The other two popular [approaches](https://blog.invisiblethings.org/2008/09/02/three-approaches-to-computer-security.html) are “Security by Correctness” and “Security by Obscurity.”
We don't believe either of these approaches are capable of providing reasonable security today, nor do we believe that they will be capable of doing so in the foreseeable future.
### How is Qubes different from other security solutions?
Please see [this article](https://blog.invisiblethings.org/2012/09/12/how-is-qubes-os-different-from.html) for a thorough discussion.
### Is Qubes just another Linux distribution?
If you really want to call it a distribution, then it's more of a "Xen distribution" than a Linux one.
But Qubes is much more than just Xen packaging.
It has its own VM management infrastructure, with support for template VMs, centralized VM updating, etc. It also has a very unique GUI virtualization infrastructure.
In short: these are non-realistic solutions today. We discuss this in further depth in our [Architecture Specification document](/attachment/wiki/QubesArchitecture/arch-spec-0.3.pdf).
We believe that this is currently the only practically viable approach to implementing strong isolation while simultaneously providing compatibility with existing applications and drivers.
In short: we believe the Xen architecture allows for the creation of more secure systems (i.e. with a much smaller TCB, which translates to a smaller attack surface).
We discuss this in much greater depth in our [Architecture Specification document](/attachment/wiki/QubesArchitecture/arch-spec-0.3.pdf).
### What about this other/new (micro)kernel/hypervisor?
Whenever starting a discussion about another (micro)kernel or hypervisor in relation to Qubes, we strongly suggest including answers to the following questions first:
1. What kinds of containers does it use for isolation? Processes? PV VMs? Fully virtualized VMs (HVMs)? And what underlying h/w technology is used (ring0/3, VT-x)?
2. Does it require specially written/built applications (e.g. patched Firefox)?
3. Does it require custom drivers, or can it use Linux/Windows ones?
4. Does it support VT-d, and does it allow for the creation of untrusted driver domains?
5. Does it support S3 sleep?
6. Does it work on multiple CPUs/Chipsets?
7. What are the performance costs, more or less? (e.g. "XYZ prevents concurrent execution of two domains/processes on shared cores of a single processor", etc.)
8. Other special features? E.g. eliminates cooperative covert channels between VMs?
Here are the answers for Xen 4.1 (which we use as of 2014-04-28):
1. PV and HVM Virtual Machines (ring0/3 for PV domains, VT-x/AMD-v for HVMs).
2. Runs unmodified usermode apps (binaries).
3. Runs unmodified Linux drivers (dom0 and driver domains). PV VMs require special written pvdrivers.
4. Full VT-d support including untrusted driver domains.
We have designed the GUI virtualization subsystem with two primary goals: security and performance.
Our GUI infrastructure introduces only about 2,500 lines of C code (LOC) into the privileged domain (Dom0), which is very little, and thus leaves little space for bugs and potential attacks.
At the same time, due to the smart use of Xen shared memory, our GUI implementation is very efficient, so most virtualized applications really feel as if they were executed natively.
### Do you recommend coreboot as an alternative to vendor BIOS?
Yes, where it is possible to use it an open source boot firmware ought to be more trustable than a closed source implementation. [coreboot](https://www.coreboot.org/) is as a result a requirement for [Qubes Certified Hardware](https://www.qubes-os.org/news/2016/07/21/new-hw-certification-for-q4/). The number of machines coreboot currently supports is limited and the use of some vendor supplied blobs is generally still required. Where coreboot does support your machine and is not already installed, you will generally need additional hardware to flash it. Please see the coreboot website / their IRC channel for further information.
Instead, if you want to obtain your own, trustworthy copy of this website in a secure way, you should clone our [website repo](https://github.com/QubesOS/qubesos.github.io), [verify the PGP signatures on the commits and/or tags](/security/verifying-signatures/#how-to-verify-qubes-repos) (signed by the [doc-signing keys](https://github.com/QubesOS/qubes-secpack/tree/master/keys/doc-signing)), then either [render the site on your local machine](https://github.com/QubesOS/qubesos.github.io/blob/master/README.md#instructions) or simply read the source, the vast majority of which was [intentionally written in Markdown so as to be readable as plain text for this very reason](/doc/doc-guidelines/#markdown-conventions).
We've gone to special effort to set all of this up so that no one has to trust the infrastructure and so that the contents of this website are maximally available and accessible.
A core tenet of the Qubes philosophy is "distrust the infrastructure," where "the infrastructure" refers to things like hosting providers, CDNs, DNS services, package repositories, email servers, PGP keyservers, etc.
As a project, we focus on securing endpoints instead of attempting to secure "the middle" (i.e., the infrastructure), since one of our primary goals is to free users from being forced to entrust their security to unknown third parties.
Instead, our aim is for users to be required to trust as few entities as possible (ideally, only themselves and any known persons whom they voluntarily decide to trust).
Users can never fully control all the infrastructure they rely upon, and they can never fully trust all the entities who do control it.
Therefore, we believe the best solution is not to attempt to make the infrastructure trustworthy, but instead to concentrate on solutions that obviate the need to do so.
We believe that many attempts to make the infrastructure appear trustworthy actually provide only the illusion of security and are ultimately a disservice to real users.
Since we don't want to encourage or endorse this, we make our distrust of the infrastructure explicit.
1. We [distrust the infrastructure](#what-does-it-mean-to-distrust-the-infrastructure), including GitHub (though there are aspects we're still [working on](https://github.com/QubesOS/qubes-issues/issues/3958)).
2. It's free (as in beer). We'd have to spend either time or money to implement a solution ourselves or pay someone to do so, and we can't spare either one right now.
3. It has low admin/overhead requirements, which is very important, given how little time we have to spare.
Also see: [Should I trust this website?](#should-i-trust-this-website)
### Why doesn't this website have security feature X?
Although we caution users against [placing undue trust in this website](#should-i-trust-this-website) because we [distrust the infrastructure](#what-does-it-mean-to-distrust-the-infrastructure), we have no objection to enabling website security features when doing so is relatively costless and provides some marginal benefit to website visitors.
### Can I run applications, like games, which require 3D support?
Those won’t fly.
We do not provide OpenGL virtualization for Qubes.
This is mostly a security decision, as implementing such a feature would most likely introduce a great deal of complexity into the GUI virtualization infrastructure.
However, Qubes does allow for the use of accelerated graphics (OpenGL) in Dom0’s Window Manager, so all the fancy desktop effects should still work.
For further discussion about the potential for GPU passthrough on Xen/Qubes, please see the following threads:
- [GPU passing to HVM](https://groups.google.com/group/qubes-devel/browse_frm/thread/31f1f2da39978573?scoring=d&q=GPU&)
- [Clarifications on GPU security](https://groups.google.com/group/qubes-devel/browse_frm/thread/31e2d8a47c8b4474?scoring=d&q=GPU&)
### Is Qubes a multi-user system?
No.
Qubes does not pretend to be a multi-user system.
Qubes assumes that the user who controls Dom0 controls the whole system.
However, in Qubes 4.x we will be implementing management functionality. See [Admin API](/news/2017/06/27/qubes-admin-api/) and [Core Stack](/news/2017/10/03/core3/) for more details.
Each qube is created from a TemplateVM and shares the root filesystem with this TemplateVM (in a read-only manner).
This means that each qube needs only as much disk space as is necessary to store its own private data.
This also means that it is possible to update the software for several qubes simultaneously by running a single update process in the TemplateVM upon which those qubes are based.
(These qubes will then have to be restarted in order for the update to take effect in them.)
Qubes 4.x requires Intel VT-x with EPT / AMD-V with RVI (SLAT) and Intel VT-d / AMD-Vi (aka AMD IOMMU) for proper functionality (see the [4.x System Requirements](/doc/system-requirements/#qubes-release-4x)). If you are receiving an error message on install saying your "hardware lacks the features required to proceed", check to make sure the virtualization options are enabled in your BIOS/UEFI configuration. You may be able to install without the required CPU features for testing purposes only, but VMs may not function correctly and there will be no security isolation. For more information, see our post on [updated requirements for Qubes-certified hardware](/news/2016/07/21/new-hw-certification-for-q4/).
You can even run a NetVM, but you will not benefit from DMA protection for driver domains.
On a system without VT-d, everything should work in the same way, except there will be no real security benefit to having a separate NetVM, as an attacker could always use a simple DMA attack to go from the NetVM to Dom0.
**Nonetheless, all of Qubes' other security mechanisms, such as qube separation, work without VT-d.
Therefore, a system running Qubes will still be significantly more secure than one running Windows, Mac, or Linux, even if it lacks VT-d.**
Without VT-d, any PCI device can access all the memory, regardless to which VM it is assigned (or if it is left in dom0).
Most PCI devices allow the driver to request an arbitrary DMA operation (like "put received network packets at this address in memory", or "get this memory area and send it to the network").
So, without VT-d, it gives unlimited access to the whole system.
Now, it is only a matter of knowing where to read/write to take over the system, instead of just crashing.
But since you can read the whole memory, it isn't that hard.
Now, how does this apply to Qubes OS?
The above attack requires access to a PCI device, which means that it can be performed only from NetVM / UsbVM, so someone must first break into one of those VMs.
But this isn't that hard, because there is a lot of complex code handling network traffic.
Most attacks on NetVM / UsbVM (but not all!) require being somewhat close to the target system - for example connected to the same WiFi network, or in the case of a UsbVM, having physical access to a USB port.
Some users have been able to do this, but it is neither recommended nor supported. Qubes should be installed bare-metal. (After all, it uses its own bare-metal hypervisor!)
A [terminal emulator](https://en.wikipedia.org/wiki/Terminal_emulator), nowadays often referred to as just a *terminal*, is a program which provides a text window.
Inside that window, a [shell](https://en.wikipedia.org/wiki/Shell_(computing)) is typically running in it.
A shell provides a [command-line interface](https://en.wikipedia.org/wiki/Command-line_interface) where the user can enter and run [commands](https://en.wikipedia.org/wiki/Command_(computing)).
See introductions on Wikibooks: [here](https://en.wikibooks.org/wiki/Fedora_And_Red_Hat_System_Administration/Shell_Basics), [here](https://en.wikibooks.org/wiki/A_Quick_Introduction_to_Unix) and [here](https://en.wikibooks.org/wiki/Bash_Shell_Scripting).
You may have an adapter (wired, wireless), that is not compatible with open-source drivers shipped by Qubes. There may be a binary blob, which provides drivers in the linux-firmware package.
Open a terminal and run `sudo dnf install linux-firmware` (or `sudo yum install linux-firmware` in Qubes versions prior to 3.2.1) in the TemplateVM upon which your NetVM is based. You have to restart the NetVM after the TemplateVM has been shut down.
For details, see the qubes-users mailing list threads [here](https://groups.google.com/d/topic/qubes-users/xPLGsAJiDW4/discussion) and [here](https://groups.google.com/d/topic/qubes-users/uN9G8hjKrGI/discussion).)
The best approach is to choose the right keyboard layout during the installation process. But if you want to change things afterwards, you can try this workaround.
Assuming XFCE desktop: in `Q` → `System Tools` → `Keyboard` → `Layout`, leave the checkbox "`Use system defaults`" checked. Do not customize the keyboard layout here.
This generates the appropriate configuration in `/etc/X11/xorg.conf.d/00-keyboard.conf`. This file is auto-generated. Do not edit it by hand, unless you know what you are doing.
More information in [this discussion](https://groups.google.com/d/topic/qubes-devel/d8ZQ_62asKI/discussion) and [this issue](https://github.com/QubesOS/qubes-issues/issues/1396).
Enable "debug mode" in the qube's settings, either by checking the box labeled "Run in debug mode" in the Qubes VM Manager qube settings menu or by running the [qvm-prefs command](/doc/dom0-tools/qvm-prefs/).)
In Qubes R2 any such errors were ignored. In Qubes R3.x they are not. In R4.x, devices that are automatically added to sys-net and sys-usb on install but do not support FLR will be attached with the no-strict-reset option, but see the related warning in the last sentence in this answer.
A device that does not support reset is not ideal and generally should not be assigned to a VM.
Alternatively you may be able to disable USB 3.0 in the BIOS.
If the BIOS does not have the option to disable USB 3.0, try running the following command in dom0 to [force USB 2.0 modes for the USB ports][force_usb2]:
* Go to the sysfs (`/sys/bus/pci`), find the right device, detach it from the pciback driver and attach back to the original driver. Replace `<BDF>` with your device, for example `00:1c.2`:
The recommended approach is to pass only the specific partition you intend to use from [`sys-usb`](/doc/usb/) to another qube via [qvm-block](/doc/dom0-tools/qvm-block/). They will show up in the destination qube as `/dev/xvd*` and must be mounted manually. Another approach is to attach the entire USB drive to your destination qube. However, this could theoretically lead to an attack because it forces the destination qube to parse the device's partition table. If you believe your device is safe, you may proceed to attach it.
In Qubes 4.0, this is accomplished with the Devices Widget located in the tool tray (default top right corner, look for an icon with a yellow square). From the top part of the list, click on the drive you want to attach, then select the qube to attach it to. Although you can also attach the entire USB device to a qube by selecting it from the bottom part of the list, in general this approach should not be used because you are exposing the target qube to unnecessary additional attack surface.
In Qubes 3.2, you can use the Qubes VM Manager. Simply insert your USB drive, right-click on the desired qube in the Qubes VM Manager list, click Attach/detach block devices, and select your desired action and device.
Although external media such as external hard drives or flash drives plugged in via USB are available in the USB qube, it is not recommended to access them directly from inside the USB qube. See ["How to attach USB drives"](/doc/usb/#how-to-attach-usb-drives) for more information.
### I keep getting "Failed to synchronize cache for repo" errors when trying to update my Fedora templates
This is general Fedora issue, not a Qubes-specific issue.
Usually, this is due to network problems (especially if downloading updates over Tor) or problems with the download mirrors.
Often, the problem can be resolved by trying again on a different connection (a different Tor circuit, if using Tor) or waiting and trying again later.
Here are some examples of non-Qubes reports about this problem:
You might even think that if a particular program or setting works so well for *you*, it would work well for *everyone*, so you'd actually be doing everyone a favor!
There is no particular configuration that will be ideal for everyone (despite how much you might feel that your preference would be better for everyone), so the best we can do is to put power in the hands of users to configure their Qubes installations the way they like (subject to security constraints, of course).
Please don't ask for your favorite program to be installed by default or for some setting that obviously varies by user preference to be changed so that it matches *your* preference.
This is an incredibly selfish attitude that demonstrates a complete lack of consideration for the thousands of other Qubes users who don't happen to share your preferences.
### Are there restrictions on the software that the Qubes developers are willing to use?
Yes.
In general, the Qubes developers will not use a piece of software unless there is an *easy* way to verify both its **integrity** and **authenticity**, preferably via PGP signatures (see [Verifying Signatures](/security/verifying-signatures/)).
Specifically:
* If PGP signatures are used, the signing key(s) should have well-publicized fingerprint(s) verifiable via multiple independent channels or be accessible to the developers through a web of trust.
* If the software is security-sensitive and requires communication with the outside world, a "split" implementation is highly preferred (for examples, see [Split GPG](/doc/split-gpg/) and [Split Bitcoin](/doc/split-bitcoin/)).
* If the software has dependencies, these should be packaged and available in repos for a [current, Qubes-supported version](/doc/supported-versions/#templatevms) of Fedora (preferred) or Debian (unless all the insecure dependencies can run in an untrusted VM in a "split" implementation).
* If the software must be built from source, the source code and any builders must be signed.
(Practically speaking, the more cumbersome and time-consuming it is to build from source, the less likely the developers are to use it.)
Since 2013 [Xen has not supported 32-bit x86 architecture](https://wiki.xenproject.org/wiki/Xen_Project_Release_Features) and Intel VT-d, which Qubes uses to isolate devices and drivers, is available on Intel 64-bit processors only.
In addition, with features like improved ASLR, it is often more difficult to exploit a bug on x64 Linux than x86 Linux.
While we designed Qubes from the beginning to limit potential attack vectors, we still realize that some of the code running in Dom0, e.g. our GUI daemon or xen-store daemon, however simple, might contain some bugs.
Plus since we haven't implemented a separate storage domain, the disk backends are in Dom0 and are "reachable" from the VMs, which adds up to the potential attack surface.
So, having faced a choice between 32-bit and 64-bit OS for Dom0, it was almost a no-brainer.
The 64-bit option provides some (little perhaps, but some) more protection against some classes of attacks, and at the same time does not have any disadvantages except the extra requirement of a 64 bit processor.
And even though Qubes now "needs" a 64 bit processor, it didn't make sense to run Qubes on a system without 3-4GB of memory, and those have 64-bit CPUs anyway.
### What is the recommended build environment for Qubes OS?
Any rpm-based, 64-bit environment, the preferred OS being Fedora.
### How do I build Qubes from sources?
See [these instructions](/doc/qubes-builder/).
### How do I submit a patch?
See the [Qubes Source Code Repositories](/doc/source-code/) article.
### What is Qubes' attitude toward changing guest distros?
We try to respect each distro's culture, where possible.
See the discussion on issue [#1014](https://github.com/QubesOS/qubes-issues/issues/1014) for an example.
The policy is there mostly to ease maintenance, on several levels:
* Less modifications means easier migration to new upstream distribution
releases.
* The upstream documentation matches the distribution running in the Qubes VM.
* We're less likely to introduce Qubes-specific issues.
* Each officially supported distribution (ideally) should offer the same set of
Qubes-specific features - a change in one supported distribution should be
followed also in others, including new future distributions.
No. Unlike many other virtualization systems, Qubes takes special effort to keep QEMU _outside_ of the TCB.
This has been achieved thanks to the careful use of Xen's stub domain feature.
For more details about how we improved on Xen's native stub domain use, see [here](https://blog.invisiblethings.org/2012/03/03/windows-support-coming-to-qubes.html).
See the Heads project [[1]](https://trmm.net/Heads) [[2]](http://osresearch.net/) for a better-designed non-UEFI-based secure boot scheme with very good support for Qubes.
Check `/usr/share/qubes/marker-vm` file existence. Additionally, its last line contains Qubes release version (`3.2`, `4.0` etc).
The file was introduced after initial Qubes 3.2 and 4.0 release. If you need to support not-fully-updated systems, check `/usr/bin/qrexec-client-vm` existence.
There is also the unofficial [ansible-qubes toolkit](https://github.com/Rudd-O/ansible-qubes).
(**Warning:** Since this is an external project that has not been reviewed or endorsed by the Qubes team, [allowing it to manage dom0 may be a security risk](/doc/security-guidelines/#dom0-precautions).)
