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ip(7)
NAME
ip - Internet Protocol (IPv4 and IPv6)
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
The following is the socket call for AF_INET sockets:
s = socket(AF_INET, SOCK_RAW, proto);
The following is the socket call for AF_INET6 sockets:
s = socket(AF_INET6, SOCK_RAW, proto);
DESCRIPTION
The Internet Protocol (IP) is the transport 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 the Transmission
Control Protocol (TCP) and the User Datagram Package (UDP)). It may also
be accessed through a raw socket when developing new protocols, or special
purpose applications.
IP_OPTIONS is used to provide IPv4 options to be transmitted in the IPv4
header of each outgoing packet. Options are set with the setsockopt()
function and examined with the getsockopt() function. The format of IPv4
options to be sent is that specified by the IPv4 specification, 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. IPv4 options may be used with any socket
type in the Internet family. Other options supported by the getsockopt()
and setsockopt() functions can be found in the <netinet/in.h> header file
for IPv4 and in the <netinet/in6.h> header file for IPv6.
Raw IP sockets are connectionless, and are normally used with the sendto()
and recvfrom() calls, though the connect() call may also be used to fix the
destination for future packets, in which case the read() or recv() and
write() or send() functions may be used.
If proto is 0 (zero), the default protocol IPPROTO_RAW is used for outgoing
packets, and only incoming packets destined for that protocol are received.
If proto is nonzero, 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 is set. IP_HDRINCL specifies whether
the IP header is provided by the sent packet. Incoming packets are
received with IP header and options intact.
Type-of-Service
The IP_TOS option sets the type-of-service (TOS) field in the IPv4 header
for a TCP or UDP socket. For example:
int tos;
setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
You can set the type-of-service to any of the defined constants in
<netinet/ip.h>. Typically used constants are: IPTOS_LOWDELAY,
IPTOS_THROUGHPUT, and IPTOS_RELIABILITY.
To determine the current value for this option, use the getsockopt call.
For example:
int tos;
getsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
Multicasting
IP multicasting is supported on AF_INET sockets of type SOCK_DGRAM and
SOCK_RAW only, and on networks only where the interface driver supports
multicasting.
For IPv4, the IP_MULTICAST_TTL option changes the time-to-live (TTL) for
outgoing multicast datagrams in order to control the scope of the
multicasts; for example:
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 net-
work, 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. For IPv4, 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));
The addr parameter specifies the local IPv4 address of the desired
interface or INADDR_ANY to specify the default interface. An interface's
local IPv4 address and multicast capability can be obtained through the
SIOCGIFCONF and SIOCGIFLAGS 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. For IPv4, the
IP_MULTICAST_LOOP option gives the sender explicit control over whether or
not subsequent datagrams are looped back, for example:
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 demon), 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 (IPv4) 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. For IPv4, to join a multicast group, use the
IP_ADD_MEMBERSHIP option, for example:
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
The mreq parameter 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 */
}
The imr_interface should be INADDR_ANY to choose the default multicast
interface, or the IPv4 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 the following:
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
The mreq parameter contains the same values as used to add the membership.
Memberships are dropped when the socket is closed or the process exits.
For IPv4, the IP_RECVDSTADDR option enables a SOCK_DGRAM socket to receive
the destination IPv4 address for a UDP datagram. The IP_RECVOPTS option
enables a SOCK_DGRAM socket to receive the Internet Protocol options.
ERRORS
If a socket operation fails, errno may be set to one of the following
values:
[EADDRNOTAVAIL]
An attempt was made to create a socket with a network address for
which no network interface exists.
[EISCONN] The socket is already connected. This error occurs when trying
to establish connection on a socket or when trying to send a
datagram with the destination address specified.
[ENOBUFS] The system ran out of memory for an internal data structure.
[ENOTCONN]
The destination address of a datagram was not specified, and the
socket has not been connected.
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.
RELATED INFORMATION
Functions: getsockopt(2), send(2), recv(2)
Network Information: netintro(7), icmp(7), inet(7), tcp(7).
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