NETINTRO(4)             NetBSD Kernel Interfaces Manual            NETINTRO(4)

NAME
     netintro -- introduction to networking facilities

SYNOPSIS
     #include <sys/types.h>
     #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.  Documentation in this part of section 4 is bro-
     ken 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 ser-
     vices may include packet fragmentation and reassembly, routing, address-
     ing, and basic transport.  A protocol family may support multiple methods
     of addressing, though the current protocol implementations do not.  A
     protocol family normally comprises 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 support-
     ing 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.  Cer-
     tain semantics of the basic socket abstractions are protocol specific.
     All protocols are expected to support the basic model for their particu-
     lar 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 pro-
     tocol families and/or address formats.  The SYNOPSIS section of each net-
     work interface entry gives a sample specification of the related drivers
     for use in providing a system description to the config(1) 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 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 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_LOCAL     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 */

ROUTING
     UNIX 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).

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 interfaces such as the
     loopback interface, lo(4), do not.

     The following ioctl(2) calls may be used to manipulate network inter-
     faces.  The ioctl(2) 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;
             void   *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 inter-
                     face.

     SIOCSIFBRDADDR  Set broadcast address for protocol family and interface.

     ioctl(2) requests to obtain addresses and requests both to set and
     retrieve other data are still fully supported and use the ifreq struc-
     ture:

     SIOCGIFADDR     Get interface address for protocol family.

     SIOCGIFDSTADDR  Get point to point address for protocol family and inter-
                     face.

     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 sepa-
                     rate calls to set destination or broadcast addresses, or
                     network masks (now an integral feature of multiple proto-
                     cols) a separate structure, ifaliasreq, is used to spec-
                     ify all three facets simultaneously (see below).  One
                     would use a slightly tailored version of this struct spe-
                     cific 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(2) identifier itself to include the total size, as
                     described in ioctl(2).

     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 pro-
                     tocols 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.

     Request making use of the ifconf structure:

     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 con-
                     tain the length, in bytes, of the configuration list.

     /*
     * Structure used in SIOC[AD]IFADDR request.
     */
     struct ifaliasreq {
             char    ifra_name[IFNAMSIZ];   /* if name, e.g. "en0" */
             struct  sockaddr        ifra_addr;
             struct  sockaddr        ifra_dstaddr;
     #define ifra_broadaddr  ifra_dstaddr
             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 {
             void    *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
     config(1), ioctl(2), socket(2), intro(4), routed(8)

HISTORY
     The netintro manual appeared in 4.3BSD-Tahoe.

NetBSD 5.0_STABLE              September 7, 2006             NetBSD 5.0_STABLE
