ip(4)

NAME

ip - Internet Protocol

SYNOPSIS

#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION

IP is the network layer protocol used by the Internet proto

col family.
Options may be set at the IP level when using higher-level

protocols that
are based on IP (such as TCP and UDP). It may also be ac

cessed through a
``raw socket'' when developing new protocols, or special

purpose applica
tions.

There are several IP-level setsockopt(2)/getsockopt(2) op

tions.
IP_OPTIONS may be used to provide IP options to be transmit

ted in the IP
header of each outgoing packet or to examine the header op

tions on incom
ing packets. IP options may be used with any socket type in

the Internet
family. The format of IP options to be sent is that speci

fied by the IP
protocol specification (RFC 791), with one exception: the

list of
addresses for Source Route options must include the first

hop gateway at
the beginning of the list of gateways. The first-hop gate

way address
will be extracted from the option list and the size adjusted

accordingly
before use. To disable previously specified options, use a

zero-length
buffer:

setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);

IP_TOS and IP_TTL may be used to set the type-of-service and

time-to-live
fields in the IP header for SOCK_STREAM and SOCK_DGRAM sock

ets. For
example,

int tos = IPTOS_LOWDELAY; /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IP, IP_TOS, tos, sizeof(tos));

int ttl = 60; /* max = 255 */
setsockopt(s, IPPROTO_IP, IP_TTL, ttl, sizeof(ttl));

IP_PORTRANGE controls how ephemeral ports are allocated for

SOCK_STREAM
and SOCK_DGRAM sockets. For example,

int range = IP_PORTRANGE_LOW; /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IP, IP_PORTRANGE, range, size

of(range));

If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM or

SOCK_RAW
socket, the recvmsg(2) call will return the destination IP

address for a
UDP datagram. The msg_control field in the msghdr structure

points to a
buffer that contains a cmsghdr structure followed by the IP

address. The
cmsghdr fields have the following values:

cmsg_len = sizeof(struct in_addr)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR

If the IP_RECVIF option is enabled on a SOCK_DGRAM or

SOCK_RAW socket,
the recvmsg(2) call will return a struct sockaddr_dl corre

sponding to the
interface on which the packet was received. the msg_control

field in the
msghdr structure points to a buffer that contains a cmsghdr

structure
followed by the struct sockaddr_dl. The cmsghdr fields have

the follow
ing values:

cmsg_len = sizeof(struct sockaddr_dl)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF

MULTICAST OPTIONS

IP multicasting is supported only on AF_INET sockets of type

SOCK_DGRAM
and SOCK_RAW, and only on networks where the interface driv

er supports
multicasting.

The IP_MULTICAST_TTL option changes the time-to-live (TTL)

for outgoing
multicast datagrams in order to control the scope of the

multicasts:

u_char ttl; /* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, ttl, size

of(ttl));

Datagrams with a TTL of 1 are not forwarded beyond the local

network.
Multicast datagrams with a TTL of 0 will not be transmitted

on any net
work, but may be delivered locally if the sending host be

longs to the
destination group and if multicast loopback has not been

disabled on the
sending socket (see below). Multicast datagrams with TTL

greater than 1
may be forwarded to other networks if a multicast router is

attached to
the local network.

For hosts with multiple interfaces, each multicast transmis

sion is sent
from the primary network interface. The IP_MULTICAST_IF op

tion overrides
the default for subsequent transmissions from a given sock

et:

struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, addr, sizeof(ad

dr));

where "addr" is the local IP address of the desired inter

face or
INADDR_ANY to specify the default interface. An interface's

local IP
address and multicast capability can be obtained via the

SIOCGIFCONF and
SIOCGIFFLAGS ioctls. Normal applications should not need to

use this
option.

If a multicast datagram is sent to a group to which the

sending host
itself belongs (on the outgoing interface), a copy of the

datagram is, by
default, looped back by the IP layer for local delivery.

The
IP_MULTICAST_LOOP option gives the sender explicit control

over whether
or not subsequent datagrams are looped back:

u_char loop; /* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, loop, size

of(loop));

This option improves performance for applications that may

have no more
than one instance on a single host (such as a router demon),

by eliminat
ing the overhead of receiving their own transmissions. It

should gener
ally not be used by applications for which there may be more

than one
instance on a single host (such as a conferencing program)

or for which
the sender does not belong to the destination group (such as

a time
querying program).

A multicast datagram sent with an initial TTL greater than 1

may be
delivered to the sending host on a different interface from

that on which
it was sent, if the host belongs to the destination group on

that other
interface. The loopback control option has no effect on

such delivery.

A host must become a member of a multicast group before it

can receive
datagrams sent to the group. To join a multicast group, use

the
IP_ADD_MEMBERSHIP option:

struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq, size

of(mreq));

where mreq is the following structure:

struct ip_mreq {

struct in_addr imr_multiaddr; /* multicast group to join

*/
struct in_addr imr_interface; /* interface to join on */

}

imr_interface should be INADDR_ANY to choose the default

multicast inter
face, or the IP address of a particular multicast-capable

interface if
the host is multihomed. Membership is associated with a

single inter
face; programs running on multihomed hosts may need to join

the same
group on more than one interface. Up to IP_MAX_MEMBERSHIPS

(currently
20) memberships may be added on a single socket.

To drop a membership, use:

struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, mreq, size

of(mreq));

where mreq contains the same values as used to add the mem

bership. Mem
berships are dropped when the socket is closed or the pro

cess exits.

RAW IP SOCKETS

Raw IP sockets are connectionless, and are normally used

with the
sendto(2) and recvfrom(2) calls, though the connect(2) call

may also be
used to fix the destination for future packets (in which

case the read(2)
or recv(2) and write(2) or send(2) system calls may be

used).

If proto is 0, the default protocol IPPROTO_RAW is used for

outgoing
packets, and only incoming packets destined for that proto

col are
received. If proto is non-zero, that protocol number will

be used on
outgoing packets and to filter incoming packets.

Outgoing packets automatically have an IP header prepended

to them (based
on the destination address and the protocol number the sock

et is created
with), unless the IP_HDRINCL option has been set. Incoming

packets are
received with IP header and options intact.

IP_HDRINCL indicates the complete IP header is included with

the data and
may be used only with the SOCK_RAW type.

#include <netinet/ip.h>

int hincl = 1; /* 1 = on, 0 = off */
setsockopt(s, IPPROTO_IP, IP_HDRINCL, hincl, sizeof(hincl));

Unlike previous BSD releases, the program must set all the

fields of the
IP header, including the following:

ip->ip_v = IPVERSION;
ip->ip_hl = hlen >> 2;
ip->ip_id = 0; /* 0 means kernel set appropriate value */
ip->ip_off = offset;

If the header source address is set to INADDR_ANY, the ker

nel will choose
an appropriate address.

DIAGNOSTICS

A socket operation may fail with one of the following errors

returned:

[EISCONN] when trying to establish a connection on a

socket which

already has one, or when trying to send a

datagram with
the destination address specified and the

socket is
already connected;

[ENOTCONN] when trying to send a datagram, but no des

tination

address is specified, and the socket hasn't

been con
nected;

[ENOBUFS] when the system runs out of memory for an

internal data

structure;

[EADDRNOTAVAIL] when an attempt is made to create a socket

with a net

work address for which no network interface

exists.

[EACCES] when an attempt is made to create a raw IP

socket by a

non-privileged process.

The following errors specific to IP may occur when setting

or getting IP
options:

[EINVAL] An unknown socket option name was given.

[EINVAL] The IP option field was improperly formed;

an option

field was shorter than the minimum value or

longer than
the option buffer provided.

SEE ALSO

getsockopt(2), recv(2), send(2), icmp(4), inet(4), intro(4)

Internet Protocol, RFC, 791, September 1981.

Host Extensions for IP Multicasting, RFC, 1112, August 1989.

Requirements for Internet Hosts -- Communication Layers,

RFC, 1122,
October 1989.

HISTORY

The ip protocol appeared in 4.2BSD.

BSD November 30, 1993

BSD