ip - Internet Protocol
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_RAW, proto);
IP is the network layer protocol used by the Internet protocol 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 accessed through a
``raw socket'' when developing new protocols, or specialpurpose applications.
There are several IP-level setsockopt(2)/getsockopt(2) options.
IP_OPTIONS may be used to provide IP options to be transmitted in the IP
header of each outgoing packet or to examine the header options on incoming
packets. IP options may be used with any socket type in
the Internet
family. The format of IP options to be sent is that specified 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 gateway 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 sockets. For example,
int tos = IPTOS_LOWDELAY; /* see <netinet/ip.h> */
setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
int ttl = 60; /* max = 255 */
setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM
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 = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR
The IP_PORTRANGE option causes the default allocation policy
for when the
kernel is asked to choose a free port number. Three choices
are available:
IP_PORTRANGE_DEFAULT The regular range of non-reserved
ports.
IP_PORTRANGE_HIGH A high range, for fun.
IP_PORTRANGE_LOW Reserved ports; between 600 and 1023.
Multicast Options [Toc] [Back]
IP multicasting is supported only on AF_INET sockets of type
SOCK_DGRAM
and SOCK_RAW, and only on networks where the interface driver 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, sizeof(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 network,
but may be delivered locally if the sending host belongs 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 transmission is sent
from the primary network interface. The IP_MULTICAST_IF option overrides
the default for subsequent transmissions from a given socket:
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
where "addr" is the local IP address of the desired interface 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, sizeof(loop));
This option improves performance for applications that may
have no more
than one instance on a single host (such as a router daemon), by eliminating
the overhead of receiving their own transmissions.
It should generally
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, sizeof(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 interface,
or the IP address of a particular multicast-capable
interface if
the host is multihomed. Membership is associated with a
single interface;
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, sizeof(mreq));
where mreq contains the same values as used to add the membership. Memberships
are dropped when the socket is closed or the process exits.
Raw IP Sockets [Toc] [Back]
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 protocol 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 socket 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 = htons(offset);
ip->ip_len = htons(len);
Additionally note that starting with OpenBSD 2.1, the ip_off
and ip_len
fields are in network byte order. If the header source address is set to
INADDR_ANY, the kernel will choose an appropriate address.
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 destination address
is specified, and the socket hasn't
been connected;
[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 network
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.
getsockopt(2), recv(2), send(2), icmp(4), inet(4), netintro(4)
The ip protocol appeared in 4.2BSD.
OpenBSD 3.6 November 30, 1993
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