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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 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 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 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(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 imple-

     #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 inter-
     faces.  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[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_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 */

     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-

     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 struc-

     SIOCGIFADDR     Get interface address for protocol family.

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

     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 struc-
                     ture, respectively.

     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 inter-
                     face 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 sepa-
                     rate calls to set destination or broadcast addresses, or
                     network masks (now an integral feature of multiple proto-
                     cols) 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-spe-
                     cific type).  Where the sockaddr itself is larger than
                     the default size, one needs to modify the ioctl() identi-
                     fier itself to include the total size, as described in

     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.

     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.

     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 bound-

     * Structure used in SIOCAIFCONF 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 */

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

     The netintro manual appeared in 4.3BSD-Tahoe.

FreeBSD 6.2                      June 18, 2004                     FreeBSD 6.2


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