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NETINTRO(4)              BSD Kernel Interfaces Manual              NETINTRO(4)

NAME
     networking -- introduction to networking facilities

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

DESCRIPTION
     This section is a general introduction to the networking facilities available in the system.  Documen-tation Documentation
     tation 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 trans-port. transport.
     port.  A protocol family may support multiple methods of addressing, though the current protocol imple-mentations implementations
     mentations 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 con-tain contain
     tain 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 out-ofband
     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.

PROTOCOLS
     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 family.

ADDRESSING
     Associated with each protocol family is an address format.  All network address 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 implementation):

     #define    AF_UNIX      1    /* local to host (pipes) */
     #define    AF_INET      2    /* IPv4: UDP, TCP, etc. */
     #define    AF_INET6     30   /* IPv6: 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 */

ROUTING
     Mac OS X 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 mes-sages messages
     sages over a special kind of socket.  This supplants fixed size ioctl(2) used in earlier releases.

     This facility is described in route(4).

INTERFACES
     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 inter-faces interfaces
     faces such as the loopback interface, lo(4), do not.

     The following ioctl 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;
             short     ifru_flags;
             int       ifru_metric;
             caddr_t   ifru_data;
         } 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_flags     ifr_ifru.ifru_flags   /* flags */
     #define ifr_metric    ifr_ifru.ifru_metric  /* metric */
     #define ifr_data      ifr_ifru.ifru_data    /* for use by interface */
     };

     Calls which are now deprecated are:

     SIOCSIFADDR     Set interface address for protocol family.  Following the address assignment, the
                     ``initialization'' routine for the interface is called.

     SIOCSIFDSTADDR  Set point to point address for protocol family and interface.

     SIOCSIFBRDADDR  Set broadcast address for protocol family and interface.

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

     SIOCGIFADDR     Get interface address for protocol family.

     SIOCGIFDSTADDR  Get point to point address for protocol family and interface.

     SIOCGIFBRDADDR  Get broadcast address for protocol family and interface.

     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.

     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 familyspecific
                     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 if_aliasreq structure to allow for the possibility of protocols allow-ing allowing
                     ing multiple masks or destination addresses, and also adopts the convention that speci-fication specification
                     fication 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.

     /*
     * 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 */
     };

SEE ALSO
     ioctl(2), socket(2), intro(4), config(5), routed(8)

HISTORY
     The netintro manual appeared in 4.3BSD-Tahoe.

4.2 Berkeley Distribution      November 30, 1993     4.2 Berkeley Distribution

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