 #include
     <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 #include <net/if_var.h>
     because it is the prerequisite of some other header file like Most
     references to those two files in particular can be replaced by

     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, 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 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.)

           if_dname         (const char *) The name of the driver.
                            (Initialized by driver.)

           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.)

           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_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_timer         (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.)

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

           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_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
           framing, e.g., Ethernet.

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

           if_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.)

           if_done()
           Not used.  We are not even sure what it was ever for.  The
           prototype 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
           flushing queues.  See the description of ifioctl() below for more
           information.

           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_RUNNING, ~IIF_OACTIVE).

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

   Interface Flags
     Interface flags are used for a number of different purposes.  Some flags
     simply indicate information about the type of interface and its
     capabilities; others are dynamically 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〉.

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

           IFF_UP           〈D〉 The interface has been configured up by the
                            user-level code.
           IFF_BROADCAST    〈S*〉 The interface supports broadcast.
           IFF_DEBUG        〈D〉 Used to enable/disable driver debugging code.
           IFF_LOOPBACK     〈S〉 The interface is a loopback device.
           IFF_POINTOPOINT  〈S*〉 The interface is point-to-point; “broadcast”
                            address is actually the address of the other end.
           IFF_RUNNING      〈D*〉 The interface has been configured and dynamic
                            resources were successfully allocated.  Probably
                            only useful internal to the interface.
           IFF_NOARP        〈D〉 Disable network address resolution on this
                            interface.
           IFF_PROMISC      〈D*〉 This interface is in promiscuous mode.
           IFF_PPROMISC     〈D〉 This interface is in the permanently
                            promiscuous mode (implies IFF_PROMISC).
           IFF_ALLMULTI     〈D*〉 This interface is in all-multicasts mode
                            (used by multicast routers).
           IFF_OACTIVE      〈D*〉 The interface’s hardware output queue (if
                            any) is full; output packets are to be queued.
           IFF_SIMPLEX      〈S*〉 The interface cannot hear its own
                            transmissions.
           IFF_LINK0
           IFF_LINK1
           IFF_LINK2        〈D〉 Control flags for the link layer.  (Currently
                            abused to select among multiple physical layers on
                            some devices.)
           IFF_MULTICAST    〈S*〉 This interface supports multicast.
           IFF_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
     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_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
                                 hardware checksum validation if the MTU is
                                 set above the hardcoded limit.

           IFCAP_TXCSUM          This interface can do checksum calculation on
                                 transmitting data.

           IFCAP_HWCSUM          A shorthand for (IFCAP_RXCSUM |
                                 IFCAP_TXCSUM).

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

           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 specification.

           IFCAP_JUMBO_MTU       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:

           CSUM_IP        The interface will compute IP checksums.

           CSUM_TCP       The interface will compute TCP checksums.

           CSUM_UDP       The interface will compute UDP checksums.

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

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

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

     The capability of a network interface to operate in polling(4) mode
     involves several flags in different global variables and per-interface
     fields.  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 if_data Structure
     In 4.4BSD, a subset of the interface information believed to be of
     interest to management stations was segregated from the ifnet structure
     and moved into its own if_data structure to facilitate its use by user
     programs.  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
                           #include <net/if_types.h>
                           and described below in the Interface Types section.

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

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

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

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

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

   Interface Types
     The header file #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
     functionality across all protocols.  Interface addresses are reference-
     counted.

     The members of struct ifaddr are as follows:

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

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

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

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

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

           ifa_rtrequest  See below.

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

           ifa_refcnt     (short) The reference count.

           ifa_metric     (int) A metric associated with this interface
                          address, for the use of some external routing
                          protocol.

     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
     indicates 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
     otherwise.

FUNCTIONS

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

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

     The elements of the structure are as follows:

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

           ifma_addr      (struct sockaddr *) A pointer to the address which
                          this record 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
           if_alloc()
           Allocate and initialize struct ifnet.  Initialization includes the
           allocation of an interface index and may include the allocation of
           a type specific structure in if_l2com.

           if_attach()
           Link the specified interface ifp into the list of network
           interfaces.  Also initialize the list of addresses on that
           interface, and create a link-layer ifaddr structure to be the first
           element in that list.  (A pointer to this address structure is
           saved in the 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
           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
                 OSIOCGIFCONF     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.)
                 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
                                  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.

                 SIOCSIFFLAGS     Change interface flags.  Caller must have
                                  appropriate privilege.  If a change to the
                                  IFF_UP flag is requested, if_up() or
                                  if_down() is called as appropriate.  Flags
                                  listed in IFF_CANTCHANGE are masked off, and
                                  the field if_flags in the interface
                                  structure is updated.  Finally, the driver
                                  if_ioctl() routine is called to perform any
                                  setup requested.

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

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

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

                 SIOCSIFDSTADDR
                 SIOCSIFADDR
                 SIOCSIFBRDADDR
                 SIOCSIFNETMASK   The socket’s protocol control routine is
                                  called to implement the requested action.

                 OSIOGIFADDR
                 OSIOCGIFDSTADDR
                 OSIOCGIFBRDADDR
                 OSIOCGIFNETMASK  The socket’s protocol control routine is
                                  called to implement the requested action.
                                  On return, 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
     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
     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.

     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, ifp, and a generic address, sa.  It also takes a pointer to a
     struct ifmultiaddr * which is filled in on successful return with the
     address of the group membership control block.  The if_addmulti()
     function performs the following four-step process:

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

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

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

           4.   If the interface’s multicast address filter needs to be
                changed because a new membership was added, call the
                interface’s if_ioctl() routine (with a cmd 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 ifmaof_ifpforaddr() function examines the membership list of
     interface ifp for an address matching addr, and returns a pointer to that
     struct ifmultiaddr if one is found, else it returns a null pointer.

SEE ALSO

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

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

AUTHORS

     This manual page was written by Garrett A. Wollman.