IP(4)                     OpenBSD Programmer's Manual                    IP(4)

NAME
     ip - Internet Protocol

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

     int
     socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION
     IP is the network 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-
     tions.

     There are several IP-level setsockopt(2)/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 ad-
     dresses 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 and SOCK_DGRAM sockets.  For ex-
     ample,

     int tos = IPTOS_LOWDELAY;       /* see <netinet/in.h> */
     setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));

     int ttl = 60;                   /* max = 255 */
     setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

     If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM socket, the
     recvmsg 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 = CMSG_LEN(sizeof(struct in_addr))
     cmsg_level = IPPROTO_IP
     cmsg_type = IP_RECVDSTADDR

     The IP_PORTRANGE option causes the default allocation policy for when the
     kernel is asked to choose a free port number.  Three choices are avail-
     able:

     IP_PORTRANGE_DEFAULT  The regular range of non-reserved ports.

     IP_PORTRANGE_HIGH     A high range, for fun.

     IP_PORTRANGE_LOW      Reserved ports; between 600 and 1023.

   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 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.  An interface's local IP ad-
     dress and multicast capability can be obtained via the SIOCGIFCONF and
     SIOCGIFFLAGS ioctls.  Normal applications should not need to use this op-
     tion.

     If a multicast datagram is sent to a group to which the sending host it-
     self 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 de-
     livered 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; /* multicast group to join */
         struct in_addr imr_interface; /* interface to join on */
     }

     imr_interface should be INADDR_ANY to choose the default multicast inter-
     face, or the IP address of a particular multicast-capable interface if
     the host is multihomed.  Membership is associated with a single inter-
     face; programs running on multihomed hosts may need to join the same
     group on more than one interface.  Up to IP_MAX_MEMBERSHIPS (currently
     20) memberships may be added on a single socket.

     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
     and recvfrom 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 re-
     ceived.  If proto is non-zero, that protocol number will be used on out-
     going 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/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 = htons(offset);
     ip->ip_len = htons(len);

     Additionally note that starting with OpenBSD 2.1, the ip_off and ip_len
     fields are in network byte order.  If the header source address is set to
     INADDR_ANY, the kernel will choose an appropriate address.

DIAGNOSTICS
     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 al-
                      ready connected;

     [ENOTCONN]       when trying to send a datagram, but no destination ad-
                      dress is specified, and the socket hasn't been connect-
                      ed;

     [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
     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.

SEE ALSO
     getsockopt(2), recv(2), send(2), icmp(4), inet(4), netintro(4)

HISTORY
     The ip protocol appeared in 4.2BSD.

OpenBSD 3.0                    November 30, 1993                             4
