Motivation

As we all know,changing time in a container is almost impossible due to docker's isolation mechanism.

You may have seen the following notice:

[root@1b4943b5b4dd /]# date -s "2022-10-25"
date: cannot set date: Operation not permitted
Tue Oct 25 00:00:00 UTC 2022
[root@1b4943b5b4dd /]# 

Of cause,you can add --cap-add SYS_TIME in command when you run the container.But this way may be unstable and may affect the host.

You may have seen the following situations:

(base) ➜  ~ docker run -itd --cap-add SYS_TIME --name testos centos:centos7 /bin/bash
b1ad49dbdbbb5c36a1f93e3f31164c872b81be9b1a16de08bbc1136e46428b14
(base) ➜  ~ docker exec -it testos /bin/bash                                         
[root@b1ad49dbdbbb /]# date -s "2022-10-25" && date
Tue Oct 25 00:00:00 UTC 2022
Tue Oct 25 00:00:00 UTC 2022
[root@b1ad49dbdbbb /]# date
Tue Oct 25 00:00:03 UTC 2022
[root@b1ad49dbdbbb /]# date
Tue Oct 25 00:00:06 UTC 2022
[root@b1ad49dbdbbb /]# date
Fri Oct 14 08:49:33 UTC 2022 #the time change back automatically after few seconds 
[root@b1ad49dbdbbb /]# 

So I think being able to freely modify the time in the container would help someone.This would be great for simulating scenarios at specific times.

Method

Thanks to wolfcw/libfaketime .This is a feasible approach to generate a fake time and intercepts all system call programs use to retrieve the time and date(detailed in libfaketime/README at master · wolfcw/libfaketime (github.com)).

Version v2:latest

I review the introduction of libfaketime.I think here is another way to change time.

steps as follow:

1. compile libfaketime with FAKETIME_COMPILE_CFLAGS="-DFAKE_SETTIME"(i.e. make FAKETIME_COMPILE_CFLAGS="-DFAKE_SETTIME" && make install)

2. create a file for storing time offset.(e.g. echo "+0" > /etc/myfaketime.rc)

3. add LD_PRELOAD=/usr/local/lib/faketime/libfaketime.so.1 FAKETIME_TIMESTAMP_FILE=/etc/myfaketime.rc FAKETIME_UPDATE_TIMESTAMP_FILE=1 FAKETIME_NO_CACHE=1 as Enviroment variables

I also provide pure version of dockerfile(i.e. remove java svn and others,only keep faketime support)

Version v1:2022-10-14

In order to achieve the same modification effect as the native command, I only used the system-wide method.

I do the following steps to achieve the effect.

1. clone [email protected]:wolfcw/libfaketime.git

2. copy the libfaketime lib into image

3. compile and install it via "make && make install"

4. add LD_PRELOAD=/usr/local/lib/faketime/libfaketime.so.1 FAKETIME_NO_CACHE=1 FAKETIME_XRESET=1 as Environment Variables

5. Finally,changing the content in /etc/faketimerc can affect the output of date

More details in Dockerfile (btw:I also install the java,svn and chinese langpack in image because I`m a developer in China^_^.So you can change that as you need.)

Usage

Step1:Clone

git clone [email protected]:KenanHuang/docker_time_modify.git

Step2:download libfaketime

Step3:put libfaketime into libs.the contents in libs as following:

(base) ➜  docker_time_modify git:(main) ls -l libs
drwxr-xr-x  17 conan  staff        544 Oct 12 15:43 libfaketime

Step4:build image

docker build -t centos7_faketime:2.0 -f ./Dockerfile-pure-v2 .

Step5:run container

docker run -it --rm centos7_faketime:2.0 /bin/bash

Step6:do whatever you want

(base) ➜  docker_time_modify git:(main) ✗ docker run -itd --name tstest centos7_faketime:2.0 /bin/bash
fa99c9cfdbb333df71c60f71538ee11f8594ebec7e0
(base) ➜  docker_time_modify git:(main) ✗ docker exec -it tstest /bin/bash
[root@fa99c9cfdbb3 /]# date
Sun Oct 16 05:30:27 UTC 2022
[root@fa99c9cfdbb3 /]# date -s "2099-11-12 12:00:00"
Thu Nov 12 12:00:00 UTC 2099
[root@fa99c9cfdbb3 /]# date
Thu Nov 12 12:00:01 UTC 2099
[root@fa99c9cfdbb3 /]# exit
(base) ➜  docker_time_modify git:(main) ✗ docker exec -it tstest /bin/bash
[root@fa99c9cfdbb3 /]# date
Thu Nov 12 12:00:12 UTC 2099
[root@fa99c9cfdbb3 /]# date -s "1993-10-25 08:00:00"
Mon Oct 25 08:00:00 UTC 1993
[root@fa99c9cfdbb3 /]# date
Mon Oct 25 08:00:01 UTC 1993

version v1 [root@df551f8e0bc4 /]# which java /usr/local/jdk1.8.0_333/bin/java [root@df551f8e0bc4 /]# which svn /usr/bin/svn [root@df551f8e0bc4 /]# date Fri Oct 14 17:41:31 CST 2022 [root@df551f8e0bc4 /]# echo "+5h" > /etc/faketimerc [root@df551f8e0bc4 /]# date Fri Oct 14 22:41:44 CST 2022

How to change time in Container

In latest version.You just need to use "date -s " command to change time.And use "echo "+0" > /etc/myfaketime.rc" to restore time.

demo:

(base) ➜  docker_time_modify git:(main) ✗ docker exec -it tstest /bin/bash
[root@fa99c9cfdbb3 /]# date
Sun Oct 16 05:30:27 UTC 2022
[root@fa99c9cfdbb3 /]# date -s "2099-11-12 12:00:00"
Thu Nov 12 12:00:00 UTC 2099
[root@fa99c9cfdbb3 /]# date
Thu Nov 12 12:00:01 UTC 2099
[root@fa99c9cfdbb3 /]# echo "+0" > /etc/myfaketime.rc
[root@fa99c9cfdbb3 /]# date
Sun Oct 16 05:30:33 UTC 2022

version v1:

Parameters you need to know in advance

The multipliers "m", "h", "d" and "y" can be used to specify the offset in minutes, hours, days and years (365 days each), respectively. -- as described in wolfcw/libfaketime

/etc/faketimerc is used to store the offset of fake time.So you just need to change it to get different time.

demo:

(base) ➜ ~ docker run -it --rm centos7_faketime_java_svn:1.0 /bin/bash [root@87238b4bc443 /]# date Fri Oct 14 17:31:50 CST 2022 [root@87238b4bc443 /]# echo "+1h" > /etc/faketimerc && date # change the time to one hour later Fri Oct 14 18:32:02 CST 2022 [root@87238b4bc443 /]# echo "+2d" > /etc/faketimerc && date Sun Oct 16 17:32:08 CST 2022 [root@87238b4bc443 /]# echo "-1y" > /etc/faketimerc && date Thu Oct 14 17:32:13 CST 2021 [root@87238b4bc443 /]# echo "+0" > /etc/faketimerc && date # restore time Fri Oct 14 17:32:17 CST 2022 [root@87238b4bc443 /]# date Fri Oct 14 17:32:19 CST 2022

Acknowledgements

Thank Wolfgang Hommel develop such fantastic job.Here is the link of libfaketime:https://github.com/wolfcw/libfaketime

PS:

Please give a star ⭐️ ! If this project could help you.Thanks a lot ! ^_^