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IP(4)			 BSD 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 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, SOCK_DGRAM and 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_IPDEFTTL can be	used to	retrieve the system wide default 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 = CMSG_LEN(sizeof(struct in_addr))
	   cmsg_level =	IPPROTO_IP
	   cmsg_type = IP_RECVDSTADDR

     If	the IP_RECVDSTPORT option is enabled on	a SOCK_DGRAM socket, the
     recvmsg(2)	call will return the destination port for a UDP	datagram.  The
     msg_control field in the msghdr structure points to a buffer that con-
     tains a cmsghdr structure followed	by the port in 16-bit network byte or-
     der.  The cmsghdr fields have the following values:

	   cmsg_len = CMSG_LEN(sizeof(u_int16_t))
	   cmsg_level =	IPPROTO_IP
	   cmsg_type = IP_RECVDSTPORT

     If	the IP_RECVTTL option is enabled on a SOCK_DGRAM or SOCK_RAW socket,
     the recvmsg(2) call will return the TTL of	the received datagram.	The
     msg_control field in the msghdr structure points to a buffer that con-
     tains a cmsghdr structure followed	by the TTL value.  The cmsghdr fields
     have the following	values:

	   cmsg_len = CMSG_LEN(sizeof(u_int8_t))
	   cmsg_level =	IPPROTO_IP
	   cmsg_type = IP_RECVTTL

     The IP_MINTTL option may be used on TCP and UDP sockets to	discard	pack-
     ets with a	TTL lower than the option value.  This can be used to imple-
     ment the Generalized TTL Security Mechanism (GTSM)	according to RFC 5082.
     To	discard	all packets with a TTL lower than 255:

	   int minttl =	255;
	   setsockopt(s, IPPROTO_IP, IP_MINTTL,	&minttl, sizeof(minttl));

     If	the IP_IPSECFLOWINFO option is enabled on a SOCK_DGRAM socket, the
     recvmsg(2)	call will return information identifying the incoming IPsec SA
     for a UDP datagram.  The msg_control field	in the msghdr structure	points
     to	a buffer that contains a cmsghdr structure followed by flow informa-
     tion in 32-bit network byte order.	 When this information is passed to a
     sendmsg(2)	call the ID of the incoming SA will be used for	looking	up the
     outgoing SA for the UDP datagram.	The cmsghdr fields for recvmsg(2) and
     sendmsg(2)	have the following values:

	   cmsg_len = CMSG_LEN(sizeof(u_int32_t))
	   cmsg_level =	IPPROTO_IP
	   cmsg_type = IP_IPSECFLOWINFO

     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.

     If	the IP_RECVRTABLE option is enabled on a SOCK_DGRAM socket, the
     recvmsg(2)	call will return the source routing domain for a UDP datagram.
     The msg_control field in the msghdr structure points to a buffer that
     contains a	cmsghdr	structure followed by the routing table	ID.  The
     cmsghdr fields have the following values:

	   cmsg_len = CMSG_LEN(sizeof(u_int))
	   cmsg_level =	IPPROTO_IP
	   cmsg_type = IP_RECVRTABLE

     When sending on a SOCK_DGRAM socket with sendmsg(2), the source address
     to	be used	can be passed 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 re-
     quested source address.  The cmsghdr fields should	have the following
     values:

	   cmsg_len = CMSG_LEN(sizeof(struct in_addr))
	   cmsg_level =	IPPROTO_IP
	   cmsg_type = IP_SENDSRCADDR

     The same checks and restrictions as for bind(2) apply, unless the socket
     is	bound to INADDR_ANY.  In this case, there is no	source address overlap
     check.

   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
     ioctl(2)'s.  Normal applications should not need to use this option.

     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
     4095) 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/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 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), ioctl(2), recv(2), send(2),	icmp(4), inet(4), netintro(4)

HISTORY
     The ip protocol appeared in 4.2BSD.

BSD				August 18, 2016				   BSD

NAME | SYNOPSIS | DESCRIPTION | DIAGNOSTICS | SEE ALSO | HISTORY

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