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

     The system keeps a linked list of interfaces using the TAILQ macros defined in queue(3);
     this list is headed by a struct ifnethead called ifnet.  The elements of this list are of
     type struct ifnet, and most kernel routines which manipulate interface as such accept or
     return pointers to these structures.  Each interface structure contains an if_data
     structure, 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 (usually via if_initname()).)

           if_dname         (const char *) The name of the driver.  (Initialized 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 number is not associated with the device.  (Initialized 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_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 generic code.)

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

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

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

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

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

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

           if_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_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 queueing mechanism is buf_ring.

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

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

           if_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 <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.

           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 capabilities; others are dynamically
     manipulated to reflect the current state of the interface.  Flags of the former kind are
     marked ⟨S⟩ in this table; the latter are marked ⟨D⟩.  Flags which begin with “IFF_DRV_” are
     stored in if_drv_flags; all other flags are stored in if_flags.

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

           IFF_UP           ⟨D⟩ The interface has been configured up by the user-level code.
           IFF_BROADCAST    ⟨S*⟩ The interface supports broadcast.
           IFF_DEBUG        ⟨D⟩ Used to enable/disable driver debugging 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_DRV_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_DRV_OACTIVE  ⟨D*⟩ The interface's hardware output queue (if any) is full; output
                            packets are to be queued.
           IFF_SIMPLEX      ⟨S*⟩ The interface cannot hear its own transmissions.
           IFF_LINK0
           IFF_LINK1
           IFF_LINK2        ⟨D⟩ Control flags for the link layer.  (Currently abused to select
                            among multiple physical layers on some devices.)
           IFF_MULTICAST    ⟨S*⟩ This interface supports multicast.
           IFF_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 <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 <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.