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IFNET(9)		 BSD Kernel Developer's Manual		      IFNET(9)

NAME
     ifnet, ifaddr, ifqueue, if_data — kernel interfaces for manipulating net‐
     work 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 partic‐
     ular 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 ker‐
     nel routines which manipulate interface as such accept or return pointers
     to these structures.  Each interface structure contains an if_data struc‐
     ture, which contains statistics and identifying information used by man‐
     agement programs, and which is exported to user programs by way of the
     ifmib(4) branch of the sysctl(3) MIB.  Each interface also has a TAILQ of
     interface addresses, described by ifaddr structures; the head of the
     queue is always an AF_LINK address (see link_addr(3)) describing the link
     layer implemented by the interface (if any).  (Some trivial interfaces do
     not provide any link layer addresses; this structure, while still
     present, serves only to identify the interface name and index.)

     Finally, those interfaces supporting reception of multicast datagrams
     have a TAILQ of multicast group memberships, described by ifmultiaddr
     structures.  These memberships are reference-counted.

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

   The ifnet structure
     The fields of struct ifnet are as follows:

	   if_softc	    (void *) A pointer to the driver's private state
			    block.  (Initialized by driver.)

	   if_l2com	    (void *) A pointer to the common data for the
			    interface's layer 2 protocol.  (Initialized by
			    if_alloc().)

	   if_link	    (TAILQ_ENTRY(ifnet)) queue(3) macro glue.

	   if_xname	    (char *) The name of the interface, (e.g., “fxp0”
			    or “lo0”).	(Initialized by driver (usually via
			    if_initname()).)

	   if_dname	    (const char *) The name of the driver.  (Initial‐
			    ized by driver (usually via if_initname()).)

	   if_dunit	    (int) A unique number assigned to each interface
			    managed by a particular driver.  Drivers may
			    choose to set this to IF_DUNIT_NONE if a unit num‐
			    ber is not associated with the device.  (Initial‐
			    ized by driver (usually via if_initname()).)

	   if_addrhead	    (struct ifaddrhead) The head of the queue(3) TAILQ
			    containing the list of addresses assigned to this
			    interface.

	   if_pcount	    (int) A count of promiscuous listeners on this
			    interface, used to reference-count the IFF_PROMISC
			    flag.

	   if_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 inter‐
			    face 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 inter‐
			    face supports (see below).

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

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

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

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

	   if_snd	    (struct ifqueue) The output queue.	(Manipulated
			    by driver.)

     There are in addition a number of function pointers which the driver must
     initialize to complete its interface with the generic interface layer:

	   if_input()
	   Pass a packet to an appropriate upper layer as determined from the
	   link-layer header of the packet.  This routine is to be called from
	   an interrupt handler or used to emulate reception of a packet on
	   this interface.  A single function implementing if_input() can be
	   shared among multiple drivers utilizing the same link-layer fram‐
	   ing, e.g., Ethernet.

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

	   if_transmit()
	   Transmit a packet on an interface or queue it if the interface is
	   in use.  This function will return ENOBUFS if the devices software
	   and hardware queues are both full.  This function must be installed
	   after if_attach() to override the default implementation.  This
	   function is exposed in order to allow drivers to manage their own
	   queues and to reduce the latency caused by a frequently gratuitous
	   enqueue / dequeue pair to ifq.  The suggested internal software
	   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 proto‐
	   type 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 capabili‐
     ties; others are dynamically manipulated to reflect the current state of
     the interface.  Flags of the former kind are marked ⟨S⟩ in this table;
     the latter are marked ⟨D⟩.	 Flags which begin with “IFF_DRV_” are stored
     in if_drv_flags; all other flags are stored in if_flags.

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

	   IFF_UP	    ⟨D⟩ The interface has been configured up by the
			    user-level code.
	   IFF_BROADCAST    ⟨S*⟩ The interface supports broadcast.
	   IFF_DEBUG	    ⟨D⟩ Used to enable/disable driver debugging code.
	   IFF_LOOPBACK	    ⟨S⟩ The interface is a loopback device.
	   IFF_POINTOPOINT  ⟨S*⟩ The interface is point-to-point; “broadcast”
			    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 promiscu‐
			    ous 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 transmis‐
			    sions.
	   IFF_LINK0
	   IFF_LINK1
	   IFF_LINK2	    ⟨D⟩ Control flags for the link layer.  (Currently
			    abused to select among multiple physical layers on
			    some devices.)
	   IFF_MULTICAST    ⟨S*⟩ This interface supports multicast.
	   IFF_POLLING	    ⟨D*⟩ The interface is in polling(4) mode.  See
			    Interface Capabilities Flags for details.

   Interface Capabilities Flags
     Interface capabilities are specialized features an interface may or may
     not support.  These capabilities are very hardware-specific and allow,
     when enabled, to offload specific network processing to the interface or
     to offer a particular feature for use by other kernel parts.

     It should be stressed that a capability can be completely uncontrolled
     (i.e., stay always enabled with no way to disable it) or allow limited
     control over itself (e.g., depend on another capability's state.)	Such
     peculiarities are determined solely by the hardware and driver of a par‐
     ticular interface.	 Only the driver possesses the knowledge on whether
     and how the interface capabilities can be controlled.  Consequently,
     capabilities flags in if_capenable should never be modified directly by
     kernel code other than the interface driver.  The command SIOCSIFCAP to
     ifioctl() is the dedicated means to attempt altering if_capenable on an
     interface.	 Userland code shall use ioctl(2).

     The following capabilities are currently supported by the system:

	   IFCAP_NETCONS	 This interface can be a network console.

	   IFCAP_POLLING	 This interface supports polling(4).  See
				 below for details.

	   IFCAP_RXCSUM		 This interface can do checksum validation on
				 receiving data.  Some interfaces do not have
				 sufficient buffer storage to store frames
				 above a certain MTU-size completely.  The
				 driver for the interface might disable hard‐
				 ware checksum validation if the MTU is set
				 above the hardcoded limit.

	   IFCAP_TXCSUM		 This interface can do checksum calculation on
				 transmitting data.

	   IFCAP_HWCSUM		 A shorthand for (IFCAP_RXCSUM |
				 IFCAP_TXCSUM).

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

	   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 some‐
				 what 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 computa‐
     tional 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 pack‐
			  ets if necessary.  The host CPU does not need to
			  care about MTU on this interface as long as a packet
			  to transmit through it is an IP one and it does not
			  exceed the size of the hardware buffer.

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

     The capability of a network interface to operate in polling(4) mode
     involves several flags in different global variables and per-interface
     fields.  First, there is a system-wide sysctl(8) master switch named
     kern.polling.enable, which can toggle polling(4) globally.	 If that vari‐
     able is set to non-zero, polling(4) will be used on those devices where
     it is enabled individually.  Otherwise, polling(4) will not be used in
     the system.  Second, the capability flag IFCAP_POLLING set in interface's
     if_capabilities indicates support for polling(4) on the particular inter‐
     face.  If set in if_capabilities, the same flag can be marked or cleared
     in the interface's if_capenable, thus initiating switch of the interface
     to polling(4) mode or interrupt mode, respectively.  The actual mode
     change will occur at an implementation-specific moment in the future,
     e.g., during the next interrupt or polling(4) cycle.  And finally, if the
     mode transition has been successful, the flag IFF_POLLING is marked or
     cleared in the interface's if_flags to indicate the current mode of the
     interface.

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

	   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 phys‐
			   ical 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 ini‐
			   tialized 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 capabili‐
			   ties to offload computational tasks for outgoing
			   packets.  The interface driver must keep this field
			   in accord with the current value of if_capenable.

	   ifi_epoch	   (time_t) The system uptime when interface was
			   attached or the statistics below were reset.	 This
			   is intended to be used to set the SNMP variable
			   ifCounterDiscontinuityTime.	It may also be used to
			   determine if two successive queries for an inter‐
			   face of the same index have returned results for
			   the same interface.

     The structure additionally contains generic statistics applicable to a
     variety of different interface types (except as noted, all members are of
     type u_long):

	   ifi_link_state  (u_char) The current link state of Ethernet inter‐
			   faces.  See the Interface Link States section for
			   possible values.

	   ifi_ipackets	   Number of packets received.

	   ifi_ierrors	   Number of receive errors detected (e.g., FCS
			   errors, DMA overruns, etc.).	 More detailed break‐
			   downs can often be had by way of a link-specific
			   MIB.

	   ifi_opackets	   Number of packets transmitted.

	   ifi_oerrors	   Number of output errors detected (e.g., late colli‐
			   sions, DMA overruns, etc.).	More detailed break‐
			   downs 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 imple‐
			   mented.

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

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

   Interface Types
     The header file <net/if_types.h> defines symbolic constants for a number
     of different types of interfaces.	The most common are:

	   IFT_OTHER	    none of the following
	   IFT_ETHER	    Ethernet
	   IFT_ISO88023	    ISO 8802-3 CSMA/CD
	   IFT_ISO88024	    ISO 8802-4 Token Bus
	   IFT_ISO88025	    ISO 8802-5 Token Ring
	   IFT_ISO88026	    ISO 8802-6 DQDB MAN
	   IFT_FDDI	    FDDI
	   IFT_PPP	    Internet Point-to-Point Protocol (ppp(8))
	   IFT_LOOP	    The loopback (lo(4)) interface
	   IFT_SLIP	    Serial Line IP
	   IFT_PARA	    Parallel-port IP (“PLIP”)
	   IFT_ATM	    Asynchronous Transfer Mode

   Interface Link States
     The following link states are currently defined:

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

   The ifaddr Structure
     Every interface is associated with a list (or, rather, a TAILQ) of
     addresses, rooted at the interface structure's if_addrlist member.	 The
     first element in this list is always an AF_LINK address representing the
     interface itself; multi-access network drivers should complete this
     structure by filling in their link-layer addresses after calling
     if_attach().  Other members of the structure represent network-layer
     addresses which have been configured by means of the SIOCAIFADDR command
     to ioctl(2), called on a socket of the appropriate protocol family.  The
     elements of this list consist of ifaddr structures.  Most protocols will
     declare their own protocol-specific interface address structures, but all
     begin with a struct ifaddr which provides the most-commonly-needed func‐
     tionality across all protocols.  Interface addresses are reference-
     counted.

     The members of struct ifaddr are as follows:

	   ifa_addr	  (struct sockaddr *) The local address of the inter‐
			  face.

	   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 struc‐
			  ture.

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

	   ifa_rtrequest  See below.

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

	   ifa_refcnt	  (short) The reference count.

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

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

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

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

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

     The elements of the structure are as follows:

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

	   ifma_addr	  (struct sockaddr *) A pointer to the address which
			  this record represents.  The memberships for various
			  address families are stored in arbitrary order.

	   ifma_lladdr	  (struct sockaddr *) A pointer to the link-layer mul‐
			  ticast 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 mem‐
			  bership also holds an invisible reference to another
			  membership for that link-layer address.

	   ifma_refcount  (u_int) A reference count of requests for this par‐
			  ticular 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 inter‐
	   faces.  Also initialize the list of addresses on that interface,
	   and create a link-layer ifaddr structure to be the first element in
	   that list.  (A pointer to this address structure is saved in the
	   global array ifnet_addrs.)  The ifp must have been allocated by
	   if_alloc().

	   if_detach()
	   Shut down and unlink the specified ifp from the interface list.

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

	   if_free_type()
	   Identical to if_free() except that the given type is used to free
	   if_l2com instead of the type in if_type.  This is intended for use
	   with drivers that change their interface type.

	   if_down()
	   Mark the interface ifp as down (i.e., IFF_UP is not set), flush its
	   output queue, notify protocols of the transition, and generate a
	   message from the route(4) routing socket.

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

	   ifpromisc()
	   Add or remove a promiscuous reference to ifp.  If pswitch is true,
	   add a reference; if it is false, remove a reference.	 On reference
	   count transitions from zero to one and one to zero, set the
	   IFF_PROMISC flag appropriately and call if_ioctl() to set up the
	   interface in the desired mode.

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

	   ifunit()
	   Return an ifnet pointer for the interface named name.

	   ifioctl()
	   Process the ioctl request cmd, issued on socket so by thread td,
	   with data parameter data.  This is the main routine for handling
	   all interface configuration requests from user mode.	 It is ordi‐
	   narily only called from the socket-layer ioctl(2) handler, and only
	   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 inter‐
				  face 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 inter‐
				  face, if_capabilities.  Requesting to enable
				  an unsupported capability is invalid.	 The
				  rest is supposed to be done by the driver,
				  which includes updating if_capenable and
				  if_data.ifi_hwassist appropriately.

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

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

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

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

		 SIOCSIFDSTADDR
		 SIOCSIFADDR
		 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 inter‐
     face ifp which matches address addr, subject to its configured netmask.
     If the interface is point-to-point, only an interface address whose
     remote address is precisely addr will be returned.

     All of these functions return a null pointer if no such address can be
     found.

   Interface Multicast Address Functions
     The if_addmulti(), if_delmulti(), and ifmaof_ifpforaddr() functions pro‐
     vide support for requesting and relinquishing multicast group member‐
     ships, and for querying an interface's membership list, respectively.
     The if_addmulti() function takes a pointer to an interface, ifp, and a
     generic address, sa.  It also takes a pointer to a struct ifmultiaddr *
     which is filled in on successful return with the address of the group
     membership control block.	The if_addmulti() function performs the fol‐
     lowing four-step process:

	   1.	Call the interface's if_resolvemulti() entry point to deter‐
		mine the link-layer address, if any, corresponding to this
		membership request, and also to give the link layer an oppor‐
		tunity to veto this membership request should it so desire.

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

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

	   4.	If the interface's multicast address filter needs to be
		changed because a new membership was added, call the inter‐
		face's if_ioctl() routine (with a cmd argument of
		SIOCADDMULTI) to request that it do so.

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

     The ifmaof_ifpforaddr() function examines the membership list of inter‐
     face 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.

BSD				March 14, 2007				   BSD
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