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Network Time Synchronization

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Introduction[edit]

Ambox warning pn.svg.png Warning: The system clock inside Whonix ™ is set to UTC to prevent against time zone leaks. This means it may be a few hours ahead or behind the user's host system clock. It is strongly recommended not to change this setting.

Warnings[edit]

A reasonably accurate host clock is crucially important. An inaccurate clock can lead to:

Therefore, at all times it is recommended to have a host clock with accuracy of up to ± 30 minutes. Clocks that are days, weeks, months or even years slow or fast can lead to many issues such as connectivity problems with Tor, inability to download operating system upgrades and inaccessible websites. [1]

Follow the platform-specific recommendations below to avoid Tor connectivity problems and to limit possible adverse anonymity impacts.

All Platforms[edit]

To protect against time zone leaks, the system clock inside Whonix ™ is set to UTC. This means it may be a few hours before or ahead of your host system clock. Do not change this setting!

If the host clock (in UTC! [2] [3]) is more than 1 hour in the past or more than 3 hours in the future, Tor cannot connect. In this case, manually fix the host clock by right-clicking on it, and also check for an empty battery. Then, power off and power on the Whonix-Gateway ™ (sys-whonix) and Tor should be able to reconnect.

Easy instructions[edit]

Non-Qubes-Whonix: It is strongly discouraged to use the pause / suspend / save / hibernate features.

Qubes-Whonix ™: It is strongly discouraged to use the pause feature of Qube Manager, but it is is safe to use the suspend or hibernate feature of dom0.

Advanced instructions[edit]

If you are interested in using the pause / suspend / save / hibernate features, please click the expand button for further instructions.

Non-Qubes-Whonix:

  • Whonix-Gateway ™: It is strongly discouraged to pause / suspend / save / hibernate the Whonix-Gateway ™, because it is difficult to restore the clock after resume. [4] If this advice is ignored, it is easiest to shutdown and restart Whonix-Gateway ™.
  • Whonix-Workstation ™: It is strongly discouraged to pause / suspend / save / hibernate the Whonix-Workstation ™. If this advice is ignored, restart sdwdate after resume. [5]

Qubes-Whonix ™:

  • Whonix-Gateway ™: It is strongly discouraged to pause Whonix-Gateway ™ (sys-whonix) using the pause feature of Qube Manager, because it is difficult to restore the clock after resume. [4] If this advice is ignored, it is easiest to shutdown and restart Whonix-Gateway ™.
  • Whonix-Workstation ™: It is strongly discouraged to pause Whonix-Workstation ™ (anon-whonix) using the pause feature of Qube Manager. If this advice is ignored, restart sdwdate after resume. [6]
  • dom0 suspend / hibernate: It is safe to use the suspend or hibernate feature of dom0 and a manual restart of sdwdate is unnecessary. [7]

Restart sdwdate[edit]

To restart sdwdate.

Start MenuApplicationsSystemTime Synchronization Monitor (sdwdate-gui)restart sdwdate

Or in a terminal. [8]

sudo /usr/lib/sdwdate/restart_fresh

Manually Set Clock Time and Date[edit]

Usually not required.

In case sdwdate fails to properly randomize the system clock, it is possible to manually set a random value.

The first step should be completed on the host to ensure the host clock is set to the correct time.

1. On the host (Qubes-Whonix ™: dom0), run the following command to report the time in UTC.

date -u

The output should be similar to the following. [9]

Oct 27 11:23:44 UTC 2021

2. Set the correct time in Whonix-Gateway ™ (sys-whonix).

Run the following command with the correct date and time parameters. [10] [11]

  • clock-random-manual-gui: a randomized clock setting (in UTC) is entered via a GUI.
  • clock-random-manual-cli: a randomized clock setting (in UTC) is entered on the command line. For example [12]:

echo "Oct 27 11:23:44 UTC 2021" | sudo clock-random-manual-cli

3. Restart sdwdate.

sudo service sdwdate restart

4. If Tor is still not functional, try restarting Tor.

sudo service tor restart

Tor should work once correct clock values are set, but that can be manually tested with whonixcheck.

Block Networking until sdwdate Finishes[edit]

sdwdate is a Tor-friendly replacement for rdate and ntpdate that sets the system's clock by communicating via end-to-end encrypted TCP with Tor onion webservers. Since timekeeping is crucial for security and anonymity, blocking network access until sdwdate succeeds is sensible. [13]

sdwdate is functional on both Whonix-Gateway ™ and Whonix-Workstation ™, but in some cases it is possible for the time to leak before it is changed. Potential leak channels include time or other servers, daemons, and client programs such as Tor Browser which are used before sdwdate successfully finishes. [14] In this case, the user is only left with the protections afforded by Boot Clock Randomization. [15]

Inside Whonix-Gateway ™ and Whonix-Workstation ™.

(Qubes-Whonix ™: It is the easiest to apply changes in the Whonix-Gateway ™ and Whonix-Workstation ™ TemplateVMs since these settings will be inherited by all TemplateBasedVMs. Alternatively, apply this setting in TemplateBasedVMs, see footnote. [16])

1. Edit the Whonix ™ firewall configuration.

In both Whonix-Gateway ™ (whonix-gw-16) and Whonix-Workstation ™ (whonix-ws-16), run.

Open file /etc/whonix_firewall.d/50_user.conf in an editor with root rights.

(Qubes-Whonix ™: In Template)

This box uses sudoedit for better security [archive]. This is an example and other tools could also achieve the same goal. If this example does not work for you or if you are not using Whonix, please refer to this link.

sudoedit /etc/whonix_firewall.d/50_user.conf

2. Copy and paste the following text.

firewall_mode=

3. Save the file and reboot.

4. Have the changes take effect.

  • Non-Qubes-Whonix ™: restart Whonix-Gateway ™ and Whonix-Workstation ™.
  • Qubes-Whonix ™: shutdown the Whonix-Gateway ™ (whonix-gw-16) and Whonix-Workstation ™ (whonix-ws-16) TemplateVMs and restart the TemplateBasedVMs (sys-whonix and anon-whonix).

NTP[edit]

Disabling NTP[edit]

If ISP tampering with NTP is ever confirmed, users are advised to disable NTP and manually update the host clock out-of-band. For example, a watch or atomic clock [archive] can be used for this purpose. If the tampering is targeted and not a widescale attack, then the user already has much bigger problems to worry about than NTP; see Confirmation Attacks.

If following the advice above -- disabling NTP on the host and adjusting the clock out-of-band -- be aware that clearnet traffic might be easier to fingerprint. [17] The reason is that it introduces a device issuing clearnet traffic (such as OS updates), but without the use of NTP. It is unknown how many people have NTP which is deactivated, broken, uninstalled, or never in fact installed in the first place. Also unknown is how many people are using alternative time synchronization methods such as authenticated NTP, tails_htp, tlsdate, sdwdate or similar. However, search engine research suggests that very few people fall into both these categories.

NTP Issues[edit]

The host system clock synchronization mechanism still uses unauthenticated NTP from a single source. This is not optimal, but there is no real solution to this problem. [18] A potential attack vector is created by this NTP behavior; the ISP and/or time server could either inadvertently or maliciously introduce a significant clock skew, or the host clock could simply malfunction.

If the host clock value is grossly inaccurate -- more than one hour in the past or more than 3 hours in future -- Tor cannot connect to the Tor network. [19] This is easily solved by manually fixing the clock on the host, then powering the Whonix-Gateway ™ (sys-whonix) off and on again.

Another side effect of a significantly inaccurate host clock concerns operating system (OS) updates and cryptographic verification on the host. Until the host clock is manually fixed, it may no longer be possible to download updates or verify SSL certificates correctly with the host browser.

Users should always check whether a host clock defect relates to an empty battery before assuming the ISP is tampering with NTP.

Spoof the Initial Virtual Hardware Clock Offset[edit]

Info Tip: Spoofing the initial virtual hardware clock offset is useful to prevent Clock Correlation Attacks.

KVM[edit]

For KVM, click on Expand on the right.

Edit the VM xml before import or edit the VM xml after import and change the following setting.

<clock offset='utc'>

To.

<clock offset='variable' adjustment='123456' basis='utc'>

The adjustment attribute takes any arbitrary value for seconds. The user must pick a random value that is unknown to others, ranging between 0 and 900 (a 15 minute range).

Qubes[edit]

TODO

Unfortunately, it is not yet possible to set a random clock offset for Qubes-Whonix ™ VMs to prevent clock correlation attacks since it is unsupported by Xen [archive]. A related issue is denying Qubes-Whonix ™ access to "clocksource=xen" [archive], which may not be possible without Linux kernel and/or Xen patches. For a detailed discussion of these issues, see here [archive].

VirtualBox[edit]

For VirtualBox, click on Expand on the right.

VirtualBox has a feature to spoof the initial virtual hardware clock offset by setting the clock X milliseconds in the future or past. The syntax is outlined below.

VBoxManage modifyvm <name> --biossystemtimeoffset -<milliseconds>
VBoxManage modifyvm <name> --biossystemtimeoffset +<milliseconds>

It is recommended to add a random delay within the following range.

VBoxManage modifyvm <name> --biossystemtimeoffset -60000
VBoxManage modifyvm <name> --biossystemtimeoffset +60000

A spoofing example is below. Users should select their own unique and random values for both the past (-) and future (+) within the specified range. Different values should be used for each distinct VM (on the host).

VBoxManage modifyvm "Whonix-Gateway" --biossystemtimeoffset -35017
VBoxManage modifyvm "Whonix-Gateway" --biossystemtimeoffset +27931

VBoxManage modifyvm "Whonix-Workstation ™" --biossystemtimeoffset -35017
VBoxManage modifyvm "Whonix-Workstation ™" --biossystemtimeoffset +27931

Apart from this small biossystemtimeoffset, a clock skew always degrades privacy. [20] [21] [22]

Summary[edit]

Table: Network Time Synchonization Summary

Platform Recommendations
All Platforms
  • Do not change system timezone.
  • Tor cannot connect if the host clock is grossly inaccurate. In this case, users should manually fix the host clock before powering the Whonix-Gateway ™ (sys-whonix) off and on again.
  • Periodically check the host clock to ensure that it is accurate or approximately so.
  • For greater security, block networking until sdwdate finishes.
Non-Qubes-Whonix
Qubes-Whonix
  • It is strongly discouraged to use the pause feature of Qube Manager.
  • it is is safe to use the suspend or hibernate feature of dom0.

Appendix[edit]

Deactivate Automatic TimeSync[edit]

Ambox warning pn.svg.png Warning: This action is recommended against and is usually not required. In all cases, first check with the Whonix ™ developers.

To deactivate sdwdate, run.

sudo service sdwdate stop

sudo systemctl mask sdwdate

Related[edit]

Footnotes[edit]

  1. Due to invalid (not yet or no longer valid) TLS certificates.
  2. To view the system time in UTC on Linux platforms, run.
    date --utc

  3. TODO: Show desktop clock in local time; keep system in UTC [archive]
  4. 4.0 4.1 This is because the clock will be incorrect after system resume. A correct clock is important for anonymity (see Dev/TimeSync to learn more). When a user suspends or saves the VM state, the clock will stop and continue after resuming, leading to a time that lags behind the correct value. This can cause later Tor connectivity problems or introduce possible adverse anonymity impacts. The Whonix-Gateway ™ state should not be suspended or saved. It is far better to power it off when it is no longer needed. If this advice is ignored, Tor can become confused if the time is more than 1 hour in the past or more than 3 hours in the future. When this happens, Tor will only reconnect to the Tor network if the clock is manually fixed or powered off and on again.
  5. Similarly, if users suspend or save the Whonix-Workstation ™ state, the clock will again lag behind the correct value. This can be manually fixed by running: Start MenuApplicationsSystemTime Synchronization Monitor (sdwdate-gui)restart sdwdate.
  6. Qubes does not dispatch the /etc/qubes/suspend-post.d / /etc/qubes/suspend-pre.d hooks upon pause / resume using Qube Manager.
  7. https://github.com/QubesOS/qubes-issues/issues/1764 [archive]
  8. Simplified in next upgrade.
    sudo sdwdate-clock-jump

  9. Mon Apr 22 04:30:44 UTC 2019

  10. A non-zero exit codes signifies an error, while 0 means it succeeded.
  11. Also see:
    man clock-random-manual-gui

    man clock-random-manual-cli

  12. echo "Sat Oct 26 07:18:25 UTC 2019" | /usr/bin/clock-random-manual-cli

  13. https://forums.whonix.org/t/testers-wanted-blocking-networking-until-sdwdate-finished-status-of-sdwdate-gui/5372 [archive]
  14. https://phabricator.whonix.org/T533 [archive]
  15. Dev/TimeSync#Block_Networking_until_sdwdate_Finishes
  16. The procedure can optionally be completed in select TemplateBasedVMs (AppVMs) like sys-whonix and anon-whonix.

    1. Edit the Whonix ™ firewall configuration.

    In the chosen Whonix ™ TemplateBasedVMs, run.

    Open file /usr/local/etc/whonix_firewall.d/*.conf in an editor with root rights.

    (Qubes-Whonix ™: In Template)

    This box uses sudoedit for better security [archive]. This is an example and other tools could also achieve the same goal. If this example does not work for you or if you are not using Whonix, please refer to this link.

    sudoedit /usr/local/etc/whonix_firewall.d/*.conf

    2. Copy and paste the following text.

    firewall_mode=

    3. Save the file and reboot.

  17. See the Fingerprint page to discover what fingerprinting means in this case.
  18. See Design: Dev/TimeSync.
  19. In this case, Tor cannot verify the Tor consensus.
  20. biossystemtimeoffset is used to unlink the virtualizer's initial clock synchronization of the VM from the host clock.
  21. After powering on a VM, it initially synchronizes the VM clock with the host clock until Whonix ™ Timesync adjusts it.
  22. Clock skews can lead to linkability, meaning the user would be pseudonymous rather than anonymous.


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