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

NAME
     networking — introduction to networking facilities

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

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
     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‐
     mentation):

     #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 */

ROUTING
     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).

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() 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 */
     };

     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() 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.

     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
		     call.

     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.

     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 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
		     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 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‐
		     aries.

     /*
     * 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 */
     };

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

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

HISTORY
     The netintro manual appeared in 4.3BSD-Tahoe.

BSD				April 14, 2010				   BSD
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