IP(4) OpenBSD Programmer's Manual IP(4)NAMEip - Internet Protocol
SYNOPSIS
#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
3682. To discard all packets with a TTL lower than 255:
int minttl = 255;
setsockopt(s, IPPROTO_IP, IP_MINTTL, &minttl, sizeof(minttl));
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
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 ALSOgetsockopt(2), ioctl(2), recv(2), send(2), icmp(4), inet(4), netintro(4)HISTORY
The ip protocol appeared in 4.2BSD.
OpenBSD 4.9 February 3, 2011 OpenBSD 4.9
