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NAME | SYNOPSIS | DESCRIPTION | ERRORS | NOTES | BUGS | SEE ALSO | COLOPHON |
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ip(7) Miscellaneous Information Manual ip(7)
ip - Linux IPv4 protocol implementation
#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);
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).
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
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 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 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) 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
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) 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) 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) and
setsockopt(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 application 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_interface 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 automatically chosen at connect(2) 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 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 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 ip_nonlocal_bind /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) 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 ip_local_port_range /proc interface
described below, for a given socket.
The option takes an uint32_t 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 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_MSFILTER_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.
Returns 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 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/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 connected 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 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)
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 useful for diagnostic tools such as
tracepath(8) 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) is an ip_mreqn or (since Linux 3.5)
ip_mreq structure 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 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) 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
accept_source_route /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) 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). 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) and
returned as the result of getsockopt(2) 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) 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 routing 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.
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) with the MSG_ERRQUEUE flag
set. The sock_extended_err 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 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_origin 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
undefined.
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
accessed with the SO_EE_OFFENDER macro. The sin_family
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), in which the kernel returns the
original destination address of the datagram being
received. The ancillary message contains a struct
sockaddr_in. 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 or CAP_NET_RAW 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 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 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.
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
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.
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 interface 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 circumstances:
• 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
previously bound;
• connect(2) was called on a socket that was not
previously bound;
• sendto(2) is called on a datagram socket that was not
previously 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
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.
ip_no_pmtu_disc (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.
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
lifetime 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 integer 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
netdevice(7).
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).
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
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), raw(7), udp(7), and socket(7).
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) 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 sockaddr_ll address structure for generic
link layer information instead of the old sockaddr_pkt.
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), 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.
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.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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Linux man-pages 6.15 2025-05-17 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)
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