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ip - Internet Protocol



      ip - Internet Protocol


      #include <sys/types.h>
      #include <sys/socket.h>
      #include <netinet/in.h>
      socket(AF_INET, SOCK_RAW, proto);


      IP is the transport layer protocol used by the Internet protocol family.
      Options may be set at the IP level when using higher-level protocols that
      are based on IP (such as TCP and UDP).  It may also be accessed through a
      “raw socket” when developing new protocols, or special-purpose applica‐
      There are several IP-level setsockopt(2) and getsockopt(2) options.
      IP_OPTIONS may be used to provide IP options to be transmitted in the IP
      header of each outgoing packet or to examine the header options on incom‐
      ing packets.  IP options may be used with any socket type in the Internet
      family.  The format of IP options to be sent is that specified by the IP
      protocol specification (RFC-791), with one exception: the list of
      addresses for Source Route options must include the first-hop gateway at
      the beginning of the list of gateways.  The first-hop gateway address
      will be extracted from the option list and the size adjusted accordingly
      before use.  To disable previously specified options, use a zero-length
      setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);
      IP_TOS and IP_TTL may be used to set the type-of-service and time-to-live
      fields in the IP header for SOCK_STREAM, SOCK_DGRAM, and certain types of
      SOCK_RAW sockets.  For example,
      int tos = IPTOS_LOWDELAY;       /* see <netinet/ip.h> */
      setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
      int ttl = 60;                   /* max = 255 */
      setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
      IP_MINTTL may be used to set the minimum acceptable TTL a packet must
      have when received on a socket.  All packets with a lower TTL are
      silently dropped.  This option is only really useful when set to 255,
      preventing packets from outside the directly connected networks reaching
      local listeners on sockets.
      IP_DONTFRAG may be used to set the Don’t Fragment flag on IP packets.
      Currently this option is respected only on udp(4) and raw ip(4) sockets,
      unless the IP_HDRINCL option has been set.  On tcp(4) sockets, the Don’t
      Fragment flag is controlled by the Path MTU Discovery option.  Sending a
      packet larger than the MTU size of the egress interface, determined by
      the destination address, returns an EMSGSIZE error.
      If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM socket, the
      recvmsg(2) call will return the destination IP address for a UDP data‐
      gram.  The msg_control field in the msghdr structure points to a buffer
      that contains a cmsghdr structure followed by the IP address.  The
      cmsghdr fields have the following values:
      cmsg_len = sizeof(struct in_addr)
      cmsg_level = IPPROTO_IP
      cmsg_type = IP_RECVDSTADDR
      The source address to be used for outgoing UDP datagrams on a socket that
      is not bound to a specific IP address can be specified as ancillary data
      with a type code of IP_SENDSRCADDR.  The msg_control field in the msghdr
      structure should point to a buffer that contains a cmsghdr structure fol‐
      lowed by the IP address.  The cmsghdr fields should have the following
      cmsg_len = sizeof(struct in_addr)
      cmsg_level = IPPROTO_IP
      cmsg_type = IP_SENDSRCADDR
      For convenience, IP_SENDSRCADDR is defined to have the same value as
      IP_RECVDSTADDR, so the IP_RECVDSTADDR control message from recvmsg(2) can
      be used directly as a control message for sendmsg(2).
      If the IP_ONESBCAST option is enabled on a SOCK_DGRAM or a SOCK_RAW
      socket, the destination address of outgoing broadcast datagrams on that
      socket will be forced to the undirected broadcast address,
      INADDR_BROADCAST, before transmission.  This is in contrast to the
      default behavior of the system, which is to transmit undirected broad‐
      casts via the first network interface with the IFF_BROADCAST flag set.
      This option allows applications to choose which interface is used to
      transmit an undirected broadcast datagram.  For example, the following
      code would force an undirected broadcast to be transmitted via the inter‐
      face configured with the broadcast address
      char msg[512];
      struct sockaddr_in sin;
      u_char onesbcast = 1;   /* 0 = disable (default), 1 = enable */
      setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
      sin.sin_addr.s_addr = inet_addr("");
      sin.sin_port = htons(1234);
      sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));
      It is the application’s responsibility to set the IP_TTL option to an
      appropriate value in order to prevent broadcast storms.  The application
      must have sufficient credentials to set the SO_BROADCAST socket level
      option, otherwise the IP_ONESBCAST option has no effect.
      If the IP_RECVTTL option is enabled on a SOCK_DGRAM socket, the
      recvmsg(2) call will return the IP TTL (time to live) field for a UDP
      datagram.  The msg_control field in the msghdr structure points to a
      buffer that contains a cmsghdr structure followed by the TTL.  The cms‐
      ghdr fields have the following values:
      cmsg_len = sizeof(u_char)
      cmsg_level = IPPROTO_IP
      cmsg_type = IP_RECVTTL
      If the IP_RECVIF option is enabled on a SOCK_DGRAM socket, the recvmsg(2)
      call returns a struct sockaddr_dl corresponding to the interface on which
      the packet was received.  The msg_control field in the msghdr structure
      points to a buffer that contains a cmsghdr structure followed by the
      struct sockaddr_dl.  The cmsghdr fields have the following values:
      cmsg_len = sizeof(struct sockaddr_dl)
      cmsg_level = IPPROTO_IP
      cmsg_type = IP_RECVIF
      IP_PORTRANGE may be used to set the port range used for selecting a local
      port number on a socket with an unspecified (zero) port number.  It has
      the following possible values:
      IP_PORTRANGE_DEFAULT  use the default range of values, normally
                            IPPORT_HIFIRSTAUTO through IPPORT_HILASTAUTO.  This
                            is adjustable through the sysctl setting:
                            net.inet.ip.portrange.first and
      IP_PORTRANGE_HIGH     use a high range of values, normally
                            IPPORT_HIFIRSTAUTO and IPPORT_HILASTAUTO.  This is
                            adjustable through the sysctl setting:
                            net.inet.ip.portrange.hifirst and
      IP_PORTRANGE_LOW      use a low range of ports, which are normally
                            restricted to privileged processes on UNIX systems.
                            The range is normally from IPPORT_RESERVED - 1 down
                            to IPPORT_RESERVEDSTART in descending order.  This
                            is adjustable through the sysctl setting:
                            net.inet.ip.portrange.lowfirst and
      The range of privileged ports which only may be opened by root-owned pro‐
      cesses may be modified by the net.inet.ip.portrange.reservedlow and
      net.inet.ip.portrange.reservedhigh sysctl settings.  The values default
      to the traditional range, 0 through IPPORT_RESERVED - 1 (0 through 1023),
      respectively.  Note that these settings do not affect and are not
      accounted for in the use or calculation of the other
      net.inet.ip.portrange values above.  Changing these values departs from
      UNIX tradition and has security consequences that the administrator
      should carefully evaluate before modifying these settings.
      Ports are allocated at random within the specified port range in order to
      increase the difficulty of random spoofing attacks.  In scenarios such as
      benchmarking, this behavior may be undesirable.  In these cases,
      net.inet.ip.portrange.randomized can be used to toggle randomization off.
      If more than net.inet.ip.portrange.randomcps ports have been allocated in
      the last second, then return to sequential port allocation.  Return to
      random allocation only once the current port allocation rate drops below
      net.inet.ip.portrange.randomcps for at least
      net.inet.ip.portrange.randomtime seconds.  The default values for
      net.inet.ip.portrange.randomcps and net.inet.ip.portrange.randomtime are
      10 port allocations per second and 45 seconds correspondingly.
    Multicast Options
      IP multicasting is supported only on AF_INET sockets of type SOCK_DGRAM
      and SOCK_RAW, and only on networks where the interface driver supports
      The IP_MULTICAST_TTL option changes the time-to-live (TTL) for outgoing
      multicast datagrams in order to control the scope of the multicasts:
      u_char ttl;     /* range: 0 to 255, default = 1 */
      setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
      Datagrams with a TTL of 1 are not forwarded beyond the local network.
      Multicast datagrams with a TTL of 0 will not be transmitted on any net‐
      work, but may be delivered locally if the sending host belongs to the
      destination group and if multicast loopback has not been disabled on the
      sending socket (see below).  Multicast datagrams with TTL greater than 1
      may be forwarded to other networks if a multicast router is attached to
      the local network.
      For hosts with multiple interfaces, each multicast transmission is sent
      from the primary network interface.  The IP_MULTICAST_IF option overrides
      the default for subsequent transmissions from a given socket:
      struct in_addr addr;
      setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
      where "addr" is the local IP address of the desired interface or
      INADDR_ANY to specify the default interface.
      To specify an interface by index, an instance of ip_mreqn should be
      passed instead.  The imr_ifindex member should be set to the index of the
      desired interface, or 0 to specify the default interface.  The kernel
      differentiates between these two structures by their size.  An inter‐
      face’s local IP address and multicast capability can be obtained via the
      SIOCGIFCONF and SIOCGIFFLAGS ioctls.  Normal applications should not need
      to use this option.
      If a multicast datagram is sent to a group to which the sending host
      itself belongs (on the outgoing interface), a copy of the datagram is, by
      default, looped back by the IP layer for local delivery.  The
      IP_MULTICAST_LOOP option gives the sender explicit control over whether
      or not subsequent datagrams are looped back:
      u_char loop;    /* 0 = disable, 1 = enable (default) */
      setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
      This option improves performance for applications that may have no more
      than one instance on a single host (such as a router daemon), by elimi‐
      nating the overhead of receiving their own transmissions.  It should gen‐
      erally not be used by applications for which there may be more than one
      instance on a single host (such as a conferencing program) or for which
      the sender does not belong to the destination group (such as a time
      querying program).
      A multicast datagram sent with an initial TTL greater than 1 may be
      delivered to the sending host on a different interface from that on which
      it was sent, if the host belongs to the destination group on that other
      interface.  The loopback control option has no effect on such delivery.
      A host must become a member of a multicast group before it can receive
      datagrams sent to the group.  To join a multicast group, use the
      IP_ADD_MEMBERSHIP option:
      struct ip_mreq mreq;
      setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
      where mreq is the following structure:
      struct ip_mreq {
          struct in_addr imr_multiaddr; /* IP multicast address of group */
          struct in_addr imr_interface; /* local IP address of interface */
      imr_interface should be set to INADDR_ANY to choose the default multicast
      interface, or the IP address of a particular multicast-capable interface
      if the host is multihomed.  Since FreeBSD 4.4, if the imr_interface mem‐
      ber is within the network range, it is treated as an interface
      index in the system interface MIB, as per the RIP Version 2 MIB Extension
      Up to IP_MAX_MEMBERSHIPS memberships may be added on a single socket.
      Membership is associated with a single interface; programs running on
      multihomed hosts may need to join the same group on more than one inter‐
      The IGMP protocol uses the primary IP address of the interface as its
      identifier for group membership.  If multiple IP aliases are configured
      on the same interface, they will be ignored.  This shortcoming was
      addressed in IPv6; MLDv2 requires that the unique link-local address for
      an interface is used to identify an MLDv2 listener.
      To drop a membership, use:
      struct ip_mreq mreq;
      setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
      where mreq contains the same values as used to add the membership.  Mem‐
      berships are dropped when the socket is closed or the process exits.
    Raw IP Sockets
      Raw IP sockets are connectionless, and are normally used with the
      sendto(2) and recvfrom(2) calls, though the connect(2) call may also be
      used to fix the destination for future packets (in which case the read(2)
      or recv(2) and write(2) or send(2) system calls may be used).
      If proto is 0, the default protocol IPPROTO_RAW is used for outgoing
      packets, and only incoming packets destined for that protocol are
      received.  If proto is non-zero, that protocol number will be used on
      outgoing packets and to filter incoming packets.
      Outgoing packets automatically have an IP header prepended to them (based
      on the destination address and the protocol number the socket is created
      with), unless the IP_HDRINCL option has been set.  Incoming packets are
      received with IP header and options intact.
      IP_HDRINCL indicates the complete IP header is included with the data and
      may be used only with the SOCK_RAW type.
      #include <netinet/in_systm.h>
      #include <netinet/ip.h>
      int hincl = 1;                  /* 1 = on, 0 = off */
      setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
      Unlike previous BSD releases, the program must set all the fields of the
      IP header, including the following:
      ip->ip_v = IPVERSION;
      ip->ip_hl = hlen >> 2;
      ip->ip_id = 0;  /* 0 means kernel set appropriate value */
      ip->ip_off = offset;
      The ip_len and ip_off fields must be provided in host byte order .  All
      other fields must be provided in network byte order.  See byteorder(3)
      for more information on network byte order.  If the ip_id field is set to
      0 then the kernel will choose an appropriate value.  If the header source
      address is set to INADDR_ANY, the kernel will choose an appropriate


      A socket operation may fail with one of the following errors returned:
      [EISCONN]          when trying to establish a connection on a socket
                         which already has one, or when trying to send a data‐
                         gram with the destination address specified and the
                         socket is already connected;
      [ENOTCONN]         when trying to send a datagram, but no destination
                         address is specified, and the socket has not been con‐
      [ENOBUFS]          when the system runs out of memory for an internal
                         data structure;
      [EADDRNOTAVAIL]    when an attempt is made to create a socket with a net‐
                         work address for which no network interface exists.
      [EACCES]           when an attempt is made to create a raw IP socket by a
                         non-privileged process.
      The following errors specific to IP may occur when setting or getting IP
      [EINVAL]           An unknown socket option name was given.
      [EINVAL]           The IP option field was improperly formed; an option
                         field was shorter than the minimum value or longer
                         than the option buffer provided.
      The following errors may occur when attempting to send IP datagrams via a
      “raw socket” with the IP_HDRINCL option set:
      [EINVAL]           The user-supplied ip_len field was not equal to the
                         length of the datagram written to the socket.
      getsockopt(2), recv(2), send(2), byteorder(3), icmp(4), inet(4),
      intro(4), multicast(4)


      The ip protocol appeared in 4.2BSD.  The ip_mreqn structure appeared in
      Linux 2.4.


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