ip (7)
NAME
ip - Linux IPv4 protocol implementation
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
tcp_socket = socket(PF_INET, SOCK_STREAM, 0);
raw_socket = socket(PF_INET, SOCK_RAW, protocol);
udp_socket = socket(PF_INET, SOCK_DGRAM, protocol);
DESCRIPTION
Linux implements the Internet Protocol, version 4, described in RFC791
and RFC1122. ip contains a level 2 multicasting implementation con-
forming to RFC1112. It also contains an IP router including a packet
filter.
The programmer's interface is BSD sockets compatible. For more infor-
mation on sockets, see socket(7).
An IP socket is created by calling the socket(2) function as
socket(PF_INET, socket_type, protocol). Valid socket types are
SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM to open a udp(7)
socket, or 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. The only valid values for protocol are 0 and IPPROTO_TCP for
TCP sockets and 0 and IPPROTO_UDP for UDP sockets. For SOCK_RAW you
may specify a valid IANA IP protocol defined in RFC1700 assigned num-
bers.
When a process wants to receive new incoming packets or connections, it
should bind a socket to a local interface address using bind(2). 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) or connect(2) are called on a
unbound socket the socket is automatically bound to a random free 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 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 */
u_int16_t sin_port; /* port in network byte order */
struct in_addr sin_addr; /* internet address */
};
below 1024 are called reserved ports. Only processes with effective
user id 0 or the CAP_NET_BIND_SERVICE capability may bind(2) to these
sockets. Note that the raw IPv4 protocol as such has no concept of a
port, they are only implemented by higher protocols like tcp(7) and
udp(7).
sin_addr is the IP host address. The addr member of struct in_addr
contains the host interface address in network order. in_addr should
be only accessed using the inet_aton(3), inet_addr(3), inet_makeaddr(3)
library functions or directly with the name resolver (see gethostby-
name(3)). IPv4 addresses are divided into unicast, broadcast and mul-
ticast 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 broad-
cast addresses can be only sent or received when the SO_BROADCAST
socket flag is set. In the current implementation connection oriented
sockets are only allowed to use unicast addresses.
Note that the address and the port are always stored in network 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 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 SOL_IP. A boolean integer flag is zero when it is false, other-
wise true.
IP_OPTIONS
Sets 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 RFC791 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 pro-
cessing of all incoming source routing options is disabled by
default and can be enabled by using the accept_source_route
sysctl. Other options like timestamps are still handled. For
datagram sockets, IP options can be only set by the local user.
Calling getsockopt(2) with IP_OPTIONS puts the current IP
options used for sending into the supplied buffer.
IP_PKTINFO
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) then the outgoing packet
will be sent over the interface specified in ipi_ifindex with
the destination address set to ipi_spec_dst
IP_RECVTOS
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
When this flag is set pass a IP_RECVTTL control message with the
time to live field of the received packet as a byte. Not sup-
ported for SOCK_STREAM sockets.
IP_RECVOPTS
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_RETOPTS
Identical to IP_RECVOPTS but returns raw unprocessed options
with timestamp and route record options not filled in for this
hop.
IP_TOS Set or receive the Type-Of-Service (TOS) field that is sent with
every IP packet originating from this socket. It is used to pri-
oritize 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 through-
put, IPTOS_RELIABILITY to optimize for reliability, IPTOS_MIN-
COST should be used for "filler data" where slow transmission
doesn't matter. At most one of these TOS values can be speci-
fied. Other bits are invalid and shall be cleared. Linux sends
IPTOS_LOWDELAY datagrams first by default, but the exact
behaviour depends on the configured queueing discipline. Some
high priority levels may require an effective user id of 0 or
the CAP_NET_ADMIN capability. The priority can also be set in a
protocol independent way by the ( SOL_SOCKET, SO_PRIORITY )
socket option (see socket(7) ).
IP_TTL Set or retrieve the current time to live field that is send in
IP_RECVERR (defined in <linux/errqueue.h>)
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 a ancillary
message with the type IP_RECVERR and the level SOL_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 {
u_int32_t ee_errno; /* error number */
u_int8_t ee_origin; /* where the error originated */
u_int8_t ee_type; /* type */
u_int8_t ee_code; /* code */
u_int8_t ee_pad;
u_int32_t ee_info; /* additional information */
u_int32_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
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 val-
ues 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 nor-
mal payload. Note that TCP has no error queue; MSG_ERRQUEUE is
IP_PMTU_DISCOVER
Sets or receives the Path MTU Discovery setting for a socket.
When enabled, Linux will perform Path MTU Discovery as defined
in RFC1191 on this socket. The don't fragment flag is set on all
outgoing datagrams. The system-wide default is controlled by
the ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and disabled
on all others. For non SOCK_STREAM sockets 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 packets
that are bigger than the known path MTU if this flag is set
(with EMSGSIZE ).
Path MTU discovery flags Meaning
IP_PMTUDISC_WANT Use per-route settings.
IP_PMTUDISC_DONT Never do Path MTU Discovery.
IP_PMTUDISC_DO Always do Path MTU Discovery.
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 a EMSGSIZE error occurred). It may change
over time. For connectionless sockets with many destinations
the new also 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 sock-
ets it is possible to start with a big datagram size (up to 64K-
headers bytes 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.
IP_MTU Retrieve the current known path MTU of the current socket. Only
valid when the socket has been connected. Returns an integer.
Only valid as a getsockopt(2).
IP_ROUTER_ALERT
Pass all to-be forwarded packets with the IP Router Alert option
set to this socket. Only valid 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 users responsibility
to send them out again. Socket binding is ignored, such packets
are only filtered by protocol. Expects an integer flag.
Sets or reads a boolean integer argument whether sent multicast
packets should be looped back to the local sockets.
IP_ADD_MEMBERSHIP
Join a multicast group. Argument is a struct 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 multi-
cast address. 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.
For compatibility, the old ip_mreq structure is still supported.
It differs from ip_mreqn only by not including the imr_ifindex
field. Only valid as a setsockopt(2).
IP_DROP_MEMBERSHIP
Leave a multicast group. Argument is an ip_mreqn or ip_mreq
structure similar to IP_ADD_MEMBERSHIP.
IP_MULTICAST_IF
Set the local device for a multicast socket. Argument is an
ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.
When an invalid socket option is passed, ENOPROTOOPT is
returned.
SYSCTLS
The IP protocol supports the sysctl interface to configure some global
options. The sysctls can be accessed by reading or writing the
/proc/sys/net/ipv4/* files or using the sysctl(2) interface.
ip_default_ttl
Set the default time-to-live value of outgoing packets. This can
be changed per socket with the IP_TTL option.
ip_forward
Enable IP forwarding with a boolean flag. IP forwarding can be
also set on a per interface basis.
ip_dynaddr
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_autoconfig
Not documented.
>4096 to avoid clashes with well known ports and to minimize
firewall problems.
ip_no_pmtu_disc
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.
ipfrag_high_thresh, ipfrag_low_thresh
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.
ip_always_defrag
[New with Kernel 2.2.13; in earlier kernel version the feature
was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG
option]
When this boolean frag 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)
before being processed, even if they are about to be forwarded.
Only enable if running either a firewall that is the sole link
to your network or a transparent proxy; never ever turn on here
for a normal router or host. Otherwise fragmented communication
may me disturbed when the fragments would 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.
neigh/*
See arp(7).
IOCTLS
All ioctls described in socket(7) apply to ip.
The ioctls to configure firewalling are documented in ipfw(7) from the
ipchains package.
Ioctls to configure generic device parameters are described in netde-
vice(7).
NOTES
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.
Some other BSD sockets implementations provide IP_RCVDSTADDR and
EINVAL Invalid argument passed. For send operations this can be caused
by sending to a blackhole route.
EMSGSIZE
Datagram is bigger than an MTU on the path and it cannot be
fragmented.
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 with-
out CAP_NET_ADMIN or effective user id 0. Binding to a reserved
port without the CAP_NET_BIND_SERVICE capacibility or effective
user id 0.
EADDRINUSE
Tried to bind to an address already in use.
ENOPROTOOPT and EOPNOTSUPP
Invalid socket option passed.
EPERM User doesn't have permission to set high priority, change con-
figuration, or send signals to the requested process or group,
EADDRNOTAVAIL
A non-existent interface was requested or the requested source
address was not local.
EAGAIN Operation on a non-blocking socket would block.
ESOCKTNOSUPPORT
The socket is not configured or an unknown socket type was
requested.
EISCONN
connect(2) was called on an already connected socket.
EALREADY
An connection operation on a non-blocking socket is already in
progress.
ECONNABORTED
A connection was closed during an accept(2).
EPIPE The connection was unexpectedly closed or shut down by the other
end.
ENOENT SIOCGSTAMP was called on a socket where no packet arrived.
EHOSTUNREACH
No valid routing table entry matches the destination address.
This error can be caused by a ICMP message from a remote router
or for the local routing table.
ENODEV Network device not available or not capable of sending IP.
raw(7), udp(7) and socket(7).
VERSIONS
IP_PKTINFO, IP_MTU, IP_PMTU_DISCOVER, IP_PKTINFO, IP_RECVERR and
IP_ROUTER_ALERT are new options in Linux 2.2. They are also all Linux
specific and should not be used in programs intended to be portable.
struct ip_mreqn is new in Linux 2.2. Linux 2.0 only supported ip_mreq.
The sysctls were introduced with Linux 2.2.
COMPATIBILITY
For compatibility with Linux 2.0, the obsolete socket(PF_INET,
SOCK_RAW, protocol) syntax is still supported to open a packet(7)
socket. This is deprecated and should be replaced by socket(PF_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.
BUGS
There are too many inconsistent error values.
The ioctls to configure IP-specific interface options and ARP tables
are not described.
Some versions of glibc forget to declare in_pktinfo. Workaround cur-
rently is to copy it into your program from this man page.
Receiving the original destination address with MSG_ERRQUEUE in
msg_name by recvmsg(2) does not work in some 2.2 kernels.
AUTHORS
This man page was written by Andi Kleen.
SEE ALSO
sendmsg(2), recvmsg(2), socket(7), netlink(7), tcp(7), udp(7), raw(7),
ipfw(7)
RFC791 for the original IP specification.
RFC1122 for the IPv4 host requirements.
RFC1812 for the IPv4 router requirements.
Linux Man Page 2001-06-19 ip(7)