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IP(4)			 BSD Kernel Interfaces Manual			 IP(4)

     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 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

     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));

     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	de-
     fault 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	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 ap-
     propriate value in	order to prevent broadcast storms.  The	application
     must have sufficient credentials to set the SO_BROADCAST socket level op-
     tion, 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 buf-
     fer 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
			   is adjustable through the sysctl setting:
			   net.inet.ip.portrange.first and

     IP_PORTRANGE_HIGH	   use a high range of values, normally
			   adjustable through the sysctl setting:
			   net.inet.ip.portrange.hifirst and

     IP_PORTRANGE_LOW	   use a low range of ports, which are normally	re-
			   stricted 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 ac-
     counted 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.

   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.  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-

     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; /* IP multicast address of group	*/
	 struct	in_addr	imr_interface; /* local	IP address of interface	*/

     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(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 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/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;

     If	the header source address is set to INADDR_ANY,	the kernel will	choose
     an	appropriate address.

     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 ad-
			dress is specified, and	the socket hasn't 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.

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

     The ip protocol appeared in 4.2BSD.

BSD				 March 3, 2001				   BSD


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