NAME
ip
—
Internet Protocol
SYNOPSIS
#include
<sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
socket
(AF_INET,
SOCK_RAW,
proto);
DESCRIPTION
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 special-purpose 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
If the IP_RECVDSTPORT
option is enabled on
a SOCK_DGRAM
socket, the
recvmsg(2) call will return the destination port for a UDP datagram.
The msg_control field in the
msghdr structure points to a buffer that contains a
cmsghdr structure followed by the port in 16-bit
network byte order. The cmsghdr fields have the
following values:
cmsg_len = CMSG_LEN(sizeof(u_int16_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTPORT
If the IP_RECVTTL
option is enabled on a
SOCK_DGRAM
or SOCK_RAW
socket, the recvmsg(2) call will return the TTL of the received datagram.
The msg_control field in the
msghdr structure points to a buffer that contains a
cmsghdr structure followed by the TTL value. The
cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(sizeof(u_int8_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVTTL
The IP_MINTTL
option may
be used on SOCK_STREAM
sockets to discard packets
with a TTL lower than the option value. This can be used to implement the
Generalized TTL
Security Mechanism (GTSM) according to RFC 5082. To discard all
packets with a TTL lower than 255:
int minttl = 255; setsockopt(s, IPPROTO_IP, IP_MINTTL, &minttl, sizeof(minttl));
If the IP_IPSECFLOWINFO
option is enabled
on a SOCK_DGRAM
socket, the
recvmsg(2) call will return information identifying the incoming
IPsec SA for a UDP datagram. The msg_control field in
the msghdr structure points to a buffer that contains
a cmsghdr structure followed by flow information in
32-bit network byte order. When this information is passed to a
sendmsg(2) call the ID of the incoming SA will be used for looking up
the outgoing SA for the UDP datagram. The cmsghdr
fields for recvmsg(2) and
sendmsg(2) have the following values:
cmsg_len = CMSG_LEN(sizeof(u_int32_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_IPSECFLOWINFO
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.
If the IP_RECVRTABLE
option is enabled on
a SOCK_DGRAM
socket, the
recvmsg(2) call will return the source routing domain for a UDP
datagram. The msg_control field in the
msghdr structure points to a buffer that contains a
cmsghdr structure followed by the routing table ID.
The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(sizeof(u_int)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVRTABLE
Multicast Options
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
ioctl(2)'s. 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 4095) 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
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.
DIAGNOSTICS
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.
SEE ALSO
getsockopt(2), ioctl(2), recv(2), send(2), icmp(4), inet(4), netintro(4)
HISTORY
The ip
protocol appeared in
4.2BSD.