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IP(4)		       FreeBSD Kernel Interfaces Manual			 IP(4)

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 applica-
     tions.

     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
     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));

     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	cms-
     ghdr fields have the following values:

     cmsg_len =	sizeof(struct in_addr)
     cmsg_level	= IPPROTO_IP
     cmsg_type = IP_RECVDSTADDR

     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_RESERVED through IPPORT_USERRESERVED.	 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.

   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
     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 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
     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;

     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	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 hasn't been con-
			nected;

     [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),	intro(4)

HISTORY
     The ip protocol appeared in 4.2BSD.

FreeBSD	9.2			 March 3, 2001			   FreeBSD 9.2

NAME | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | HISTORY

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