bsd/man/man4/ip.4

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  34 .\"     @(#)ip.4        8.2 (Berkeley) 11/30/93
  35 .\"
  36 .Dd November 30, 1993
  37 .Dt IP 4
  38 .Os BSD 4.2
  39 .Sh NAME
  40 .Nm ip
  41 .Nd Internet Protocol
  42 .Sh SYNOPSIS
  43 .Fd #include <sys/socket.h>
  44 .Fd #include <netinet/in.h>
  45 .Ft int
  46 .Fn socket AF_INET SOCK_RAW proto
  47 .Sh DESCRIPTION
  48 .Tn IP
  49 is the transport layer protocol used
  50 by the Internet protocol family.
  51 Options may be set at the
  52 .Tn IP
  53 level
  54 when using higher-level protocols that are based on
  55 .Tn IP
  56 (such as
  57 .Tn TCP
  58 and
  59 .Tn UDP ) .
  60 It may also be accessed
  61 through a
  62 .Dq raw socket
  63 when developing new protocols, or
  64 special-purpose applications.
  65 .Pp
  66 There are several
  67 .Tn IP-level
  68 .Xr setsockopt 2 / Ns
  69 .Xr getsockopt 2
  70 options.
  71 .Dv IP_OPTIONS
  72 may be used to provide
  73 .Tn IP
  74 options to be transmitted in the
  75 .Tn IP
  76 header of each outgoing packet
  77 or to examine the header options on incoming packets.
  78 .Tn IP
  79 options may be used with any socket type in the Internet family.
  80 The format of
  81 .Tn IP
  82 options to be sent is that specified by the
  83 .Tn IP
  84 protocol specification (RFC-791), with one exception:
  85 the list of addresses for Source Route options must include the first-hop
  86 gateway at the beginning of the list of gateways.
  87 The first-hop gateway address will be extracted from the option list
  88 and the size adjusted accordingly before use.
  89 To disable previously specified options,
  90 use a zero-length buffer:
  91 .Bd -literal
  92 setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);
  93 .Ed
  94 .Pp
  95 .Dv IP_TOS
  96 and
  97 .Dv IP_TTL
  98 may be used to set the type-of-service and time-to-live
  99 fields in the
 100 .Tn IP
 101 header for
 102 .Dv SOCK_STREAM
 103 and
 104 .Dv SOCK_DGRAM
 105 sockets. For example,
 106 .Bd -literal
 107 int tos = IPTOS_LOWDELAY;       /* see <netinet/in.h> */
 108 setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
 109
 110 int ttl = 60;                   /* max = 255 */
 111 setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
 112 .Ed
 113 .Pp
 114 If the
 115 .Dv IP_RECVDSTADDR
 116 option is enabled on a
 117 .Dv SOCK_DGRAM
 118 socket,
 119 the
 120 .Xr recvmsg
 121 call will return the destination
 122 .Tn IP
 123 address for a
 124 .Tn UDP
 125 datagram.
 126 The msg_control field in the msghdr structure points to a buffer
 127 that contains a cmsghdr structure followed by the
 128 .Tn IP
 129 address.
 130 The cmsghdr fields have the following values:
 131 .Bd -literal
 132 cmsg_len = CMSG_LEN(sizeof(struct in_addr))
 133 cmsg_level = IPPROTO_IP
 134 cmsg_type = IP_RECVDSTADDR
 135 .Ed
 136 .Pp
 137 If the
 138 .Dv IP_RECVTOS
 139 option is enabled on a
 140 .Dv SOCK_DGRAM
 141 or
 142 .Dv SOCK_RAW
 143 socket,
 144 the
 145 .Xr recvmsg
 146 call will return the TOS (type of service) field of the IP header.
 147 The msg_control field in the msghdr structure points to a buffer
 148 that contains a cmsghdr structure followed by the TOS.
 149 The cmsghdr fields have the following values:
 150 .Bd -literal
 151 cmsg_len = CMSG_LEN(sizeof(u_char))
 152 cmsg_level = IPPROTO_IP
 153 cmsg_type = IP_RECVTOS
 154 .Ed
 155 .Ss "Multicast Options"
 156 .Pp
 157 .Tn IP
 158 multicasting is supported only on
 159 .Dv AF_INET
 160 sockets of type
 161 .Dv SOCK_DGRAM
 162 and
 163 .Dv SOCK_RAW,
 164 and only on networks where the interface
 165 driver supports multicasting.
 166 .Pp
 167 The
 168 .Dv IP_MULTICAST_TTL
 169 option changes the time-to-live (TTL)
 170 for outgoing multicast datagrams
 171 in order to control the scope of the multicasts:
 172 .Bd -literal
 173 u_char ttl;     /* range: 0 to 255, default = 1 */
 174 setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
 175 .Ed
 176 .sp
 177 Datagrams with a TTL of 1 are not forwarded beyond the local network.
 178 Multicast datagrams with a TTL of 0 will not be transmitted on any network,
 179 but may be delivered locally if the sending host belongs to the destination
 180 group and if multicast loopback has not been disabled on the sending socket
 181 (see below).  Multicast datagrams with TTL greater than 1 may be forwarded
 182 to other networks if a multicast router is attached to the local network.
 183 .Pp
 184 For hosts with multiple interfaces, each multicast transmission is
 185 sent from the primary network interface.
 186 The
 187 .Dv IP_MULTICAST_IF
 188 option overrides the default for
 189 subsequent transmissions from a given socket:
 190 .Bd -literal
 191 struct in_addr addr;
 192 setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
 193 .Ed
 194 .sp
 195 where "addr" is the local
 196 .Tn IP
 197 address of the desired interface or
 198 .Dv INADDR_ANY
 199 to specify the default interface.
 200 An interface's local IP address and multicast capability can
 201 be obtained via the
 202 .Dv SIOCGIFCONF
 203 and
 204 .Dv SIOCGIFFLAGS
 205 ioctls.
 206 Normal applications should not need to use this option.
 207 .Pp
 208 If a multicast datagram is sent to a group to which the sending host itself
 209 belongs (on the outgoing interface), a copy of the datagram is, by default,
 210 looped back by the IP layer for local delivery.
 211 The
 212 .Dv IP_MULTICAST_LOOP
 213 option gives the sender explicit control
 214 over whether or not subsequent datagrams are looped back:
 215 .Bd -literal
 216 u_char loop;    /* 0 = disable, 1 = enable (default) */
 217 setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
 218 .Ed
 219 .sp
 220 This option
 221 improves performance for applications that may have no more than one
 222 instance on a single host (such as a router demon), by eliminating
 223 the overhead of receiving their own transmissions.  It should generally not
 224 be used by applications for which there may be more than one instance on a
 225 single host (such as a conferencing program) or for which the sender does
 226 not belong to the destination group (such as a time querying program).
 227 .Pp
 228 A multicast datagram sent with an initial TTL greater than 1 may be delivered
 229 to the sending host on a different interface from that on which it was sent,
 230 if the host belongs to the destination group on that other interface.  The
 231 loopback control option has no effect on such delivery.
 232 .Pp
 233 A host must become a member of a multicast group before it can receive
 234 datagrams sent to the group.  To join a multicast group, use the
 235 .Dv IP_ADD_MEMBERSHIP
 236 option:
 237 .Bd -literal
 238 struct ip_mreq mreq;
 239 setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
 240 .Ed
 241 .sp
 242 where
 243 .Fa mreq
 244 is the following structure:
 245 .Bd -literal
 246 struct ip_mreq {
 247     struct in_addr imr_multiaddr; /* multicast group to join */
 248     struct in_addr imr_interface; /* interface to join on */
 249 }
 250 .Ed
 251 .sp
 252 .Dv imr_interface
 253 should
 254 be
 255 .Dv INADDR_ANY
 256 to choose the default multicast interface,
 257 or the
 258 .Tn IP
 259 address of a particular multicast-capable interface if
 260 the host is multihomed.
 261 Membership is associated with a single interface;
 262 programs running on multihomed hosts may need to
 263 join the same group on more than one interface.
 264 Up to
 265 .Dv IP_MAX_MEMBERSHIPS
 266 (currently 20) memberships may be added on a
 267 single socket.
 268 .Pp
 269 To drop a membership, use:
 270 .Bd -literal
 271 struct ip_mreq mreq;
 272 setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
 273 .Ed
 274 .sp
 275 where
 276 .Fa mreq
 277 contains the same values as used to add the membership.
 278 Memberships are dropped when the socket is closed or the process exits.
 279 .\"-----------------------
 280 .Ss "Raw IP Sockets"
 281 .Pp
 282 Raw
 283 .Tn IP
 284 sockets are connectionless,
 285 and are normally used with the
 286 .Xr sendto
 287 and
 288 .Xr recvfrom
 289 calls, though the
 290 .Xr connect 2
 291 call may also be used to fix the destination for future
 292 packets (in which case the
 293 .Xr read 2
 294 or
 295 .Xr recv 2
 296 and
 297 .Xr write 2
 298 or
 299 .Xr send 2
 300 system calls may be used).
 301 .Pp
 302 If
 303 .Fa proto
 304 is 0, the default protocol
 305 .Dv IPPROTO_RAW
 306 is used for outgoing
 307 packets, and only incoming packets destined for that protocol
 308 are received.
 309 If
 310 .Fa proto
 311 is non-zero, that protocol number will be used on outgoing packets
 312 and to filter incoming packets.
 313 .Pp
 314 Outgoing packets automatically have an
 315 .Tn IP
 316 header prepended to
 317 them (based on the destination address and the protocol
 318 number the socket is created with),
 319 unless the
 320 .Dv IP_HDRINCL
 321 option has been set.
 322 Incoming packets are received with
 323 .Tn IP
 324 header and options intact.
 325 .Pp
 326 .Dv IP_HDRINCL
 327 indicates the complete IP header is included with the data
 328 and may be used only with the
 329 .Dv SOCK_RAW
 330 type.
 331 .Bd -literal
 332 #include <netinet/ip.h>
 333
 334 int hincl = 1;                  /* 1 = on, 0 = off */
 335 setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
 336 .Ed
 337 .sp
 338 Unlike previous
 339 .Tn BSD
 340 releases, the program must set all
 341 the fields of the IP header, including the following:
 342 .Bd -literal
 343 ip->ip_v = IPVERSION;
 344 ip->ip_hl = hlen >> 2;
 345 ip->ip_id = 0;  /* 0 means kernel set appropriate value */
 346 ip->ip_off = offset;
 347 ip->ip_len = len;
 348 .Ed
 349 .sp .5
 350 .Pp
 351 Note that
 352 the ip_off and ip_len fields are in host byte order.
 353 .Pp
 354 If the header source address is set to
 355 .Dv INADDR_ANY,
 356 the kernel will choose an appropriate address.
 357 .Sh DIAGNOSTICS
 358 A socket operation may fail with one of the following errors returned:
 359 .Bl -tag -width [EADDRNOTAVAIL]
 360 .It Bq Er EISCONN
 361 when trying to establish a connection on a socket which
 362 already has one, or when trying to send a datagram with the destination
 363 address specified and the socket is already connected;
 364 .It Bq Er ENOTCONN
 365 when trying to send a datagram, but
 366 no destination address is specified, and the socket hasn't been
 367 connected;
 368 .It Bq Er ENOBUFS
 369 when the system runs out of memory for
 370 an internal data structure;
 371 .It Bq Er EADDRNOTAVAIL
 372 when an attempt is made to create a
 373 socket with a network address for which no network interface
 374 exists.
 375 .It Bq Er EACESS
 376 when an attempt is made to create
 377 a raw IP socket by a non-privileged process.
 378 .El
 379 .Pp
 380 The following errors specific to
 381 .Tn IP
 382 may occur when setting or getting
 383 .Tn IP
 384 options:
 385 .Bl -tag -width EADDRNOTAVAILxx
 386 .It Bq Er EINVAL
 387 An unknown socket option name was given.
 388 .It Bq Er EINVAL
 389 The IP option field was improperly formed;
 390 an option field was shorter than the minimum value
 391 or longer than the option buffer provided.
 392 .El
 393 .Sh SEE ALSO
 394 .Xr getsockopt 2 ,
 395 .Xr recv 2 ,
 396 .Xr send 2 ,
 397 .Xr icmp 4 ,
 398 .Xr inet 4 ,
 399 .Xr intro 4
 400 .Sh HISTORY
 401 The
 402 .Nm
 403 protocol appeared in
 404 .Bx 4.2 .