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INET(4)                FreeBSD Kernel Interfaces Manual                INET(4)

     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 and i386
     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 broadcast.

     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

           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_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 packets.

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

     IPCTL_SOURCEROUTE        (ip.sourceroute) Boolean: enable/disable
                              forwarding of source-routed IP packets (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.rfc6864               Boolean: control IP IDs generation behaviour.
                              True value enables RFC6864 support, which
                              specifies that IP ID field of atomic datagrams
                              can be set to any value.  The
                              FreeBSD implementation sets it to zero. Enabled
                              by default.

     ip.random_id             Boolean: control IP IDs generation behaviour.
                              Setting this sysctl(8) to 1 causes the ID field
                              in non-atomic IP datagrams (or all IP datagrams,
                              if ip.rfc6864 is disabled) 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.  At 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

     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 exported.

FreeBSD 11.1-RELEASE-p4        February 4, 2016        FreeBSD 11.1-RELEASE-p4
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