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NAME

     ip - Internet Protocol

SYNOPSIS

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

     int
     socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION

     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
     applications.

     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
     incoming 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
     buffer:

     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
     datagram.  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
     followed by the IP address.  The cmsghdr fields should have the following
     values:

     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
     broadcasts 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
     interface configured with the broadcast address 192.168.2.255:

     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("192.168.2.255");
     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
     cmsghdr 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
                           net.inet.ip.portrange.last.

     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
                           net.inet.ip.portrange.hilast.

     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
                           net.inet.ip.portrange.lowlast.

     The range of privileged ports which only may be opened by root-owned
     processes 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
     multicasting.

     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
     network, 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
     interface’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
     eliminating the overhead of receiving their own transmissions.  It should
     generally 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
     member is within the network range 0.0.0.0/8, it is treated as an
     interface index in the system interface MIB, as per the RIP Version 2 MIB
     Extension (RFC-1724).

     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
     interface.

     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.
     Memberships 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
     address.

ERRORS

     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
                        datagram 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
                        connected;

     [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
                        network 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
     options:

     [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.

SEE ALSO

     getsockopt(2), recv(2), send(2), byteorder(3), icmp(4), inet(4),
     intro(4), multicast(4)

HISTORY

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