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

     netintro -- introduction to networking facilities

     #include <&lt;sys/socket.h>&gt;
     #include <&lt;net/route.h>&gt;
     #include <&lt;net/if.h>&gt;

     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.

     Network interfaces may be collected together into interface groups.  An
     interface group is a container that can be used generically when refer-
     ring to any interface related by some criteria.  When an action is per-
     formed on an interface group, such as packet filtering by the pf(4) sub-
     system, the operation will be applied to each member interface in the
     group, if supported by the subsystem.  The ifconfig(8) utility can be
     used to view and assign membership of an interface to an interface group
     with the group modifier.

     The system currently supports the Internet protocols (IPv4 and IPv6),
     MPLS, and a few others.  Raw socket interfaces are provided to the IP
     protocol layer of the Internet.  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 a finer, more specific structure,
     generally renaming the variant, which is discussed in the protocol family
     manual page alluded to above.

           struct sockaddr {
                   u_int8_t        sa_len;         /* total length */
                   sa_family_t     sa_family;      /* address family */
                   char            sa_data[14];    /* actually longer */

     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_LOCAL        1       /* local to host */
     #define AF_INET         2       /* internetwork: UDP, TCP, etc. */
     #define AF_INET6        24      /* IPv6 */
     #define AF_MPLS         33      /* Multiprotocol Label Switching */

     The sa_data field contains the actual address value.  Note that it may be
     longer than 14 bytes.

     OpenBSD 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)s 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(2) is made on a socket (typically of type SOCK_DGRAM)
     in the desired domain.  Most of the requests take an ifreq structure
     pointer as their parameter.  This structure is as follows:

     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   /* p-to-p peer */
     #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_mtu         ifr_ifru.ifru_metric    /* mtu (overload) */
     #define ifr_media       ifr_ifru.ifru_metric    /* media options */
     #define ifr_data        ifr_ifru.ifru_data      /* used by interface */

     The supported ioctl(2) requests are:

     SIOCSIFADDR struct ifreq *
             Set the interface address for a protocol family.  Following the
             address assignment, the ``initialization'' routine for the inter-
             face is called.

             This call has been deprecated and superseded by the SIOCAIFADDR
             call, described below.

     SIOCSIFDSTADDR struct ifreq *
             Set the point-to-point address for a protocol family and inter-

             This call has been deprecated and superseded by the SIOCAIFADDR
             call, described below.

     SIOCSIFBRDADDR struct ifreq *
             Set the broadcast address for a protocol family and interface.

             This call has been deprecated and superseded by the SIOCAIFADDR
             call, described below.

     SIOCGIFADDR struct ifreq *
             Get the interface address for a protocol family.

     SIOCGIFDSTADDR struct ifreq *
             Get the point-to-point address for a protocol family and inter-

     SIOCGIFBRDADDR struct ifreq *
             Get the broadcast address for a protocol family and interface.

     SIOCGIFDESCR struct ifreq *
             Get the interface description, returned in the ifru_data field.

     SIOCSIFDESCR struct ifreq *
             Set the interface description to the value of the ifru_data
             field, limited to the size of IFDESCRSIZE.

     SIOCSIFFLAGS struct ifreq *
             Set the interface flags.  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

     SIOCGIFFLAGS struct ifreq *
             Get the interface flags.

     SIOCGIFXFLAGS struct ifreq *
             Get the extended interface flags.

     SIOCGIFMTU struct ifreq *
             Get the current MTU of the interface.

     SIOCGIFHARDMTU struct ifreq *
             Get the maximum hardware MTU of the interface.

     SIOCSIFMEDIA struct ifreq *
             Set the interface media settings.  See ifmedia(4) for possible

     SIOCGIFMEDIA struct ifmediareq *
             Get the interface media settings.  The ifmediareq structure is as

             struct ifmediareq {
                     char     ifm_name[IFNAMSIZ];    /* if name, e.g. "en0" */
                     int      ifm_current;   /* current media options */
                     int      ifm_mask;      /* don't care mask */
                     int      ifm_status;    /* media status */
                     int      ifm_active;    /* active options */
                     int      ifm_count;     /* #entries in ifm_ulist array */
                     int     *ifm_ulist;     /* media words */

             See ifmedia(4) for interpreting this value.

     SIOCSIFMETRIC struct ifreq *
             Set the interface routing metric.  The metric is used only by
             user-level routers.

     SIOCGIFMETRIC struct ifreq *
             Get the interface metric.

     SIOCSIFPRIORITY struct ifreq *
             Set the interface routing priority.  The interface routing prior-
             ity influences the resulting routing priority of new static
             routes added to the kernel using the specified interface.  The
             value is in the range of 0 to 16 with smaller numbers being bet-

     SIOCGIFPRIORITY struct ifreq *
             Get the interface priority.

     SIOCGIFRDOMAIN struct ifreq *
             Get the interface routing domain.  This identifies which routing
             table is used for the interface.

     SIOCAIFADDR struct ifaliasreq *
             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, ifaliasreq, is used to specify
             all three facets simultaneously (see below).  One would use a
             slightly tailored version of this structure specific to each fam-
             ily (replacing each sockaddr by one of the family-specific type).
             One should always set the length of a sockaddr, as described in

             The ifaliasreq structure is as follows:

             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;

     SIOCDIFADDR struct ifreq *
             This request deletes the specified address from the list associ-
             ated with an interface.  It also uses the ifaliasreq structure to
             allow for the possibility of protocols allowing multiple masks or
             destination addresses, and also adopts the convention that speci-
             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 struct ifconf *
             Get the 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.

             Alternately, if the ifc_len passed in is set to 0, SIOCGIFCONF
             will set ifc_len to the size that ifc_buf needs to be to fit the
             entire configuration list and will not fill in the other parame-
             ters.  This is useful for determining the exact size that ifc_buf
             needs to be in advance.  Note, however, that this is an extension
             that not all operating systems support.

             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 ret'd */

     SIOCIFCREATE struct ifreq *
             Attempt to create the specified interface.

     SIOCIFDESTROY struct ifreq *
             Attempt to destroy the specified interface.

     SIOCIFGCLONERS struct if_clonereq *
             Get the list of clonable interfaces.  This request takes an
             if_clonereq structure pointer (see below) as a value-result
             parameter.  The ifcr_count field should be set to the number of
             IFNAMSIZ-sized strings that can 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 boundaries.

             The if_clonereq structure is as follows:

             struct if_clonereq {
                     int   ifcr_total;  /* total cloners (out) */
                     int   ifcr_count;  /* room for this many in user buf */
                     char *ifcr_buffer; /* buffer for cloner names */

     SIOCAIFGROUP struct ifgroupreq *
             Associate the interface named by ifgr_name with the interface
             group named by ifgr_group.  The ifgroupreq structure is as fol-

             struct ifg_req {
                     char                     ifgrq_group[IFNAMSIZ];

             struct ifgroupreq {
                     char    ifgr_name[IFNAMSIZ];
                     u_int   ifgr_len;
                     union {
                             char    ifgru_group[IFNAMSIZ];
                             struct  ifg_req *ifgru_groups;
                     } ifgr_ifgru;
             #define ifgr_group      ifgr_ifgru.ifgru_group
             #define ifgr_groups     ifgr_ifgru.ifgru_groups

     SIOCGIFGROUP struct ifgroupreq *
             Retrieve the list of groups for which an interface is a member.
             The interface is named by ifgr_name.  On enter, the amount of
             memory in which the group names will be written is stored in
             ifgr_len, and the group names themselves will be written to the
             memory pointed to by ifgr_groups.  On return, the amount of mem-
             ory actually written is returned in ifgr_len.

             Alternately, if the ifgr_len passed in is set to 0, SIOCGIFGROUP
             will set ifgr_len to the size that ifgr_groups needs to be to fit
             the entire group list and will not fill in the other parameters.
             This is useful for determining the exact size that ifgr_groups
             needs to be in advance.

     SIOCDIFGROUP struct ifgroupreq *
             Remove the membership of the interface named by ifgr_name from
             the group ifgr_group.

     netstat(1), ioctl(2), socket(2), arp(4), bridge(4), ifmedia(4), inet(4),
     intro(4), ip(4), ip6(4), lo(4), mpe(4), pf(4), tcp(4), udp(4), hosts(5),
     networks(5), bgpd(8), config(8), ifconfig(8), mrouted(8), netstart(8),
     ospfd(8), ripd(8), route(8)

     The netintro manual appeared in 4.3BSD-Tahoe.

BSD                             March 30, 2017                             BSD