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IP(4)                    BSD Kernel Interfaces 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 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) /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 and SOCK_DGRAM sockets. For example,

     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 desti-nation destination
     nation 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

   Multicast Options
     IP multicasting is supported only on AF_INET sockets of type SOCK_DGRAM and SOCK_RAW, and only on net-works networks
     works 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 inter-face. interface.
     face.  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 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 demon), 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; /* multicast group to join */
         struct in_addr imr_interface; /* interface to join on */
     }

     imr_interface should be INADDR_ANY to choose the default multicast interface, or the IP address of a
     particular multicast-capable interface if the host is multihomed.  Membership is associated with a sin-gle single
     gle interface; 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.  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 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 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.  Incom-ing Incoming
     ing 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 fol-lowing: following:
     lowing:

     ip->ip_v = IPVERSION;
     ip->ip_hl = hlen >> 2;
     ip->ip_id = 0;  /* 0 means kernel set appropriate value */
     ip->ip_off = offset;
     ip->ip_len = len;

     Note that the ip_off and ip_len fields are in host 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 try-ing trying
                      ing 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 hasn't 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.

     [EACESS]         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 mini-mum minimum
                      mum value or longer than the option buffer provided.

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

HISTORY
     The ip protocol appeared in 4.2BSD.

4.2 Berkeley Distribution      November 30, 1993     4.2 Berkeley Distribution

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