man/man4/ip.4

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.\"     @(#)ip.4	8.2 (Berkeley) 11/30/93
.\" $FreeBSD$
.\"
.Dd September 1, 2014
.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 transport 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
and
.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 , SOCK_DGRAM ,
and certain types of
.Dv SOCK_RAW
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_MINTTL
may be used to set the minimum acceptable TTL a packet must have when
received on a socket.
All packets with a lower TTL are silently dropped.
This option is only really useful when set to 255, preventing packets
from outside the directly connected networks reaching local listeners
on sockets.
.Pp
.Dv IP_DONTFRAG
may be used to set the Don't Fragment flag on IP packets.
Currently this option is respected only on
.Xr udp 4
and raw
.Xr ip 4
sockets, unless the
.Dv IP_HDRINCL
option has been set.
On
.Xr tcp 4
sockets, the Don't Fragment flag is controlled by the Path
MTU Discovery option.
Sending a packet larger than the MTU size of the egress interface,
determined by the destination address, returns an
.Er EMSGSIZE
error.
.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
.Vt 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
cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR
.Ed
.Pp
The source address to be used for outgoing
.Tn UDP
datagrams on a socket can be specified as ancillary data with a type code of
.Dv IP_SENDSRCADDR .
The msg_control field in the msghdr structure should point to a buffer
that contains a
.Vt cmsghdr
structure followed by the
.Tn IP
address.
The cmsghdr fields should have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_SENDSRCADDR
.Ed
.Pp
The socket should be either bound to
.Dv INADDR_ANY
and a local port, and the address supplied with
.Dv IP_SENDSRCADDR
should't be
.Dv INADDR_ANY ,
or the socket should be bound to a local address and the address supplied with
.Dv IP_SENDSRCADDR
should be
.Dv INADDR_ANY .
In the latter case bound address is overriden via generic source address
selection logic, which would choose IP address of interface closest to
destination.
.Pp
For convenience,
.Dv IP_SENDSRCADDR
is defined to have the same value as
.Dv IP_RECVDSTADDR ,
so the
.Dv IP_RECVDSTADDR
control message from
.Xr recvmsg 2
can be used directly as a control message for
.Xr sendmsg 2 .
.\"
.Pp
If the
.Dv IP_ONESBCAST
option is enabled on a
.Dv SOCK_DGRAM
or a
.Dv SOCK_RAW
socket, the destination address of outgoing
broadcast datagrams on that socket will be forced
to the undirected broadcast address,
.Dv INADDR_BROADCAST ,
before transmission.
This is in contrast to the default behavior of the
system, which is to transmit undirected broadcasts
via the first network interface with the
.Dv IFF_BROADCAST
flag set.
.Pp
This option allows applications to choose which
interface is used to transmit an undirected broadcast
datagram.
For example, the following code would force an
undirected broadcast to be transmitted via the interface
configured with the broadcast address 192.168.2.255:
.Bd -literal
char msg[512];
struct sockaddr_in sin;
int onesbcast = 1;	/* 0 = disable (default), 1 = enable */

setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
sin.sin_addr.s_addr = inet_addr("192.168.2.255");
sin.sin_port = htons(1234);
sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));
.Ed
.Pp
It is the application's responsibility to set the
.Dv IP_TTL
option
to an appropriate value in order to prevent broadcast storms.
The application must have sufficient credentials to set the
.Dv SO_BROADCAST
socket level option, otherwise the
.Dv IP_ONESBCAST
option has no effect.
.Pp
If the
.Dv IP_BINDANY
option is enabled on a
.Dv SOCK_STREAM ,
.Dv SOCK_DGRAM
or a
.Dv SOCK_RAW
socket, one can
.Xr bind 2
to any address, even one not bound to any available network interface in the
system.
This functionality (in conjunction with special firewall rules) can be used for
implementing a transparent proxy.
The
.Dv PRIV_NETINET_BINDANY
privilege is needed to set this option.
.Pp
If the
.Dv IP_RECVTTL
option is enabled on a
.Dv SOCK_DGRAM
socket, the
.Xr recvmsg 2
call will return the
.Tn IP
.Tn TTL
(time to live) field 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 TTL .
The cmsghdr fields have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(u_char))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTTL
.Ed
.\"
.Pp
If the
.Dv IP_RECVTOS
option is enabled on a
.Dv SOCK_DGRAM
socket, the
.Xr recvmsg 2
call will return the
.Tn IP
.Tn TOS
(type of service) field 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 TOS .
The cmsghdr fields have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(sizeof(u_char))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTOS
.Ed
.\"
.Pp
If the
.Dv IP_RECVIF
option is enabled on a
.Dv SOCK_DGRAM
socket, the
.Xr recvmsg 2
call returns a
.Vt "struct sockaddr_dl"
corresponding to the interface on which the
packet was received.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by the
.Vt "struct sockaddr_dl" .
The
.Vt 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
.Dv IP_PORTRANGE
may be used to set the port range used for selecting a local port number
on a socket with an unspecified (zero) port number.
It has the following
possible values:
.Bl -tag -width IP_PORTRANGE_DEFAULT
.It Dv IP_PORTRANGE_DEFAULT
use the default range of values, normally
.Dv IPPORT_HIFIRSTAUTO
through
.Dv IPPORT_HILASTAUTO .
This is adjustable through the sysctl setting:
.Va net.inet.ip.portrange.first
and
.Va net.inet.ip.portrange.last .
.It Dv IP_PORTRANGE_HIGH
use a high range of values, normally
.Dv IPPORT_HIFIRSTAUTO
and
.Dv IPPORT_HILASTAUTO .
This is adjustable through the sysctl setting:
.Va net.inet.ip.portrange.hifirst
and
.Va net.inet.ip.portrange.hilast .
.It Dv IP_PORTRANGE_LOW
use a low range of ports, which are normally restricted to
privileged processes on
.Ux
systems.
The range is normally from
.Dv IPPORT_RESERVED
\- 1 down to
.Li IPPORT_RESERVEDSTART
in descending order.
This is adjustable through the sysctl setting:
.Va net.inet.ip.portrange.lowfirst
and
.Va net.inet.ip.portrange.lowlast .
.El
.Pp
The range of privileged ports which only may be opened by
root-owned processes may be modified by the
.Va net.inet.ip.portrange.reservedlow
and
.Va net.inet.ip.portrange.reservedhigh
sysctl settings.
The values default to the traditional range,
0 through
.Dv IPPORT_RESERVED
\- 1
(0 through 1023), respectively.
Note that these settings do not affect and are not accounted for in the
use or calculation of the other
.Va net.inet.ip.portrange
values above.
Changing these values departs from
.Ux
tradition and has security
consequences that the administrator should carefully evaluate before
modifying these settings.
.Pp
Ports are allocated at random within the specified port range in order
to increase the difficulty of random spoofing attacks.
In scenarios such as benchmarking, this behavior may be undesirable.
In these cases,
.Va net.inet.ip.portrange.randomized
can be used to toggle randomization off.
If more than
.Va net.inet.ip.portrange.randomcps
ports have been allocated in the last second, then return to sequential
port allocation.
Return to random allocation only once the current port allocation rate
drops below
.Va net.inet.ip.portrange.randomcps
for at least
.Va net.inet.ip.portrange.randomtime
seconds.
The default values for
.Va net.inet.ip.portrange.randomcps
and
.Va net.inet.ip.portrange.randomtime
are 10 port allocations per second and 45 seconds correspondingly.
.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, where an interface has not
been specified for a multicast group membership,
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.
.Pp
To specify an interface by index, an instance of
.Vt ip_mreqn
may be passed instead.
The
.Vt imr_ifindex
member should be set to the index of the desired interface,
or 0 to specify the default interface.
The kernel differentiates between these two structures by their size.
.Pp
The use of
.Vt IP_MULTICAST_IF
is
.Em not recommended ,
as multicast memberships are scoped to each
individual interface.
It is supported for legacy use only by applications,
such as routing daemons, which expect to
be able to transmit link-local IPv4 multicast datagrams (224.0.0.0/24)
on multiple interfaces,
without requesting an individual membership for each interface.
.Pp
.\"
An interface's local IP address and multicast capability can
be obtained via the
.Dv SIOCGIFCONF
and
.Dv SIOCGIFFLAGS
ioctls.
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
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 routing 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
The sysctl setting
.Va net.inet.ip.mcast.loop
controls the default setting of the
.Dv IP_MULTICAST_LOOP
socket option for new sockets.
.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; /* IP multicast address of group */
    struct in_addr imr_interface; /* local IP address of interface */
}
.Ed
.Pp
.Va imr_interface
should be set to the
.Tn IP
address of a particular multicast-capable interface if
the host is multihomed.
It may be set to
.Dv INADDR_ANY
to choose the default interface, although this is not recommended;
this is considered to be the first interface corresponding
to the default route.
Otherwise, the first multicast-capable interface
configured in the system will be used.
.Pp
Prior to
.Fx 7.0 ,
if the
.Va imr_interface
member is within the network range
.Li 0.0.0.0/8 ,
it is treated as an interface index in the system interface MIB,
as per the RIP Version 2 MIB Extension (RFC-1724).
In versions of
.Fx
since 7.0, this behavior is no longer supported.
Developers should
instead use the RFC 3678 multicast source filter APIs; in particular,
.Dv MCAST_JOIN_GROUP .
.Pp
Up to
.Dv IP_MAX_MEMBERSHIPS
memberships may be added on a single socket.
Membership is associated with a single interface;
programs running on multihomed hosts may need to
join the same group on more than one interface.
.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.
.\" TODO: Update this piece when IPv4 source-address selection is implemented.
.Pp
The IGMP protocol uses the primary IP address of the interface
as its identifier for group membership.
This is the first IP address configured on the interface.
If this address is removed or changed, the results are
undefined, as the IGMP membership state will then be inconsistent.
If multiple IP aliases are configured on the same interface,
they will be ignored.
.Pp
This shortcoming was addressed in IPv6; MLDv2 requires
that the unique link-local address for an interface is
used to identify an MLDv2 listener.
.Ss "Source-Specific Multicast Options"
Since
.Fx 8.0 ,
the use of Source-Specific Multicast (SSM) is supported.
These extensions require an IGMPv3 multicast router in order to
make best use of them.
If a legacy multicast router is present on the link,
.Fx
will simply downgrade to the version of IGMP spoken by the router,
and the benefits of source filtering on the upstream link
will not be present, although the kernel will continue to
squelch transmissions from blocked sources.
.Pp
Each group membership on a socket now has a filter mode:
.Bl -tag -width MCAST_EXCLUDE
.It Dv MCAST_EXCLUDE
Datagrams sent to this group are accepted,
unless the source is in a list of blocked source addresses.
.It Dv MCAST_INCLUDE
Datagrams sent to this group are accepted
only if the source is in a list of accepted source addresses.
.El
.Pp
Groups joined using the legacy
.Dv IP_ADD_MEMBERSHIP
option are placed in exclusive-mode,
and are able to request that certain sources are blocked or allowed.
This is known as the
.Em delta-based API .
.Pp
To block a multicast source on an existing group membership:
.Bd -literal
struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_BLOCK_SOURCE, &mreqs, sizeof(mreqs));
.Ed
.Pp
where
.Fa mreqs
is the following structure:
.Bd -literal
struct ip_mreq_source {
    struct in_addr imr_multiaddr; /* IP multicast address of group */
    struct in_addr imr_sourceaddr; /* IP address of source */
    struct in_addr imr_interface; /* local IP address of interface */
}
.Ed
.Va imr_sourceaddr
should be set to the address of the source to be blocked.
.Pp
To unblock a multicast source on an existing group:
.Bd -literal
struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_UNBLOCK_SOURCE, &mreqs, sizeof(mreqs));
.Ed
.Pp
The
.Dv IP_BLOCK_SOURCE
and
.Dv IP_UNBLOCK_SOURCE
options are
.Em not permitted
for inclusive-mode group memberships.
.Pp
To join a multicast group in
.Dv MCAST_INCLUDE
mode with a single source,
or add another source to an existing inclusive-mode membership:
.Bd -literal
struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_ADD_SOURCE_MEMBERSHIP, &mreqs, sizeof(mreqs));
.Ed
.Pp
To leave a single source from an existing group in inclusive mode:
.Bd -literal
struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_DROP_SOURCE_MEMBERSHIP, &mreqs, sizeof(mreqs));
.Ed
If this is the last accepted source for the group, the membership
will be dropped.
.Pp
The
.Dv IP_ADD_SOURCE_MEMBERSHIP
and
.Dv IP_DROP_SOURCE_MEMBERSHIP
options are
.Em not accepted
for exclusive-mode group memberships.
However, both exclusive and inclusive mode memberships
support the use of the
.Em full-state API
documented in RFC 3678.
For management of source filter lists using this API,
please refer to
.Xr sourcefilter 3 .
.Pp
The sysctl settings
.Va net.inet.ip.mcast.maxsocksrc
and
.Va net.inet.ip.mcast.maxgrpsrc
are used to specify an upper limit on the number of per-socket and per-group
source filter entries which the kernel may allocate.
.\"-----------------------
.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.
Unlike in previous
.Bx
releases, incoming packets are received with
.Tn IP
header and options intact, leaving all fields in network byte order.
.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/in_systm.h>
#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 = htons(offset);
ip->ip_len = htons(len);
.Ed
.Pp
The packet should be provided as is to be sent over wire.
This implies all fields, including
.Va ip_len
and
.Va ip_off
to be in network byte order.
See
.Xr byteorder 3
for more information on network byte order.
If the
.Va ip_id
field is set to 0 then the kernel will choose an
appropriate value.
If the header source address is set to
.Dv INADDR_ANY ,
the kernel will choose an appropriate address.
.Sh ERRORS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width Er
.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 has not 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 Er
.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
.Pp
The following errors may occur when attempting to send
.Tn IP
datagrams via a
.Dq raw socket
with the
.Dv IP_HDRINCL
option set:
.Bl -tag -width Er
.It Bq Er EINVAL
The user-supplied
.Va ip_len
field was not equal to the length of the datagram written to the socket.
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr byteorder 3 ,
.Xr icmp 4 ,
.Xr igmp 4 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr multicast 4 ,
.Xr sourcefilter 3
.Rs
.%A D. Thaler
.%A B. Fenner
.%A B. Quinn
.%T "Socket Interface Extensions for Multicast Source Filters"
.%N RFC 3678
.%D Jan 2004
.Re
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.2 .
The
.Vt ip_mreqn
structure appeared in
.Tn Linux 2.4 .
.Sh BUGS
Before
.Fx 10.0
packets received on raw IP sockets had the
.Va ip_hl
subtracted from the
.Va ip_len
field.
.Pp
Before
.Fx 11.0
packets received on raw IP sockets had the
.Va ip_len
and
.Va ip_off
fields converted to host byte order.
Packets written to raw IP sockets were expected to have
.Va ip_len
and
.Va ip_off
in host byte order.