ip(7) - Linux manual page (original) (raw)
ip(7) Miscellaneous Information Manual ip(7)
NAME top
ip - Linux IPv4 protocol implementationSYNOPSIS top
**#include <sys/socket.h>**
**#include <netinet/in.h>**
**#include <netinet/ip.h>** /* superset of previous */
_tcpsocket_ **= socket(AF_INET, SOCK_STREAM, 0);**
_udpsocket_ **= socket(AF_INET, SOCK_DGRAM, 0);**
_rawsocket_ **= socket(AF_INET, SOCK_RAW,** _protocol_**);**DESCRIPTION top
Linux implements the Internet Protocol, version 4, described in
RFC 791 and RFC 1122. **ip** contains a level 2 multicasting
implementation conforming to RFC 1112. It also contains an IP
router including a packet filter.
The programming interface is BSD-sockets compatible. For more
information on sockets, see [socket(7)](../man7/socket.7.html).
An IP socket is created using [socket(2)](../man2/socket.2.html):
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)](../man7/raw.7.html) 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)](../man7/tcp.7.html) stream sockets;
• 0 and **IPPROTO_UDP** for [udp(7)](../man7/udp.7.html) datagram sockets;
• **IPPROTO_SCTP** for [sctp(7)](../man7/sctp.7.html) stream sockets; and
• **IPPROTO_UDPLITE** for [udplite(7)](../man7/udplite.7.html) 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)](../man2/bind.2.html). 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)](../man2/listen.2.html) 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)](../man2/connect.2.html) 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
**INADDR_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 address 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 sinport 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 */
};
_sinfamily_ is always set to **AF_INET**. This is required; in Linux
2.2 most networking functions return **EINVAL** when this setting is
missing. _sinport_ contains the port in network byte order. The
port numbers below 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)](../man2/bind.2.html) 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)](../man7/tcp.7.html) and
[udp(7)](../man7/udp.7.html).
_sinaddr_ is the IP host address. The _saddr_ member of _struct_
_inaddr_ contains the host interface address in network byte order.
_inaddr_ should be assigned one of the **INADDR_*** values (e.g.,
**INADDR_LOOPBACK**) using [htonl(3)](../man3/htonl.3.html) or set using the [inet_aton(3)](../man3/inet%5Faton.3.html),
[inet_addr(3)](../man3/inet%5Faddr.3.html), [inet_makeaddr(3)](../man3/inet%5Fmakeaddr.3.html) library functions or directly with
the name resolver (see [gethostbyname(3)](../man3/gethostbyname.3.html)).
IPv4 addresses are divided into unicast, broadcast, and multicast
addresses. Unicast addresses specify a single interface of a
host, broadcast addresses specify all hosts on a network, and
multicast addresses 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 sockets 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)](../man3/htons.3.html) on the number that is assigned to a port. All
address/port manipulation functions in the standard library work
in network byte order.Special and reserved addresses 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 socket binding;
**INADDR_BROADCAST** (255.255.255.255)
has the same effect on [bind(2)](../man2/bind.2.html) as **INADDR_ANY** for historical
reasons. A packet addressed to **INADDR_BROADCAST** through a
socket which has **SO_BROADCAST** set will be broadcast to all
hosts on the local network segment, as long as the link is
broadcast-capable.
Highest-numbered address
Lowest-numbered address
On any locally-attached non-point-to-point IP subnet with a
link type that supports broadcasts, the highest-numbered
address (e.g., the .255 address on a subnet with netmask
255.255.255.0) is designated as a broadcast address. It
cannot usefully be assigned to an individual interface, and
can only be addressed with a socket on which the
**SO_BROADCAST** option has been set. Internet standards have
historically also reserved the lowest-numbered address
(e.g., the .0 address on a subnet with netmask
255.255.255.0) for broadcast, though they call it
"obsolete" for this purpose. (Some sources also refer to
this as the "network address.") Since Linux 5.14, it is
treated as an ordinary unicast address and can be assigned
to an interface.
Internet standards have traditionally also reserved various
addresses for particular uses, though Linux no longer treats some
of these specially.
[0.0.0.1, 0.255.255.255]
[240.0.0.0, 255.255.255.254]
Addresses in these ranges (0/8 and 240/4) are reserved
globally. Since Linux 5.3 and Linux 2.6.25, respectively,
the 0/8 and 240/4 addresses, other than **INADDR_ANY** and
**INADDR_BROADCAST**, are treated as ordinary unicast
addresses. Systems that follow the traditional behaviors
may not interoperate with these historically reserved
addresses.
[127.0.0.1, 127.255.255.254]
Addresses in this range (127/8) are treated as loopback
addresses akin to the standardized local loopback address
**INADDR_LOOPBACK** (127.0.0.1);
[224.0.0.0, 239.255.255.255]
Addresses in this range (224/4) are dedicated to multicast
use.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)](../man2/getsockopt.2.html) and
[setsockopt(2)](../man2/setsockopt.2.html) fail with the error **ENOPROTOOPT**.
**IP_ADD_MEMBERSHIP** (since Linux 1.2)
Join a multicast group. Argument is an _ipmreqn_ 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 */
};
_imrmultiaddr_ contains the address of the multicast group
the application wants to join or leave. It must be a valid
multicast address (or [setsockopt(2)](../man2/setsockopt.2.html) fails with the error
**EINVAL**). _imraddress_ 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. _imrifindex_ is the interface index
of the interface that should join/leave the _imrmultiaddr_
group, or 0 to indicate any interface.
The _ipmreqn_ structure is available only since Linux 2.2.
For compatibility, the old _ipmreq_ structure (present since
Linux 1.2) is still supported; it differs from _ipmreqn_
only by not including the _imrifindex_ 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)](../man2/setsockopt.2.html).
**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 _ipmreqsource_ 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 _ipmreqsource_ structure is similar to _ipmreqn_
described under **IP_ADD_MEMBERSHIP**. The _imrmultiaddr_ field
contains the address of the multicast group the application
wants to join or leave. The _imrinterface_ field is the
address of the local interface with which the system should
join the multicast group. Finally, the _imrsourceaddr_
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)](../man2/bind.2.html) with a port number of 0. The port will later
be automatically chosen at [connect(2)](../man2/connect.2.html) time, in a way that
allows sharing 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 _ipmreqsource_ structure as described under
**IP_ADD_SOURCE_MEMBERSHIP**.
**IP_DROP_MEMBERSHIP** (since Linux 1.2)
Leave a multicast group. Argument is an _ipmreqn_ or
_ipmreq_ 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 _ipmreqsource_ 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 listening on a socket, without requiring the
underlying network interface 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 _ipnonlocalbind /proc_ interface 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)](../man7/raw.7.html) for
more information. When this flag is enabled, the values
set by **IP_OPTIONS**, **IP_TTL**, and **IP_TOS** are ignored.
**IP_LOCAL_PORT_RANGE** (since Linux 6.3)
Set or get the per-socket default local port range. This
option can be used to clamp down the global local port
range, defined by the _iplocalportrange /proc_ interface
described below, for a given socket.
The option takes an _uint32t_ value with the high 16 bits
set to the upper range bound, and the low 16 bits set to
the lower range bound. Range bounds are inclusive. The
16-bit values should be in host byte order.
The lower bound has to be less than the upper bound when
both bounds are not zero. Otherwise, setting the option
fails with EINVAL.
If either bound is outside of the global local port range,
or is zero, then that bound has no effect.
To reset the setting, pass zero as both the upper and the
lower bound.
**IP_MSFILTER** (since Linux 2.4.22 / 2.5.68)
This option provides access to the advanced full-state
filtering API. Argument is an _ipmsfilter_ 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_MSFILTER_SIZE**(n) macro exists to
determine how much memory is needed to store _ipmsfilter_
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.
Returns an integer.
**IP_MTU** is valid only for [getsockopt(2)](../man2/getsockopt.2.html) 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** sockets, **IP_PMTUDISC_DO** forces the don't-
fragment flag to be set on all outgoing packets. It is the
user's responsibility to packetize 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/ipnopmtudisc_ 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 connected to a specific peer with [connect(2)](../man2/connect.2.html), 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 destinations, the new MTU
for a given destination can also be accessed 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
datagram sockets may be dropped. Applications using UDP
should be aware of this and not take it into account for
their packet retransmit strategy.
To bootstrap the path MTU discovery process on unconnected
sockets, 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)](../man2/connect.2.html)
and retrieve the MTU by calling [getsockopt(2)](../man2/getsockopt.2.html) 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 useful for diagnostic tools such as
[tracepath(8)](../man8/tracepath.8.html) that wish to deliberately 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
multicast messages. 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 explicitly 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)](../man2/setsockopt.2.html) is an _ipmreqn_ or (since Linux 3.5)
_ipmreq_ structure similar to **IP_ADD_MEMBERSHIP**, or an
_inaddr_ structure. (The kernel determines which structure
is being passed based on the size passed in _optlen_.) For
[getsockopt(2)](../man2/getsockopt.2.html), the argument is an _inaddr_ 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 sockets.
**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 containing the options and the option length. The
[setsockopt(2)](../man2/setsockopt.2.html) call sets the IP options associated with a
socket. The maximum option size for IPv4 is 40 bytes. See
RFC 791 for the allowed options. 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
processing of all incoming source routing options is
disabled by default and can be enabled by using the
_acceptsourceroute /proc_ interface. Other options like
timestamps are still handled. For datagram sockets, IP
options can be set only by the local user. Calling
[getsockopt(2)](../man2/getsockopt.2.html) with **IP_OPTIONS** puts the current 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)](../man2/recvmsg.2.html). This
option is supported 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)](../man2/setsockopt.2.html) and
returned as the result of [getsockopt(2)](../man2/getsockopt.2.html) is an integer
boolean flag.
The security context returned in the **SCM_SECURITY** ancillary
message 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 namespace 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 argument 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)](../man2/recvmsg.2.html) or [sendmsg(2)](../man2/sendmsg.2.html).
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 */
};
_ipiifindex_ is the unique index of the interface the packet
was received on. _ipispecdst_ is the local address of the
packet and _ipiaddr_ is the destination address in the
packet header. If **IP_PKTINFO** is passed to [sendmsg(2)](../man2/sendmsg.2.html) and
_ipispecdst_ is not zero, then it is used as the local
source address for the routing table lookup and for setting
up IP source route options. When _ipiifindex_ is not zero,
the primary local address of the interface specified by the
index overwrites _ipispecdst_ for the routing table lookup.
Not supported for **SOCK_STREAM** sockets.
**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)](../man2/recvmsg.2.html) with the **MSG_ERRQUEUE** flag
set. The _sockextendederr_ structure describing the error
will be passed in an ancillary message with the type
**IP_RECVERR** and the level **IPPROTO_IP**. This is useful for
reliable error handling on unconnected sockets. The
received data portion of the error queue contains the error
packet.
The **IP_RECVERR** control message contains a _sockextendederr_
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 *);
_eeerrno_ contains the _[errno](../man3/errno.3.html)_ number of the queued error.
_eeorigin_ 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 _safamily_ member of the _sockaddr_ contains
**AF_UNSPEC** and the other fields of the _sockaddr_ are
undefined.
IP uses the _sockextendederr_ structure as follows:
_eeorigin_ 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.
_eetype_ and _eecode_ are set from the type and code fields
of the ICMP header. _eeinfo_ contains the discovered MTU
for **EMSGSIZE** errors. The message also contains the
_sockaddrin of the node_ caused the error, which can be
accessed with the **SO_EE_OFFENDER** macro. The _sinfamily_
field of the **SO_EE_OFFENDER** address is **AF_UNSPEC** when the
source was unknown. When the error originated from the
network, all IP options (**IP_OPTIONS**, **IP_TTL**, etc.) enabled
on the socket and contained 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_ERROR** 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**
defaults 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)](../man2/recvmsg.2.html), in which the kernel returns the
original destination address of the datagram being
received. The ancillary message contains a _struct_
_sockaddrin_. Not supported for **SOCK_STREAM** sockets.
**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 Service/Precedence field of the packet header.
Expects a boolean integer flag. Not supported for
**SOCK_STREAM** sockets.
**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. Not supported for **SOCK_STREAM** sockets.
**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 useful, for instance, for user-space RSVP daemons.
The tapped packets are not forwarded by the kernel; it is
the user's responsibility 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 delays for interactive traffic,
**IPTOS_THROUGHPUT** to optimize throughput, **IPTOS_RELIABILITY**
to optimize for reliability, **IPTOS_MINCOST** should be used
for "filler data" where slow transmission 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_TRANSPARENT** socket
option). Enabling this socket option requires superuser
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
**EADDRNOTAVAIL** when given source is not being blocked.
Argument is an _ipmreqsource_ 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)](../man2/getsockopt.2.html) 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)](../man2/getsockopt.2.html) returns -1, sets _[errno](../man3/errno.3.html)_ to **ERANGE**, and returns
the required length via _optlen_. The caller should allocate
at least **NAME_MAX** bytes for the buffer 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
character 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 guaranteed 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
security 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. Labeled IPSEC always
passes full security contexts as part of establishing 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 writing 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.
_ipalwaysdefrag_ (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
fragments (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) before 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. Defragmentation also has a large memory
and CPU time cost.
This is automagically turned on when masquerading or
transparent proxying are configured.
_ipautoconfig_ (since Linux 2.2 to Linux 2.6.17)
Not documented.
_ipdefaultttl_ (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.
_ipdynaddr_ (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 interface with changing IP addresses. 0 means no
rewriting, 1 turns it on and 2 enables verbose mode.
_ipforward_ (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.
_iplocalportrange_ (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 circumstances:
• the port number in a socket address is specified as 0
when calling [bind(2)](../man2/bind.2.html);
• [listen(2)](../man2/listen.2.html) is called on a stream socket that was not
previously bound;
• [connect(2)](../man2/connect.2.html) was called on a socket that was not
previously bound;
• [sendto(2)](../man2/sendto.2.html) is called on a datagram socket that was not
previously bound.
Allocation of ephemeral ports starts with the first number
in _iplocalportrange_ 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 _iplocalportrange_ should not
conflict 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.
_ipnopmtudisc_ (Boolean; default: disabled; since Linux 2.2)
If enabled, don't do Path MTU Discovery for TCP sockets by
default. 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.
_ipnonlocalbind_ (Boolean; default: disabled; since Linux 2.4)
If set, allows processes to [bind(2)](../man2/bind.2.html) to nonlocal IP
addresses, which can be quite useful, but may break some
applications.
_ip6fragtime_ (integer; default: 30)
Time in seconds to keep an IPv6 fragment in memory.
_ip6fragsecretinterval_ (integer; default: 600)
Regeneration interval (in seconds) of the hash secret (or
lifetime for the hash secret) for IPv6 fragments.
_ipfraghighthresh_ (integer)
_ipfraglowthresh_ (integer)
If the amount of queued IP fragments reaches
_ipfraghighthresh_, the queue is pruned down to
_ipfraglowthresh_. Contains an integer with the number of
bytes.
_neigh/*_
See [arp(7)](../man7/arp.7.html).Ioctls All ioctls described in socket(7) apply to ip.
Ioctls to configure generic device parameters are described in
[netdevice(7)](../man7/netdevice.7.html).ERRORS top
**EACCES** The user tried to execute an operation without the
necessary permissions. These include: sending a packet to
a broadcast address without having the **SO_BROADCAST** flag
set; sending a packet via a _prohibit_ route; modifying
firewall settings without superuser 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)](../man2/accept.2.html).
**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)](../man2/connect.2.html) 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
allocation 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**
**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
configuration, 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
requested.
Other errors may be generated by the overlaying protocols; see
[tcp(7)](../man7/tcp.7.html), [raw(7)](../man7/raw.7.html), [udp(7)](../man7/udp.7.html), and [socket(7)](../man7/socket.7.html).NOTES top
**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 instead of broadcasting. Broadcasting is
discouraged. See RFC 6762 for an example of a protocol (mDNS)
using the more modern multicast approach to communicating with an
open-ended group of hosts on the local network.
Some other BSD sockets implementations provide **IP_RCVDSTADDR** and
**IP_RECVIF** socket options to get the destination address and the
interface 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)](../man3/htonl.3.html) 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 sockaddrll address structure for generic link layer information instead of the old sockaddr_pkt.
BUGS top
There are too many inconsistent error values.
The error used to diagnose exhaustion of the ephemeral port range
differs across the various system calls ([connect(2)](../man2/connect.2.html), [bind(2)](../man2/bind.2.html),
[listen(2)](../man2/listen.2.html), [sendto(2)](../man2/sendto.2.html)) 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
_msgname_ by [recvmsg(2)](../man2/recvmsg.2.html) does not work in some Linux 2.2 kernels.SEE ALSO top
[recvmsg(2)](../man2/recvmsg.2.html), [sendmsg(2)](../man2/sendmsg.2.html), [byteorder(3)](../man3/byteorder.3.html), [capabilities(7)](../man7/capabilities.7.html), [icmp(7)](../man7/icmp.7.html),
[ipv6(7)](../man7/ipv6.7.html), [netdevice(7)](../man7/netdevice.7.html), [netlink(7)](../man7/netlink.7.html), [raw(7)](../man7/raw.7.html), [socket(7)](../man7/socket.7.html), [tcp(7)](../man7/tcp.7.html),
[udp(7)](../man7/udp.7.html), [ip(8)](../man8/ip.8.html)
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.COLOPHON top
This page is part of the _man-pages_ (Linux kernel and C library
user-space interface documentation) project. Information about
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part of the original manual page), send a mail to
man-pages@man7.orgLinux man-pages 6.10 2024-06-28 ip(7)
Pages that refer to this page:bind(2), connect(2), getpeername(2), getsockname(2), getsockopt(2), listen(2), recv(2), send(2), socket(2), getaddrinfo(3), getaddrinfo_a(3), sctp_connectx(3), sd_is_fifo(3), proc_sys_net(5), systemd.socket(5), address_families(7), arp(7), icmp(7), ipv6(7), netdevice(7), packet(7), raw(7), rtnetlink(7), sctp(7), socket(7), tcp(7), udp(7), udplite(7)