IP
Section: Linux Programmer's Manual (7)
Updated: 2010-09-11
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NAME
ip - Linux IPv4 protocol implementation
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h> /* superset of previous */
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
raw_socket = socket(AF_INET, SOCK_RAW, protocol);
DESCRIPTION
Linux implements the Internet Protocol, version 4,
described in RFC 791 and RFC 1122.
ip
contains a level 2 multicasting implementation 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 by calling the
socket(2)
function as
socket(AF_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
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).
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 an unbound socket, it 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 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 privileged processes (i.e., those having 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
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_ANY)
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 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 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.
There are several special addresses:
INADDR_LOOPBACK
(127.0.0.1)
always refers to the local host via the loopback device;
INADDR_ANY
(0.0.0.0)
means any address for binding;
INADDR_BROADCAST
(255.255.255.255)
means any host and has the same effect on bind as
INADDR_ANY
for historical reasons.
Socket Options
IP supports some protocol-specific socket options that can be set with
setsockopt(2)
and read with
getsockopt(2).
The socket option level for IP is
IPPROTO_IP.
A boolean integer flag is zero when it is false, otherwise true.
- 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
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.
Only valid as a
setsockopt(2).
- 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_HDRINCL (since Linux 2.0)
-
If enabled,
the user supplies an IP header in front of the user data.
Only valid 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_MTU (since Linux 2.2)
-
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_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 this socket.
The don't-fragment flag is set on all outgoing datagrams.
The system-wide default is controlled by the
/proc/sys/net/ipv4/ip_no_pmtu_disc
file 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.
|
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 a
EMSGSIZE
error occurred).
It 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
(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.
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_IF (since Linux 1.2)
-
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.
- 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.
This option is only valid for
SOCK_RAW
sockets.
The argument is an integer.
- 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 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 (since Linux 2.2)
-
Pass an
IP_PKTINFO
ancillary message that contains a
pktinfo
structure that supplies some information about the incoming packet.
This only works 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 only be sent/retrieved
as 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.
- 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 only reported 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_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.
- 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 byte.
Not supported for
SOCK_STREAM
sockets.
- IP_RETOPTS (since Linux 2.2)
-
Identical to
IP_RECVOPTS,
but returns raw unprocessed options with timestamp and route record
options not filled in for this hop.
- IP_ROUTER_ALERT (since Linux 2.2)
-
Pass all to-be forwarded packets with the
IP Router Alert option set to this socket.
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 user's responsibility to send them out again.
Socket binding is ignored,
such packets are only filtered 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).
The priority can also be set in a protocol independent way by the
(SOL_SOCKET, SO_PRIORITY)
socket option (see
socket(7)).
- 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.
/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 kernel 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 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.
Only enable 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 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)
-
Contains two integers that define the default local port range
allocated to sockets.
Allocation starts with the first number and ends with the second number.
Note that these 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.
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).
ERRORS
- 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
-
An 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 a 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 and EOPNOTSUPP
-
Invalid socket option passed.
- ENOTCONN
-
The operation is only defined 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).
NOTES
IP_MTU,
IP_MTU_DISCOVER,
IP_PKTINFO,
IP_RECVERR
and
IP_ROUTER_ALERT
are Linux-specific and should not be used in
programs intended to be portable.
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
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
SOL_IP
socket options level isn't portable, BSD-based stacks use
IPPROTO_IP
level.
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.
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 currently 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.
SEE ALSO
recvmsg(2),
sendmsg(2),
byteorder(3),
ipfw(4),
capabilities(7),
netlink(7),
raw(7),
socket(7),
tcp(7),
udp(7)
RFC 791 for the original IP specification.
RFC 1122 for the IPv4 host requirements.
RFC 1812 for the IPv4 router requirements.
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Address Format
-
- Socket Options
-
- /proc interfaces
-
- Ioctls
-
- ERRORS
-
- NOTES
-
- Compatibility
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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Time: 19:49:29 GMT, April 27, 2011