IP(4) BSD 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 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 TCP and UDP). It may also be accessed through a
``raw socket'' when developing new protocols, or special-purpose applica-
tions.
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 incom-
ing 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 ad-
dresses 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 exam-
ple,
int tos = IPTOS_LOWDELAY; /* see <netinet/in.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 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
The IP_ONESBCAST option allows broadcast datagrams to be sent with an all
ones destination address (ie: both the host and network portions of the
IP address). This option is only useful with the UDP protocol.
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 net-
work, 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 ad-
dress and multicast capability can be obtained via the SIOCGIFCONF and
SIOCGIFFLAGS ioctls. Normal applications should not need to use this op-
tion.
If a multicast datagram is sent to a group to which the sending host it-
self 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 eliminat-
ing the overhead of receiving their own transmissions. It should gener-
ally not be used by applications for which there may be more than one in-
stance 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 de-
livered 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 inter-
face, or the IP address of a particular multicast-capable interface if
the host is multihomed. Membership is associated with a single inter-
face; 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. Mem-
berships 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
and recvfrom 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 re-
ceived. If proto is non-zero, that protocol number will be used on out-
going 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;
The ip_len and ip_off fields must be provided in host byte order. All
other fields must be provided in network byte order. See byteorder(4)
for more information on network byte order. If the header source address
is set to INADDR_ANY, the kernel will choose an appropriate address.
SYSCTL VARIABLES
Some IP options can be read or written via the sysctl(3) facility. Vari-
ables specific to IP are:
CTL_NET, PF_LINK, CTL_LINK_PROTOTYPE, PF_INET
These variables are used to get or set various Internet specific inter-
face data. The Fifth- through Eighth-level names specify the first
through fourth bytes of an Internet interface address. Specifying an In-
ternet address of zero indicates that non-address specific interface pa-
rameters are to be queried or set.
The non-address specific interface parameters are:
Fourth level name Type Changeable
IPIFCTL_GEN_MULTIADDRS struct in_multiaddr no
IPIFCTL_GEN_MULITADDRS
Returns a chain of struct in_multiaddr structures (as defined in
/usr/include/netinet/in.h) with the chain pointers corrected for
user space. Each struct in_multiaddr describes one multicast
group membership, the number of requests for group membership,
the state of the group membership and the current value of the
IGMP membership report timer.
No address specific Internet interface sysctls are implemented at this
time.
CTL_NET, PF_INET
These variables are used to get or set various Internet specific data.
The third level name is the protocol.
IPPROTO_IP
Fourth level name Type Changeable
IPCTL_DEFTTL integer yes
IPCTL_FORWARDING integer yes
IPCTL_FORWSRCRT integer yes
IPCTL_MAXFRAGPACKETS integer yes
IPCTL_MRSTATS struct no
IPCTL_MRTPROTO integer no
IPCTL_SENDREDIRECTS integer yes
IPCTL_SOURCECHECK integer yes
IPCTL_SOURCECHECK_LOGINT integer yes
IPCTL_STATS struct no
IPCTL_DEFTTL
The maximum time-to-live (hop count) value for an IP
packet originated by the system. This value applies to
normal transport protocols, not to ICMP.
IPCTL_FORWARDING
Returns 1 when IP forwarding is enabled for the host,
meaning that the host is acting as a router.
IPCTL_FORWSRCRT
Returns 1 when IP forwarding is enabled for source routed
packets.
IPCTL_MAXFRAGPACKETS
Returns the maximum number of fragmented IP packets that
will be maintained in reassembly queue. When this limit
is reached, fragments of packets not already in the re-
assembly queue will be discarded. A value of 0 indicates
that the reassembly of fragmented IP packets is disabled.
A value of -1 removes any limit on the number of packets
in the reassembly queue.
IPCTL_MRSTATS
Returns the struct mrtstat structure containing statis-
tics relating to the IP multicast forwarding level.
IPCTL_MRTPROTO
Returns the value of the IP multicast routing protocol
for which the kernel is configured.
IPCTL_SENDREDIRECTS
Returns 1 when ICMP redirects may be sent by the host.
This option is ignored unless the host is routing IP
packets, and should normally be enabled on all systems.
IPCTL_SOURCECHECK
Returns 1 if the source of received unicast and broadcast
IP packets will be verified. A route to the source of
these packets must exist, and they must have arrived on
the interface specified in this route. Packets that do
not meet this criterion will be discarded.
IPCTL_SOURCECHECK_LOGINT
Returns the minimum number of seconds between log mes-
sages warning of packets discarded because IP source
checking is enabled. A value of zero indicates that
there is no minimum time interval.
IPCTL_STATS
Returns the struct ipstat structure containing statistics
relating to the IP level.
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 al-
ready connected;
[ENOTCONN] when trying to send a datagram, but no destination ad-
dress is specified, and the socket hasn't been connect-
ed;
[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 net-
work address for which no network interface exists.
[EACESS] 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), send(2), recv(2), byteorder(3), sysctl(3), intro(4),
icmp(4), igmp(4), inet(4)HISTORY
The ip protocol appeared in 4.2BSD.
4.2 Berkeley Distribution November 30, 1993 5
