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32 .\" @(#)ip.4 8.2 (Berkeley) 11/30/93
35 .Dd September 26, 2005
46 .Fn socket AF_INET SOCK_RAW proto
49 is the transport layer protocol used
50 by the Internet protocol family.
51 Options may be set at the
54 when using higher-level protocols that are based on
60 It may also be accessed
63 when developing new protocols, or
64 special-purpose applications.
73 may be used to provide
75 options to be transmitted in the
77 header of each outgoing packet
78 or to examine the header options on incoming packets.
80 options may be used with any socket type in the Internet family.
83 options to be sent is that specified by the
85 protocol specification (RFC-791), with one exception:
86 the list of addresses for Source Route options must include the first-hop
87 gateway at the beginning of the list of gateways.
88 The first-hop gateway address will be extracted from the option list
89 and the size adjusted accordingly before use.
90 To disable previously specified options,
91 use a zero-length buffer:
93 setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);
99 may be used to set the type-of-service and time-to-live
103 .Dv SOCK_STREAM , SOCK_DGRAM ,
109 int tos = IPTOS_LOWDELAY; /* see <netinet/ip.h> */
110 setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
112 int ttl = 60; /* max = 255 */
113 setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
117 may be used to set the minimum acceptable TTL a packet must have when
118 received on a socket.
119 All packets with a lower TTL are silently dropped.
120 This option is only really useful when set to 255, preventing packets
121 from outside the directly connected networks reaching local listeners
125 may be used to set the Don't Fragment flag on IP packets.
126 Currently this option is respected only on
135 sockets, the Don't Fragment flag is controlled by the Path
136 MTU Discovery option.
137 Sending a packet larger than the MTU size of the egress interface,
138 determined by the destination address, returns an
144 option is enabled on a
149 call will return the destination
158 structure points to a buffer
161 structure followed by the
166 fields have the following values:
168 cmsg_len = sizeof(struct in_addr)
169 cmsg_level = IPPROTO_IP
170 cmsg_type = IP_RECVDSTADDR
173 The source address to be used for outgoing
175 datagrams on a socket that is not bound to a specific
177 address can be specified as ancillary data with a type code of
179 The msg_control field in the msghdr structure should point to a buffer
182 structure followed by the
185 The cmsghdr fields should have the following values:
187 cmsg_len = sizeof(struct in_addr)
188 cmsg_level = IPPROTO_IP
189 cmsg_type = IP_SENDSRCADDR
194 is defined to have the same value as
200 can be used directly as a control message for
205 option is enabled on a
209 socket, the destination address of outgoing
210 broadcast datagrams on that socket will be forced
211 to the undirected broadcast address,
212 .Dv INADDR_BROADCAST ,
214 This is in contrast to the default behavior of the
215 system, which is to transmit undirected broadcasts
216 via the first network interface with the
217 .Dv IFF_BROADCAST flag set.
219 This option allows applications to choose which
220 interface is used to transmit an undirected broadcast
222 For example, the following code would force an
223 undirected broadcast to be transmitted via the interface
224 configured with the broadcast address 192.168.2.255:
227 struct sockaddr_in sin;
228 u_char onesbcast = 1; /* 0 = disable (default), 1 = enable */
230 setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
231 sin.sin_addr.s_addr = inet_addr("192.168.2.255");
232 sin.sin_port = htons(1234);
233 sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));
236 It is the application's responsibility to set the
238 to an appropriate value in order to prevent broadcast storms.
239 The application must have sufficient credentials to set the
241 socket level option, otherwise the
242 .Dv IP_ONESBCAST option has no effect.
246 option is enabled on a
253 (time to live) field for a
256 The msg_control field in the msghdr structure points to a buffer
257 that contains a cmsghdr structure followed by the
259 The cmsghdr fields have the following values:
261 cmsg_len = sizeof(u_char)
262 cmsg_level = IPPROTO_IP
263 cmsg_type = IP_RECVTTL
268 option is enabled on a
273 .Vt "struct sockaddr_dl"
274 corresponding to the interface on which the
280 structure points to a buffer that contains a
282 structure followed by the
283 .Vt "struct sockaddr_dl" .
286 fields have the following values:
288 cmsg_len = sizeof(struct sockaddr_dl)
289 cmsg_level = IPPROTO_IP
290 cmsg_type = IP_RECVIF
294 may be used to set the port range used for selecting a local port number
295 on a socket with an unspecified (zero) port number.
298 .Bl -tag -width IP_PORTRANGE_DEFAULT
299 .It Dv IP_PORTRANGE_DEFAULT
300 use the default range of values, normally
301 .Dv IPPORT_HIFIRSTAUTO
303 .Dv IPPORT_HILASTAUTO .
304 This is adjustable through the sysctl setting:
305 .Va net.inet.ip.portrange.first
307 .Va net.inet.ip.portrange.last .
308 .It Dv IP_PORTRANGE_HIGH
309 use a high range of values, normally
310 .Dv IPPORT_HIFIRSTAUTO
312 .Dv IPPORT_HILASTAUTO .
313 This is adjustable through the sysctl setting:
314 .Va net.inet.ip.portrange.hifirst
316 .Va net.inet.ip.portrange.hilast .
317 .It Dv IP_PORTRANGE_LOW
318 use a low range of ports, which are normally restricted to
319 privileged processes on
322 The range is normally from
325 .Li IPPORT_RESERVEDSTART
327 This is adjustable through the sysctl setting:
328 .Va net.inet.ip.portrange.lowfirst
330 .Va net.inet.ip.portrange.lowlast .
333 The range of privileged ports which only may be opened by
334 root-owned processes may be modified by the
335 .Va net.inet.ip.portrange.reservedlow
337 .Va net.inet.ip.portrange.reservedhigh
339 The values default to the traditional range,
343 (0 through 1023), respectively.
344 Note that these settings do not affect and are not accounted for in the
345 use or calculation of the other
346 .Va net.inet.ip.portrange
348 Changing these values departs from
350 tradition and has security
351 consequences that the administrator should carefully evaluate before
352 modifying these settings.
354 Ports are allocated at random within the specified port range in order
355 to increase the difficulty of random spoofing attacks.
356 In scenarios such as benchmarking, this behavior may be undesirable.
358 .Va net.inet.ip.portrange.randomized
359 can be used to toggle randomization off.
361 .Va net.inet.ip.portrange.randomcps
362 ports have been allocated in the last second, then return to sequential
364 Return to random allocation only once the current port allocation rate
366 .Va net.inet.ip.portrange.randomcps
368 .Va net.inet.ip.portrange.randomtime
370 The default values for
371 .Va net.inet.ip.portrange.randomcps
373 .Va net.inet.ip.portrange.randomtime
374 are 10 port allocations per second and 45 seconds correspondingly.
375 .Ss "Multicast Options"
378 multicasting is supported only on
384 and only on networks where the interface
385 driver supports multicasting.
389 option changes the time-to-live (TTL)
390 for outgoing multicast datagrams
391 in order to control the scope of the multicasts:
393 u_char ttl; /* range: 0 to 255, default = 1 */
394 setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
397 Datagrams with a TTL of 1 are not forwarded beyond the local network.
398 Multicast datagrams with a TTL of 0 will not be transmitted on any network,
399 but may be delivered locally if the sending host belongs to the destination
400 group and if multicast loopback has not been disabled on the sending socket
402 Multicast datagrams with TTL greater than 1 may be forwarded
403 to other networks if a multicast router is attached to the local network.
405 For hosts with multiple interfaces, each multicast transmission is
406 sent from the primary network interface.
409 option overrides the default for
410 subsequent transmissions from a given socket:
413 setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
416 where "addr" is the local
418 address of the desired interface or
420 to specify the default interface.
421 An interface's local IP address and multicast capability can
427 Normal applications should not need to use this option.
429 If a multicast datagram is sent to a group to which the sending host itself
430 belongs (on the outgoing interface), a copy of the datagram is, by default,
431 looped back by the IP layer for local delivery.
433 .Dv IP_MULTICAST_LOOP
434 option gives the sender explicit control
435 over whether or not subsequent datagrams are looped back:
437 u_char loop; /* 0 = disable, 1 = enable (default) */
438 setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
442 improves performance for applications that may have no more than one
443 instance on a single host (such as a router daemon), by eliminating
444 the overhead of receiving their own transmissions.
445 It should generally not
446 be used by applications for which there may be more than one instance on a
447 single host (such as a conferencing program) or for which the sender does
448 not belong to the destination group (such as a time querying program).
450 A multicast datagram sent with an initial TTL greater than 1 may be delivered
451 to the sending host on a different interface from that on which it was sent,
452 if the host belongs to the destination group on that other interface.
453 The loopback control option has no effect on such delivery.
455 A host must become a member of a multicast group before it can receive
456 datagrams sent to the group.
457 To join a multicast group, use the
458 .Dv IP_ADD_MEMBERSHIP
462 setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
467 is the following structure:
470 struct in_addr imr_multiaddr; /* IP multicast address of group */
471 struct in_addr imr_interface; /* local IP address of interface */
478 to choose the default multicast interface,
481 address of a particular multicast-capable interface if
482 the host is multihomed.
487 member is within the network range
489 it is treated as an interface index in the system interface MIB,
490 as per the RIP Version 2 MIB Extension (RFC-1724).
492 Membership is associated with a single interface;
493 programs running on multihomed hosts may need to
494 join the same group on more than one interface.
496 .Dv IP_MAX_MEMBERSHIPS
497 (currently 20) memberships may be added on a
500 To drop a membership, use:
503 setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
508 contains the same values as used to add the membership.
509 Memberships are dropped when the socket is closed or the process exits.
510 .\"-----------------------
515 sockets are connectionless,
516 and are normally used with the
522 call may also be used to fix the destination for future
523 packets (in which case the
531 system calls may be used).
535 is 0, the default protocol
538 packets, and only incoming packets destined for that protocol
542 is non-zero, that protocol number will be used on outgoing packets
543 and to filter incoming packets.
545 Outgoing packets automatically have an
548 them (based on the destination address and the protocol
549 number the socket is created with),
553 Incoming packets are received with
555 header and options intact.
558 indicates the complete IP header is included with the data
559 and may be used only with the
563 #include <netinet/in_systm.h>
564 #include <netinet/ip.h>
566 int hincl = 1; /* 1 = on, 0 = off */
567 setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
572 releases, the program must set all
573 the fields of the IP header, including the following:
575 ip->ip_v = IPVERSION;
576 ip->ip_hl = hlen >> 2;
577 ip->ip_id = 0; /* 0 means kernel set appropriate value */
587 be provided in host byte order .
588 All other fields must be provided in network byte order.
591 for more information on network byte order.
594 field is set to 0 then the kernel will choose an
596 If the header source address is set to
598 the kernel will choose an appropriate address.
600 A socket operation may fail with one of the following errors returned:
603 when trying to establish a connection on a socket which
604 already has one, or when trying to send a datagram with the destination
605 address specified and the socket is already connected;
607 when trying to send a datagram, but
608 no destination address is specified, and the socket has not been
611 when the system runs out of memory for
612 an internal data structure;
613 .It Bq Er EADDRNOTAVAIL
614 when an attempt is made to create a
615 socket with a network address for which no network interface
618 when an attempt is made to create
619 a raw IP socket by a non-privileged process.
622 The following errors specific to
624 may occur when setting or getting
629 An unknown socket option name was given.
631 The IP option field was improperly formed;
632 an option field was shorter than the minimum value
633 or longer than the option buffer provided.
636 The following errors may occur when attempting to send
647 field was not equal to the length of the datagram written to the socket.