IP(7) UNIX System V(Internet Utilities) IP(7)
NAME
IP - Internet Protocol
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
s = socket(AFINET, SOCKRAW, proto);
t = topen ("/dev/rawip", ORDWR);
d = open ("/dev/ip", ORDWR);
DESCRIPTION
IP is the internetwork datagram delivery protocol that is central to the
Internet protocol family. Programs may use IP through higher-level
protocols such as the Transmission Control Protocol (TCP) or the User
Datagram Protocol (UDP), or may interface directly to IP. See tcp(7) and
udp(7). Direct access may be via the socket interface (using a raw
socket) or the Transport Level Interface (TLI). The protocol options
defined in the IP specification may be set in outgoing datagrams.
The STREAMS driver /dev/rawip is the TLI transport provider that provides
raw access to IP. The device /dev/ip is the multiplexing STREAMS driver
that implements the protocol processing of IP. The latter connects below
to datalink providers [interface drivers, see if(3N)], and above to
transport providers such as TCP and UDP.
Raw IP sockets are connectionless and are normally used with the sendto()
and recvfrom() calls, [(see send(2) and recv(2)] although the connect(2)
call may also be used to fix the destination for future datagrams [in
which case the read(2) or recv(2) and write(2) or send(2) calls may be
used]. If proto is zero, the default protocol, IPPROTORAW, is used. If
proto is non-zero, that protocol number will be set in outgoing datagrams
and will be used to filter incoming datagrams. An IP header will be
generated and prepended to each outgoing datagram; received datagrams are
returned with the IP header and options intact.
A single socket option, IPOPTIONS, is supported at the IP level. This
socket option may be used to set IP options to be included in each
outgoing datagram. IP options to be sent are set with setsockopt() [see
getsockopt(2)]. The getsockopt(2) call returns the IP options set in the
last setsockopt() call. IP options on received datagrams are visible to
user programs only using raw IP sockets. The format of IP options given
in setsockopt() matches those defined in the IP specification with one
exception: the list of addresses for the source routing 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. IP options may be used with
any socket type in the Internet family.
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At the socket level, the socket option SODONTROUTE may be applied. This
option forces datagrams being sent to bypass the routing step in output.
Normally, IP selects a network interface to send the datagram, and
possibly an intermediate gateway, based on an entry in the routing table.
See routing(4). When SODONTROUTE is set, the datagram will be sent
using the interface whose network number or full IP address matches the
destination address. If no interface matches, the error ENETUNRCH will
be returned.
Raw IP datagrams can also be sent and received using the TLI
connectionless primitives.
Datagrams flow through the IP layer in two directions: from the network
up to user processes and from user processes down to the network. Using
this orientation, IP is layered above the network interface drivers and
below the transport protocols such as UDP and TCP. The Internet Control
Message Protocol (ICMP) is logically a part of IP. See icmp(7).
IP provides for a checksum of the header part, but not the data part of
the datagram. The checksum value is computed and set in the process of
sending datagrams and checked when receiving datagrams. IP header
checksumming may be disabled for debugging purposes by patching the
kernel variable ipcksum to have the value zero.
IP options in received datagrams are processed in the IP layer according
to the protocol specification. Currently recognized IP options include:
security, loose source and record route (LSRR), strict source and record
route (SSRR), record route, stream identifier, and internet timestamp.
The IP layer will normally forward received datagrams that are not
addressed to it. Forwarding is under the control of the kernel variable
ipforwarding: if ipforwarding is zero, IP datagrams will not be
forwarded; if ipforwarding is one, IP datagrams will be forwarded.
ipforwarding is usually set to one only in machines with more than one
network interface (internetwork routers). This kernel variable can be
patched to enable or disable forwarding.
The IP layer will send an ICMP message back to the source host in many
cases when it receives a datagram that can not be handled. A time
exceeded ICMP message will be sent if the time to live field in the IP
header drops to zero in the process of forwarding a datagram. A
destination unreachable message will be sent if a datagram can not be
forwarded because there is no route to the final destination, or if it
can not be fragmented. If the datagram is addressed to the local host
but is destined for a protocol that is not supported or a port that is
not in use, a destination unreachable message will also be sent. The IP
layer may send an ICMP source quench message if it is receiving datagrams
too quickly. ICMP messages are only sent for the first fragment of a
fragmented datagram and are never returned in response to errors in other
ICMP messages.
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The IP layer supports fragmentation and reassembly. Datagrams are
fragmented on output if the datagram is larger than the maximum
transmission unit (MTU) of the network interface. Fragments of received
datagrams are dropped from the reassembly queues if the complete datagram
is not reconstructed within a short time period.
Errors in sending discovered at the network interface driver layer are
passed by IP back up to the user process.
SEE ALSO
read(2), write(2), connect(3N), getsockopt(3N), recv(3N), send(3N),
routing(4), icmp(7), inet(7) tcp(7), udp(7)
Postel, Jon, Internet Protocol - DARPA Internet Program Protocol
Specification, RFC 791, Network Information Center, SRI International,
Menlo Park, Calif., September 1981
DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
EACCESS A IP broadcast destination address was specified and
the caller was not the privileged user.
EISCONN An attempt was made to establish a connection on a
socket which already had one, or to send a datagram
with the destination address specified and the socket
was already connected.
EMSGSIZE An attempt was made to send a datagram that was too
large for an interface, but was not allowed to be
fragmented (such as broadcasts).
ENETUNREACH An attempt was made to establish a connection or send
a datagram, where there was no matching entry in the
routing table, or if an ICMP destination unreachable
message was received.
ENOTCONN A datagram was sent, but no destination address was
specified, and the socket had not been connected.
ENOBUFS The system ran out of memory for fragmentation
buffers or other internal data structure.
EADDRNOTAVAIL An attempt was made to create a socket with a local
address that did not match any network interface, or
an IP broadcast destination address was specified and
the network interface does not support broadcast.
The following errors may occur when setting or getting IP options:
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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.
NOTES
Raw sockets should receive ICMP error packets relating to the protocol;
currently such packets are simply discarded.
Users of higher-level protocols such as TCP and UDP should be able to see
received IP options.
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