Provided by: freebsd-manpages_8.0-1_all
ifnet, ifaddr, ifqueue, if_data - kernel interfaces for manipulating
Interface Manipulation Functions
struct ifnet *
if_attach(struct ifnet *ifp);
if_detach(struct ifnet *ifp);
if_free(struct ifnet *ifp);
if_free_type(struct ifnet *ifp, u_char type);
if_down(struct ifnet *ifp);
ifioctl(struct socket *so, u_long cmd, caddr_t data, struct thread *td);
ifpromisc(struct ifnet *ifp, int pswitch);
if_allmulti(struct ifnet *ifp, int amswitch);
struct ifnet *
ifunit(const char *name);
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);
ifafree(struct ifaddr *ifa);
IFAFREE(struct ifaddr *ifa);
Interface Multicast Address Functions
if_addmulti(struct ifnet *ifp, struct sockaddr *sa,
struct ifmultiaddr **ifmap);
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
(*if_input)(struct ifnet *ifp, struct mbuf *m);
(*if_output)(struct ifnet *ifp, struct mbuf *m, struct sockaddr *dst,
struct rtentry *rt);
(*if_start)(struct ifnet *ifp);
(*if_transmit)(struct ifnet *ifp, struct mbuf *m);
(*if_qflush)(struct ifnet *ifp);
(*if_ioctl)(struct ifnet *ifp, int cmd, caddr_t data);
(*if_watchdog)(struct ifnet *ifp);
(*if_resolvemulti)(struct ifnet *ifp, struct sockaddr **retsa,
struct sockaddr *addr);
struct ifaddr member function
(*ifa_rtrequest)(int cmd, struct rtentry *rt, struct sockaddr *dst);
extern struct ifnethead ifnet;
extern struct ifaddr **ifnet_addrs;
extern int if_index;
extern int ifqmaxlen;
The kernel mechanisms for handling network interfaces reside primarily in
the ifnet, if_data, ifaddr, and ifmultiaddr structures in #include
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
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_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
if_pcount (int) A count of promiscuous listeners on this
interface, used to reference-count the IFF_PROMISC
if_bpf (struct bpf_if *) Opaque per-interface data for
the packet filter, bpf(4). (Initialized by
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
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
if_linkmiblen (size_t) The size of said structure. (Initialized
if_data (struct if_data) More statistics and information;
see The if_data structure, below. (Initialized by
driver, manipulated by both driver and generic
if_snd (struct ifqueue) The output queue. (Manipulated
There are in addition a number of function pointers which the driver must
initialize to complete its interface with the generic interface layer:
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.
Output a packet on interface ifp, or queue it on the output queue
if the interface is already active.
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
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
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.) Please note that this function will soon be
Not used. We are not even sure what it was ever for. The
prototype is faked.
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
Routine called by the generic code when the watchdog timer,
if_timer, expires. Usually this will reset the interface.
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).
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 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
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
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
IFF_LINK2 〈D〉 Control flags for the link layer. (Currently
abused to select among multiple physical layers on
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
IFCAP_HWCSUM A shorthand for (IFCAP_RXCSUM |
IFCAP_VLAN_HWTAGGING This interface can do VLAN tagging on output
and demultiplex frames by their VLAN tag on
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
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
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
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
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
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
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-
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-
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-
ifi_omcasts Number of packets sent by link-layer multicast.
ifi_iqdrops Number of packets dropped on input. Rarely
ifi_noproto Number of packets received for unknown network-
ifi_lastchange (struct timeval) The time of the last
administrative change to the interface (as required
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_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_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-
The members of struct ifaddr are as follows:
ifa_addr (struct sockaddr *) The local address of the
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_netmask (struct sockaddr *) The network mask for multi-
access interfaces, and the confusion generator for
ifa_ifp (struct ifnet *) A link back to the interface
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
ifa_refcnt (short) The reference count.
ifa_metric (int) A metric associated with this interface
address, for the use of some external routing
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
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
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
Interface Manipulation Functions
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.
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().
Shut down and unlink the specified ifp from the interface list.
Free the given ifp back to the system. The interface must have
been previously detached if it was ever attached.
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.
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.
Mark the interface ifp as up, notify protocols of the transition,
and generate a message from the route(4) routing socket.
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.
As ifpromisc(), but for the all-multicasts (IFF_ALLMULTI) flag
instead of the promiscuous flag.
Return an ifnet pointer for the interface named name.
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():
OSIOCGIFCONF Get interface configuration. (No call-down
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.)
SIOCGIFPHYS Get interface capabilities, flags, metric,
MTU, medium selection. (No call-down to
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
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.
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.
SIOCSIFNETMASK The socket’s protocol control routine is
called to implement the requested action.
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()
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
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
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
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.
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.
This manual page was written by Garrett A. Wollman.