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.\"     @(#)ip.4	8.2 (Berkeley) 11/30/93
.\"
.Dd December 31, 2017
.Dt IP 4
.Os
.Sh NAME
.Nm ip
.Nd Internet Protocol
.Sh SYNOPSIS
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET SOCK_RAW proto
.Sh DESCRIPTION
.Tn IP
is the network layer protocol used by the Internet protocol family.
Options may be set at the
.Tn IP
level when using higher-level protocols that are based on
.Tn IP
(such as
.Tn TCP
and
.Tn UDP ) .
It may also be accessed through a
.Dq raw socket
when developing new protocols, or special-purpose applications.
.Pp
There are several
.Tn IP-level
.Xr setsockopt 2 Ns / Ns Xr getsockopt 2
options.
.Dv IP_OPTIONS
may be used to provide
.Tn IP
options to be transmitted in the
.Tn IP
header of each outgoing packet
or to examine the header options on incoming packets.
.Tn IP
options may be used with any socket type in the Internet family.
The format of
.Tn IP
options to be sent is that specified by the
.Tn 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:
.Bd -literal
setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);
.Ed
.Pp
.Dv IP_TOS
and
.Dv IP_TTL
may be used to set the type-of-service and time-to-live fields in the
.Tn IP
header for
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
sockets.
For example,
.Bd -literal
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));
.Ed
.Pp
.Dv IP_IPSEC_POLICY
controls IPSec policy for sockets.
For example,
.Bd -literal
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));
.Ed
.Pp
The
.Dv 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
.Vt struct in_pktinfo
structure, which contains
.Bd -literal
	struct in_addr ipi_addr;	/* the source or destination address */
	unsigned int ipi_ifindex;	/* the interface index */
.Ed
.Pp
and added to the control portion of the message:
The cmsghdr fields have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo))
cmsg_level = IPPROTO_IP
cmsg_type = IP_PKTINFO
.Ed
.Pp
For
.Xr sendmsg 2 ,
the source address or output interface can be specified by adding an
.Dv IP_PKTINFO
message to the control part of the message on a
.Dv SOCK_DGRAM
or
.Dv 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.
.Pp
Setting the
.Dv IP_PKTINFO
option on a socket, with the same
.Vt 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.
.Pp
The
.Dv IP_PORTALGO
can be used to randomize the port selection.
Valid algorithms are described in
.Xr rfc6056 7
and their respective constants are in
.In netinet/portalgo.h .
For example,
.Bd -literal
int algo = PORTALGO_ALGO_RANDOM_PICK;       /* see <netinet/portalgo.h> */
setsockopt(s, IPPROTO_IP, IP_PORTALGO, &algo, sizeof(algo));
.Ed
.Pp
The port selection can be also viewed and controlled at a global level for all
.Tn IP
sockets using the following
.Xr sysctl 7
variables:
.Dv net.inet.ip.anonportalgo.available
and
.Dv net.inet.ip.anonportalgo.selected .
.Pp
.Dv IP_PORTRANGE
controls how ephemeral ports are allocated for
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
sockets.
For example,
.Bd -literal
int range = IP_PORTRANGE_LOW;       /* see <netinet/in.h> */
setsockopt(s, IPPROTO_IP, IP_PORTRANGE, &range, sizeof(range));
.Ed
.Pp
If the
.Dv IP_RECVDSTADDR
option is enabled on a
.Dv SOCK_DGRAM
or
.Dv SOCK_RAW
socket,
the
.Xr recvmsg 2
call will return the destination
.Tn IP
address for a
.Tn UDP
datagram.
The msg_control field in the msghdr structure points to a buffer
that contains a cmsghdr structure followed by the
.Tn IP
address.
The cmsghdr fields have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR
.Ed
.Pp
For
.Xr sendmsg 2 ,
the source address can be specified by adding
.Dv IP_SENDSRCADDR
to the control part of the message on a
.Dv SOCK_DGRAM
or
.Dv 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.
.Pp
If the
.Dv IP_RECVIF
option is enabled on a
.Dv SOCK_DGRAM
or
.Dv SOCK_RAW
socket,
the
.Xr 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:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(struct sockaddr_dl))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF
.Ed
.Pp
If the
.Dv IP_RECVTTL
option is enabled on a
.Dv SOCK_DGRAM
socket, the
.Xr recvmsg 2
call will return the
.Tn 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
.Tn TTL
value.
The cmsghdr fields have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(uint8_t))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTTL
.Ed
.Pp
The
.Dv IP_MINTTL
option may be used on
.Dv SOCK_DGRAM
or
.Dv SOCK_STREAM
sockets to discard packets with a TTL lower than the option value.
This can be used to implement the
.Em Generalized TTL Security Mechanism (GTSM)
according to RFC 3682.
To discard all packets with a TTL lower than 255:
.Bd -literal -offset indent
int minttl = 255;
setsockopt(s, IPPROTO_IP, IP_MINTTL, &minttl, sizeof(minttl));
.Ed
.Ss MULTICAST OPTIONS
.Tn IP
multicasting is supported only on
.Dv AF_INET
sockets of type
.Dv SOCK_DGRAM
and
.Dv SOCK_RAW ,
and only on networks where the interface driver supports multicasting.
.Pp
The
.Dv IP_MULTICAST_TTL
option changes the time-to-live (TTL) for outgoing multicast datagrams
in order to control the scope of the multicasts:
.Bd -literal
u_char ttl;	/* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
.Ed
.Pp
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.
.Pp
For hosts with multiple interfaces, each multicast transmission is
sent from the primary network interface.
The
.Dv IP_MULTICAST_IF
option overrides the default for
subsequent transmissions from a given socket:
.Bd -literal
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
.Ed
.Pp
where "addr" is the local
.Tn IP
address of the desired interface or
.Dv INADDR_ANY
to specify the default interface.
An interface's local IP address and multicast capability can
be obtained via the
.Dv SIOCGIFCONF
and
.Dv SIOCGIFFLAGS
ioctls.
An application may also specify an alternative to the default network interface
by index:
.Bd -literal
struct uint32_t idx = htonl(ifindex);
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &idx, sizeof(idx));
.Ed
.Pp
where "ifindex" is an interface index as returned by
.Xr if_nametoindex 3 .
.Pp
Normal applications should not need to use
.Dv IP_MULTICAST_IF .
.Pp
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
.Dv IP_MULTICAST_LOOP
option gives the sender explicit control
over whether or not subsequent datagrams are looped back:
.Bd -literal
u_char loop;	/* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
.Ed
.Pp
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).
.Pp
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.
.Pp
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
.Dv IP_ADD_MEMBERSHIP
option:
.Bd -literal
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
.Ed
.Pp
where
.Fa mreq
is the following structure:
.Bd -literal
struct ip_mreq {
    struct in_addr imr_multiaddr; /* multicast group to join */
    struct in_addr imr_interface; /* interface to join on */
}
.Ed
.Pp
.Dv imr_interface
should be
.Dv INADDR_ANY
to choose the default multicast interface, or the
.Tn 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
.Dv IP_MAX_MEMBERSHIPS
(currently 20) memberships may be added on a single socket.
.Pp
To drop a membership, use:
.Bd -literal
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
.Ed
.Pp
where
.Fa mreq
contains the same values as used to add the membership.
Memberships are dropped when the socket is closed or the process exits.
.\"-----------------------
.Ss RAW IP SOCKETS
Raw
.Tn IP
sockets are connectionless, and are normally used with the
.Xr sendto 2
and
.Xr recvfrom 2
calls, though the
.Xr connect 2
call may also be used to fix the destination for future
packets (in which case the
.Xr read 2
or
.Xr recv 2
and
.Xr write 2
or
.Xr send 2
system calls may be used).
.Pp
If
.Fa proto
is 0, the default protocol
.Dv IPPROTO_RAW
is used for outgoing packets, and only incoming packets destined
for that protocol are received.
If
.Fa proto
is non-zero, that protocol number will be used on outgoing packets
and to filter incoming packets.
.Pp
Outgoing packets automatically have an
.Tn IP
header prepended to them (based on the destination address and the
protocol number the socket is created with), unless the
.Dv IP_HDRINCL
option has been set.
Incoming packets are received with
.Tn IP
header and options intact.
.Pp
.Dv IP_HDRINCL
indicates the complete IP header is included with the data and may
be used only with the
.Dv SOCK_RAW
type.
.Bd -literal
#include <netinet/ip.h>

int hincl = 1;                  /* 1 = on, 0 = off */
setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));
.Ed
.Pp
Unlike previous
.Bx
releases, the program must set all
the fields of the IP header, including the following:
.Bd -literal
ip->ip_v = IPVERSION;
ip->ip_hl = hlen >> 2;
ip->ip_id = 0;  /* 0 means kernel set appropriate value */
ip->ip_off = offset;
.Ed
.Pp
If the header source address is set to
.Dv INADDR_ANY ,
the kernel will choose an appropriate address.
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er 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;
.It Bq Er ENOTCONN
when trying to send a datagram, but no destination address is
specified, and the socket hasn't been connected;
.It Bq Er ENOBUFS
when the system runs out of memory for an internal data structure;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a socket with a network address
for which no network interface exists.
.It Bq Er EACCES
when an attempt is made to create a raw IP socket by a non-privileged process.
.El
.Pp
The following errors specific to
.Tn IP
may occur when setting or getting
.Tn IP
options:
.Bl -tag -width EADDRNOTAVAILxx
.It Bq Er EINVAL
An unknown socket option name was given.
.It Bq Er EINVAL
The IP option field was improperly formed; an option field was
shorter than the minimum value or longer than the option buffer provided.
.El
.Sh COMPATIBILITY
The
.Dv IP_RECVPKTINFO
option is used because it is directly compatible with Solaris, AIX, etc.,
and the
.Dv IP_PKTINFO
option is intended to be used in their manner, to set the default source
address for outgoing packets on a
.Dv SOCK_DGRAM
or
.Dv SOCK_RAW
socket.
For compatibility with Linux, however, if you attempt to set the
.Dv IP_PKTINFO
option, using an integer parameter as a boolean value, this will
transparently manipulate the
.Dv IP_RECVPKTINFO
option instead.
Source code compatbility with both environments is thus maintained.
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr CMSG_DATA 3 ,
.Xr ipsec_set_policy 3 ,
.Xr icmp 4 ,
.Xr inet 4 ,
.Xr intro 4
.Rs
.%R RFC
.%N 791
.%D September 1981
.%T "Internet Protocol"
.Re
.Rs
.%R RFC
.%N 1112
.%D August 1989
.%T "Host Extensions for IP Multicasting"
.Re
.Rs
.%R RFC
.%N 1122
.%D October 1989
.%T "Requirements for Internet Hosts -- Communication Layers"
.Re
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.2 .