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NETINTRO(4)            FreeBSD Kernel Interfaces Manual            NETINTRO(4)

     networking - introduction to networking facilities

     #include <sys/types.h>
     #include <sys/time.h>
     #include <sys/socket.h>
     #include <net/if.h>
     #include <net/route.h>

     This section is a general introduction to the networking facilities
     available in the system.  Documentation in this part of section 4 is
     broken up into three areas: protocol families (domains), protocols, and
     network interfaces.

     All network protocols are associated with a specific protocol family.  A
     protocol family provides basic services to the protocol implementation to
     allow it to function within a specific network environment.  These
     services may include packet fragmentation and reassembly, routing,
     addressing, and basic transport.  A protocol family may support multiple
     methods of addressing, though the current protocol implementations do
     not.  A protocol family is normally comprised of a number of protocols,
     one per socket(2) type.  It is not required that a protocol family
     support all socket types.  A protocol family may contain multiple
     protocols supporting the same socket abstraction.

     A protocol supports one of the socket abstractions detailed in socket(2).
     A specific protocol may be accessed either by creating a socket of the
     appropriate type and protocol family, or by requesting the protocol
     explicitly when creating a socket.  Protocols normally accept only one
     type of address format, usually determined by the addressing structure
     inherent in the design of the protocol family/network architecture.
     Certain semantics of the basic socket abstractions are protocol specific.
     All protocols are expected to support the basic model for their
     particular socket type, but may, in addition, provide non-standard
     facilities or extensions to a mechanism.  For example, a protocol
     supporting the SOCK_STREAM abstraction may allow more than one byte of
     out-of-band data to be transmitted per out-of-band message.

     A network interface is similar to a device interface.  Network interfaces
     comprise the lowest layer of the networking subsystem, interacting with
     the actual transport hardware.  An interface may support one or more
     protocol families and/or address formats.  The SYNOPSIS section of each
     network interface entry gives a sample specification of the related
     drivers for use in providing a system description to the config(8)
     program.  The DIAGNOSTICS section lists messages which may appear on the
     console and/or in the system error log, /var/log/messages (see
     syslogd(8)), due to errors in device operation.

     The system currently supports the Internet protocols, the Xerox Network
     Systems(tm) protocols, and some of the ISO OSI protocols.  Raw socket
     interfaces are provided to the IP protocol layer of the Internet, and to
     the IDP protocol of Xerox NS.  Consult the appropriate manual pages in
     this section for more information regarding the support for each protocol

     Associated with each protocol family is an address format.  All network
     addresses adhere to a general structure, called a sockaddr, described
     below.  However, each protocol imposes finer and more specific structure,
     generally renaming the variant, which is discussed in the protocol family
     manual page alluded to above.

           struct sockaddr {
               u_char      sa_len;
               u_char      sa_family;
               char        sa_data[14];

     The field sa_len contains the total length of the structure, which may
     exceed 16 bytes.  The following address values for sa_family are known to
     the system (and additional formats are defined for possible future

     #define    AF_UNIX      1    /* local to host (pipes, portals) */
     #define    AF_INET      2    /* internetwork: UDP, TCP, etc. */
     #define    AF_NS        6    /* Xerox NS protocols */
     #define    AF_CCITT     10   /* CCITT protocols, X.25 etc */
     #define    AF_HYLINK    15   /* NSC Hyperchannel */
     #define    AF_ISO       18   /* ISO protocols */

     FreeBSD provides some packet routing facilities.  The kernel maintains a
     routing information database, which is used in selecting the appropriate
     network interface when transmitting packets.

     A user process (or possibly multiple co-operating processes) maintains
     this database by sending messages over a special kind of socket.  This
     supplants fixed size ioctl(2) used in earlier releases.

     This facility is described in route(4).

     Each network interface in a system corresponds to a path through which
     messages may be sent and received.  A network interface usually has a
     hardware device associated with it, though certain interfaces such as the
     loopback interface, lo(4), do not.

     The following ioctl(2) calls may be used to manipulate network
     interfaces.  The ioctl() is made on a socket (typically of type
     SOCK_DGRAM) in the desired domain.  Most of the requests supported in
     earlier releases take an ifreq structure as its parameter.  This
     structure has the form

     struct  ifreq {
     #define    IFNAMSIZ    16
         char    ifr_name[IFNAMSIZ];        /* if name, e.g. "en0" */
         union {
             struct    sockaddr ifru_addr;
             struct    sockaddr ifru_dstaddr;
             struct    sockaddr ifru_broadaddr;
             struct    ifreq_buffer ifru_buffer;
             short     ifru_flags[2];
             short     ifru_index;
             int       ifru_metric;
             int       ifru_mtu;
             int       ifru_phys;
             int       ifru_media;
             caddr_t   ifru_data;
             int       ifru_cap[2];
         } ifr_ifru;
     #define ifr_addr      ifr_ifru.ifru_addr      /* address */
     #define ifr_dstaddr   ifr_ifru.ifru_dstaddr   /* other end of p-to-p link */
     #define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */
     #define ifr_buffer    ifr_ifru.ifru_buffer    /* user supplied buffer with its length */
     #define ifr_flags     ifr_ifru.ifru_flags[0]  /* flags (low 16 bits) */
     #define ifr_flagshigh ifr_ifru.ifru_flags[1]  /* flags (high 16 bits) */
     #define ifr_metric    ifr_ifru.ifru_metric    /* metric */
     #define ifr_mtu       ifr_ifru.ifru_mtu       /* mtu */
     #define ifr_phys      ifr_ifru.ifru_phys      /* physical wire */
     #define ifr_media     ifr_ifru.ifru_media     /* physical media */
     #define ifr_data      ifr_ifru.ifru_data      /* for use by interface */
     #define ifr_reqcap    ifr_ifru.ifru_cap[0]    /* requested capabilities */
     #define ifr_curcap    ifr_ifru.ifru_cap[1]    /* current capabilities */
     #define ifr_index     ifr_ifru.ifru_index     /* interface index */

     Ioctl() requests to obtain addresses and requests both to set and
     retrieve other data are still fully supported and use the ifreq

     SIOCGIFADDR     Get interface address for protocol family.

     SIOCGIFDSTADDR  Get point to point address for protocol family and

     SIOCGIFBRDADDR  Get broadcast address for protocol family and interface.

     SIOCSIFCAP      Attempt to set the enabled capabilities field for the
                     interface to the value of the ifr_reqcap field of the
                     ifreq structure.  Note that, depending on the particular
                     interface features, some capabilities may appear hard-
                     coded to enabled, or toggling a capability may affect the
                     status of other ones.  The supported capabilities field
                     is read-only, and the ifr_curcap field is unused by this

     SIOCGIFCAP      Get the interface capabilities fields.  The values for
                     supported and enabled capabilities will be returned in
                     the ifr_reqcap and ifr_curcap fields of the ifreq
                     structure, respectively.

     SIOCGIFDESCR    Get the interface description, returned in the buffer
                     field of ifru_buffer struct.  The user supplied buffer
                     length should be defined in the length field of
                     ifru_buffer struct passed in as parameter, and the length
                     would include the terminating nul character.  If there is
                     not enough space to hold the interface length, no copy
                     would be done and the buffer field of ifru_buffer would
                     be set to NULL.  The kernel will store the buffer length
                     in the length field upon return, regardless whether the
                     buffer itself is sufficient to hold the data.

     SIOCSIFDESCR    Set the interface description to the value of the buffer
                     field of ifru_buffer struct, with length field specifying
                     its length (counting the terminating nul).

     SIOCSIFFLAGS    Set interface flags field.  If the interface is marked
                     down, any processes currently routing packets through the
                     interface are notified; some interfaces may be reset so
                     that incoming packets are no longer received.  When
                     marked up again, the interface is reinitialized.

     SIOCGIFFLAGS    Get interface flags.

     SIOCSIFMETRIC   Set interface routing metric.  The metric is used only by
                     user-level routers.

     SIOCGIFMETRIC   Get interface metric.

     SIOCIFCREATE    Attempt to create the specified interface.  If the
                     interface name is given without a unit number the system
                     will attempt to create a new interface with an arbitrary
                     unit number.  On successful return the ifr_name field
                     will contain the new interface name.

     SIOCIFDESTROY   Attempt to destroy the specified interface.

     There are two requests that make use of a new structure:

     SIOCAIFADDR     An interface may have more than one address associated
                     with it in some protocols.  This request provides a means
                     to add additional addresses (or modify characteristics of
                     the primary address if the default address for the
                     address family is specified).  Rather than making
                     separate calls to set destination or broadcast addresses,
                     or network masks (now an integral feature of multiple
                     protocols) a separate structure is used to specify all
                     three facets simultaneously (see below).  One would use a
                     slightly tailored version of this struct specific to each
                     family (replacing each sockaddr by one of the family-
                     specific type).  Where the sockaddr itself is larger than
                     the default size, one needs to modify the ioctl()
                     identifier itself to include the total size, as described
                     in ioctl().

     SIOCDIFADDR     This requests deletes the specified address from the list
                     associated with an interface.  It also uses the
                     ifaliasreq structure to allow for the possibility of
                     protocols allowing multiple masks or destination
                     addresses, and also adopts the convention that
                     specification of the default address means to delete the
                     first address for the interface belonging to the address
                     family in which the original socket was opened.

     SIOCGIFCONF     Get interface configuration list.  This request takes an
                     ifconf structure (see below) as a value-result parameter.
                     The ifc_len field should be initially set to the size of
                     the buffer pointed to by ifc_buf.  On return it will
                     contain the length, in bytes, of the configuration list.

     SIOCIFGCLONERS  Get list of clonable interfaces.  This request takes an
                     if_clonereq structure (see below) as a value-result
                     parameter.  The ifcr_count field should be set to the
                     number of IFNAMSIZ sized strings that can be fit in the
                     buffer pointed to by ifcr_buffer.  On return, ifcr_total
                     will be set to the number of clonable interfaces and the
                     buffer pointed to by ifcr_buffer will be filled with the
                     names of clonable interfaces aligned on IFNAMSIZ

     * Structure used in SIOCAIFADDR request.
     struct ifaliasreq {
             char    ifra_name[IFNAMSIZ];   /* if name, e.g. "en0" */
             struct  sockaddr        ifra_addr;
             struct  sockaddr        ifra_broadaddr;
             struct  sockaddr        ifra_mask;

     * Structure used in SIOCGIFCONF request.
     * Used to retrieve interface configuration
     * for machine (useful for programs which
     * must know all networks accessible).
     struct ifconf {
         int   ifc_len;              /* size of associated buffer */
         union {
             caddr_t    ifcu_buf;
             struct     ifreq *ifcu_req;
         } ifc_ifcu;
     #define ifc_buf ifc_ifcu.ifcu_buf /* buffer address */
     #define ifc_req ifc_ifcu.ifcu_req /* array of structures returned */

     /* Structure used in SIOCIFGCLONERS request. */
     struct if_clonereq {
             int     ifcr_total;     /* total cloners (out) */
             int     ifcr_count;     /* room for this many in user buffer */
             char    *ifcr_buffer;   /* buffer for cloner names */

     /* Structure used in SIOCGIFDESCR and SIOCSIFDESCR requests */
     struct ifreq_buffer {
             size_t  length;         /* length of the buffer */
             void   *buffer;         /* pointer to userland space buffer */

     ioctl(2), socket(2), intro(4), config(8), routed(8), ifnet(9)

     The netintro manual appeared in 4.3BSD-Tahoe.

FreeBSD 11.1-RELEASE-p4        January 26, 2012        FreeBSD 11.1-RELEASE-p4
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