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

     networking — introduction to networking facilities

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

     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).

     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 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[IFNAMSIZE];	     /* 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 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 structure:

     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.

     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 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
		     if_aliasreq structure to allow for the possibility of
		     protocols allowing multiple masks or destination
		     addresses, and also adopts the convention that specifica‐
		     tion 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.

     * 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 */

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

     The netintro manual appeared in 4.3BSD-Tahoe.

4.2 Berkeley Distribution      November 30, 1993     4.2 Berkeley Distribution

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