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.\"     @(#)ip.4	8.2 (Berkeley) 11/30/93
.\"
.Dd $Mdocdate: March 31 2022 $
.Dt IP 4
.Os
.Sh NAME
.Nm ip
.Nd Internet Protocol
.Sh SYNOPSIS
.In sys/types.h
.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 -offset indent
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 ,
.Dv SOCK_DGRAM
and
.Dv SOCK_RAW
sockets.
For example,
.Bd -literal -offset indent
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_IPDEFTTL
can be used to retrieve the system wide default TTL.
.Pp
If the
.Dv IP_RECVDSTADDR
option is enabled on a
.Dv SOCK_DGRAM
socket,
the
.Xr recvmsg 2
call will return the destination
.Tn IP
address for a
.Tn UDP
datagram.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by the
.Tn IP
address.
The
.Vt cmsghdr
fields have the following values:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR
.Ed
.Pp
If the
.Dv IP_RECVDSTPORT
option is enabled on a
.Dv SOCK_DGRAM
socket,
the
.Xr recvmsg 2
call will return the destination
port for a
.Tn UDP
datagram.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by the port in 16-bit network byte order.
The
.Vt cmsghdr
fields have the following values:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(u_int16_t))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTPORT
.Ed
.Pp
If the
.Dv IP_RECVTTL
option is enabled on a
.Dv SOCK_DGRAM
or
.Dv SOCK_RAW
socket, the
.Xr recvmsg 2
call will return the
.Tn TTL
of the received datagram.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by the
.Tn TTL
value.
The
.Vt cmsghdr
fields have the following values:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(u_int8_t))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTTL
.Ed
.Pp
The
.Dv IP_MINTTL
option may be used on TCP and UDP 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 5082.
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
.Pp
If the
.Dv IP_IPSECFLOWINFO
option is enabled on a
.Dv SOCK_DGRAM
socket,
the
.Xr recvmsg 2
call will return information identifying the incoming
IPsec SA for a
.Tn UDP
datagram.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by flow information in 32-bit network byte order.
When this information is passed to a
.Xr sendmsg 2
call, the ID of the incoming SA will be used for looking up the
outgoing SA for the
.Tn UDP
datagram.
The
.Vt cmsghdr
fields for
.Xr recvmsg 2
and
.Xr sendmsg 2
have the following values:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(u_int32_t))
cmsg_level = IPPROTO_IP
cmsg_type = IP_IPSECFLOWINFO
.Ed
.Pp
The
.Dv IP_PORTRANGE
option causes the default allocation policy for when the kernel is asked
to choose a free port number.
Three choices are available:
.Pp
.Bl -tag -width IP_PORTRANGE_DEFAULT -compact -offset indent
.It Dv IP_PORTRANGE_DEFAULT
The regular range of non-reserved ports.
.It Dv IP_PORTRANGE_HIGH
A high range, for fun.
.It Dv IP_PORTRANGE_LOW
Reserved ports; between 600 and 1023.
.El
.Pp
If the
.Dv IP_RECVRTABLE
option is enabled on a
.Dv SOCK_DGRAM
socket,
the
.Xr recvmsg 2
call will return the source routing domain for a
.Tn UDP
datagram.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by the routing table ID.
The
.Vt cmsghdr
fields have the following values:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(u_int))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVRTABLE
.Ed
.Pp
When sending on a
.Dv SOCK_DGRAM
socket with
.Xr sendmsg 2 ,
the source address to be used can be passed as ancillary data
with a type code of
.Dv IP_SENDSRCADDR .
The
.Va msg_control
field in the
.Vt msghdr
structure should point to a buffer that contains a
.Vt cmsghdr
structure followed by the requested source address.
The
.Vt cmsghdr
fields should have the following values:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_SENDSRCADDR
.Ed
.Pp
The same checks and restrictions as for
.Xr bind 2
apply, unless the socket is bound to
.Dv INADDR_ANY .
In this case, there is no source address overlap check.
.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 -offset indent
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 -offset indent
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
.Ed
.Pp
where
.Va 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
.Xr ioctl 2 Ns 's .
Normal applications should not need to use this option.
.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 -offset indent
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 daemon), 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 -offset indent
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
.Ed
.Pp
where
.Fa mreq
is either of the following structures:
.Bd -literal -offset indent
struct ip_mreq {
    struct in_addr imr_multiaddr; /* multicast group to join */
    struct in_addr imr_interface; /* interface to join on */
}

struct ip_mreqn {
    struct in_addr imr_multiaddr; /* multicast group to join */
    struct in_addr imr_address;   /* local IP address of interface */
    int            imr_ifindex;   /* interface index to join */
};
.Ed
.Pp
.Va 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.
The
.Va imr_ifindex
element of
.Va struct ip_mreqn
can be set to the interface index instead of specifying the
.Tn IP
address of a  particular multicast-capable interface.
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 4095) memberships may be added on a
single socket.
.Pp
To drop a membership, use:
.Bd -literal -offset indent
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 -offset indent
#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 -offset indent
ip->ip_v = IPVERSION;
ip->ip_hl = hlen >> 2;
ip->ip_id = 0;  /* 0 means kernel set appropriate value */
ip->ip_off = htons(offset);
ip->ip_len = htons(len);
.Ed
.Pp
Additionally note that starting with
.Ox 2.1 ,
the
.Va ip_off
and
.Va ip_len
fields are in network byte order.
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 SEE ALSO
.Xr getsockopt 2 ,
.Xr ioctl 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr icmp 4 ,
.Xr inet 4 ,
.Xr netintro 4
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.2 .