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Index for Section 7 |
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Alphabetical listing for I |
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ip(7)
NAME
ip - Internet Protocol
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
s = socket(AF_INET, 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 IP options to be transmitted in the IP header
of each outgoing packet. Options are set with the setsockopt() function
and examined with the getsockopt() function. The format of IP options to
be sent is that specified by the IP 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. IP options may be used with any socket type in the
Internet family. Other options supported by the getsocket() and
setsocket() functions can be found in the <netinet/in.h> header file.
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.
Multicasting
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; 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. 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 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 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. 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 greater than 1 may be de-
livered 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, 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 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 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.
The IP_RECVDSTADDR option enables a SOCK_DGRAM socket to receive the
destination IP 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:
[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.
[ENOTCONN]
The destination address of a datagram was not specified, and the
socket has not been connected.
[ENOBUFS] The system ran out of memory for an internal data structure.
[EADDRNOTAVAIL]
An attempt was made to create a socket with a network address for
which no network interface exists.
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)