Provided by: freebsd-manpages_12.0-1_all bug

NAME

     inet — Internet protocol family

SYNOPSIS

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

DESCRIPTION

     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.

ADDRESSING

     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.

PROTOCOLS

     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.
           SIOCGIFADDR
           SIOCGIFBRDADDR
           SIOCGIFDSTADDR
           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 false).

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

     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 random.

     ip.maxfrags              Integer: maximum number of fragments the host will accept and
                              simultaneously hold across all reassembly queues in all VNETs.  If
                              set to 0, reassembly is disabled.  If set to -1, this limit is not
                              applied.  This limit is recalculated when the number of mbuf
                              clusters is changed.  This is a global limit.

     ip.maxfragpackets        Integer: maximum number of fragmented packets the host will accept
                              and simultaneously hold in the reassembly queue for a particular
                              VNET.  0 means that the host will not accept any fragmented packets
                              for that VNET.  -1 means that the host will not apply this limit
                              for that VNET.  This limit is recalculated when the number of mbuf
                              clusters is changed.  This is a per-VNET limit.

     ip.maxfragbucketsize     Integer: maximum number of reassembly queues per bucket.
                              Fragmented packets are hashed to buckets.  Each bucket has a list
                              of reassembly queues.  The system must compare the incoming packets
                              to the existing reassembly queues in the bucket to find a matching
                              reassembly queue.  To preserve system resources, the system limits
                              the number of reassembly queues allowed in each bucket.  This limit
                              is recalculated when the number of mbuf clusters is changed or when
                              the value of ip.maxfragpackets changes.  This is a per-VNET limit.

     ip.maxfragsperpacket     Integer: maximum number of fragments the host will accept and hold
                              in the reassembly queue for a packet.  0 means that the host will
                              not accept any fragmented packets for the VNET.  This is a per-VNET
                              limit.

SEE ALSO

     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.

HISTORY

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

CAVEATS

     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.