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Version 0.8 adds:

• support for fractional time offsets, such as "+1,5y" (one and a half years)
• support for clock speed changes (speed up / slow down), such as "x2,0" or "x0,5" to make time pass twice or half as fast.
• support for fstatat() and fstatat64(), i.e. new system calls introduced in Linux 2.6.16 and used in recent coreutils.
• a wrapper script "faketime" for eased invocation and a man page.

Previous changes in 0.7:

• phtread synchronization support
• support for "start at" time specifications
• options to dynamically disable features at runtime
• enhanced compatibility for system-level applications, such as Java Virtual Machines

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 usage options and caveats
   f) Faking the date and time system-wide
   g) Using the faketime wrapper script
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 (e.g. /usr/local/lib).

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.

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 usage options 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.

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!

5. License

FTPL has been released under the GNU Public License, GPL. Please see the 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

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Keywords: UNIX, Linux, system administration, debugging, legacy software, year 2038 compliance, preload, function interposition, fake date.
License: GNU public license (GPL), version 2