libfaketime/README

      =====================================================
      FakeTime Preload Library, version 0.9 (February 2011)
      =====================================================


Content of this file:
---------------------

1. Introduction
2. Compatibility issues
3. Installation
4. Usage
   a) Basics
   b) Using absolute dates
   c) Using 'start at' dates
   d) Using offsets for relative dates
   e) Advanced features and caveats
   f) Faking the date and time system-wide
   g) Using the "faketime" wrapper script
   h) "Limiting" libfaketime
   i) Spawning an external process
   j) Saving timestamps to file, loading them from file
5. License
6. Contact



1. Introduction
---------------

FTPL intercepts various system calls which programs use to retrieve the
current date and time. It can then report faked dates and times (as
specified by you, the user) to these programs. This means you can modify
the system time a program sees without having to change the time system-
wide.

FTPL allows you to specify both absolute dates (e.g., 01/01/2004) and
relative dates (e.g., 10 days ago).

FTPL might be used for various purposes, for example

- running legacy software with y2k bugs
- testing software for year-2038 compliance
- debugging time-related issues, such as expired SSL certificates
- running software which ceases to run outside a certain timeframe
- using different system-wide date and time settings, e.g. on OpenVZ-
  based virtual machines running on the same host



2. Compatibility issues
-----------------------

* FTPL has been designed on and for Linux 2.x, but is supposed and has been
  reported to work on other *NIXes as well.

* FTPL uses the library preload mechanism and thus cannot work with statically
  linked binaries or binaries that have the setuid-flag set (e.g., suidroot
  programs like "ping" or "passwd").

* As of version 0.7, support has been added for use in a pthreads environment. A
  separate library is built (libfaketimeMT.so.1) which contains the pthread
  synchronization calls. This library also single-threads calls through the
  time() intercept, because several variables are statically cached by the
  library and could cause issues when accessed without synchronization. However,
  the performance penalty for this might be an issue for some applications. If
  this is the case, you can try using an unsynchronized time() intercept by
  removing the -DPTHREAD_SINGLETHREADED_TIME from the Makefile and rebuilding
  libfaketimeMT.so.1 . Thanks to David North, TDI!

* If and only if you want to run Java programs with faked times in the future
  (not in the past) on Linux, you also should set the environment variable
  LD_ASSUME_KERNEL=2.4.19 before running the appropriate "java" command. This
  fixes an occasional bug where Java locks up at exiting. Again, this is only
  required for Java with faked times in the future. Thanks to Jamie Cameron for
  reporting this issue and finding a workaround!


3. Installation
---------------

Running "make" should compile both library versions and a test program, which
it then also executes.

If the test works fine, you should copy the FTPL libraries (libfaketime.so.1,
and libfaketimeMT.so.1) to the place you want them in. Running "make install"
will attempt to place them in /usr/local/lib/faketime and will install the wrapper
shell script "faketime" in /usr/local/bin, both of which most likely will require
root privileges; however, from a technical point of view, there is no
necessity for a system-wide installation, so you can use FTPL also on machines
where you do not have root privileges. You may want to adjust the PREFIX
variable in the Makefiles accordingly.

Since version 0.6, system calls to file timestamps are also intercepted now,
thanks to a contribution by Philipp Hachtmann. This is especially useful in
combination with relative time offsets as explained in section 4d) below, if a
program writes and reads files whose timestamps also shall be faked. If you do
not need this feature or if it confuses the application you want to use FTPL
with, define the environment variable NO_FAKE_STAT, and the intercepted stat
calls will be passed through unaltered.

On MacOS, it is necessary to compile differently, due to the different
behavior dyld has. Use the Makefile.MacOS provided to compile
libfaketime.dylib.1. Additionally, instead of using LD_PRELOAD,
the variable DYLD_INSERT_LIBRARIES should be set to the path to
libfaketime.dylib.1, and the variable DYLD_FORCE_FLAT_NAMESPACE should be
set (to anything).

4. Usage
--------

4a) Usage basics
----------------

Using FTPL on a program of your choice consists of two steps:

1. Making sure FTPL gets loaded.
2. Specify the faked time.


As an example, we want the "date" command to report our faked time. To do so,
we could use the following command line:

user@host> date
Tue Nov 23 12:01:05 CEST 2007

user@host> LD_PRELOAD=/usr/local/lib/libfaketime.so.1 FAKETIME="-15d" date
Mon Nov  8 12:01:12 CEST 2007


The basic way of running any command/program with FTPL enabled is to make sure
the environment variable LD_PRELOAD contains the full path and filename of the
FTPL library. This can either be done by setting it once beforehand:

export LD_PRELOAD=/path/to/libfaketime.so.1
(now run any command you want)


Or it can be done by specifying it on the command line itself:

LD_PRELOAD=/path/to/libfaketime.so.1 your_command_here

(These examples are for the bash shell; how environment variables are set may
vary on your system.)


However, also the faked time should be specified; otherwise, FTPL will be
loaded, but just report the real system time. There are three ways to specify
the faked time:

a) By setting the environment variable FAKETIME.
b) By using the file .faketimerc in your home directory.
c) By using the file /etc/faketimerc for a system-wide default.

If you want to use b) or c), $HOME/.faketimerc or /etc/faketimerc consist of
only one line of text with exactly the same content as the FAKETIME environment
variable, which is described below. Note that /etc/faketimerc will only be used
if there is no $HOME/.faketimerc, and the FAKETIME environment variable always
has priority over the files.


4b) Using absolute dates
------------------------

The format which _must_ be used for _absolute_ dates is "YYYY-MM-DD hh:mm:ss".
For example, the 24th of December, 2002, 8:30 PM would have to be specified as
FAKETIME="2002-12-24 20:30:00".


4c) Using 'start at' dates
--------------------------

(Thanks to a major contribution by David North, TDI in version 0.7)

The format which _must_ be used for _start_at_ dates is "@YYYY-MM-DD hh:mm:ss".
For example, the 24th of December, 2002, 8:30 PM would have to be specified as
FAKETIME="@2002-12-24 20:30:00".

The absolute dates described in 4b simulate a STOPPED system clock at the
specified absolute time. The 'start at' format allows a 'relative' clock
operation as described below in section 4d, but using a 'start at' time
instead of an offset time.


4d) Using offsets for relative dates
------------------------------------

Relative date offsets can be positive or negative, thus what you put into
FAKETIME _must_ either start with a + or a -, followed by a number, and
optionally followed by a multiplier:

- by default, the offset you specify is in seconds. Example:

  export FAKETIME="-120" will set the faked time 2 minutes (120 seconds) behind
  the real time.

- the multipliers "m", "h", "d" and "y" can be used to specify the offset in
  minutes, hours, days and years (365 days each), respectively. Examples:

  export FAKETIME="-10m" sets the faked time 10 minutes behind the real time.
  export FAKETIME="+14d" sets the faked time to 14 days in the future.


You now should understand the complete example we've used before:

LD_PRELOAD=/usr/local/lib/libfaketime.so.1 FAKETIME="-15d" date

This command line makes sure FTPL gets loaded and sets the faked time to
15 days in the past.

Moreno Baricevic has contributed support for the FAKETIME_FMT environment
variable, which allows to optionally set the strptime() format:

Some simple examples:
LD_PRELOAD=./libfaketime.so.1 FAKETIME_FMT=%s FAKETIME="`date +%s -d'1 year ago'`" date
LD_PRELOAD=./libfaketime.so.1 FAKETIME_FMT=%s FAKETIME="`stat -c %Y somefile`" date
LD_PRELOAD=./libfaketime.so.1 FAKETIME_FMT=%c FAKETIME="`date`" date


4e) Advanced features and caveats
---------------------------------

Advanced time specification options:
------------------------------------

Since version 0.8, thanks to a contribution by Karl Chen, fractions can be used
in the specification of time offsets. For example,

FAKETIME="+1,5h"

is equivalent to FAKETIME="+90m". Please be aware that the fraction delimiter
depends on your locale settings, so actually you might need to use

FAKETIME="+1.5h"

You should figure out the proper delimiter, e.g. by using FTPL on a command
like /bin/date where you immediately can verify whether it worked as expected.

Also contributed by Karl Chen in v0.8 is the option to speed up or slow down
the wall clock time for the program which is executed using FTPL. For example,

FAKETIME="+1y x2"

will set the faked time one year into the future and will make the clock run
twice as fast. Similarly,

FAKETIME="+1y x0,5"

will make the clock run only half as fast. As stated above, the fraction
delimiter depends on your locale.

FAKETIME="+1y i2,0"

will make the clock step two seconds per each time(), etc. call, running
completely independently from the system clock. It helps running programs
with some determinism. In this single case all spawned processes will use
the same global clock without restaring it at the start of each process.

For testing, your should run a command like

LD_PRELOAD=./libfaketime.so.1 FAKETIME="+1,5y x10,0" \
/bin/bash -c 'while true; do echo $SECONDS ; sleep 1 ; done'

For each second that the endless loop sleeps, the executed bash shell will
think that 10 seconds have passed ($SECONDS is a bash-internal variable
measuring the time since the shell was started).

(Please note that replacing "echo $SECONDS" e.g. with a call to "/bin/date"
will not give the expected result, since /bin/date will always be started
as a new process for which also FTPL will be re-initialized. It will show
the correct offset (1.5 years in the future), but no speed-ups or
slow-downs.)


Caveats:
--------

Whenever possible, you should use relative offsets or 'start at' dates, and not
use absolute dates.

Why? Because the absolute date/time you set is fixed, i.e. if a program
retrieves the current time, and retrieves the current time again 5 minutes
later, it will still get the same result twice. This is likely to break
programs which measure the time passing by (e.g. a mail program which checks
for new mail every X minutes).

Using relative offsets or 'start at' dates solves this problem. FTPL then will
always report the faked time based on the real current time and the offset
you've specified.

Please also note that your specification of the fake time is cached for 10
seconds in order to enhance the library's performance. Thus, if you change the
content of $HOME/.faketimerc or /etc/faketimerc while a program is running, it
may take up to 10 seconds before the new fake time is applied. If this is a
problem in your scenario, you can disable caching at compile time by adding the
command line option -DNO_CACHING to this library's Makefile.


4f) Faking the date and time system-wide
----------------------------------------

David Burley of SourceForge, Inc. reported an interesting use case of applying
FTPL system-wide: Currently, all virtual machines running inside an OpenVZ host
have the same system date and time. In order to use multiple sandboxes with
different system dates, the FTPL library can be put into /etc/ld.so.preload;
it will then be applied to all commands and programs automatically. This is
of course best used with a system-wide /etc/faketimerc file. Kudos to
SourceForge, Inc. for providing the patch!


4g) Using the "faketime" wrapper script
---------------------------------------

As of version 0.8, FTPL provides a shell script named "faketime" which is
placed into /usr/bin by "make install". It spares the hassle of setting
the LD_PRELOAD and FAKETIME environment variables manually, but only exposes
a subset of FTPL's functionality. On the other hand, it uses the date
interpretation function by /bin/date in order to provide higher flexibility
regarding the specification of the faked date and time. For example, you
can use

faketime 'last Friday 5 pm' /your/command/here

Of course, also absolute dates can be used, such as in

faketime '2008-12-24 08:15:42' /bin/date

Thanks to Daniel Kahn Gillmor for providing these suggestions!


4h) "Limiting" libfaketime
--------------------------

Starting with version 0.9, libfaketime can be configured to not be continuously
active, but only during a certain time interval.

For example, you might want to start a program with the real current time, but
after 5 minutes of usage, you might want it to see a faked time, e.g., a year
in the future.

Dynamic changes to the faked time are alternatively possible by

- changing the FAKETIME environment variable at run-time; this is the preferred
  way if you use libfaketime for debugging and testing as a programmer, as it
  gives you the most direct control of libfaketime without any performance
  penalities.

- not using the FAKETIME environment variable, but specifying the fake time in a
  file (such as ~/.faketimerc). You can change the content of this file at
  run-time. This works best with caching disabled (see Makefile), but comes at a
  performance cost because this file has to be read and evaluated each time.

The feature described here works based on two pairs of environment variables,

  FAKETIME_START_AFTER_SECONDS and FAKETIME_STOP_AFTER_SECONDS, and
  FAKETIME_START_AFTER_NUMCALLS and FAKETIME_STOP_AFTER_NUMCALLS

The default value for each of these environment variables is -1, which means
"ignore this value".

If you want libfaketime to be only active during the run-time minutes 2 to 5
of your application, you would set

  FAKETIME_START_AFTER_SECONDS=60
  FAKETIME_STOP_AFTER_SECONDS=300

This means that your application will work with the real time from start (second
0) up to second 60. It will then see a faked time from run-time seconds 60 to
300 (minutes 2, 3, 4, and 5). After run-time second 600, it will again see the
real (not-faked) time.

This approach is not as flexible as changing the FAKETIME environment variable
during runtime, but may be easier to use, works on a per-program (and not a
per-user or system-wide) scope, and has only a minor performance overhead.

Using the other pair of environment variables, you can limit the activity time
of libfaketime not based on wall-clock seconds, but on the number of
time-related function calls the started program performs. This alternative is
probably only suitable for programmers who either know the code of the program
in order to determine useful start/stop values or want to perform fuzzing
tests.

Both pairs of environment variables can be combined to further restrict
libfaketime activity, although this is only useful in very few scenarios.

Limiting libfaketime activity in this way is not recommended in general. Many
programs will break when they are subject to sudden changes in time, especially
if they are started using the current (real) time and are then sent back into
the past after, e.g., 5 minutes. For example, they may appear to be frozen or
stuck because they are waiting until a certain point in time that, however, is
never reached due to the delayed libfaketime activity. Avoid using this
functionality unless you are sure you really need it and know what you are
doing.


4i) Spawning an external process
--------------------------------

From version 0.9 on, libfaketime can execute a shell command once after an
arbitrary number of seconds or number of time-related system calls of the
program started. This has two limitations one needs to be aware of:

* Spawning the external process happens during a time-related system call
  of the original program. If you want the external process to be started
  5 seconds after program start, but this program does not make any time-
  related system calls before run-time second 8, the start of your external
  process will be delayed until run-time second 8.

* The original program is blocked until the external process is finished,
  because the intercepting time-related system call will not return earlier. If
  you need to start a long-running external process, make sure it forks into the
  background.

Spawning the external process is controlled using three environment variables:
FAKETIME_SPAWN_TARGET, FAKETIME_SPAWN_SECONDS, FAKETIME_SPAWN_NUMCALLS.

Example (using bash on Linux):

(... usual libfaketime setup here, setting LD_PRELOAD and FAKETIME ...)
export FAKETIME_SPAWN_TARGET="/bin/echo 'Hello world'"
export FAKETIME_SPAWN_SECONDS=5
/opt/local/bin/myprogram

This will run the "echo" command with the given parameter during the first
time-related system function call that "myprogram" performs after running for 5
seconds.

4j) Saving timestamps to file, loading them from file
--------------------------------

Faketime can save faked timestamps to a file specified by FAKETIME_SAVE_FILE
environment variable. It can also use the file specified by FAKETIME_LOAD_FILE
to replay timestamps from it. After consuming the whole file faketime returns
to using the rule set in FAKETIME variable, but the timestamp processes will
start counting from will be the last timestamp in the file.

The file stores each timestamp in a stream of saved_timestamp structs
without any metadata or padding:

/** Storage format for timestamps written to file. Big endian.*/
struct saved_timestamp {
  int64_t sec;
  uint64_t nsec;
};


Faketime needs to be run using the faketime wrapper to use the files.

5. License
----------

FTPL has been released under the GNU Public License, GPL. Please see xthe
included COPYING file.



6. Contact
-----------

Bug reports, feature suggestions, success reports and patches are highly
appreciated.

Please send an email to: wolf /at/ code-wizards.com

git pull requests are welcome, see https://github.com/wolfcw/libfaketime