ip - Internet Protocol


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


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
There are several IP-level setsockopt(2)/getsockopt(2) op
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
before use. To disable previously specified options, use a
setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);
IP_TOS and IP_TTL may be used to set the type-of-service and
fields in the IP header for SOCK_STREAM and SOCK_DGRAM sock
ets. For
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
and SOCK_DGRAM sockets. For example,
int range = IP_PORTRANGE_LOW; /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IP, IP_PORTRANGE, range, size
If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM or
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
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
IP multicasting is supported only on AF_INET sockets of type
and SOCK_RAW, and only on networks where the interface driv
er supports
The IP_MULTICAST_TTL option changes the time-to-live (TTL)
for outgoing
multicast datagrams in order to control the scope of the
u_char ttl; /* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, ttl, size
Datagrams with a TTL of 1 are not forwarded beyond the local
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
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, addr, sizeof(ad
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
SIOCGIFFLAGS ioctls. Normal applications should not need to
use this
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.
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
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
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq, size
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
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
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 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
If proto is 0, the default protocol IPPROTO_RAW is used for
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.


A socket operation may fail with one of the following errors
[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
address is specified, and the socket hasn't
been con
[ENOBUFS] when the system runs out of memory for an
internal data
[EADDRNOTAVAIL] when an attempt is made to create a socket
with a net
work address for which no network interface
[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
[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.


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.


The ip protocol appeared in 4.2BSD.
BSD November 30, 1993
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