1IP(7) Linux Programmer's Manual IP(7)
2
6 ip - Linux IPv4 protocol implementation
7
9 #include <sys/socket.h>
10 #include <netinet/in.h>
11 #include <netinet/ip.h> /* superset of previous */
12 tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
14 udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
15 raw_socket = socket(AF_INET, SOCK_RAW, protocol);
16
18 Linux implements the Internet Protocol, version 4, described in RFC 791
19 and RFC 1122. ip contains a level 2 multicasting implementation con‐
20 forming to RFC 1112. It also contains an IP router including a packet
21 filter.
22 The programming interface is BSD-sockets compatible. For more informa‐
24 tion on sockets, see socket(7).
25 An IP socket is created using socket(2):
27 socket(AF_INET, socket_type, protocol);
29 Valid socket types are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM
31 to open a udp(7) socket, or SOCK_RAW to open a raw(7) socket to access
32 the IP protocol directly. protocol is the IP protocol in the IP header
33 to be received or sent. The only valid values for protocol are 0 and
34 IPPROTO_TCP for TCP sockets, and 0 and IPPROTO_UDP for UDP sockets.
35 For SOCK_RAW you may specify a valid IANA IP protocol defined in
36 RFC 1700 assigned numbers.
37 When a process wants to receive new incoming packets or connections, it
39 should bind a socket to a local interface address using bind(2). In
40 this case, only one IP socket may be bound to any given local (address,
41 port) pair. When INADDR_ANY is specified in the bind call, the socket
42 will be bound to all local interfaces. When listen(2) is called on an
43 unbound socket, the socket is automatically bound to a random free port
44 with the local address set to INADDR_ANY. When connect(2) is called on
45 an unbound socket, the socket is automatically bound to a random free
46 port or to a usable shared port with the local address set to
47 INADDR_ANY.
48 A TCP local socket address that has been bound is unavailable for some
50 time after closing, unless the SO_REUSEADDR flag has been set. Care
51 should be taken when using this flag as it makes TCP less reliable.
52 Address format
54 An IP socket address is defined as a combination of an IP interface
55 address and a 16-bit port number. The basic IP protocol does not sup‐
56 ply port numbers, they are implemented by higher level protocols like
57 udp(7) and tcp(7). On raw sockets sin_port is set to the IP protocol.
58 struct sockaddr_in {
60 sa_family_t sin_family; /* address family: AF_INET */
61 in_port_t sin_port; /* port in network byte order */
62 struct in_addr sin_addr; /* internet address */
63 };
64 /* Internet address. */
66 struct in_addr {
67 uint32_t s_addr; /* address in network byte order */
68 };
69 sin_family is always set to AF_INET. This is required; in Linux 2.2
71 most networking functions return EINVAL when this setting is missing.
72 sin_port contains the port in network byte order. The port numbers
73 below 1024 are called privileged ports (or sometimes: reserved ports).
74 Only a privileged process (on Linux: a process that has the
75 CAP_NET_BIND_SERVICE capability in the user namespace governing its
76 network namespace) may bind(2) to these sockets. Note that the raw
77 IPv4 protocol as such has no concept of a port, they are implemented
78 only by higher protocols like tcp(7) and udp(7).
79 sin_addr is the IP host address. The s_addr member of struct in_addr
81 contains the host interface address in network byte order. in_addr
82 should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK)
83 using htonl(3) or set using the inet_aton(3), inet_addr(3),
84 inet_makeaddr(3) library functions or directly with the name resolver
85 (see gethostbyname(3)).
86 IPv4 addresses are divided into unicast, broadcast, and multicast
88 addresses. Unicast addresses specify a single interface of a host,
89 broadcast addresses specify all hosts on a network, and multicast
90 addresses address all hosts in a multicast group. Datagrams to broad‐
91 cast addresses can be sent or received only when the SO_BROADCAST
92 socket flag is set. In the current implementation, connection-oriented
93 sockets are allowed to use only unicast addresses.
94 Note that the address and the port are always stored in network byte
96 order. In particular, this means that you need to call htons(3) on the
97 number that is assigned to a port. All address/port manipulation func‐
98 tions in the standard library work in network byte order.
99 There are several special addresses: INADDR_LOOPBACK (127.0.0.1) always
101 refers to the local host via the loopback device; INADDR_ANY (0.0.0.0)
102 means any address for binding; INADDR_BROADCAST (255.255.255.255) means
103 any host and has the same effect on bind as INADDR_ANY for historical
104 reasons.
105 Socket options
107 IP supports some protocol-specific socket options that can be set with
108 setsockopt(2) and read with getsockopt(2). The socket option level for
109 IP is IPPROTO_IP. A boolean integer flag is zero when it is false,
110 otherwise true.
111 When an invalid socket option is specified, getsockopt(2) and setsock‐
113 opt(2) fail with the error ENOPROTOOPT.
114 IP_ADD_MEMBERSHIP (since Linux 1.2)
116 Join a multicast group. Argument is an ip_mreqn structure.
117 struct ip_mreqn {
119 struct in_addr imr_multiaddr; /* IP multicast group
120 address */
121 struct in_addr imr_address; /* IP address of local
122 interface */
123 int imr_ifindex; /* interface index */
124 };
125 imr_multiaddr contains the address of the multicast group the applica‐
127 tion wants to join or leave. It must be a valid multicast address (or
128 setsockopt(2) fails with the error EINVAL). imr_address is the address
129 of the local interface with which the system should join the multicast
130 group; if it is equal to INADDR_ANY, an appropriate interface is chosen
131 by the system. imr_ifindex is the interface index of the interface
132 that should join/leave the imr_multiaddr group, or 0 to indicate any
133 interface.
134 The ip_mreqn structure is available only since Linux 2.2. For
136 compatibility, the old ip_mreq structure (present since Linux
137 1.2) is still supported; it differs from ip_mreqn only by not
138 including the imr_ifindex field. (The kernel determines which
139 structure is being passed based on the size passed in optlen.)
140 IP_ADD_MEMBERSHIP is valid only for setsockopt(2).
142 IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
144 Join a multicast group and allow receiving data only from a
145 specified source. Argument is an ip_mreq_source structure.
146 struct ip_mreq_source {
148 struct in_addr imr_multiaddr; /* IP multicast group
149 address */
150 struct in_addr imr_interface; /* IP address of local
151 interface */
152 struct in_addr imr_sourceaddr; /* IP address of
153 multicast source */
154 };
155 The ip_mreq_source structure is similar to ip_mreqn described under
157 IP_ADD_MEMBERSIP. The imr_multiaddr field contains the address of the
158 multicast group the application wants to join or leave. The imr_inter‐
159 face field is the address of the local interface with which the system
160 should join the multicast group. Finally, the imr_sourceaddr field
161 contains the address of the source the application wants to receive
162 data from.
163 This option can be used multiple times to allow receiving data
165 from more than one source.
166 IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
168 Inform the kernel to not reserve an ephemeral port when using
169 bind(2) with a port number of 0. The port will later be auto‐
170 matically chosen at connect(2) time, in a way that allows shar‐
171 ing a source port as long as the 4-tuple is unique.
172 IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
174 Stop receiving multicast data from a specific source in a given
175 group. This is valid only after the application has subscribed
176 to the multicast group using either IP_ADD_MEMBERSHIP or
177 IP_ADD_SOURCE_MEMBERSHIP.
178 Argument is an ip_mreq_source structure as described under
180 IP_ADD_SOURCE_MEMBERSHIP.
181 IP_DROP_MEMBERSHIP (since Linux 1.2)
183 Leave a multicast group. Argument is an ip_mreqn or ip_mreq
184 structure similar to IP_ADD_MEMBERSHIP.
185 IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
187 Leave a source-specific group—that is, stop receiving data from
188 a given multicast group that come from a given source. If the
189 application has subscribed to multiple sources within the same
190 group, data from the remaining sources will still be delivered.
191 To stop receiving data from all sources at once, use
192 IP_DROP_MEMBERSHIP.
193 Argument is an ip_mreq_source structure as described under
195 IP_ADD_SOURCE_MEMBERSHIP.
196 IP_FREEBIND (since Linux 2.4)
198 If enabled, this boolean option allows binding to an IP address
199 that is nonlocal or does not (yet) exist. This permits listen‐
200 ing on a socket, without requiring the underlying network inter‐
201 face or the specified dynamic IP address to be up at the time
202 that the application is trying to bind to it. This option is
203 the per-socket equivalent of the ip_nonlocal_bind /proc inter‐
204 face described below.
205 IP_HDRINCL (since Linux 2.0)
207 If enabled, the user supplies an IP header in front of the user
208 data. Valid only for SOCK_RAW sockets; see raw(7) for more
209 information. When this flag is enabled, the values set by
210 IP_OPTIONS, IP_TTL, and IP_TOS are ignored.
211 IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
213 This option provides access to the advanced full-state filtering
214 API. Argument is an ip_msfilter structure.
215 struct ip_msfilter {
217 struct in_addr imsf_multiaddr; /* IP multicast group
218 address */
219 struct in_addr imsf_interface; /* IP address of local
220 interface */
221 uint32_t imsf_fmode; /* Filter-mode */
222 uint32_t imsf_numsrc; /* Number of sources in
224 the following array */
225 struct in_addr imsf_slist[1]; /* Array of source
226 addresses */
227 };
228 There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can be
230 used to specify the filtering mode. Additionally, the IP_MSFIL‐
231 TER_SIZE(n) macro exists to determine how much memory is needed to
232 store ip_msfilter structure with n sources in the source list.
233 For the full description of multicast source filtering refer to
235 RFC 3376.
236 IP_MTU (since Linux 2.2)
238 Retrieve the current known path MTU of the current socket.
239 Returns an integer.
240 IP_MTU is valid only for getsockopt(2) and can be employed only
242 when the socket has been connected.
243 IP_MTU_DISCOVER (since Linux 2.2)
245 Set or receive the Path MTU Discovery setting for a socket.
246 When enabled, Linux will perform Path MTU Discovery as defined
247 in RFC 1191 on SOCK_STREAM sockets. For non-SOCK_STREAM sock‐
248 ets, IP_PMTUDISC_DO forces the don't-fragment flag to be set on
249 all outgoing packets. It is the user's responsibility to packe‐
250 tize the data in MTU-sized chunks and to do the retransmits if
251 necessary. The kernel will reject (with EMSGSIZE) datagrams
252 that are bigger than the known path MTU. IP_PMTUDISC_WANT will
253 fragment a datagram if needed according to the path MTU, or will
254 set the don't-fragment flag otherwise.
255 The system-wide default can be toggled between IP_PMTUDISC_WANT
257 and IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
258 values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.
259 Path MTU discovery value Meaning
261 IP_PMTUDISC_WANT Use per-route settings.
262 IP_PMTUDISC_DONT Never do Path MTU Discovery.
263 IP_PMTUDISC_DO Always do Path MTU Discovery.
264 IP_PMTUDISC_PROBE Set DF but ignore Path MTU.
265 When PMTU discovery is enabled, the kernel automatically keeps
267 track of the path MTU per destination host. When it is con‐
268 nected to a specific peer with connect(2), the currently known
269 path MTU can be retrieved conveniently using the IP_MTU socket
270 option (e.g., after an EMSGSIZE error occurred). The path MTU
271 may change over time. For connectionless sockets with many des‐
272 tinations, the new MTU for a given destination can also be
273 accessed using the error queue (see IP_RECVERR). A new error
274 will be queued for every incoming MTU update.
275 While MTU discovery is in progress, initial packets from data‐
277 gram sockets may be dropped. Applications using UDP should be
278 aware of this and not take it into account for their packet
279 retransmit strategy.
280 To bootstrap the path MTU discovery process on unconnected sock‐
282 ets, it is possible to start with a big datagram size (headers
283 up to 64 kilobytes long) and let it shrink by updates of the
284 path MTU.
285 To get an initial estimate of the path MTU, connect a datagram
287 socket to the destination address using connect(2) and retrieve
288 the MTU by calling getsockopt(2) with the IP_MTU option.
289 It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
291 or SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE
292 (available since Linux 2.6.22). This is also particularly use‐
293 ful for diagnostic tools such as tracepath(8) that wish to
294 deliberately send probe packets larger than the observed Path
295 MTU.
296 IP_MULTICAST_ALL (since Linux 2.6.31)
298 This option can be used to modify the delivery policy of multi‐
299 cast messages to sockets bound to the wildcard INADDR_ANY
300 address. The argument is a boolean integer (defaults to 1). If
301 set to 1, the socket will receive messages from all the groups
302 that have been joined globally on the whole system. Otherwise,
303 it will deliver messages only from the groups that have been
304 explicitly joined (for example via the IP_ADD_MEMBERSHIP option)
305 on this particular socket.
306 IP_MULTICAST_IF (since Linux 1.2)
308 Set the local device for a multicast socket. The argument for
309 setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq struc‐
310 ture similar to IP_ADD_MEMBERSHIP, or an in_addr structure.
311 (The kernel determines which structure is being passed based on
312 the size passed in optlen.) For getsockopt(2), the argument is
313 an in_addr structure.
314 IP_MULTICAST_LOOP (since Linux 1.2)
316 Set or read a boolean integer argument that determines whether
317 sent multicast packets should be looped back to the local sock‐
318 ets.
319 IP_MULTICAST_TTL (since Linux 1.2)
321 Set or read the time-to-live value of outgoing multicast packets
322 for this socket. It is very important for multicast packets to
323 set the smallest TTL possible. The default is 1 which means
324 that multicast packets don't leave the local network unless the
325 user program explicitly requests it. Argument is an integer.
326 IP_NODEFRAG (since Linux 2.6.36)
328 If enabled (argument is nonzero), the reassembly of outgoing
329 packets is disabled in the netfilter layer. The argument is an
330 integer.
331 This option is valid only for SOCK_RAW sockets.
333 IP_OPTIONS (since Linux 2.0)
335 Set or get the IP options to be sent with every packet from this
336 socket. The arguments are a pointer to a memory buffer contain‐
337 ing the options and the option length. The setsockopt(2) call
338 sets the IP options associated with a socket. The maximum
339 option size for IPv4 is 40 bytes. See RFC 791 for the allowed
340 options. When the initial connection request packet for a
341 SOCK_STREAM socket contains IP options, the IP options will be
342 set automatically to the options from the initial packet with
343 routing headers reversed. Incoming packets are not allowed to
344 change options after the connection is established. The pro‐
345 cessing of all incoming source routing options is disabled by
346 default and can be enabled by using the accept_source_route
347 /proc interface. Other options like timestamps are still han‐
348 dled. For datagram sockets, IP options can be only set by the
349 local user. Calling getsockopt(2) with IP_OPTIONS puts the cur‐
350 rent IP options used for sending into the supplied buffer.
351 IP_PKTINFO (since Linux 2.2)
353 Pass an IP_PKTINFO ancillary message that contains a pktinfo
354 structure that supplies some information about the incoming
355 packet. This only works for datagram oriented sockets. The
356 argument is a flag that tells the socket whether the IP_PKTINFO
357 message should be passed or not. The message itself can only be
358 sent/retrieved as control message with a packet using recvmsg(2)
359 or sendmsg(2).
360 struct in_pktinfo {
362 unsigned int ipi_ifindex; /* Interface index */
363 struct in_addr ipi_spec_dst; /* Local address */
364 struct in_addr ipi_addr; /* Header Destination
365 address */
366 };
367 ipi_ifindex is the unique index of the interface the packet was
369 received on. ipi_spec_dst is the local address of the packet
370 and ipi_addr is the destination address in the packet header.
371 If IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst is not
372 zero, then it is used as the local source address for the rout‐
373 ing table lookup and for setting up IP source route options.
374 When ipi_ifindex is not zero, the primary local address of the
375 interface specified by the index overwrites ipi_spec_dst for the
376 routing table lookup.
377 IP_RECVERR (since Linux 2.2)
379 Enable extended reliable error message passing. When enabled on
380 a datagram socket, all generated errors will be queued in a per-
381 socket error queue. When the user receives an error from a
382 socket operation, the errors can be received by calling
383 recvmsg(2) with the MSG_ERRQUEUE flag set. The
384 sock_extended_err structure describing the error will be passed
385 in an ancillary message with the type IP_RECVERR and the level
386 IPPROTO_IP. This is useful for reliable error handling on
387 unconnected sockets. The received data portion of the error
388 queue contains the error packet.
389 The IP_RECVERR control message contains a sock_extended_err
391 structure:
392 #define SO_EE_ORIGIN_NONE 0
394 #define SO_EE_ORIGIN_LOCAL 1
395 #define SO_EE_ORIGIN_ICMP 2
396 #define SO_EE_ORIGIN_ICMP6 3
397 struct sock_extended_err {
399 uint32_t ee_errno; /* error number */
400 uint8_t ee_origin; /* where the error originated */
401 uint8_t ee_type; /* type */
402 uint8_t ee_code; /* code */
403 uint8_t ee_pad;
404 uint32_t ee_info; /* additional information */
405 uint32_t ee_data; /* other data */
406 /* More data may follow */
407 };
408 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
410 ee_errno contains the errno number of the queued error. ee_ori‐
412 gin is the origin code of where the error originated. The other
413 fields are protocol-specific. The macro SO_EE_OFFENDER returns
414 a pointer to the address of the network object where the error
415 originated from given a pointer to the ancillary message. If
416 this address is not known, the sa_family member of the sockaddr
417 contains AF_UNSPEC and the other fields of the sockaddr are
418 undefined.
419 IP uses the sock_extended_err structure as follows: ee_origin is
421 set to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet,
422 or SO_EE_ORIGIN_LOCAL for locally generated errors. Unknown
423 values should be ignored. ee_type and ee_code are set from the
424 type and code fields of the ICMP header. ee_info contains the
425 discovered MTU for EMSGSIZE errors. The message also contains
426 the sockaddr_in of the node caused the error, which can be
427 accessed with the SO_EE_OFFENDER macro. The sin_family field of
428 the SO_EE_OFFENDER address is AF_UNSPEC when the source was
429 unknown. When the error originated from the network, all IP
430 options (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and
431 contained in the error packet are passed as control messages.
432 The payload of the packet causing the error is returned as nor‐
433 mal payload. Note that TCP has no error queue; MSG_ERRQUEUE is
434 not permitted on SOCK_STREAM sockets. IP_RECVERR is valid for
435 TCP, but all errors are returned by socket function return or
436 SO_ERROR only.
437 For raw sockets, IP_RECVERR enables passing of all received ICMP
439 errors to the application, otherwise errors are only reported on
440 connected sockets
441 It sets or retrieves an integer boolean flag. IP_RECVERR
443 defaults to off.
444 IP_RECVOPTS (since Linux 2.2)
446 Pass all incoming IP options to the user in a IP_OPTIONS control
447 message. The routing header and other options are already
448 filled in for the local host. Not supported for SOCK_STREAM
449 sockets.
450 IP_RECVORIGDSTADDR (since Linux 2.6.29)
452 This boolean option enables the IP_ORIGDSTADDR ancillary message
453 in recvmsg(2), in which the kernel returns the original destina‐
454 tion address of the datagram being received. The ancillary mes‐
455 sage contains a struct sockaddr_in.
456 IP_RECVTOS (since Linux 2.2)
458 If enabled, the IP_TOS ancillary message is passed with incoming
459 packets. It contains a byte which specifies the Type of Ser‐
460 vice/Precedence field of the packet header. Expects a boolean
461 integer flag.
462 IP_RECVTTL (since Linux 2.2)
464 When this flag is set, pass a IP_TTL control message with the
465 time-to-live field of the received packet as a byte. Not sup‐
466 ported for SOCK_STREAM sockets.
467 IP_RETOPTS (since Linux 2.2)
469 Identical to IP_RECVOPTS, but returns raw unprocessed options
470 with timestamp and route record options not filled in for this
471 hop.
472 IP_ROUTER_ALERT (since Linux 2.2)
474 Pass all to-be forwarded packets with the IP Router Alert option
475 set to this socket. Valid only for raw sockets. This is use‐
476 ful, for instance, for user-space RSVP daemons. The tapped
477 packets are not forwarded by the kernel; it is the user's
478 responsibility to send them out again. Socket binding is
479 ignored, such packets are only filtered by protocol. Expects an
480 integer flag.
481 IP_TOS (since Linux 1.0)
483 Set or receive the Type-Of-Service (TOS) field that is sent with
484 every IP packet originating from this socket. It is used to
485 prioritize packets on the network. TOS is a byte. There are
486 some standard TOS flags defined: IPTOS_LOWDELAY to minimize
487 delays for interactive traffic, IPTOS_THROUGHPUT to optimize
488 throughput, IPTOS_RELIABILITY to optimize for reliability,
489 IPTOS_MINCOST should be used for "filler data" where slow trans‐
490 mission doesn't matter. At most one of these TOS values can be
491 specified. Other bits are invalid and shall be cleared. Linux
492 sends IPTOS_LOWDELAY datagrams first by default, but the exact
493 behavior depends on the configured queueing discipline. Some
494 high-priority levels may require superuser privileges (the
495 CAP_NET_ADMIN capability).
496 IP_TRANSPARENT (since Linux 2.6.24)
498 Setting this boolean option enables transparent proxying on this
499 socket. This socket option allows the calling application to
500 bind to a nonlocal IP address and operate both as a client and a
501 server with the foreign address as the local endpoint. NOTE:
502 this requires that routing be set up in a way that packets going
503 to the foreign address are routed through the TProxy box (i.e.,
504 the system hosting the application that employs the IP_TRANSPAR‐
505 ENT socket option). Enabling this socket option requires supe‐
506 ruser privileges (the CAP_NET_ADMIN capability).
507 TProxy redirection with the iptables TPROXY target also requires
509 that this option be set on the redirected socket.
510 IP_TTL (since Linux 1.0)
512 Set or retrieve the current time-to-live field that is used in
513 every packet sent from this socket.
514 IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
516 Unblock previously blocked multicast source. Returns EADDRNO‐
517 TAVAIL when given source is not being blocked.
518 Argument is an ip_mreq_source structure as described under
520 IP_ADD_SOURCE_MEMBERSHIP.
521 /proc interfaces
523 The IP protocol supports a set of /proc interfaces to configure some
524 global parameters. The parameters can be accessed by reading or writ‐
525 ing files in the directory /proc/sys/net/ipv4/. Interfaces described
526 as Boolean take an integer value, with a nonzero value ("true") meaning
527 that the corresponding option is enabled, and a zero value ("false")
528 meaning that the option is disabled.
529 ip_always_defrag (Boolean; since Linux 2.2.13)
531 [New with kernel 2.2.13; in earlier kernel versions this feature
532 was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG
533 option; this option is not present in 2.4.x and later]
534 When this boolean flag is enabled (not equal 0), incoming frag‐
536 ments (parts of IP packets that arose when some host between
537 origin and destination decided that the packets were too large
538 and cut them into pieces) will be reassembled (defragmented)
539 before being processed, even if they are about to be forwarded.
540 Enable only if running either a firewall that is the sole link
542 to your network or a transparent proxy; never ever use it for a
543 normal router or host. Otherwise, fragmented communication can
544 be disturbed if the fragments travel over different links.
545 Defragmentation also has a large memory and CPU time cost.
546 This is automagically turned on when masquerading or transparent
548 proxying are configured.
549 ip_autoconfig (since Linux 2.2 to 2.6.17)
551 Not documented.
552 ip_default_ttl (integer; default: 64; since Linux 2.2)
554 Set the default time-to-live value of outgoing packets. This
555 can be changed per socket with the IP_TTL option.
556 ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
558 Enable dynamic socket address and masquerading entry rewriting
559 on interface address change. This is useful for dialup inter‐
560 face with changing IP addresses. 0 means no rewriting, 1 turns
561 it on and 2 enables verbose mode.
562 ip_forward (Boolean; default: disabled; since Linux 1.2)
564 Enable IP forwarding with a boolean flag. IP forwarding can be
565 also set on a per-interface basis.
566 ip_local_port_range (since Linux 2.2)
568 This file contains two integers that define the default local
569 port range allocated to sockets that are not explicitly bound to
570 a port number—that is, the range used for ephemeral ports. An
571 ephemeral port is allocated to a socket in the following circum‐
572 stances:
573 * the port number in a socket address is specified as 0 when
575 calling bind(2);
576 * listen(2) is called on a stream socket that was not previ‐
578 ously bound;
579 * connect(2) was called on a socket that was not previously
581 bound;
582 * sendto(2) is called on a datagram socket that was not previ‐
584 ously bound.
585 Allocation of ephemeral ports starts with the first number in
587 ip_local_port_range and ends with the second number. If the
588 range of ephemeral ports is exhausted, then the relevant system
589 call returns an error (but see BUGS).
590 Note that the port range in ip_local_port_range should not con‐
592 flict with the ports used by masquerading (although the case is
593 handled). Also, arbitrary choices may cause problems with some
594 firewall packet filters that make assumptions about the local
595 ports in use. The first number should be at least greater than
596 1024, or better, greater than 4096, to avoid clashes with well
597 known ports and to minimize firewall problems.
598 ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
600 If enabled, don't do Path MTU Discovery for TCP sockets by
601 default. Path MTU discovery may fail if misconfigured firewalls
602 (that drop all ICMP packets) or misconfigured interfaces (e.g.,
603 a point-to-point link where the both ends don't agree on the
604 MTU) are on the path. It is better to fix the broken routers on
605 the path than to turn off Path MTU Discovery globally, because
606 not doing it incurs a high cost to the network.
607 ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
609 If set, allows processes to bind(2) to nonlocal IP addresses,
610 which can be quite useful, but may break some applications.
611 ip6frag_time (integer; default: 30)
613 Time in seconds to keep an IPv6 fragment in memory.
614 ip6frag_secret_interval (integer; default: 600)
616 Regeneration interval (in seconds) of the hash secret (or life‐
617 time for the hash secret) for IPv6 fragments.
618 ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
620 If the amount of queued IP fragments reaches ipfrag_high_thresh,
621 the queue is pruned down to ipfrag_low_thresh. Contains an
622 integer with the number of bytes.
623 neigh/*
625 See arp(7).
626 Ioctls
628 All ioctls described in socket(7) apply to ip.
629 Ioctls to configure generic device parameters are described in netde‐
631 vice(7).
632
634 EACCES The user tried to execute an operation without the necessary
635 permissions. These include: sending a packet to a broadcast
636 address without having the SO_BROADCAST flag set; sending a
637 packet via a prohibit route; modifying firewall settings without
638 superuser privileges (the CAP_NET_ADMIN capability); binding to
639 a privileged port without superuser privileges (the
640 CAP_NET_BIND_SERVICE capability).
641 EADDRINUSE
643 Tried to bind to an address already in use.
644 EADDRNOTAVAIL
646 A nonexistent interface was requested or the requested source
647 address was not local.
648 EAGAIN Operation on a nonblocking socket would block.
650 EALREADY
652 A connection operation on a nonblocking socket is already in
653 progress.
654 ECONNABORTED
656 A connection was closed during an accept(2).
657 EHOSTUNREACH
659 No valid routing table entry matches the destination address.
660 This error can be caused by an ICMP message from a remote router
661 or for the local routing table.
662 EINVAL Invalid argument passed. For send operations this can be caused
664 by sending to a blackhole route.
665 EISCONN
667 connect(2) was called on an already connected socket.
668 EMSGSIZE
670 Datagram is bigger than an MTU on the path and it cannot be
671 fragmented.
672 ENOBUFS, ENOMEM
674 Not enough free memory. This often means that the memory allo‐
675 cation is limited by the socket buffer limits, not by the system
676 memory, but this is not 100% consistent.
677 ENOENT SIOCGSTAMP was called on a socket where no packet arrived.
679 ENOPKG A kernel subsystem was not configured.
681 ENOPROTOOPT and EOPNOTSUPP
683 Invalid socket option passed.
684 ENOTCONN
686 The operation is defined only on a connected socket, but the
687 socket wasn't connected.
688 EPERM User doesn't have permission to set high priority, change con‐
690 figuration, or send signals to the requested process or group.
691 EPIPE The connection was unexpectedly closed or shut down by the other
693 end.
694 ESOCKTNOSUPPORT
696 The socket is not configured or an unknown socket type was
697 requested.
698 Other errors may be generated by the overlaying protocols; see tcp(7),
700 raw(7), udp(7), and socket(7).
701
703 IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER, IP_RECVORIGDSTADDR,
704 IP_PKTINFO, IP_RECVERR, IP_ROUTER_ALERT, and IP_TRANSPARENT are Linux-
705 specific.
706 Be very careful with the SO_BROADCAST option - it is not privileged in
708 Linux. It is easy to overload the network with careless broadcasts.
709 For new application protocols it is better to use a multicast group
710 instead of broadcasting. Broadcasting is discouraged.
711 Some other BSD sockets implementations provide IP_RCVDSTADDR and
713 IP_RECVIF socket options to get the destination address and the inter‐
714 face of received datagrams. Linux has the more general IP_PKTINFO for
715 the same task.
716 Some BSD sockets implementations also provide an IP_RECVTTL option, but
718 an ancillary message with type IP_RECVTTL is passed with the incoming
719 packet. This is different from the IP_TTL option used in Linux.
720 Using the SOL_IP socket options level isn't portable; BSD-based stacks
722 use the IPPROTO_IP level.
723 INADDR_ANY (0.0.0.0) and INADDR_BROADCAST (255.255.255.255) are byte-
725 order-neutral.
726 This means htonl(3) has no effect on them.
727 Compatibility
729 For compatibility with Linux 2.0, the obsolete socket(AF_INET,
730 SOCK_PACKET, protocol) syntax is still supported to open a packet(7)
731 socket. This is deprecated and should be replaced by socket(AF_PACKET,
732 SOCK_RAW, protocol) instead. The main difference is the new sock‐
733 addr_ll address structure for generic link layer information instead of
734 the old sockaddr_pkt.
735
737 There are too many inconsistent error values.
738 The error used to diagnose exhaustion of the ephemeral port range dif‐
740 fers across the various system calls (connect(2), bind(2), listen(2),
741 sendto(2)) that can assign ephemeral ports.
742 The ioctls to configure IP-specific interface options and ARP tables
744 are not described.
745 Receiving the original destination address with MSG_ERRQUEUE in
747 msg_name by recvmsg(2) does not work in some 2.2 kernels.
748
750 recvmsg(2), sendmsg(2), byteorder(3), ipfw(4), capabilities(7),
751 icmp(7), ipv6(7), netlink(7), raw(7), socket(7), tcp(7), udp(7), ip(8)
752 RFC 791 for the original IP specification. RFC 1122 for the IPv4 host
754 requirements. RFC 1812 for the IPv4 router requirements.
755
757 This page is part of release 4.15 of the Linux man-pages project. A
758 description of the project, information about reporting bugs, and the
759 latest version of this page, can be found at
760 https://www.kernel.org/doc/man-pages/.
761Linux 2018-02-02 IP(7)