1ip(7) Miscellaneous Information 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 include SOCK_STREAM to open a stream socket,
31 SOCK_DGRAM to open a datagram socket, and SOCK_RAW to open a raw(7)
32 socket to access the IP protocol directly.
33 protocol is the IP protocol in the IP header to be received or sent.
35 Valid values for protocol include:
36 • 0 and IPPROTO_TCP for tcp(7) stream sockets;
38 • 0 and IPPROTO_UDP for udp(7) datagram sockets;
40 • IPPROTO_SCTP for sctp(7) stream sockets; and
42 • IPPROTO_UDPLITE for udplite(7) datagram sockets.
44 For SOCK_RAW you may specify a valid IANA IP protocol defined in
46 RFC 1700 assigned numbers.
47 When a process wants to receive new incoming packets or connections, it
49 should bind a socket to a local interface address using bind(2). In
50 this case, only one IP socket may be bound to any given local (address,
51 port) pair. When INADDR_ANY is specified in the bind call, the socket
52 will be bound to all local interfaces. When listen(2) is called on an
53 unbound socket, the socket is automatically bound to a random free port
54 with the local address set to INADDR_ANY. When connect(2) is called on
55 an unbound socket, the socket is automatically bound to a random free
56 port or to a usable shared port with the local address set to IN‐
57 ADDR_ANY.
58 A TCP local socket address that has been bound is unavailable for some
60 time after closing, unless the SO_REUSEADDR flag has been set. Care
61 should be taken when using this flag as it makes TCP less reliable.
62 Address format
64 An IP socket address is defined as a combination of an IP interface ad‐
65 dress and a 16-bit port number. The basic IP protocol does not supply
66 port numbers, they are implemented by higher level protocols like
67 udp(7) and tcp(7). On raw sockets sin_port is set to the IP protocol.
68 struct sockaddr_in {
70 sa_family_t sin_family; /* address family: AF_INET */
71 in_port_t sin_port; /* port in network byte order */
72 struct in_addr sin_addr; /* internet address */
73 };
74 /* Internet address */
76 struct in_addr {
77 uint32_t s_addr; /* address in network byte order */
78 };
79 sin_family is always set to AF_INET. This is required; in Linux 2.2
81 most networking functions return EINVAL when this setting is missing.
82 sin_port contains the port in network byte order. The port numbers be‐
83 low 1024 are called privileged ports (or sometimes: reserved ports).
84 Only a privileged process (on Linux: a process that has the
85 CAP_NET_BIND_SERVICE capability in the user namespace governing its
86 network namespace) may bind(2) to these sockets. Note that the raw
87 IPv4 protocol as such has no concept of a port, they are implemented
88 only by higher protocols like tcp(7) and udp(7).
89 sin_addr is the IP host address. The s_addr member of struct in_addr
91 contains the host interface address in network byte order. in_addr
92 should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK)
93 using htonl(3) or set using the inet_aton(3), inet_addr(3),
94 inet_makeaddr(3) library functions or directly with the name resolver
95 (see gethostbyname(3)).
96 IPv4 addresses are divided into unicast, broadcast, and multicast ad‐
98 dresses. Unicast addresses specify a single interface of a host,
99 broadcast addresses specify all hosts on a network, and multicast ad‐
100 dresses address all hosts in a multicast group. Datagrams to broadcast
101 addresses can be sent or received only when the SO_BROADCAST socket
102 flag is set. In the current implementation, connection-oriented sock‐
103 ets are allowed to use only unicast addresses.
104 Note that the address and the port are always stored in network byte
106 order. In particular, this means that you need to call htons(3) on the
107 number that is assigned to a port. All address/port manipulation func‐
108 tions in the standard library work in network byte order.
109 There are several special addresses: INADDR_LOOPBACK (127.0.0.1) always
111 refers to the local host via the loopback device; INADDR_ANY (0.0.0.0)
112 means any address for binding; INADDR_BROADCAST (255.255.255.255) means
113 any host and has the same effect on bind as INADDR_ANY for historical
114 reasons.
115 Socket options
117 IP supports some protocol-specific socket options that can be set with
118 setsockopt(2) and read with getsockopt(2). The socket option level for
119 IP is IPPROTO_IP. A boolean integer flag is zero when it is false,
120 otherwise true.
121 When an invalid socket option is specified, getsockopt(2) and setsock‐
123 opt(2) fail with the error ENOPROTOOPT.
124 IP_ADD_MEMBERSHIP (since Linux 1.2)
126 Join a multicast group. Argument is an ip_mreqn structure.
127 struct ip_mreqn {
129 struct in_addr imr_multiaddr; /* IP multicast group
130 address */
131 struct in_addr imr_address; /* IP address of local
132 interface */
133 int imr_ifindex; /* interface index */
134 };
135 imr_multiaddr contains the address of the multicast group the applica‐
137 tion wants to join or leave. It must be a valid multicast address (or
138 setsockopt(2) fails with the error EINVAL). imr_address is the address
139 of the local interface with which the system should join the multicast
140 group; if it is equal to INADDR_ANY, an appropriate interface is chosen
141 by the system. imr_ifindex is the interface index of the interface
142 that should join/leave the imr_multiaddr group, or 0 to indicate any
143 interface.
144 The ip_mreqn structure is available only since Linux 2.2. For
146 compatibility, the old ip_mreq structure (present since Linux
147 1.2) is still supported; it differs from ip_mreqn only by not
148 including the imr_ifindex field. (The kernel determines which
149 structure is being passed based on the size passed in optlen.)
150 IP_ADD_MEMBERSHIP is valid only for setsockopt(2).
152 IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / Linux 2.5.68)
154 Join a multicast group and allow receiving data only from a
155 specified source. Argument is an ip_mreq_source structure.
156 struct ip_mreq_source {
158 struct in_addr imr_multiaddr; /* IP multicast group
159 address */
160 struct in_addr imr_interface; /* IP address of local
161 interface */
162 struct in_addr imr_sourceaddr; /* IP address of
163 multicast source */
164 };
165 The ip_mreq_source structure is similar to ip_mreqn described under
167 IP_ADD_MEMBERSHIP. The imr_multiaddr field contains the address of the
168 multicast group the application wants to join or leave. The imr_inter‐
169 face field is the address of the local interface with which the system
170 should join the multicast group. Finally, the imr_sourceaddr field
171 contains the address of the source the application wants to receive
172 data from.
173 This option can be used multiple times to allow receiving data
175 from more than one source.
176 IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
178 Inform the kernel to not reserve an ephemeral port when using
179 bind(2) with a port number of 0. The port will later be auto‐
180 matically chosen at connect(2) time, in a way that allows shar‐
181 ing a source port as long as the 4-tuple is unique.
182 IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
184 Stop receiving multicast data from a specific source in a given
185 group. This is valid only after the application has subscribed
186 to the multicast group using either IP_ADD_MEMBERSHIP or
187 IP_ADD_SOURCE_MEMBERSHIP.
188 Argument is an ip_mreq_source structure as described under
190 IP_ADD_SOURCE_MEMBERSHIP.
191 IP_DROP_MEMBERSHIP (since Linux 1.2)
193 Leave a multicast group. Argument is an ip_mreqn or ip_mreq
194 structure similar to IP_ADD_MEMBERSHIP.
195 IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
197 Leave a source-specific group—that is, stop receiving data from
198 a given multicast group that come from a given source. If the
199 application has subscribed to multiple sources within the same
200 group, data from the remaining sources will still be delivered.
201 To stop receiving data from all sources at once, use
202 IP_DROP_MEMBERSHIP.
203 Argument is an ip_mreq_source structure as described under
205 IP_ADD_SOURCE_MEMBERSHIP.
206 IP_FREEBIND (since Linux 2.4)
208 If enabled, this boolean option allows binding to an IP address
209 that is nonlocal or does not (yet) exist. This permits listen‐
210 ing on a socket, without requiring the underlying network inter‐
211 face or the specified dynamic IP address to be up at the time
212 that the application is trying to bind to it. This option is
213 the per-socket equivalent of the ip_nonlocal_bind /proc inter‐
214 face described below.
215 IP_HDRINCL (since Linux 2.0)
217 If enabled, the user supplies an IP header in front of the user
218 data. Valid only for SOCK_RAW sockets; see raw(7) for more in‐
219 formation. When this flag is enabled, the values set by IP_OP‐
220 TIONS, IP_TTL, and IP_TOS are ignored.
221 IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
223 This option provides access to the advanced full-state filtering
224 API. Argument is an ip_msfilter structure.
225 struct ip_msfilter {
227 struct in_addr imsf_multiaddr; /* IP multicast group
228 address */
229 struct in_addr imsf_interface; /* IP address of local
230 interface */
231 uint32_t imsf_fmode; /* Filter-mode */
232 uint32_t imsf_numsrc; /* Number of sources in
234 the following array */
235 struct in_addr imsf_slist[1]; /* Array of source
236 addresses */
237 };
238 There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can be
240 used to specify the filtering mode. Additionally, the IP_MSFIL‐
241 TER_SIZE(n) macro exists to determine how much memory is needed to
242 store ip_msfilter structure with n sources in the source list.
243 For the full description of multicast source filtering refer to
245 RFC 3376.
246 IP_MTU (since Linux 2.2)
248 Retrieve the current known path MTU of the current socket. Re‐
249 turns an integer.
250 IP_MTU is valid only for getsockopt(2) and can be employed only
252 when the socket has been connected.
253 IP_MTU_DISCOVER (since Linux 2.2)
255 Set or receive the Path MTU Discovery setting for a socket.
256 When enabled, Linux will perform Path MTU Discovery as defined
257 in RFC 1191 on SOCK_STREAM sockets. For non-SOCK_STREAM sock‐
258 ets, IP_PMTUDISC_DO forces the don't-fragment flag to be set on
259 all outgoing packets. It is the user's responsibility to packe‐
260 tize the data in MTU-sized chunks and to do the retransmits if
261 necessary. The kernel will reject (with EMSGSIZE) datagrams
262 that are bigger than the known path MTU. IP_PMTUDISC_WANT will
263 fragment a datagram if needed according to the path MTU, or will
264 set the don't-fragment flag otherwise.
265 The system-wide default can be toggled between IP_PMTUDISC_WANT
267 and IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
268 values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.
269 Path MTU discovery value Meaning
271 IP_PMTUDISC_WANT Use per-route settings.
272 IP_PMTUDISC_DONT Never do Path MTU Discovery.
273 IP_PMTUDISC_DO Always do Path MTU Discovery.
274 IP_PMTUDISC_PROBE Set DF but ignore Path MTU.
275 When PMTU discovery is enabled, the kernel automatically keeps
277 track of the path MTU per destination host. When it is con‐
278 nected to a specific peer with connect(2), the currently known
279 path MTU can be retrieved conveniently using the IP_MTU socket
280 option (e.g., after an EMSGSIZE error occurred). The path MTU
281 may change over time. For connectionless sockets with many des‐
282 tinations, the new MTU for a given destination can also be ac‐
283 cessed using the error queue (see IP_RECVERR). A new error will
284 be queued for every incoming MTU update.
285 While MTU discovery is in progress, initial packets from data‐
287 gram sockets may be dropped. Applications using UDP should be
288 aware of this and not take it into account for their packet re‐
289 transmit strategy.
290 To bootstrap the path MTU discovery process on unconnected sock‐
292 ets, it is possible to start with a big datagram size (headers
293 up to 64 kilobytes long) and let it shrink by updates of the
294 path MTU.
295 To get an initial estimate of the path MTU, connect a datagram
297 socket to the destination address using connect(2) and retrieve
298 the MTU by calling getsockopt(2) with the IP_MTU option.
299 It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
301 or SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE
302 (available since Linux 2.6.22). This is also particularly use‐
303 ful for diagnostic tools such as tracepath(8) that wish to de‐
304 liberately send probe packets larger than the observed Path MTU.
305 IP_MULTICAST_ALL (since Linux 2.6.31)
307 This option can be used to modify the delivery policy of multi‐
308 cast messages. The argument is a boolean integer (defaults to
309 1). If set to 1, the socket will receive messages from all the
310 groups that have been joined globally on the whole system. Oth‐
311 erwise, it will deliver messages only from the groups that have
312 been explicitly joined (for example via the IP_ADD_MEMBERSHIP
313 option) on this particular socket.
314 IP_MULTICAST_IF (since Linux 1.2)
316 Set the local device for a multicast socket. The argument for
317 setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq struc‐
318 ture similar to IP_ADD_MEMBERSHIP, or an in_addr structure.
319 (The kernel determines which structure is being passed based on
320 the size passed in optlen.) For getsockopt(2), the argument is
321 an in_addr structure.
322 IP_MULTICAST_LOOP (since Linux 1.2)
324 Set or read a boolean integer argument that determines whether
325 sent multicast packets should be looped back to the local sock‐
326 ets.
327 IP_MULTICAST_TTL (since Linux 1.2)
329 Set or read the time-to-live value of outgoing multicast packets
330 for this socket. It is very important for multicast packets to
331 set the smallest TTL possible. The default is 1 which means
332 that multicast packets don't leave the local network unless the
333 user program explicitly requests it. Argument is an integer.
334 IP_NODEFRAG (since Linux 2.6.36)
336 If enabled (argument is nonzero), the reassembly of outgoing
337 packets is disabled in the netfilter layer. The argument is an
338 integer.
339 This option is valid only for SOCK_RAW sockets.
341 IP_OPTIONS (since Linux 2.0)
343 Set or get the IP options to be sent with every packet from this
344 socket. The arguments are a pointer to a memory buffer contain‐
345 ing the options and the option length. The setsockopt(2) call
346 sets the IP options associated with a socket. The maximum op‐
347 tion size for IPv4 is 40 bytes. See RFC 791 for the allowed op‐
348 tions. When the initial connection request packet for a
349 SOCK_STREAM socket contains IP options, the IP options will be
350 set automatically to the options from the initial packet with
351 routing headers reversed. Incoming packets are not allowed to
352 change options after the connection is established. The pro‐
353 cessing of all incoming source routing options is disabled by
354 default and can be enabled by using the accept_source_route
355 /proc interface. Other options like timestamps are still han‐
356 dled. For datagram sockets, IP options can be set only by the
357 local user. Calling getsockopt(2) with IP_OPTIONS puts the cur‐
358 rent IP options used for sending into the supplied buffer.
359 IP_PASSSEC (since Linux 2.6.17)
361 If labeled IPSEC or NetLabel is configured on the sending and
362 receiving hosts, this option enables receiving of the security
363 context of the peer socket in an ancillary message of type
364 SCM_SECURITY retrieved using recvmsg(2). This option is sup‐
365 ported only for UDP sockets; for TCP or SCTP sockets, see the
366 description of the SO_PEERSEC option below.
367 The value given as an argument to setsockopt(2) and returned as
369 the result of getsockopt(2) is an integer boolean flag.
370 The security context returned in the SCM_SECURITY ancillary mes‐
372 sage is of the same format as the one described under the
373 SO_PEERSEC option below.
374 Note: the reuse of the SCM_SECURITY message type for the
376 IP_PASSSEC socket option was likely a mistake, since other IP
377 control messages use their own numbering scheme in the IP name‐
378 space and often use the socket option value as the message type.
379 There is no conflict currently since the IP option with the same
380 value as SCM_SECURITY is IP_HDRINCL and this is never used for a
381 control message type.
382 IP_PKTINFO (since Linux 2.2)
384 Pass an IP_PKTINFO ancillary message that contains a pktinfo
385 structure that supplies some information about the incoming
386 packet. This works only for datagram oriented sockets. The ar‐
387 gument is a flag that tells the socket whether the IP_PKTINFO
388 message should be passed or not. The message itself can be
389 sent/retrieved only as a control message with a packet using
390 recvmsg(2) or sendmsg(2).
391 struct in_pktinfo {
393 unsigned int ipi_ifindex; /* Interface index */
394 struct in_addr ipi_spec_dst; /* Local address */
395 struct in_addr ipi_addr; /* Header Destination
396 address */
397 };
398 ipi_ifindex is the unique index of the interface the packet was
400 received on. ipi_spec_dst is the local address of the packet
401 and ipi_addr is the destination address in the packet header.
402 If IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst is not
403 zero, then it is used as the local source address for the rout‐
404 ing table lookup and for setting up IP source route options.
405 When ipi_ifindex is not zero, the primary local address of the
406 interface specified by the index overwrites ipi_spec_dst for the
407 routing table lookup.
408 IP_RECVERR (since Linux 2.2)
410 Enable extended reliable error message passing. When enabled on
411 a datagram socket, all generated errors will be queued in a per-
412 socket error queue. When the user receives an error from a
413 socket operation, the errors can be received by calling
414 recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_ex‐
415 tended_err structure describing the error will be passed in an
416 ancillary message with the type IP_RECVERR and the level IP‐
417 PROTO_IP. This is useful for reliable error handling on uncon‐
418 nected sockets. The received data portion of the error queue
419 contains the error packet.
420 The IP_RECVERR control message contains a sock_extended_err
422 structure:
423 #define SO_EE_ORIGIN_NONE 0
425 #define SO_EE_ORIGIN_LOCAL 1
426 #define SO_EE_ORIGIN_ICMP 2
427 #define SO_EE_ORIGIN_ICMP6 3
428 struct sock_extended_err {
430 uint32_t ee_errno; /* error number */
431 uint8_t ee_origin; /* where the error originated */
432 uint8_t ee_type; /* type */
433 uint8_t ee_code; /* code */
434 uint8_t ee_pad;
435 uint32_t ee_info; /* additional information */
436 uint32_t ee_data; /* other data */
437 /* More data may follow */
438 };
439 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
441 ee_errno contains the errno number of the queued error. ee_ori‐
443 gin is the origin code of where the error originated. The other
444 fields are protocol-specific. The macro SO_EE_OFFENDER returns
445 a pointer to the address of the network object where the error
446 originated from given a pointer to the ancillary message. If
447 this address is not known, the sa_family member of the sockaddr
448 contains AF_UNSPEC and the other fields of the sockaddr are un‐
449 defined.
450 IP uses the sock_extended_err structure as follows: ee_origin is
452 set to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet,
453 or SO_EE_ORIGIN_LOCAL for locally generated errors. Unknown
454 values should be ignored. ee_type and ee_code are set from the
455 type and code fields of the ICMP header. ee_info contains the
456 discovered MTU for EMSGSIZE errors. The message also contains
457 the sockaddr_in of the node caused the error, which can be ac‐
458 cessed with the SO_EE_OFFENDER macro. The sin_family field of
459 the SO_EE_OFFENDER address is AF_UNSPEC when the source was un‐
460 known. When the error originated from the network, all IP op‐
461 tions (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and con‐
462 tained in the error packet are passed as control messages. The
463 payload of the packet causing the error is returned as normal
464 payload. Note that TCP has no error queue; MSG_ERRQUEUE is not
465 permitted on SOCK_STREAM sockets. IP_RECVERR is valid for TCP,
466 but all errors are returned by socket function return or SO_ER‐
467 ROR only.
468 For raw sockets, IP_RECVERR enables passing of all received ICMP
470 errors to the application, otherwise errors are reported only on
471 connected sockets
472 It sets or retrieves an integer boolean flag. IP_RECVERR de‐
474 faults to off.
475 IP_RECVOPTS (since Linux 2.2)
477 Pass all incoming IP options to the user in a IP_OPTIONS control
478 message. The routing header and other options are already
479 filled in for the local host. Not supported for SOCK_STREAM
480 sockets.
481 IP_RECVORIGDSTADDR (since Linux 2.6.29)
483 This boolean option enables the IP_ORIGDSTADDR ancillary message
484 in recvmsg(2), in which the kernel returns the original destina‐
485 tion address of the datagram being received. The ancillary mes‐
486 sage contains a struct sockaddr_in.
487 IP_RECVTOS (since Linux 2.2)
489 If enabled, the IP_TOS ancillary message is passed with incoming
490 packets. It contains a byte which specifies the Type of Ser‐
491 vice/Precedence field of the packet header. Expects a boolean
492 integer flag.
493 IP_RECVTTL (since Linux 2.2)
495 When this flag is set, pass a IP_TTL control message with the
496 time-to-live field of the received packet as a 32 bit integer.
497 Not supported for SOCK_STREAM sockets.
498 IP_RETOPTS (since Linux 2.2)
500 Identical to IP_RECVOPTS, but returns raw unprocessed options
501 with timestamp and route record options not filled in for this
502 hop.
503 IP_ROUTER_ALERT (since Linux 2.2)
505 Pass all to-be forwarded packets with the IP Router Alert option
506 set to this socket. Valid only for raw sockets. This is use‐
507 ful, for instance, for user-space RSVP daemons. The tapped
508 packets are not forwarded by the kernel; it is the user's re‐
509 sponsibility to send them out again. Socket binding is ignored,
510 such packets are filtered only by protocol. Expects an integer
511 flag.
512 IP_TOS (since Linux 1.0)
514 Set or receive the Type-Of-Service (TOS) field that is sent with
515 every IP packet originating from this socket. It is used to
516 prioritize packets on the network. TOS is a byte. There are
517 some standard TOS flags defined: IPTOS_LOWDELAY to minimize de‐
518 lays for interactive traffic, IPTOS_THROUGHPUT to optimize
519 throughput, IPTOS_RELIABILITY to optimize for reliability, IP‐
520 TOS_MINCOST should be used for "filler data" where slow trans‐
521 mission doesn't matter. At most one of these TOS values can be
522 specified. Other bits are invalid and shall be cleared. Linux
523 sends IPTOS_LOWDELAY datagrams first by default, but the exact
524 behavior depends on the configured queueing discipline. Some
525 high-priority levels may require superuser privileges (the
526 CAP_NET_ADMIN capability).
527 IP_TRANSPARENT (since Linux 2.6.24)
529 Setting this boolean option enables transparent proxying on this
530 socket. This socket option allows the calling application to
531 bind to a nonlocal IP address and operate both as a client and a
532 server with the foreign address as the local endpoint. NOTE:
533 this requires that routing be set up in a way that packets going
534 to the foreign address are routed through the TProxy box (i.e.,
535 the system hosting the application that employs the IP_TRANSPAR‐
536 ENT socket option). Enabling this socket option requires supe‐
537 ruser privileges (the CAP_NET_ADMIN capability).
538 TProxy redirection with the iptables TPROXY target also requires
540 that this option be set on the redirected socket.
541 IP_TTL (since Linux 1.0)
543 Set or retrieve the current time-to-live field that is used in
544 every packet sent from this socket.
545 IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
547 Unblock previously blocked multicast source. Returns EADDRNO‐
548 TAVAIL when given source is not being blocked.
549 Argument is an ip_mreq_source structure as described under
551 IP_ADD_SOURCE_MEMBERSHIP.
552 SO_PEERSEC (since Linux 2.6.17)
554 If labeled IPSEC or NetLabel is configured on both the sending
555 and receiving hosts, this read-only socket option returns the
556 security context of the peer socket connected to this socket.
557 By default, this will be the same as the security context of the
558 process that created the peer socket unless overridden by the
559 policy or by a process with the required permissions.
560 The argument to getsockopt(2) is a pointer to a buffer of the
562 specified length in bytes into which the security context string
563 will be copied. If the buffer length is less than the length of
564 the security context string, then getsockopt(2) returns -1, sets
565 errno to ERANGE, and returns the required length via optlen.
566 The caller should allocate at least NAME_MAX bytes for the buf‐
567 fer initially, although this is not guaranteed to be sufficient.
568 Resizing the buffer to the returned length and retrying may be
569 necessary.
570 The security context string may include a terminating null char‐
572 acter in the returned length, but is not guaranteed to do so: a
573 security context "foo" might be represented as either
574 {'f','o','o'} of length 3 or {'f','o','o','\0'} of length 4,
575 which are considered to be interchangeable. The string is
576 printable, does not contain non-terminating null characters, and
577 is in an unspecified encoding (in particular, it is not guaran‐
578 teed to be ASCII or UTF-8).
579 The use of this option for sockets in the AF_INET address family
581 is supported since Linux 2.6.17 for TCP sockets, and since Linux
582 4.17 for SCTP sockets.
583 For SELinux, NetLabel conveys only the MLS portion of the secu‐
585 rity context of the peer across the wire, defaulting the rest of
586 the security context to the values defined in the policy for the
587 netmsg initial security identifier (SID). However, NetLabel can
588 be configured to pass full security contexts over loopback. La‐
589 beled IPSEC always passes full security contexts as part of es‐
590 tablishing the security association (SA) and looks them up based
591 on the association for each packet.
592 /proc interfaces
594 The IP protocol supports a set of /proc interfaces to configure some
595 global parameters. The parameters can be accessed by reading or writ‐
596 ing files in the directory /proc/sys/net/ipv4/. Interfaces described
597 as Boolean take an integer value, with a nonzero value ("true") meaning
598 that the corresponding option is enabled, and a zero value ("false")
599 meaning that the option is disabled.
600 ip_always_defrag (Boolean; since Linux 2.2.13)
602 [New with Linux 2.2.13; in earlier kernel versions this feature
603 was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG
604 option; this option is not present in Linux 2.4.x and later]
605 When this boolean flag is enabled (not equal 0), incoming frag‐
607 ments (parts of IP packets that arose when some host between
608 origin and destination decided that the packets were too large
609 and cut them into pieces) will be reassembled (defragmented) be‐
610 fore being processed, even if they are about to be forwarded.
611 Enable only if running either a firewall that is the sole link
613 to your network or a transparent proxy; never ever use it for a
614 normal router or host. Otherwise, fragmented communication can
615 be disturbed if the fragments travel over different links. De‐
616 fragmentation also has a large memory and CPU time cost.
617 This is automagically turned on when masquerading or transparent
619 proxying are configured.
620 ip_autoconfig (since Linux 2.2 to Linux 2.6.17)
622 Not documented.
623 ip_default_ttl (integer; default: 64; since Linux 2.2)
625 Set the default time-to-live value of outgoing packets. This
626 can be changed per socket with the IP_TTL option.
627 ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
629 Enable dynamic socket address and masquerading entry rewriting
630 on interface address change. This is useful for dialup inter‐
631 face with changing IP addresses. 0 means no rewriting, 1 turns
632 it on and 2 enables verbose mode.
633 ip_forward (Boolean; default: disabled; since Linux 1.2)
635 Enable IP forwarding with a boolean flag. IP forwarding can be
636 also set on a per-interface basis.
637 ip_local_port_range (since Linux 2.2)
639 This file contains two integers that define the default local
640 port range allocated to sockets that are not explicitly bound to
641 a port number—that is, the range used for ephemeral ports. An
642 ephemeral port is allocated to a socket in the following circum‐
643 stances:
644 • the port number in a socket address is specified as 0 when
646 calling bind(2);
647 • listen(2) is called on a stream socket that was not previ‐
649 ously bound;
650 • connect(2) was called on a socket that was not previously
652 bound;
653 • sendto(2) is called on a datagram socket that was not previ‐
655 ously bound.
656 Allocation of ephemeral ports starts with the first number in
658 ip_local_port_range and ends with the second number. If the
659 range of ephemeral ports is exhausted, then the relevant system
660 call returns an error (but see BUGS).
661 Note that the port range in ip_local_port_range should not con‐
663 flict with the ports used by masquerading (although the case is
664 handled). Also, arbitrary choices may cause problems with some
665 firewall packet filters that make assumptions about the local
666 ports in use. The first number should be at least greater than
667 1024, or better, greater than 4096, to avoid clashes with well
668 known ports and to minimize firewall problems.
669 ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
671 If enabled, don't do Path MTU Discovery for TCP sockets by de‐
672 fault. Path MTU discovery may fail if misconfigured firewalls
673 (that drop all ICMP packets) or misconfigured interfaces (e.g.,
674 a point-to-point link where the both ends don't agree on the
675 MTU) are on the path. It is better to fix the broken routers on
676 the path than to turn off Path MTU Discovery globally, because
677 not doing it incurs a high cost to the network.
678 ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
680 If set, allows processes to bind(2) to nonlocal IP addresses,
681 which can be quite useful, but may break some applications.
682 ip6frag_time (integer; default: 30)
684 Time in seconds to keep an IPv6 fragment in memory.
685 ip6frag_secret_interval (integer; default: 600)
687 Regeneration interval (in seconds) of the hash secret (or life‐
688 time for the hash secret) for IPv6 fragments.
689 ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
691 If the amount of queued IP fragments reaches ipfrag_high_thresh,
692 the queue is pruned down to ipfrag_low_thresh. Contains an in‐
693 teger with the number of bytes.
694 neigh/*
696 See arp(7).
697 Ioctls
699 All ioctls described in socket(7) apply to ip.
700 Ioctls to configure generic device parameters are described in netde‐
702 vice(7).
703
705 EACCES The user tried to execute an operation without the necessary
706 permissions. These include: sending a packet to a broadcast ad‐
707 dress without having the SO_BROADCAST flag set; sending a packet
708 via a prohibit route; modifying firewall settings without supe‐
709 ruser privileges (the CAP_NET_ADMIN capability); binding to a
710 privileged port without superuser privileges (the
711 CAP_NET_BIND_SERVICE capability).
712 EADDRINUSE
714 Tried to bind to an address already in use.
715 EADDRNOTAVAIL
717 A nonexistent interface was requested or the requested source
718 address was not local.
719 EAGAIN Operation on a nonblocking socket would block.
721 EALREADY
723 A connection operation on a nonblocking socket is already in
724 progress.
725 ECONNABORTED
727 A connection was closed during an accept(2).
728 EHOSTUNREACH
730 No valid routing table entry matches the destination address.
731 This error can be caused by an ICMP message from a remote router
732 or for the local routing table.
733 EINVAL Invalid argument passed. For send operations this can be caused
735 by sending to a blackhole route.
736 EISCONN
738 connect(2) was called on an already connected socket.
739 EMSGSIZE
741 Datagram is bigger than an MTU on the path and it cannot be
742 fragmented.
743 ENOBUFS, ENOMEM
745 Not enough free memory. This often means that the memory allo‐
746 cation is limited by the socket buffer limits, not by the system
747 memory, but this is not 100% consistent.
748 ENOENT SIOCGSTAMP was called on a socket where no packet arrived.
750 ENOPKG A kernel subsystem was not configured.
752 ENOPROTOOPT and EOPNOTSUPP
754 Invalid socket option passed.
755 ENOTCONN
757 The operation is defined only on a connected socket, but the
758 socket wasn't connected.
759 EPERM User doesn't have permission to set high priority, change con‐
761 figuration, or send signals to the requested process or group.
762 EPIPE The connection was unexpectedly closed or shut down by the other
764 end.
765 ESOCKTNOSUPPORT
767 The socket is not configured or an unknown socket type was re‐
768 quested.
769 Other errors may be generated by the overlaying protocols; see tcp(7),
771 raw(7), udp(7), and socket(7).
772
774 IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER, IP_RECVORIGDSTADDR,
775 IP_PASSSEC, IP_PKTINFO, IP_RECVERR, IP_ROUTER_ALERT, and IP_TRANSPARENT
776 are Linux-specific.
777 Be very careful with the SO_BROADCAST option - it is not privileged in
779 Linux. It is easy to overload the network with careless broadcasts.
780 For new application protocols it is better to use a multicast group in‐
781 stead of broadcasting. Broadcasting is discouraged.
782 Some other BSD sockets implementations provide IP_RCVDSTADDR and
784 IP_RECVIF socket options to get the destination address and the inter‐
785 face of received datagrams. Linux has the more general IP_PKTINFO for
786 the same task.
787 Some BSD sockets implementations also provide an IP_RECVTTL option, but
789 an ancillary message with type IP_RECVTTL is passed with the incoming
790 packet. This is different from the IP_TTL option used in Linux.
791 Using the SOL_IP socket options level isn't portable; BSD-based stacks
793 use the IPPROTO_IP level.
794 INADDR_ANY (0.0.0.0) and INADDR_BROADCAST (255.255.255.255) are byte-
796 order-neutral. This means htonl(3) has no effect on them.
797 Compatibility
799 For compatibility with Linux 2.0, the obsolete socket(AF_INET,
800 SOCK_PACKET, protocol) syntax is still supported to open a packet(7)
801 socket. This is deprecated and should be replaced by socket(AF_PACKET,
802 SOCK_RAW, protocol) instead. The main difference is the new sock‐
803 addr_ll address structure for generic link layer information instead of
804 the old sockaddr_pkt.
805
807 There are too many inconsistent error values.
808 The error used to diagnose exhaustion of the ephemeral port range dif‐
810 fers across the various system calls (connect(2), bind(2), listen(2),
811 sendto(2)) that can assign ephemeral ports.
812 The ioctls to configure IP-specific interface options and ARP tables
814 are not described.
815 Receiving the original destination address with MSG_ERRQUEUE in
817 msg_name by recvmsg(2) does not work in some Linux 2.2 kernels.
818
820 recvmsg(2), sendmsg(2), byteorder(3), capabilities(7), icmp(7),
821 ipv6(7), netdevice(7), netlink(7), raw(7), socket(7), tcp(7), udp(7),
822 ip(8)
823 The kernel source file Documentation/networking/ip-sysctl.txt.
825 RFC 791 for the original IP specification. RFC 1122 for the IPv4 host
827 requirements. RFC 1812 for the IPv4 router requirements.
828Linux man-pages 6.04 2023-03-17 ip(7)