IFNET(9)

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

SYNOPSIS
#include <sys/types.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
void
if_attach(struct ifnet *ifp);
void
if_down(struct ifnet *ifp);
int
ifioctl(struct socket *so, u_long cmd, caddr_t data, struct proc *p);
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);
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
int
(*if_output)(struct ifnet *ifp, struct mbuf *m, struct sockaddr *dst, struct rtentry *rt);
void
(*if_input)(struct ifnet *ifp, struct mbuf *m, const struct pktinfo *pi, int cpuid);
void
(*if_start)(struct ifnet *ifp, struct ifaltq_subque *ifsq);
int
(*if_ioctl)(struct ifnet *ifp, u_long command, caddr_t data, struct ucred *cr);
void
(*if_watchdog)(struct ifnet *ifp);
void
(*if_init)(void *if_softc);
int
(*if_resolvemulti)(struct ifnet *ifp, struct sockaddr **retsa, struct sockaddr *addr);
void
(*if_poll)(struct ifnet *ifp, enum poll_cmd cmd, int count);

  struct ifaddr member function
void
(*ifa_rtrequest)(int cmd, struct rtentry *rt, struct sockaddr *dst);

  Global Variables
extern struct ifnethead ifnet;
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 LIST 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_link
    (TAILQ_ENTRY(ifnet)) queue(3) macro glue.

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

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

if_dunit
    (int) A unique number assigned to each interface managed by a particular driver. Drivers may choose to set this to IF_DUNIT_NONE if a unit number is not associated with the device. (Initialized by driver.)

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

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

if_carp
    (struct carp_if *) Per-interface data for carp(4).

if_bpf
    (struct bpf_if *) Opaque per-interface data for the packet filter, bpf(4). (Initialized by bpfattach().)

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)).

if_timer
    (short) Number of seconds until the watchdog timer if_watchdog() is called, or zero if the timer is disabled. (Set by driver, decremented by generic watchdog code.)

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

if_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 ifaltq) The output queue including altq(4). (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_output()
Output a packet on interface ifp, or queue it on the output queue if the interface is already active.

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

if_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_RUNNING, ~IFF_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>.

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

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 listing above.

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 initialize 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_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_link_state
    (u_long) The link state of the interface, either LINK_STATE_UNKNOWN, LINK_STATE_DOWN, or LINK_STATE_UP.

ifi_baudrate
    (uint64_t) The line rate of the interface, in bits per second.

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_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

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_containers
    (struct ifaddr_container *) A pointer to an array of ifaddr_container structures which hold per-CPU data.

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_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 of the according ifa_containers structure (such as by calling the IFAREF() macro). References are released by calling 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-NULL, 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_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.)

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

SIOCGIFFLAGS
SIOCGIFMETRIC
SIOCGIFMTU
SIOCGIFPHYS
    Get interface flags, metric, MTU, medium selection. (No call-down to driver.)

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 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.

if_down(), ifioctl(), ifpromisc(), and if_up() must be called inside a critical section.

  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), ppp(8), rtentry(9) Gary R. Wright, 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.