IP(4) | Device Drivers Manual | IP(4) |
ip
—
#include <sys/socket.h>
#include <netinet/in.h>
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);
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));
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));
The IP_RECVPKTINFO
option can be used to
turn on receiving of information about the destination address of the
packet, and the interface index. The information is passed in a
struct in_pktinfo structure, which contains
struct in_addr ipi_addr; /* the source or destination address */ unsigned int ipi_ifindex; /* the interface index */
and added to the control portion of the message: The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo)) cmsg_level = IPPROTO_IP cmsg_type = IP_PKTINFO
For sendmsg(2), the
source address or output interface can be specified by adding an
IP_PKTINFO
message to the control part of the
message on a SOCK_DGRAM
or
SOCK_RAW
socket. Setting ipi_ifindex will cause the
primary address of that interface to be used; setting ipi_addr will directly
choose that address. The IP_PKTINFO
cmsghdr
structure from a received message may be used unchanged, in which case the
outgoing message will be sent from the address the incoming message was
received on.
Setting the IP_PKTINFO
option on a socket,
with the same struct in_pktinfo structure, will set
the default source address to be used until set again, unless explicitly
overridden on a per-packet basis, as above.
The IP_PORTALGO
can be used to randomize
the port selection. Valid algorithms are described in
rfc6056(7) and their
respective constants are in
<netinet/portalgo.h>
. For
example,
int algo = PORTALGO_ALGO_RANDOM_PICK; /* see <netinet/portalgo.h> */ setsockopt(s, IPPROTO_IP, IP_PORTALGO, &algo, sizeof(algo));
The port selection can be also viewed and controlled at a global
level for all IP sockets using the following
sysctl(7) variables:
net.inet.ip.anonportalgo.available
and
net.inet.ip.anonportalgo.selected
.
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 = CMSG_LEN(sizeof(struct in_addr)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTADDR
For sendmsg(2), the
source address can be specified by adding
IP_SENDSRCADDR
to the control part of the message on
a SOCK_DGRAM
or SOCK_RAW
socket. The IP_RECVDSTADDR
cmsghdr structure from a
received message may be used unchanged, in which case the outgoing message
will be sent from the address the incoming message was received on.
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 = CMSG_LEN(sizeof(struct sockaddr_dl)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVIF
If the IP_RECVTTL
option is enabled on a
SOCK_DGRAM
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(uint8_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVTTL
The IP_MINTTL
option may be used on
SOCK_DGRAM
or 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));
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.
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 = offset;
If the header source address is set to
INADDR_ANY
, the kernel will choose an appropriate
address.
EACCES
]EADDRNOTAVAIL
]EISCONN
]ENOBUFS
]ENOTCONN
]The following errors specific to IP may occur when setting or getting IP options:
EINVAL
]IP_RECVPKTINFO
option is used because it is directly
compatible with Solaris, AIX, etc., and the IP_PKTINFO
option is intended to be used in their manner, to set the default source
address for outgoing packets on a SOCK_DGRAM
or
SOCK_RAW
socket. For compatibility with Linux,
however, if you attempt to set the IP_PKTINFO
option,
using an integer parameter as a boolean value, this will transparently
manipulate the IP_RECVPKTINFO
option instead. Source
code compatbility with both environments is thus maintained.
Internet Protocol, RFC, 791, September 1981.
Host Extensions for IP Multicasting, RFC, 1112, August 1989.
Requirements for Internet Hosts -- Communication Layers, RFC, 1122, October 1989.
ip
protocol appeared in
4.2BSD.
December 31, 2017 | NetBSD 9.4 |