ip(7) Miscellaneous Information Manual ip(7)
NAME
ip - Linux IPv4 protocol implementation
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h> /* superset of previous */
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
raw_socket = socket(AF_INET, SOCK_RAW, protocol);
DESCRIPTION
Linux implements the Internet Protocol, version 4, described in RFC 791
and RFC 1122. ip contains a level 2 multicasting implementation con-
forming to RFC 1112. It also contains an IP router including a packet
filter.
The programming interface is BSD-sockets compatible. For more informa-
tion on sockets, see socket(7).
An IP socket is created using socket(2):
socket(AF_INET, socket_type, protocol);
Valid socket types include SOCK_STREAM to open a stream socket,
SOCK_DGRAM to open a datagram socket, and SOCK_RAW to open a raw(7)
socket to access the IP protocol directly.
protocol is the IP protocol in the IP header to be received or sent.
Valid values for protocol include:
• 0 and IPPROTO_TCP for tcp(7) stream sockets;
• 0 and IPPROTO_UDP for udp(7) datagram sockets;
• IPPROTO_SCTP for sctp(7) stream sockets; and
• IPPROTO_UDPLITE for udplite(7) datagram sockets.
For SOCK_RAW you may specify a valid IANA IP protocol defined in
RFC 1700 assigned numbers.
When a process wants to receive new incoming packets or connections, it
should bind a socket to a local interface address using bind(2). In
this case, only one IP socket may be bound to any given local (address,
port) pair. When INADDR_ANY is specified in the bind call, the socket
will be bound to all local interfaces. When listen(2) is called on an
unbound socket, the socket is automatically bound to a random free port
with the local address set to INADDR_ANY. When connect(2) is called on
an unbound socket, the socket is automatically bound to a random free
port or to a usable shared port with the local address set to IN-
ADDR_ANY.
A TCP local socket address that has been bound is unavailable for some
time after closing, unless the SO_REUSEADDR flag has been set. Care
should be taken when using this flag as it makes TCP less reliable.
Address format
An IP socket address is defined as a combination of an IP interface ad-
dress and a 16-bit port number. The basic IP protocol does not supply
port numbers, they are implemented by higher level protocols like
udp(7) and tcp(7). On raw sockets sin_port is set to the IP protocol.
struct sockaddr_in {
sa_family_t sin_family; /* address family: AF_INET */
in_port_t sin_port; /* port in network byte order */
struct in_addr sin_addr; /* internet address */
};
/* Internet address */
struct in_addr {
uint32_t s_addr; /* address in network byte order */
};
sin_family is always set to AF_INET. This is required; in Linux 2.2
most networking functions return EINVAL when this setting is missing.
sin_port contains the port in network byte order. The port numbers be-
low 1024 are called privileged ports (or sometimes: reserved ports).
Only a privileged process (on Linux: a process that has the
CAP_NET_BIND_SERVICE capability in the user namespace governing its
network namespace) may bind(2) to these sockets. Note that the raw
IPv4 protocol as such has no concept of a port, they are implemented
only by higher protocols like tcp(7) and udp(7).
sin_addr is the IP host address. The s_addr member of struct in_addr
contains the host interface address in network byte order. in_addr
should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK)
using htonl(3) or set using the inet_aton(3), inet_addr(3),
inet_makeaddr(3) library functions or directly with the name resolver
(see gethostbyname(3)).
IPv4 addresses are divided into unicast, broadcast, and multicast ad-
dresses. Unicast addresses specify a single interface of a host,
broadcast addresses specify all hosts on a network, and multicast ad-
dresses address all hosts in a multicast group. Datagrams to broadcast
addresses can be sent or received only when the SO_BROADCAST socket
flag is set. In the current implementation, connection-oriented sock-
ets are allowed to use only unicast addresses.
Note that the address and the port are always stored in network byte
order. In particular, this means that you need to call htons(3) on the
number that is assigned to a port. All address/port manipulation func-
tions in the standard library work in network byte order.
There are several special addresses: INADDR_LOOPBACK (127.0.0.1) always
refers to the local host via the loopback device; INADDR_ANY (0.0.0.0)
means any address for binding; INADDR_BROADCAST (255.255.255.255) means
any host and has the same effect on bind as INADDR_ANY for historical
reasons.
Socket options
IP supports some protocol-specific socket options that can be set with
setsockopt(2) and read with getsockopt(2). The socket option level for
IP is IPPROTO_IP. A boolean integer flag is zero when it is false,
otherwise true.
When an invalid socket option is specified, getsockopt(2) and setsock-
opt(2) fail with the error ENOPROTOOPT.
IP_ADD_MEMBERSHIP (since Linux 1.2)
Join a multicast group. Argument is an ip_mreqn structure.
struct ip_mreqn {
struct in_addr imr_multiaddr; /* IP multicast group
address */
struct in_addr imr_address; /* IP address of local
interface */
int imr_ifindex; /* interface index */
};
imr_multiaddr contains the address of the multicast group the applica-
tion wants to join or leave. It must be a valid multicast address (or
setsockopt(2) fails with the error EINVAL). imr_address is the address
of the local interface with which the system should join the multicast
group; if it is equal to INADDR_ANY, an appropriate interface is chosen
by the system. imr_ifindex is the interface index of the interface
that should join/leave the imr_multiaddr group, or 0 to indicate any
interface.
The ip_mreqn structure is available only since Linux 2.2. For
compatibility, the old ip_mreq structure (present since Linux
1.2) is still supported; it differs from ip_mreqn only by not
including the imr_ifindex field. (The kernel determines which
structure is being passed based on the size passed in optlen.)
IP_ADD_MEMBERSHIP is valid only for setsockopt(2).
IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / Linux 2.5.68)
Join a multicast group and allow receiving data only from a
specified source. Argument is an ip_mreq_source structure.
struct ip_mreq_source {
struct in_addr imr_multiaddr; /* IP multicast group
address */
struct in_addr imr_interface; /* IP address of local
interface */
struct in_addr imr_sourceaddr; /* IP address of
multicast source */
};
The ip_mreq_source structure is similar to ip_mreqn described under
IP_ADD_MEMBERSHIP. The imr_multiaddr field contains the address of the
multicast group the application wants to join or leave. The imr_inter-
face field is the address of the local interface with which the system
should join the multicast group. Finally, the imr_sourceaddr field
contains the address of the source the application wants to receive
data from.
This option can be used multiple times to allow receiving data
from more than one source.
IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
Inform the kernel to not reserve an ephemeral port when using
bind(2) with a port number of 0. The port will later be auto-
matically chosen at connect(2) time, in a way that allows shar-
ing a source port as long as the 4-tuple is unique.
IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
Stop receiving multicast data from a specific source in a given
group. This is valid only after the application has subscribed
to the multicast group using either IP_ADD_MEMBERSHIP or
IP_ADD_SOURCE_MEMBERSHIP.
Argument is an ip_mreq_source structure as described under
IP_ADD_SOURCE_MEMBERSHIP.
IP_DROP_MEMBERSHIP (since Linux 1.2)
Leave a multicast group. Argument is an ip_mreqn or ip_mreq
structure similar to IP_ADD_MEMBERSHIP.
IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
Leave a source-specific group—that is, stop receiving data from
a given multicast group that come from a given source. If the
application has subscribed to multiple sources within the same
group, data from the remaining sources will still be delivered.
To stop receiving data from all sources at once, use
IP_DROP_MEMBERSHIP.
Argument is an ip_mreq_source structure as described under
IP_ADD_SOURCE_MEMBERSHIP.
IP_FREEBIND (since Linux 2.4)
If enabled, this boolean option allows binding to an IP address
that is nonlocal or does not (yet) exist. This permits listen-
ing on a socket, without requiring the underlying network inter-
face or the specified dynamic IP address to be up at the time
that the application is trying to bind to it. This option is
the per-socket equivalent of the ip_nonlocal_bind /proc inter-
face described below.
IP_HDRINCL (since Linux 2.0)
If enabled, the user supplies an IP header in front of the user
data. Valid only for SOCK_RAW sockets; see raw(7) for more in-
formation. When this flag is enabled, the values set by IP_OP-
TIONS, IP_TTL, and IP_TOS are ignored.
IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
This option provides access to the advanced full-state filtering
API. Argument is an ip_msfilter structure.
struct ip_msfilter {
struct in_addr imsf_multiaddr; /* IP multicast group
address */
struct in_addr imsf_interface; /* IP address of local
interface */
uint32_t imsf_fmode; /* Filter-mode */
uint32_t imsf_numsrc; /* Number of sources in
the following array */
struct in_addr imsf_slist[1]; /* Array of source
addresses */
};
There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can be
used to specify the filtering mode. Additionally, the IP_MSFIL-
TER_SIZE(n) macro exists to determine how much memory is needed to
store ip_msfilter structure with n sources in the source list.
For the full description of multicast source filtering refer to
RFC 3376.
IP_MTU (since Linux 2.2)
Retrieve the current known path MTU of the current socket. Re-
turns an integer.
IP_MTU is valid only for getsockopt(2) and can be employed only
when the socket has been connected.
IP_MTU_DISCOVER (since Linux 2.2)
Set or receive the Path MTU Discovery setting for a socket.
When enabled, Linux will perform Path MTU Discovery as defined
in RFC 1191 on SOCK_STREAM sockets. For non-SOCK_STREAM sock-
ets, IP_PMTUDISC_DO forces the don't-fragment flag to be set on
all outgoing packets. It is the user's responsibility to packe-
tize the data in MTU-sized chunks and to do the retransmits if
necessary. The kernel will reject (with EMSGSIZE) datagrams
that are bigger than the known path MTU. IP_PMTUDISC_WANT will
fragment a datagram if needed according to the path MTU, or will
set the don't-fragment flag otherwise.
The system-wide default can be toggled between IP_PMTUDISC_WANT
and IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.
Path MTU discovery value Meaning
IP_PMTUDISC_WANT Use per-route settings.
IP_PMTUDISC_DONT Never do Path MTU Discovery.
IP_PMTUDISC_DO Always do Path MTU Discovery.
IP_PMTUDISC_PROBE Set DF but ignore Path MTU.
When PMTU discovery is enabled, the kernel automatically keeps
track of the path MTU per destination host. When it is con-
nected to a specific peer with connect(2), the currently known
path MTU can be retrieved conveniently using the IP_MTU socket
option (e.g., after an EMSGSIZE error occurred). The path MTU
may change over time. For connectionless sockets with many des-
tinations, the new MTU for a given destination can also be ac-
cessed using the error queue (see IP_RECVERR). A new error will
be queued for every incoming MTU update.
While MTU discovery is in progress, initial packets from data-
gram sockets may be dropped. Applications using UDP should be
aware of this and not take it into account for their packet re-
transmit strategy.
To bootstrap the path MTU discovery process on unconnected sock-
ets, it is possible to start with a big datagram size (headers
up to 64 kilobytes long) and let it shrink by updates of the
path MTU.
To get an initial estimate of the path MTU, connect a datagram
socket to the destination address using connect(2) and retrieve
the MTU by calling getsockopt(2) with the IP_MTU option.
It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
or SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE
(available since Linux 2.6.22). This is also particularly use-
ful for diagnostic tools such as tracepath(8) that wish to de-
liberately send probe packets larger than the observed Path MTU.
IP_MULTICAST_ALL (since Linux 2.6.31)
This option can be used to modify the delivery policy of multi-
cast messages to sockets bound to the wildcard INADDR_ANY ad-
dress. The argument is a boolean integer (defaults to 1). If
set to 1, the socket will receive messages from all the groups
that have been joined globally on the whole system. Otherwise,
it will deliver messages only from the groups that have been ex-
plicitly joined (for example via the IP_ADD_MEMBERSHIP option)
on this particular socket.
IP_MULTICAST_IF (since Linux 1.2)
Set the local device for a multicast socket. The argument for
setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq struc-
ture similar to IP_ADD_MEMBERSHIP, or an in_addr structure.
(The kernel determines which structure is being passed based on
the size passed in optlen.) For getsockopt(2), the argument is
an in_addr structure.
IP_MULTICAST_LOOP (since Linux 1.2)
Set or read a boolean integer argument that determines whether
sent multicast packets should be looped back to the local sock-
ets.
IP_MULTICAST_TTL (since Linux 1.2)
Set or read the time-to-live value of outgoing multicast packets
for this socket. It is very important for multicast packets to
set the smallest TTL possible. The default is 1 which means
that multicast packets don't leave the local network unless the
user program explicitly requests it. Argument is an integer.
IP_NODEFRAG (since Linux 2.6.36)
If enabled (argument is nonzero), the reassembly of outgoing
packets is disabled in the netfilter layer. The argument is an
integer.
This option is valid only for SOCK_RAW sockets.
IP_OPTIONS (since Linux 2.0)
Set or get the IP options to be sent with every packet from this
socket. The arguments are a pointer to a memory buffer contain-
ing the options and the option length. The setsockopt(2) call
sets the IP options associated with a socket. The maximum op-
tion size for IPv4 is 40 bytes. See RFC 791 for the allowed op-
tions. When the initial connection request packet for a
SOCK_STREAM socket contains IP options, the IP options will be
set automatically to the options from the initial packet with
routing headers reversed. Incoming packets are not allowed to
change options after the connection is established. The pro-
cessing of all incoming source routing options is disabled by
default and can be enabled by using the accept_source_route
/proc interface. Other options like timestamps are still han-
dled. For datagram sockets, IP options can be set only by the
local user. Calling getsockopt(2) with IP_OPTIONS puts the cur-
rent IP options used for sending into the supplied buffer.
IP_PASSSEC (since Linux 2.6.17)
If labeled IPSEC or NetLabel is configured on the sending and
receiving hosts, this option enables receiving of the security
context of the peer socket in an ancillary message of type
SCM_SECURITY retrieved using recvmsg(2). This option is sup-
ported only for UDP sockets; for TCP or SCTP sockets, see the
description of the SO_PEERSEC option below.
The value given as an argument to setsockopt(2) and returned as
the result of getsockopt(2) is an integer boolean flag.
The security context returned in the SCM_SECURITY ancillary mes-
sage is of the same format as the one described under the
SO_PEERSEC option below.
Note: the reuse of the SCM_SECURITY message type for the
IP_PASSSEC socket option was likely a mistake, since other IP
control messages use their own numbering scheme in the IP name-
space and often use the socket option value as the message type.
There is no conflict currently since the IP option with the same
value as SCM_SECURITY is IP_HDRINCL and this is never used for a
control message type.
IP_PKTINFO (since Linux 2.2)
Pass an IP_PKTINFO ancillary message that contains a pktinfo
structure that supplies some information about the incoming
packet. This works only for datagram oriented sockets. The ar-
gument is a flag that tells the socket whether the IP_PKTINFO
message should be passed or not. The message itself can be
sent/retrieved only as a control message with a packet using
recvmsg(2) or sendmsg(2).
struct in_pktinfo {
unsigned int ipi_ifindex; /* Interface index */
struct in_addr ipi_spec_dst; /* Local address */
struct in_addr ipi_addr; /* Header Destination
address */
};
ipi_ifindex is the unique index of the interface the packet was
received on. ipi_spec_dst is the local address of the packet
and ipi_addr is the destination address in the packet header.
If IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst is not
zero, then it is used as the local source address for the rout-
ing table lookup and for setting up IP source route options.
When ipi_ifindex is not zero, the primary local address of the
interface specified by the index overwrites ipi_spec_dst for the
routing table lookup.
IP_RECVERR (since Linux 2.2)
Enable extended reliable error message passing. When enabled on
a datagram socket, all generated errors will be queued in a per-
socket error queue. When the user receives an error from a
socket operation, the errors can be received by calling
recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_ex-
tended_err structure describing the error will be passed in an
ancillary message with the type IP_RECVERR and the level IP-
PROTO_IP. This is useful for reliable error handling on uncon-
nected sockets. The received data portion of the error queue
contains the error packet.
The IP_RECVERR control message contains a sock_extended_err
structure:
#define SO_EE_ORIGIN_NONE 0
#define SO_EE_ORIGIN_LOCAL 1
#define SO_EE_ORIGIN_ICMP 2
#define SO_EE_ORIGIN_ICMP6 3
struct sock_extended_err {
uint32_t ee_errno; /* error number */
uint8_t ee_origin; /* where the error originated */
uint8_t ee_type; /* type */
uint8_t ee_code; /* code */
uint8_t ee_pad;
uint32_t ee_info; /* additional information */
uint32_t ee_data; /* other data */
/* More data may follow */
};
struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
ee_errno contains the errno number of the queued error. ee_ori-
gin is the origin code of where the error originated. The other
fields are protocol-specific. The macro SO_EE_OFFENDER returns
a pointer to the address of the network object where the error
originated from given a pointer to the ancillary message. If
this address is not known, the sa_family member of the sockaddr
contains AF_UNSPEC and the other fields of the sockaddr are un-
defined.
IP uses the sock_extended_err structure as follows: ee_origin is
set to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet,
or SO_EE_ORIGIN_LOCAL for locally generated errors. Unknown
values should be ignored. ee_type and ee_code are set from the
type and code fields of the ICMP header. ee_info contains the
discovered MTU for EMSGSIZE errors. The message also contains
the sockaddr_in of the node caused the error, which can be ac-
cessed with the SO_EE_OFFENDER macro. The sin_family field of
the SO_EE_OFFENDER address is AF_UNSPEC when the source was un-
known. When the error originated from the network, all IP op-
tions (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and con-
tained in the error packet are passed as control messages. The
payload of the packet causing the error is returned as normal
payload. Note that TCP has no error queue; MSG_ERRQUEUE is not
permitted on SOCK_STREAM sockets. IP_RECVERR is valid for TCP,
but all errors are returned by socket function return or SO_ER-
ROR only.
For raw sockets, IP_RECVERR enables passing of all received ICMP
errors to the application, otherwise errors are reported only on
connected sockets
It sets or retrieves an integer boolean flag. IP_RECVERR de-
faults to off.
IP_RECVOPTS (since Linux 2.2)
Pass all incoming IP options to the user in a IP_OPTIONS control
message. The routing header and other options are already
filled in for the local host. Not supported for SOCK_STREAM
sockets.
IP_RECVORIGDSTADDR (since Linux 2.6.29)
This boolean option enables the IP_ORIGDSTADDR ancillary message
in recvmsg(2), in which the kernel returns the original destina-
tion address of the datagram being received. The ancillary mes-
sage contains a struct sockaddr_in.
IP_RECVTOS (since Linux 2.2)
If enabled, the IP_TOS ancillary message is passed with incoming
packets. It contains a byte which specifies the Type of Ser-
vice/Precedence field of the packet header. Expects a boolean
integer flag.
IP_RECVTTL (since Linux 2.2)
When this flag is set, pass a IP_TTL control message with the
time-to-live field of the received packet as a 32 bit integer.
Not supported for SOCK_STREAM sockets.
IP_RETOPTS (since Linux 2.2)
Identical to IP_RECVOPTS, but returns raw unprocessed options
with timestamp and route record options not filled in for this
hop.
IP_ROUTER_ALERT (since Linux 2.2)
Pass all to-be forwarded packets with the IP Router Alert option
set to this socket. Valid only for raw sockets. This is use-
ful, for instance, for user-space RSVP daemons. The tapped
packets are not forwarded by the kernel; it is the user's re-
sponsibility to send them out again. Socket binding is ignored,
such packets are filtered only by protocol. Expects an integer
flag.
IP_TOS (since Linux 1.0)
Set or receive the Type-Of-Service (TOS) field that is sent with
every IP packet originating from this socket. It is used to
prioritize packets on the network. TOS is a byte. There are
some standard TOS flags defined: IPTOS_LOWDELAY to minimize de-
lays for interactive traffic, IPTOS_THROUGHPUT to optimize
throughput, IPTOS_RELIABILITY to optimize for reliability, IP-
TOS_MINCOST should be used for "filler data" where slow trans-
mission doesn't matter. At most one of these TOS values can be
specified. Other bits are invalid and shall be cleared. Linux
sends IPTOS_LOWDELAY datagrams first by default, but the exact
behavior depends on the configured queueing discipline. Some
high-priority levels may require superuser privileges (the
CAP_NET_ADMIN capability).
IP_TRANSPARENT (since Linux 2.6.24)
Setting this boolean option enables transparent proxying on this
socket. This socket option allows the calling application to
bind to a nonlocal IP address and operate both as a client and a
server with the foreign address as the local endpoint. NOTE:
this requires that routing be set up in a way that packets going
to the foreign address are routed through the TProxy box (i.e.,
the system hosting the application that employs the IP_TRANSPAR-
ENT socket option). Enabling this socket option requires supe-
ruser privileges (the CAP_NET_ADMIN capability).
TProxy redirection with the iptables TPROXY target also requires
that this option be set on the redirected socket.
IP_TTL (since Linux 1.0)
Set or retrieve the current time-to-live field that is used in
every packet sent from this socket.
IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
Unblock previously blocked multicast source. Returns EADDRNO-
TAVAIL when given source is not being blocked.
Argument is an ip_mreq_source structure as described under
IP_ADD_SOURCE_MEMBERSHIP.
SO_PEERSEC (since Linux 2.6.17)
If labeled IPSEC or NetLabel is configured on both the sending
and receiving hosts, this read-only socket option returns the
security context of the peer socket connected to this socket.
By default, this will be the same as the security context of the
process that created the peer socket unless overridden by the
policy or by a process with the required permissions.
The argument to getsockopt(2) is a pointer to a buffer of the
specified length in bytes into which the security context string
will be copied. If the buffer length is less than the length of
the security context string, then getsockopt(2) returns -1, sets
errno to ERANGE, and returns the required length via optlen.
The caller should allocate at least NAME_MAX bytes for the buf-
fer initially, although this is not guaranteed to be sufficient.
Resizing the buffer to the returned length and retrying may be
necessary.
The security context string may include a terminating null char-
acter in the returned length, but is not guaranteed to do so: a
security context "foo" might be represented as either
{'f','o','o'} of length 3 or {'f','o','o','\0'} of length 4,
which are considered to be interchangeable. The string is
printable, does not contain non-terminating null characters, and
is in an unspecified encoding (in particular, it is not guaran-
teed to be ASCII or UTF-8).
The use of this option for sockets in the AF_INET address family
is supported since Linux 2.6.17 for TCP sockets, and since Linux
4.17 for SCTP sockets.
For SELinux, NetLabel conveys only the MLS portion of the secu-
rity context of the peer across the wire, defaulting the rest of
the security context to the values defined in the policy for the
netmsg initial security identifier (SID). However, NetLabel can
be configured to pass full security contexts over loopback. La-
beled IPSEC always passes full security contexts as part of es-
tablishing the security association (SA) and looks them up based
on the association for each packet.
/proc interfaces
The IP protocol supports a set of /proc interfaces to configure some
global parameters. The parameters can be accessed by reading or writ-
ing files in the directory /proc/sys/net/ipv4/. Interfaces described
as Boolean take an integer value, with a nonzero value ("true") meaning
that the corresponding option is enabled, and a zero value ("false")
meaning that the option is disabled.
ip_always_defrag (Boolean; since Linux 2.2.13)
[New with Linux 2.2.13; in earlier kernel versions this feature
was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG
option; this option is not present in Linux 2.4.x and later]
When this boolean flag is enabled (not equal 0), incoming frag-
ments (parts of IP packets that arose when some host between
origin and destination decided that the packets were too large
and cut them into pieces) will be reassembled (defragmented) be-
fore being processed, even if they are about to be forwarded.
Enable only if running either a firewall that is the sole link
to your network or a transparent proxy; never ever use it for a
normal router or host. Otherwise, fragmented communication can
be disturbed if the fragments travel over different links. De-
fragmentation also has a large memory and CPU time cost.
This is automagically turned on when masquerading or transparent
proxying are configured.
ip_autoconfig (since Linux 2.2 to Linux 2.6.17)
Not documented.
ip_default_ttl (integer; default: 64; since Linux 2.2)
Set the default time-to-live value of outgoing packets. This
can be changed per socket with the IP_TTL option.
ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
Enable dynamic socket address and masquerading entry rewriting
on interface address change. This is useful for dialup inter-
face with changing IP addresses. 0 means no rewriting, 1 turns
it on and 2 enables verbose mode.
ip_forward (Boolean; default: disabled; since Linux 1.2)
Enable IP forwarding with a boolean flag. IP forwarding can be
also set on a per-interface basis.
ip_local_port_range (since Linux 2.2)
This file contains two integers that define the default local
port range allocated to sockets that are not explicitly bound to
a port number—that is, the range used for ephemeral ports. An
ephemeral port is allocated to a socket in the following circum-
stances:
• the port number in a socket address is specified as 0 when
calling bind(2);
• listen(2) is called on a stream socket that was not previ-
ously bound;
• connect(2) was called on a socket that was not previously
bound;
• sendto(2) is called on a datagram socket that was not previ-
ously bound.
Allocation of ephemeral ports starts with the first number in
ip_local_port_range and ends with the second number. If the
range of ephemeral ports is exhausted, then the relevant system
call returns an error (but see BUGS).
Note that the port range in ip_local_port_range should not con-
flict with the ports used by masquerading (although the case is
handled). Also, arbitrary choices may cause problems with some
firewall packet filters that make assumptions about the local
ports in use. The first number should be at least greater than
1024, or better, greater than 4096, to avoid clashes with well
known ports and to minimize firewall problems.
ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
If enabled, don't do Path MTU Discovery for TCP sockets by de-
fault. Path MTU discovery may fail if misconfigured firewalls
(that drop all ICMP packets) or misconfigured interfaces (e.g.,
a point-to-point link where the both ends don't agree on the
MTU) are on the path. It is better to fix the broken routers on
the path than to turn off Path MTU Discovery globally, because
not doing it incurs a high cost to the network.
ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
If set, allows processes to bind(2) to nonlocal IP addresses,
which can be quite useful, but may break some applications.
ip6frag_time (integer; default: 30)
Time in seconds to keep an IPv6 fragment in memory.
ip6frag_secret_interval (integer; default: 600)
Regeneration interval (in seconds) of the hash secret (or life-
time for the hash secret) for IPv6 fragments.
ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
If the amount of queued IP fragments reaches ipfrag_high_thresh,
the queue is pruned down to ipfrag_low_thresh. Contains an in-
teger with the number of bytes.
neigh/*
See arp(7).
Ioctls
All ioctls described in socket(7) apply to ip.
Ioctls to configure generic device parameters are described in netde-
vice(7).
ERRORS
EACCES The user tried to execute an operation without the necessary
permissions. These include: sending a packet to a broadcast ad-
dress without having the SO_BROADCAST flag set; sending a packet
via a prohibit route; modifying firewall settings without supe-
ruser privileges (the CAP_NET_ADMIN capability); binding to a
privileged port without superuser privileges (the
CAP_NET_BIND_SERVICE capability).
EADDRINUSE
Tried to bind to an address already in use.
EADDRNOTAVAIL
A nonexistent interface was requested or the requested source
address was not local.
EAGAIN Operation on a nonblocking socket would block.
EALREADY
A connection operation on a nonblocking socket is already in
progress.
ECONNABORTED
A connection was closed during an accept(2).
EHOSTUNREACH
No valid routing table entry matches the destination address.
This error can be caused by an ICMP message from a remote router
or for the local routing table.
EINVAL Invalid argument passed. For send operations this can be caused
by sending to a blackhole route.
EISCONN
connect(2) was called on an already connected socket.
EMSGSIZE
Datagram is bigger than an MTU on the path and it cannot be
fragmented.
ENOBUFS, ENOMEM
Not enough free memory. This often means that the memory allo-
cation is limited by the socket buffer limits, not by the system
memory, but this is not 100% consistent.
ENOENT SIOCGSTAMP was called on a socket where no packet arrived.
ENOPKG A kernel subsystem was not configured.
ENOPROTOOPT and EOPNOTSUPP
Invalid socket option passed.
ENOTCONN
The operation is defined only on a connected socket, but the
socket wasn't connected.
EPERM User doesn't have permission to set high priority, change con-
figuration, or send signals to the requested process or group.
EPIPE The connection was unexpectedly closed or shut down by the other
end.
ESOCKTNOSUPPORT
The socket is not configured or an unknown socket type was re-
quested.
Other errors may be generated by the overlaying protocols; see tcp(7),
raw(7), udp(7), and socket(7).
NOTES
IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER, IP_RECVORIGDSTADDR,
IP_PASSSEC, IP_PKTINFO, IP_RECVERR, IP_ROUTER_ALERT, and IP_TRANSPARENT
are Linux-specific.
Be very careful with the SO_BROADCAST option - it is not privileged in
Linux. It is easy to overload the network with careless broadcasts.
For new application protocols it is better to use a multicast group in-
stead of broadcasting. Broadcasting is discouraged.
Some other BSD sockets implementations provide IP_RCVDSTADDR and
IP_RECVIF socket options to get the destination address and the inter-
face of received datagrams. Linux has the more general IP_PKTINFO for
the same task.
Some BSD sockets implementations also provide an IP_RECVTTL option, but
an ancillary message with type IP_RECVTTL is passed with the incoming
packet. This is different from the IP_TTL option used in Linux.
Using the SOL_IP socket options level isn't portable; BSD-based stacks
use the IPPROTO_IP level.
INADDR_ANY (0.0.0.0) and INADDR_BROADCAST (255.255.255.255) are byte-
order-neutral. This means htonl(3) has no effect on them.
Compatibility
For compatibility with Linux 2.0, the obsolete socket(AF_INET,
SOCK_PACKET, protocol) syntax is still supported to open a packet(7)
socket. This is deprecated and should be replaced by socket(AF_PACKET,
SOCK_RAW, protocol) instead. The main difference is the new sock-
addr_ll address structure for generic link layer information instead of
the old sockaddr_pkt.
BUGS
There are too many inconsistent error values.
The error used to diagnose exhaustion of the ephemeral port range dif-
fers across the various system calls (connect(2), bind(2), listen(2),
sendto(2)) that can assign ephemeral ports.
The ioctls to configure IP-specific interface options and ARP tables
are not described.
Receiving the original destination address with MSG_ERRQUEUE in
msg_name by recvmsg(2) does not work in some Linux 2.2 kernels.
SEE ALSO
recvmsg(2), sendmsg(2), byteorder(3), capabilities(7), icmp(7),
ipv6(7), netdevice(7), netlink(7), raw(7), socket(7), tcp(7), udp(7),
ip(8)
The kernel source file Documentation/networking/ip-sysctl.txt.
RFC 791 for the original IP specification. RFC 1122 for the IPv4 host
requirements. RFC 1812 for the IPv4 router requirements.
Linux man-pages 6.03 2023-02-05 ip(7)
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