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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)). You can also
access the protocol through a raw socket when developing new protocols, or
special purpose applications. Both IP version 4 and IP version 6 are
supported.
IP-level options are set with the setsockopt() function and examined with
the getsockopt() function. 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.
IP_OPTIONS
Provides IPv4 options to be transmitted in the IPv4 header of each
outgoing packet or examines IPv4 header options on incoming packets.
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.
IP_RECVDSTADDR
Enables a SOCK_DGRAM socket to receive the destination IPv4 address for
a UDP datagram.
IP_RECVOPTS
Enables a SOCK_DGRAM socket to receive the IPv4 options for a UDP
datagram.
IP_TOS
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));
IP_TTL
Sets the time-to-live (TTL) field in the IPv4 header for a TCP or UDP
socket, by default the values are 60 and 30, respectively, for example:
int ttl;
setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
To determine the current value for this option, use the getsockopt
call, for example:
int ttl;
getsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
IPV6_RECVDSTOPTS
Enables a SOCK_RAW socket to receive IPv6 destination options, for
example:
int on = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_RECVDSTOPTS,
&on, sizeof(on)) == -1)
perror("setsockopt IPV6_RECVDSTOPTS");
IPV6_RECVHOPLIMIT
Enables a SOCK_RAW socket to receive the IPv6 hop limit, for example:
int on = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_RECVHOPLIMIT,
&on, sizeof(on)) == -1)
perror("setsockopt IPV6_RECVHOPLIMIT");
To determine the current value for this option, use the getsockopt
call.
IPV6_RECVHOPOPTS
Enables a SOCK_RAW socket to receive the IPv6 Hop-by-Hop options
header, for example:
int on = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_RECVHOPOPTS,
&on, sizeof(on)) == -1)
perror("setsockopt IPV6_RECVHOPOPTS");
IPV6_RECVPKTINFO
Enables a SOCK_RAW socket to receive the send and receive interfaces
and the source and destination addresses, for example:
int on = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_RECVPKTINFO,
&on, sizeof(on)) == -1)
perror("setsockopt IPV6_RECVPKTINFO");
IPV6_RECVRTHDR
Enables a SOCK_RAW socket to receive the IPv6 routing header, for
example:
int on = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_RECVRTHDR,
&on, sizeof(on)) == -1)
perror("setsockopt IPV6_RECVRTHDR");
IPV6_UNICAST_HOPS
Sets the hop limit for all subsequent IPv6 unicast packets sent on a
specified socket. For example:
int hoplimit = 10;
if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, sizeof(hoplimit)) == -1)
perror("setsockopt IPV6_UNICAST_HOPS");
To determine the current value for this option, use the getsockopt
call. For example:
int hoplimit;
size_t len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, &len) == -1);
perror("getsockopt IPV6_UNICAST_HOPS");
else
printf("Using %d for hop limit.\n", hoplimit);
IPV6_V6ONLY
Specifies whether to restrict AF_INET6 sockets to sending and receiving
only IPv6 packets. For example:
int on = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY,
(char *)&on, sizeof(on)) == -1)
perror("setsockopt IPV6_V6ONLY");
else
printf("IPV6_V6ONLY option set");
If set, operations with IPv4-mapped IPv6 addresses are not allowed.
This is a Boolean option. The default (0) is to accept both IPv4 and
IPv6 packets.
MULTICASTING
IP multicasting is supported on AF_INET and AF_INET6 sockets of type
SOCK_DGRAM and SOCK_RAW only, and only on networks where the interface
driver supports multicasting. By default, 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 looped back by the IP layer for local
delivery.
A host must become a member of a multicast group before it can receive
datagrams sent to the group. Memberships in multicast groups are dropped
when the socket is closed or the process exits.
IPv4 Multicasting
The following IPv4 multicasting options are supported:
IP_ADD_MEMBERSHIP
Joins an IPv4 multicast group, 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 on which to join */
}
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.
IP_DROP_MEMBERSHIP
Drops a membership in an IPv4 multicast group, for example:
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.
IP_MULTICAST_IF
Overrides the default for subsequent IPv4 transmissions from a given
socket. By default, for hosts with multiple interfaces, each multicast
transmission is sent from the primary network interface.
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.
IP_MULTICAST_LOOP
Specifies whether or not subsequent IPv6 datagrams are looped back,
giving the sender explicit control, 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 daemon), by
eliminating the overhead of receiving their own transmissions.
However, 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) generally should not use this option.
An IPv4 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.
IP_MULTICAST_TTL
Changes the time-to-live (TTL) in the header for outgoing IPv4
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
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 the IP_MULTICAST_LOOP option). Multicast
datagrams with TTL greater than 1 may be forwarded to other networks if
a multicast router is attached to the local network.
IPv6 Multicasting
The following IPv6 multicasting options are supported:
IPV6_JOIN_GROUP
Joins an IPv6 multicast group on the specified interface, for example:
#include <netinet/in.h>
struct ipv6_mreq imr6;
imr6.ipv6mr_interface = if_index;
if (setsockopt(s, IPPROTO_IPV6, IPV6_JOIN_GROUP,
(char *)&imr6, sizeof(imr6)) < 0) {
perror("setsockopt IPV6_JOIN_GROUP");
}
The ipv6_mreq parameter is the following structure:
struct ipv6_mreq {
struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast address */
unsigned int ipv6mr_interface; /* interface index; 0 =>
* pick a default interface
* that supports IP multicasts
*/
};
The ipv6mr_interface should be either zero to choose the default
multicast interface or the interface index of a particular 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 IPV6_MAX_MEMBERSHIPS
(currently 20) may be added to a single socket.
IPv4-mapped IPv6 addresses are supported as parameters for this option.
IPV6_LEAVE_GROUP
Drops a membership in an IPv6 multicast group, for example:
#include <netinet/in.h>
struct ipv6_mreq imr6;
if (setsockopt(s, IPPROTO_IPV6, IPV6_LEAVE_GROUP,
(char *)&imr6, sizeof(imr6)) < 0) {
perror("setsockopt IPV6_LEAVE_GROUP");
}
The imr6 parameter contains the same values as used to add the
membership. IPv4-mapped IPv6 addresses are supported as valid
parameters for this option.
IPV6_MULTICAST_HOPS
Changes the hop limit for outgoing IPv6 multicast packets; for example:
int hoplimit = 255; /* range 0 to 255; hop limit = -1 sets the
* hoplimit to the default value of 1
*/
if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_HOPS,
&hoplimit, sizeof(hoplimit)) < 0) {
perror("setsockopt IPV6_MULTICAST_HOPS");
}
IPV6_MULTICAST_IF
Sets the interface to use for outgoing IPv6 multicast packets:
u_int if_index = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_IF,
&if_index, sizeof(ifindex)) < 0)
perror("setsockopt IPV6_MULTICAST_IF");
}
The if_index parameter specifies the interface index of the desired
interface or 0 to select a default interface. You can use the
if_nametoindex() routine to find the interface index.
IPV6_MULTICAST_LOOP
Specifies whether or not subsequent IPv6 datagrams are looped back, for
example:
u_int on = 0; /* 0 = disable, 1 = enable; default = 1 */
if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_LOOP,
&on, sizeof(on)) < 0){
perror("setsockopt IPV6_MULTICAST_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.
However, 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) generally should not use this option.
An IPv6 multicast datagram sent with an initial hop limit 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.
Raw IP Sockets
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.
For IPv4, 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. The IP_HDRINCL
option specifies whether the IP header is provided by the sent packet.
Incoming packets are received with IP header and options intact.
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.
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
Functions: getsockopt(2), send(2), recv(2)
Network Information: netintro(7), icmp(7), inet(7), tcp(7)
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