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 = sizeof(struct in_addr)
 133 cmsg_level = IPPROTO_IP
 134 cmsg_type = IP_RECVDSTADDR
 135 .Ed
 136 .Ss "Multicast Options"
 137 .Pp
 138 .Tn IP
 139 multicasting is supported only on
 140 .Dv AF_INET
 141 sockets of type
 142 .Dv SOCK_DGRAM
 143 and
 144 .Dv SOCK_RAW,
 145 and only on networks where the interface
 146 driver supports multicasting.
 147 .Pp
 148 The
 149 .Dv IP_MULTICAST_TTL
 150 option changes the time-to-live (TTL)
 151 for outgoing multicast datagrams
 152 in order to control the scope of the multicasts:
 153 .Bd -literal
 154 u_char ttl;     /* range: 0 to 255, default = 1 */
 155 setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
 156 .Ed
 157 .sp
 158 Datagrams with a TTL of 1 are not forwarded beyond the local network.
 159 Multicast datagrams with a TTL of 0 will not be transmitted on any network,
 160 but may be delivered locally if the sending host belongs to the destination
 161 group and if multicast loopback has not been disabled on the sending socket
 162 (see below).  Multicast datagrams with TTL greater than 1 may be forwarded
 163 to other networks if a multicast router is attached to the local network.
 164 .Pp
 165 For hosts with multiple interfaces, each multicast transmission is
 166 sent from the primary network interface.
 167 The
 168 .Dv IP_MULTICAST_IF
 169 option overrides the default for
 170 subsequent transmissions from a given socket:
 171 .Bd -literal
 172 struct in_addr addr;
 173 setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
 174 .Ed
 175 .sp
 176 where "addr" is the local
 177 .Tn IP
 178 address of the desired interface or
 179 .Dv INADDR_ANY
 180 to specify the default interface.
 181 An interface's local IP address and multicast capability can
 182 be obtained via the
 183 .Dv SIOCGIFCONF
 184 and
 185 .Dv SIOCGIFFLAGS
 186 ioctls.
 187 Normal applications should not need to use this option.
 188 .Pp
 189 If a multicast datagram is sent to a group to which the sending host itself
 190 belongs (on the outgoing interface), a copy of the datagram is, by default,
 191 looped back by the IP layer for local delivery.
 192 The
 193 .Dv IP_MULTICAST_LOOP
 194 option gives the sender explicit control
 195 over whether or not subsequent datagrams are looped back:
 196 .Bd -literal
 197 u_char loop;    /* 0 = disable, 1 = enable (default) */
 198 setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
 199 .Ed
 200 .sp
 201 This option
 202 improves performance for applications that may have no more than one
 203 instance on a single host (such as a router demon), by eliminating
 204 the overhead of receiving their own transmissions.  It should generally not
 205 be used by applications for which there may be more than one instance on a
 206 single host (such as a conferencing program) or for which the sender does
 207 not belong to the destination group (such as a time querying program).
 208 .Pp
 209 A multicast datagram sent with an initial TTL greater than 1 may be delivered
 210 to the sending host on a different interface from that on which it was sent,
 211 if the host belongs to the destination group on that other interface.  The
 212 loopback control option has no effect on such delivery.
 213 .Pp
 214 A host must become a member of a multicast group before it can receive
 215 datagrams sent to the group.  To join a multicast group, use the
 216 .Dv IP_ADD_MEMBERSHIP
 217 option:
 218 .Bd -literal
 219 struct ip_mreq mreq;
 220 setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
 221 .Ed
 222 .sp
 223 where
 224 .Fa mreq
 225 is the following structure:
 226 .Bd -literal
 227 struct ip_mreq {
 228     struct in_addr imr_multiaddr; /* multicast group to join */
 229     struct in_addr imr_interface; /* interface to join on */
 230 }
 231 .Ed
 232 .sp
 233 .Dv imr_interface
 234 should
 235 be
 236 .Dv INADDR_ANY
 237 to choose the default multicast interface,
 238 or the
 239 .Tn IP
 240 address of a particular multicast-capable interface if
 241 the host is multihomed.
 242 Membership is associated with a single interface;
 243 programs running on multihomed hosts may need to
 244 join the same group on more than one interface.
 245 Up to
 246 .Dv IP_MAX_MEMBERSHIPS
 247 (currently 20) memberships may be added on a
 248 single socket.
 249 .Pp
 250 To drop a membership, use:
 251 .Bd -literal
 252 struct ip_mreq mreq;
 253 setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
 254 .Ed
 255 .sp
 256 where
 257 .Fa mreq
 258 contains the same values as used to add the membership.
 259 Memberships are dropped when the socket is closed or the process exits.
 260 .\"-----------------------
 261 .Ss "Raw IP Sockets"
 262 .Pp
 263 Raw
 264 .Tn IP
 265 sockets are connectionless,
 266 and are normally used with the
 267 .Xr sendto
 268 and
 269 .Xr recvfrom
 270 calls, though the
 271 .Xr connect 2
 272 call may also be used to fix the destination for future
 273 packets (in which case the
 274 .Xr read 2
 275 or
 276 .Xr recv 2
 277 and
 278 .Xr write 2
 279 or
 280 .Xr send 2
 281 system calls may be used).
 282 .Pp
 283 If
 284 .Fa proto
 285 is 0, the default protocol
 286 .Dv IPPROTO_RAW
 287 is used for outgoing
 288 packets, and only incoming packets destined for that protocol
 289 are received.
 290 If
 291 .Fa proto
 292 is non-zero, that protocol number will be used on outgoing packets
 293 and to filter incoming packets.
 294 .Pp
 295 Outgoing packets automatically have an
 296 .Tn IP
 297 header prepended to
 298 them (based on the destination address and the protocol
 299 number the socket is created with),
 300 unless the
 301 .Dv IP_HDRINCL
 302 option has been set.
 303 Incoming packets are received with
 304 .Tn IP
 305 header and options intact.
 306 .Pp
 307 .Dv IP_HDRINCL
 308 indicates the complete IP header is included with the data
 309 and may be used only with the
 310 .Dv SOCK_RAW
 311 type.
 312 .Bd -literal
 313 #include <netinet/ip.h>
 314
 315 int hincl = 1;                  /* 1 = on, 0 = off */
 316 setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
 317 .Ed
 318 .sp
 319 Unlike previous
 320 .Tn BSD
 321 releases, the program must set all
 322 the fields of the IP header, including the following:
 323 .Bd -literal
 324 ip->ip_v = IPVERSION;
 325 ip->ip_hl = hlen >> 2;
 326 ip->ip_id = 0;  /* 0 means kernel set appropriate value */
 327 ip->ip_off = offset;
 328 ip->ip_len = len;
 329 .Ed
 330 .sp .5
 331 .Pp
 332 Note that
 333 the ip_off and ip_len fields are in host byte order.
 334 .Pp
 335 If the header source address is set to
 336 .Dv INADDR_ANY,
 337 the kernel will choose an appropriate address.
 338 .Sh DIAGNOSTICS
 339 A socket operation may fail with one of the following errors returned:
 340 .Bl -tag -width [EADDRNOTAVAIL]
 341 .It Bq Er EISCONN
 342 when trying to establish a connection on a socket which
 343 already has one, or when trying to send a datagram with the destination
 344 address specified and the socket is already connected;
 345 .It Bq Er ENOTCONN
 346 when trying to send a datagram, but
 347 no destination address is specified, and the socket hasn't been
 348 connected;
 349 .It Bq Er ENOBUFS
 350 when the system runs out of memory for
 351 an internal data structure;
 352 .It Bq Er EADDRNOTAVAIL
 353 when an attempt is made to create a
 354 socket with a network address for which no network interface
 355 exists.
 356 .It Bq Er EACESS
 357 when an attempt is made to create
 358 a raw IP socket by a non-privileged process.
 359 .El
 360 .Pp
 361 The following errors specific to
 362 .Tn IP
 363 may occur when setting or getting
 364 .Tn IP
 365 options:
 366 .Bl -tag -width EADDRNOTAVAILxx
 367 .It Bq Er EINVAL
 368 An unknown socket option name was given.
 369 .It Bq Er EINVAL
 370 The IP option field was improperly formed;
 371 an option field was shorter than the minimum value
 372 or longer than the option buffer provided.
 373 .El
 374 .Sh SEE ALSO
 375 .Xr getsockopt 2 ,
 376 .Xr recv 2 ,
 377 .Xr send 2 ,
 378 .Xr icmp 4 ,
 379 .Xr inet 4 ,
 380 .Xr intro 4
 381 .Sh HISTORY
 382 The
 383 .Nm
 384 protocol appeared in
 385 .Bx 4.2 .