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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
130 sockets, unless the IP_HDRINCL option has been set.
133 sockets the Don't Fragment flag is controlled by the Path
134 MTU Discovery option.
135 Sending a packet larger than the MTU size of the egress interface,
136 determined by the destination address, returns an EMSGSIZE error.
140 option is enabled on a
145 call will return the destination
154 structure points to a buffer
157 structure followed by the
162 fields have the following values:
164 cmsg_len = sizeof(struct in_addr)
165 cmsg_level = IPPROTO_IP
166 cmsg_type = IP_RECVDSTADDR
169 The source address to be used for outgoing
171 datagrams on a socket that is not bound to a specific
173 address can be specified as ancillary data with a type code of
175 The msg_control field in the msghdr structure should point to a buffer
178 structure followed by the
181 The cmsghdr fields should have the following values:
183 cmsg_len = sizeof(struct in_addr)
184 cmsg_level = IPPROTO_IP
185 cmsg_type = IP_SENDSRCADDR
190 is defined to have the same value as
196 can be used directly as a control message for
201 option is enabled on a
205 socket, the destination address of outgoing
206 broadcast datagrams on that socket will be forced
207 to the undirected broadcast address,
208 .Dv INADDR_BROADCAST ,
210 This is in contrast to the default behavior of the
211 system, which is to transmit undirected broadcasts
212 via the first network interface with the
213 .Dv IFF_BROADCAST flag set.
215 This option allows applications to choose which
216 interface is used to transmit an undirected broadcast
218 For example, the following code would force an
219 undirected broadcast to be transmitted via the interface
220 configured with the broadcast address 192.168.2.255:
223 struct sockaddr_in sin;
224 u_char onesbcast = 1; /* 0 = disable (default), 1 = enable */
226 setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
227 sin.sin_addr.s_addr = inet_addr("192.168.2.255");
228 sin.sin_port = htons(1234);
229 sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));
232 It is the application's responsibility to set the
234 to an appropriate value in order to prevent broadcast storms.
235 The application must have sufficient credentials to set the
237 socket level option, otherwise the
238 .Dv IP_ONESBCAST option has no effect.
242 option is enabled on a
249 (time to live) field for a
252 The msg_control field in the msghdr structure points to a buffer
253 that contains a cmsghdr structure followed by the
255 The cmsghdr fields have the following values:
257 cmsg_len = sizeof(u_char)
258 cmsg_level = IPPROTO_IP
259 cmsg_type = IP_RECVTTL
264 option is enabled on a
269 .Vt "struct sockaddr_dl"
270 corresponding to the interface on which the
276 structure points to a buffer that contains a
278 structure followed by the
279 .Vt "struct sockaddr_dl" .
282 fields have the following values:
284 cmsg_len = sizeof(struct sockaddr_dl)
285 cmsg_level = IPPROTO_IP
286 cmsg_type = IP_RECVIF
290 may be used to set the port range used for selecting a local port number
291 on a socket with an unspecified (zero) port number.
294 .Bl -tag -width IP_PORTRANGE_DEFAULT
295 .It Dv IP_PORTRANGE_DEFAULT
296 use the default range of values, normally
297 .Dv IPPORT_HIFIRSTAUTO
299 .Dv IPPORT_HILASTAUTO .
300 This is adjustable through the sysctl setting:
301 .Va net.inet.ip.portrange.first
303 .Va net.inet.ip.portrange.last .
304 .It Dv IP_PORTRANGE_HIGH
305 use a high range of values, normally
306 .Dv IPPORT_HIFIRSTAUTO
308 .Dv IPPORT_HILASTAUTO .
309 This is adjustable through the sysctl setting:
310 .Va net.inet.ip.portrange.hifirst
312 .Va net.inet.ip.portrange.hilast .
313 .It Dv IP_PORTRANGE_LOW
314 use a low range of ports, which are normally restricted to
315 privileged processes on
318 The range is normally from
321 .Li IPPORT_RESERVEDSTART
323 This is adjustable through the sysctl setting:
324 .Va net.inet.ip.portrange.lowfirst
326 .Va net.inet.ip.portrange.lowlast .
329 The range of privileged ports which only may be opened by
330 root-owned processes may be modified by the
331 .Va net.inet.ip.portrange.reservedlow
333 .Va net.inet.ip.portrange.reservedhigh
335 The values default to the traditional range,
339 (0 through 1023), respectively.
340 Note that these settings do not affect and are not accounted for in the
341 use or calculation of the other
342 .Va net.inet.ip.portrange
344 Changing these values departs from
346 tradition and has security
347 consequences that the administrator should carefully evaluate before
348 modifying these settings.
350 Ports are allocated at random within the specified port range in order
351 to increase the difficulty of random spoofing attacks.
352 In scenarios such as benchmarking, this behavior may be undesirable.
354 .Va net.inet.ip.portrange.randomized
355 can be used to toggle randomization off.
357 .Va net.inet.ip.portrange.randomcps
358 ports have been allocated in the last second, then return to sequential
360 Return to random allocation only once the current port allocation rate
362 .Va net.inet.ip.portrange.randomcps
364 .Va net.inet.ip.portrange.randomtime
366 The default values for
367 .Va net.inet.ip.portrange.randomcps
369 .Va net.inet.ip.portrange.randomtime
370 are 10 port allocations per second and 45 seconds correspondingly.
371 .Ss "Multicast Options"
374 multicasting is supported only on
380 and only on networks where the interface
381 driver supports multicasting.
385 option changes the time-to-live (TTL)
386 for outgoing multicast datagrams
387 in order to control the scope of the multicasts:
389 u_char ttl; /* range: 0 to 255, default = 1 */
390 setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
393 Datagrams with a TTL of 1 are not forwarded beyond the local network.
394 Multicast datagrams with a TTL of 0 will not be transmitted on any network,
395 but may be delivered locally if the sending host belongs to the destination
396 group and if multicast loopback has not been disabled on the sending socket
398 Multicast datagrams with TTL greater than 1 may be forwarded
399 to other networks if a multicast router is attached to the local network.
401 For hosts with multiple interfaces, each multicast transmission is
402 sent from the primary network interface.
405 option overrides the default for
406 subsequent transmissions from a given socket:
409 setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
412 where "addr" is the local
414 address of the desired interface or
416 to specify the default interface.
417 An interface's local IP address and multicast capability can
423 Normal applications should not need to use this option.
425 If a multicast datagram is sent to a group to which the sending host itself
426 belongs (on the outgoing interface), a copy of the datagram is, by default,
427 looped back by the IP layer for local delivery.
429 .Dv IP_MULTICAST_LOOP
430 option gives the sender explicit control
431 over whether or not subsequent datagrams are looped back:
433 u_char loop; /* 0 = disable, 1 = enable (default) */
434 setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
438 improves performance for applications that may have no more than one
439 instance on a single host (such as a router daemon), by eliminating
440 the overhead of receiving their own transmissions.
441 It should generally not
442 be used by applications for which there may be more than one instance on a
443 single host (such as a conferencing program) or for which the sender does
444 not belong to the destination group (such as a time querying program).
446 A multicast datagram sent with an initial TTL greater than 1 may be delivered
447 to the sending host on a different interface from that on which it was sent,
448 if the host belongs to the destination group on that other interface.
449 The loopback control option has no effect on such delivery.
451 A host must become a member of a multicast group before it can receive
452 datagrams sent to the group.
453 To join a multicast group, use the
454 .Dv IP_ADD_MEMBERSHIP
458 setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
463 is the following structure:
466 struct in_addr imr_multiaddr; /* IP multicast address of group */
467 struct in_addr imr_interface; /* local IP address of interface */
474 to choose the default multicast interface,
477 address of a particular multicast-capable interface if
478 the host is multihomed.
483 member is within the network range
485 it is treated as an interface index in the system interface MIB,
486 as per the RIP Version 2 MIB Extension (RFC-1724).
488 Membership is associated with a single interface;
489 programs running on multihomed hosts may need to
490 join the same group on more than one interface.
492 .Dv IP_MAX_MEMBERSHIPS
493 (currently 20) memberships may be added on a
496 To drop a membership, use:
499 setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
504 contains the same values as used to add the membership.
505 Memberships are dropped when the socket is closed or the process exits.
506 .\"-----------------------
511 sockets are connectionless,
512 and are normally used with the
518 call may also be used to fix the destination for future
519 packets (in which case the
527 system calls may be used).
531 is 0, the default protocol
534 packets, and only incoming packets destined for that protocol
538 is non-zero, that protocol number will be used on outgoing packets
539 and to filter incoming packets.
541 Outgoing packets automatically have an
544 them (based on the destination address and the protocol
545 number the socket is created with),
549 Incoming packets are received with
551 header and options intact.
554 indicates the complete IP header is included with the data
555 and may be used only with the
559 #include <netinet/in_systm.h>
560 #include <netinet/ip.h>
562 int hincl = 1; /* 1 = on, 0 = off */
563 setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
568 releases, the program must set all
569 the fields of the IP header, including the following:
571 ip->ip_v = IPVERSION;
572 ip->ip_hl = hlen >> 2;
573 ip->ip_id = 0; /* 0 means kernel set appropriate value */
583 be provided in host byte order .
584 All other fields must be provided in network byte order.
587 for more information on network byte order.
590 field is set to 0 then the kernel will choose an
592 If the header source address is set to
594 the kernel will choose an appropriate address.
596 A socket operation may fail with one of the following errors returned:
599 when trying to establish a connection on a socket which
600 already has one, or when trying to send a datagram with the destination
601 address specified and the socket is already connected;
603 when trying to send a datagram, but
604 no destination address is specified, and the socket has not been
607 when the system runs out of memory for
608 an internal data structure;
609 .It Bq Er EADDRNOTAVAIL
610 when an attempt is made to create a
611 socket with a network address for which no network interface
614 when an attempt is made to create
615 a raw IP socket by a non-privileged process.
618 The following errors specific to
620 may occur when setting or getting
625 An unknown socket option name was given.
627 The IP option field was improperly formed;
628 an option field was shorter than the minimum value
629 or longer than the option buffer provided.
632 The following errors may occur when attempting to send
643 field was not equal to the length of the datagram written to the socket.