int
socket(
AF_INET
, SOCK_RAW
, proto
)
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);
int ttl = 60; /* max = 255 */
setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
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));
IP_IPSEC_POLICY
controls IPSec policy for sockets.
For example,
const char *policy = "in ipsec ah/transport//require";
char *buf = ipsec_set_policy(policy, strlen(policy));
setsockopt(s, IPPROTO_IP, IP_IPSEC_POLICY, buf, ipsec_get_policylen(buf));
IP_PORTRANGE
controls how ephemeral ports are allocated for
SOCK_STREAM
and
SOCK_DGRAM
sockets. For example,
int range = IP_PORTRANGE_LOW; /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IP, IP_PORTRANGE, &range, sizeof(range));
If the
IP_RECVDSTADDR
option is enabled on a
SOCK_DGRAM
or
SOCK_RAW
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 = sizeof(struct in_addr)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR
If the
IP_RECVIF
option is enabled on a
SOCK_DGRAM
or
SOCK_RAW
socket,
the
recvmsg(2)
call will return a struct sockaddr_dl corresponding to
the interface on which the packet was received.
the msg_control field in the msghdr structure points to a buffer
that contains a cmsghdr structure followed by the struct sockaddr_dl.
The cmsghdr fields have the following values:
cmsg_len = sizeof(struct sockaddr_dl)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF
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
ioctls.
An application may also specify an alternative to the default network interface
by index:
struct uint32_t idx = htonl(ifindex);
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &idx, sizeof(idx));
where "ifindex" is an interface index as returned by if_nametoindex(3).
Normal applications should not need to use
IP_MULTICAST_IF
.
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 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 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 20) 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.
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.
int hincl = 1; /* 1 = on, 0 = off */
setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
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>
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 = offset;
If the header source address is set to
INADDR_ANY
,
the kernel will choose an appropriate address.
EISCONN
]
ENOTCONN
]
ENOBUFS
]
EADDRNOTAVAIL
]
EACCES
]The following errors specific to IP may occur when setting or getting IP options:
EINVAL
]
EINVAL
]