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     inet — Internet protocol family


     #include <sys/types.h>
     #include <netinet/in.h>


     The Internet protocol family is a collection of protocols layered atop the Internet Protocol
     (IP) transport layer, and utilizing the Internet address format.  The Internet family
     provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the
     SOCK_RAW interface provides access to the IP protocol.


     Internet addresses are four byte quantities, stored in network standard format (on little
     endian machines, such as the alpha, amd64, i386 and ia64 these are word and byte reversed).
     The include file <netinet/in.h> defines this address as a discriminated union.

     Sockets bound to the Internet protocol family utilize the following addressing structure,

           struct sockaddr_in {
                   uint8_t         sin_len;
                   sa_family_t     sin_family;
                   in_port_t       sin_port;
                   struct in_addr  sin_addr;
                   char            sin_zero[8];

     Sockets may be created with the local address INADDR_ANY to affect “wildcard” matching on
     incoming messages.  The address in a connect(2) or sendto(2) call may be given as INADDR_ANY
     to mean “this host”.  The distinguished address INADDR_BROADCAST is allowed as a shorthand
     for the broadcast address on the primary network if the first network configured supports


     The Internet protocol family is comprised of the IP network protocol, Internet Control
     Message Protocol (ICMP), Internet Group Management Protocol (IGMP), Transmission Control
     Protocol (TCP), and User Datagram Protocol (UDP).  TCP is used to support the SOCK_STREAM
     abstraction while UDP is used to support the SOCK_DGRAM abstraction.  A raw interface to IP
     is available by creating an Internet socket of type SOCK_RAW.  The ICMP message protocol is
     accessible from a raw socket.

     The inet address on an interface consist of the address itself, the netmask, either
     broadcast address in case of a broadcast interface or peers address in case of point-to-
     point interface.  The following ioctl(2) commands are provided for a datagram socket in the
     Internet domain:

           SIOCAIFADDR     Add address to an interface.  The command requires struct in_aliasreq
                           as argument.
           SIOCDIFADDR     Delete address from an interface.  The command requires struct ifreq
                           as argument.
           SIOCGIFNETMASK  Return address information from interface. The returned value is in
                           struct ifreq.  This way of address information retrieval is obsoleted,
                           a preferred way is to use getifaddrs(3) API.

   MIB Variables
     A number of variables are implemented in the net.inet branch of the sysctl(3) MIB.  In
     addition to the variables supported by the transport protocols (for which the respective
     manual pages may be consulted), the following general variables are defined:

     IPCTL_FORWARDING      (ip.forwarding) Boolean: enable/disable forwarding of IP packets.
                           Defaults to off.

     IPCTL_FASTFORWARDING  (ip.fastforwarding) Boolean: enable/disable the use of fast IP
                           forwarding code.  Defaults to off.  When fast IP forwarding is
                           enabled, IP packets are forwarded directly to the appropriate network
                           interface with direct processing to completion, which greatly improves
                           the throughput.  All packets for local IP addresses, non-unicast, or
                           with IP options are handled by the normal IP input processing path.
                           All features of the normal (slow) IP forwarding path are supported
                           including firewall (through pfil(9) hooks) checking, except ipsec(4)
                           tunnel brokering.  The IP fastforwarding path does not generate ICMP
                           redirect or source quench messages.

     IPCTL_SENDREDIRECTS   (ip.redirect) Boolean: enable/disable sending of ICMP redirects in
                           response to IP packets for which a better, and for the sender directly
                           reachable, route and next hop is known.  Defaults to on.

     IPCTL_DEFTTL          (ip.ttl) Integer: default time-to-live (“TTL”) to use for outgoing IP

                           (ip.accept_sourceroute) Boolean: enable/disable accepting of source-
                           routed IP packets (default false).

     IPCTL_SOURCEROUTE     (ip.sourceroute) Boolean: enable/disable forwarding of source-routed
                           IP packets (default false).

     IPCTL_RTEXPIRE        (ip.rtexpire) Integer: lifetime in seconds of protocol-cloned IP
                           routes after the last reference drops (default one hour).  This value
                           varies dynamically as described above.

     IPCTL_RTMINEXPIRE     (ip.rtminexpire) Integer: minimum value of ip.rtexpire (default ten
                           seconds).  This value has no effect on user modifications, but
                           restricts the dynamic adaptation described above.

     IPCTL_RTMAXCACHE      (ip.rtmaxcache) Integer: trigger level of cached, unreferenced,
                           protocol-cloned routes which initiates dynamic adaptation (default

     ip.process_options    Integer: control IP options processing.  By setting this variable to
                           0, all IP options in the incoming packets will be ignored, and the
                           packets will be passed unmodified.  By setting to 1, IP options in the
                           incoming packets will be processed accordingly.  By setting to 2, an
                           ICMP “prohibited by filter” message will be sent back in response to
                           incoming packets with IP options.  Default is 1.  This sysctl(8)
                           variable affects packets destined for a local host as well as packets
                           forwarded to some other host.

     ip.random_id          Boolean: control IP IDs generation behaviour.  Setting this sysctl(8)
                           to non-zero causes the ID field in IP packets to be randomized instead
                           of incremented by 1 with each packet generated.  This closes a minor
                           information leak which allows remote observers to determine the rate
                           of packet generation on the machine by watching the counter.  In the
                           same time, on high-speed links, it can decrease the ID reuse cycle
                           greatly.  Default is 0 (sequential IP IDs).  IPv6 flow IDs and
                           fragment IDs are always random.

     ip.maxfragpackets     Integer: maximum number of fragmented packets the host will accept and
                           hold in the reassembling queue simultaneously.  0 means that the host
                           will not accept any fragmented packets.  -1 means that the host will
                           accept as many fragmented packets as it receives.

     ip.maxfragsperpacket  Integer: maximum number of fragments the host will accept and hold in
                           the reassembling queue for a packet.  0 means that the host will not
                           accept any fragmented packets.


     ioctl(2), socket(2), getifaddrs(3), sysctl(3), icmp(4), intro(4), ip(4), ipfirewall(4),
     route(4), tcp(4), udp(4), pfil(9)

     "An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.

     "An Advanced 4.3 BSD Interprocess Communication Tutorial", PS1, 8.


     The inet protocol interface appeared in 4.2BSD.  The “protocol cloning” code appeared in
     FreeBSD 2.1.


     The Internet protocol support is subject to change as the Internet protocols develop.  Users
     should not depend on details of the current implementation, but rather the services