inet

INET(4)                   BSD Kernel Interfaces Manual                   INET(4)

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.

BSD                              August 14, 2018                             BSD