General Host Security
- 1 apt-transport-tor
- 2 DMZ
- 3 Hardware Security
- 4 Key Hardening Steps
- 5 Layered Defense
- 6 One VM Whonix ™ Configuration
- 7 Separate VirtualBox User Account
- 8 Tor
- 9 Torify apt-get Traffic
- 10 Tor Traffic Whitelisting Gateway
- 11 Virtualization Platform
- 12 Footnotes
apt-transport-tor is a package that allows host operating systems or non-Whonix-Workstation ™ VMs that are not behind a torifying gateway (like Whonix-Gateway ™) to torify their apt-get traffic for individual repositories.
With non-Whonix ™ systems in mind, for security reasons apt-get blocks clearnet connections to
.onion domains by default. apt-get developers want to protect users from accidentally trying to use
.onion repositories without using Tor. Otherwise, a rouge DNS server could redirect users to a false domain and trick them into thinking they are using Tor when they are not.
Strictly speaking, there is no need to use apt-transport-tor inside Whonix ™ VMs since all traffic is already routed over Tor. apt-get is stream-isolated using a pre-configured uwt wrapper. In other words, apt-get in Whonix ™ is already talking to a Tor
SocksPort. Nevertheless, apt-transport-tor (
tor+http) is the default from Whonix ™ 14 onward because it provides better error handling and stream isolation.   
A properly configured DMZ restricts Whonix-Gateway ™ (
sys-whonix) from accessing, and being accessible to, other nodes on the network like printers, phones, computers and laptops. This is true even if root access is somehow achieved.
Should an incursion take place, a DMZ prevents an adversary from exploring other systems and possibly compromising them. However, in this case a DMZ does not protect the user's anonymity, since the adversary could just ping a remote server and discover the real IP address. Another benefit of a DMZ is that should other systems be compromised, it is more difficult to compromise Whonix-Gateway ™ (
Trusted computer hardware is fundamental to anonymity and security. It is recommended to purchase and use "clean" computers that have components manufactured by reputable companies. It is preferable to pay in cash so hardware IDs do not leak your identity.
As outlined in the System Configuration and Access entry, it is safest to purchase a computer that is solely used for Whonix ™ activities because this minimizes the risk of a prior hardware compromise.
Key Hardening Steps
For greater security, advanced users should harden the host OS as much as is practicably possible. This includes, but is not limited to applying relevant steps from the System Hardening Checklist and instructions found throughout this section:
- Disable and minimize hardware risks.
- Securely download files and install software on the host.
- Avoid insecure dual-boot arrangements [archive].
- Choose a host OS that is compatible with privacy and security goals.
- Harden the host (Debian) GNU/Linux OS:
- Apply instructions to harden networking, including:
- Spoof MAC addresses if necessary.
- Disable TCP and ICMP timestamps.
- Remove the NTP client and disable the NTP synchronization feature.
Attack Surface Reduction
In addition to the checklist above, it is suggested to also follow the principles of minimizing the attack surface [archive] of the OS, and securely configuring services -- for example when using SSH, implementing Fail2ban [archive] so only key authentication is allowed.
The attack surface concept deserves more consideration. Simply put, it is the sum of different attack vectors (aggregate of vulnerabilities) where an unauthorized user can try to enter or extract data from an environment.  To reduce the attack surface and mitigate risks, it is necessary to: 
- Enforce least privilege for all executed processes and reduce entry points for untrusted users.
- Control system and network segment access across the network, for example, reduce (unauthenticated) access to network endpoints.
- Minimize exposed system targets by reducing the amount of code running and removing unnecessary functionality.
- Remove or shutdown software and services (channels, protocols) that are infrequently or rarely used.
- Frequently patch security vulnerabilities.
This includes, but is not limited to:
- Compile time hardening [archive]
- Intrusion Prevention Systems [archive]
- Mandatory Access Control [archive]
The usefulness of this approach is limited because it does not prevent security breaches; it can only help in making future breaches less probable:
- Anti-virus and anti-malware programs.
- Intrusion Detection System (IDS) [archive].
- Rootkit Hunter (rkhunter) [archive].
- Snort network intrusion prevention system [archive].
- sxid file and directory change tracker [archive].
- TIGER security auditing program [archive].
The programs listed in this section are only a very brief introduction to this topic. If interested, users should research these topics in depth because they are beyond the scope of this guide.
One VM Whonix ™ Configuration
This platform was developed and tested successfully for Whonix ™ v0.1.3.
Basically, it is possible to use one VM instead of two, with Tor running on the host OS and a single client VM routing activities via Tor. This configuration has several advantages and disadvantages relating to security and other matters. For further information, see OneVM.
Separate VirtualBox User Account
Security-wise, it makes sense to create a separate user account solely for using VirtualBox, which is not in the admin/sudo group.
Torify apt-get Traffic
It is recommended to torrify APT's traffic on the host for several reasons:
- Each machine has its own unique package selection. This allows location tracking, because systems can be fingerprinted across physical networks as system updates are performed.
- System updates leak sensitive security information like package versions and the varying patch levels. This information aids targeted attacks.
Follow the instructions below to torify APT traffic in Debian. 
Other URL Configurations
Alternatively, the tor+http:// URL scheme is possible. apt-transport-tor can also be combined with apt-transport-https, leading to the tor+https:// URL scheme. 
Note that changing
http.debian.net picks a mirror near to whichever Tor exit node is being used. Throughput is surprisingly fast.  Also be aware that all public-facing debian.org FTP services were shut down on November 1, 2017 [archive]. 
Debian URLs can also be pointed to the available onion services
http://vwakviie2ienjx6t.onion [archive] and
http://sgvtcaew4bxjd7ln.onion [archive]. This is the most secure option, as no package metadata ever leaves Tor.    This URL scheme also protects from system compromise in the event APT has a critical security bug. The following entries should work in the sources list:
deb tor+http://vwakviie2ienjx6t.onion/debian stretch main deb tor+http://vwakviie2ienjx6t.onion/debian stretch-updates main deb tor+http://sgvtcaew4bxjd7ln.onion/debian-security stretch/updates main #deb tor+http://vwakviie2ienjx6t.onion/debian stretch-backports main
Tor Traffic Whitelisting Gateway
It is possible to configure Whonix-Gateway ™ (
sys-whonix) to use corridor as a local proxy to establish the following tunnel:
This approach is not necessarily more anonymous, but it is an additional fail-safe since a Tor traffic whitelisting gateway can help protect from accidental clearnet leaks.
VirtualBox is developed by Oracle, a company which has a reputation of not being very "open". In the past, concerns have been raised about how they announce security issues in their products and how well they communicate with each other, leading to a negative perception by the security community.
VirtualBox is primarily a simple, "user-friendly", desktop solution and is most certainly not designed with the Whonix ™ threat model in mind. Development is reported to be at a standstill and the author is not aware of any serious code audits having been completed.  Whonix ™ developers would like to recommend a different VM solution at least as an alternative, but many popular, open source options like KVM and Xen are not cross-platform. Further, the latter examples seem to still lack a reliable "internal networking" feature, which Whonix ™ heavily depends upon. Any readers who have in-depth knowledge of this issue are encouraged to edit this paragraph accordingly.
Users that have a strong preference for security should strongly consider using Qubes-Whonix ™, if they have suitably modern hardware. In short, Qubes-Whonix ™ is more secure than the default Whonix ™ configuration using a Type 2 hypervisor like VirtualBox.
Related VirtualBox Links:
- Virtualization Platform [archive] for licensing issues.
- Why Use KVM Over VirtualBox?
- Why use Qubes over other Virtualizers?
When running guest operating systems in full screen mode, the guest operating system usually has control over the whole screen. This could present a security risk as the guest operating system might fool the user into thinking that it is either a different system (which might have a higher security level) or it might present messages on the screen that appear to stem from the host operating system.
In order to protect the user against the above mentioned security risks, the secure labeling feature has been developed. Secure labeling is currently available only for VBoxSDL. When enabled, a portion of the display area is reserved for a label in which a user defined message is displayed. The label height in set to 20 pixels in VBoxSDL. The label font color and background color can be optionally set as hexadecimal RGB color values.
Any readers who are knowledgeable in this area are encouraged to share their expertise and edit this section accordingly.
Before this feature could be implemented in Whonix ™, one prerequisite is that users do not end up with a non-standard desktop resolution, as this degrades anonymity as per Protocol Leak Protection and Fingerprinting Protection.
As noted in the Virtualization Platform Security entry, there are two platforms providing greater security than the standard host OS / Type 2 hypervisor Whonix ™ configuration:
- Qubes-Whonix ™.
- Whonix ™ with Physical Isolation, set up using two different computers and virtualization.
In contrast to Qubes-Whonix ™, physical isolation is:
- Difficult to set up.
- Inconvenient and still experimental.
- Requires a significant time investment.
- Not clearly superior to Qubes' compartmentalized software approach [archive].
- Does not support Qubes features like:
- A USB VM.
- Secure copy and paste operations.
- Secure copying and transfer of files.
- PDF/image sanitization.
- An ephemeral Whonix-Gateway ™ ProxyVM and/or Whonix-Workstation ™ AppVM. 
- source: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=754242#54 [archive]
That said, it might make sense to use a-t-tor anyhow even if not strictly needed as it will deal better with certain tor anomalies given that it knows tor is involved reporting better errors (like telling you that the .onion address you typo'ed is too long/short; saying "unreachable host" if a service is… well, not reachable, instead of saying "TTL expired" which is reported by Tor and technically more correct but unhelpful), will use different circuits for different sources and stuff.
- For instance it reports if the .onion address is too long or short, and will use different circuits for different sources.
- apt-transport-tor will not result in Tor over Tor scenarios due to built-in Whonix ™ settings preventing this [archive].
- Wikipedia: Attack surface [archive]
- http://resources.infosecinstitute.com/attack-surface-reduction/ [archive]
- https://packages.debian.org/apt-transport-tor [archive]
- https://lwn.net/Articles/672350/ [archive]
- https://retout.co.uk/blog/2014/07/21/apt-transport-tor [archive]
- ftp://ftp.debian.org and ftp://security.debian.org
- http://richardhartmann.de/blog/posts/2015/08/24-Tor-enabled_Debian_mirror/ [archive]
- https://onion.debian.org [archive]
- https://onion.torproject.org [archive]
- https://developers.slashdot.org/story/15/01/30/1530245/virtualbox-development-at-a-standstill/ [archive]
- Partially because it is not available on the macOS platform.
- In Qubes R4.
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