Qubes network server

This software lets you turn your Qubes OS 4.2 machine into a network server, enjoying all the benefits of Qubes OS (isolation, secure inter-VM process communication, ease of use) with none of the drawbacks of setting up your own Xen server.

This release is only intended for use with Qubes OS 4.2. Older Qubes OS releases will not support it. For Qubes OS 4.1, check branch r4.1.

Important note about upgrades: when you upgrade your system from Qubes OS 4.1 to Qubes OS 4.2, if you have this package installed in your template, the template will likely fail to update. Please consult our upgrade instructions for information on how to proceed.

Why?

Qubes OS is a magnificent operating system. That said, there are many use cases its networking model does not work well for:

• Remote management of Qubes OS instances. Vanilla Qubes OS cannot easily be managed remotely. A better networking model would allow for orchestration tools — such as Ansible Qubes — to manage entire Qubes OS deployments, all of their VMs, and even minutiae within each VM.
• Anything that involves a secure server, serving data to people or machines, simply cannot be done under vanilla Qubes OS.
• As an automated integration testing system. Qubes OS would be phenomenal for this, and its automation tools would make it extremely easy to bring up and tear down entire environments. If only those environments could network with each other securely!

The traditional Qubes networking model

The traditional Qubes OS networking model contemplates a client-only use case. User VMs (AppVMs or StandaloneVMs) are attached to ProxyVMs, which give the user control over outbound connections taking place from user VMs. ProxyVMs in turn attach to NetVMs, which provide outbound connectivity for ProxyVMs and other user VMs alike.

No provision is made for running a server in a virtualized environment, such that the server's ports are accessible by (a) other VMs (b) machines beyond the perimeter of the NetVM. By default, firewall rules in NetVMs prevent traffic from reaching any VM attached to them. Furthermore, even with custom, permissive firewall rules, the IP addresses of VMs attached to any NetVM are not visible "on the other side of the NetVM", so firewall rules can only help with something like DNAT. Finally, such custom firewalls require careful thought not to open the system to unexpected attack vectors. The Qubes OS user interface provides no means to set this up either.

The Qubes network server networking model

Qubes network server builds on the Qubes security model and enhances it to optionally permit traffic to user VMs.

With the Qubes network server software, it becomes possible to make network servers in user VMs available to other machines, be them peer VMs in the same Qubes OS system or machines connected to a physical link shared by a NetVM. Those network server VMs also obey the Qubes OS outbound firewall rules controls, letting you run services with outbound connections restricted using the standard Qubes OS firewall system.

This is all, of course, opt-in, so the standard Qubes OS network security model remains in effect until you decide to enable the feature on any particular VM.

The only drawback of this method is that it requires you to attach VMs meant to be exposed to the network directly to a NetVM, rather than through a ProxyVM. VMs exposed through a ProxyVM will not be visible to machines on the same network as the NetVM.

How to use this software

Once installed (see below for installation instructions), usage of the software is straightforward.

These sample instructions assume:

• the software is properly installed (see below),
• you understand the distinction between dom0 and qubes,
• you already have an AppVM VM set up, named testvm, and
• your sys-net VM is attached to a network with subnet 192.168.16.0/24 — this, of course, may vary depending on your local router configuration.

Do not proceed any further if you do not yet meet these requirements.

First, attach the VM you want to expose to the network to a NetVM that has an active network connection:

qvm-prefs -s testvm netvm sys-net

Set an IP address on testvm belonging to the same LAN as sys-net:

qvm-prefs -s testvm ip 192.168.16.25

Restart the testvm VM if it was already running.

Configure routing method; to enable the network server feature for your testvm VM, all you have to do in your AdminVM (dom0) is run the following command:

qvm-features testvm routing-method forward

Now testvm is exposed to the network with address 192.168.16.25, as well as to other VMs attached to NetVM.

Finally, adjust input firewall rules on testvm to permit traffic coming from machines in your LAN. testvm will have the standard Qubes OS firewall rules stopping inbound traffic. To solve that issue, you can use a sample rule in testvm:

sudo nft add rule qubes custom-input ip saddr 192.168.16.0/24 ct state new,established,related counter accept

You can make these rules persistent by following instructions on the Qubes OS firewall documentation page. Note that you do not need to change the custom-forward chain at all on any qube -- Qubes network server manages that for you transparently in your NetVM.

Here are documents that will help you take advantage of Qubes network server:

• [Setting up your first server](doc/Setting up your first server.md)
• [Setting up an SSH server](doc/Setting up an SSH server.md)

Setup

Package installation consists of two steps (the package creation instructions are below):

1. Deploy the qubes-core-admin-addon-network-server RPM to your dom0.
2. Deploy the qubes-network-server RPM to the TemplateVM backing your NetVM (which must be a Fedora instance). If your NetVM is a StandaloneVM, then you must deploy this RPM to the NetVM directly.

After that, to make it all take effect:

1. Power off the TemplateVM.
2. Reboot the NetVM.

You're done. You can verify that the necessary component is running by launching a terminal in your NetVM, then typing the following:

systemctl status qubes-routing-manager.service

The routing manager should show as enabled and active in the terminal output, with no errors. You can now follow the usage instructions above.

How to build the packages to install

You will first build the qubes-core-admin-addon-network-server RPM.

To build this package, you will need to use a chroot jail containing a Fedora installation of the exact same release as your dom0 (Fedora 37 for Qubes 4.2). You can do this using toolbox (for maximum safety within a disposable qube):

dnf install -y toolbox
toolbox create -r 37
toolbox enter fedora-toolbox-37
# Bam!  You have a shell in an isolated Fedora 37 instance now.

Within the toolbox, all your normal files from your home directory are visible. Change into the directory that contains this source code, then type make rpm. You may have to install some packages within your toolbox -- use dnf install git rpm-build make coreutils tar gawk findutils systemd systemd-rpm-macros to get the minimum dependency set installed. Don't worry -- nothing within the toolbox affects

Once built, in the source directory you will find the qubes-core-admin-addon-network-server RPM built for your dom0.

To build the qubes-network-server RPM that goes in your template, you can simply use a DisposableVM running the same Fedora release as your NetVM. Build said package as follows:

# Dependencies
dnf install git rpm-build make coreutils tar gawk findutils systemd systemd-rpm-macros
# Source code
cd /path/to/qubes-network-server
# <make sure you are in the correct branch now>
make rpm

The process will output a qubes-network-server-*.noarch.rpm in the directory where it ran. Fish it out and copy it into the template where you'll install it.

You can power off the DisposableVM now.

Upgrading to new / bug-fixing releases

Follow the same procedures as above, but when asked to install the packages with rpm -ivh, change it to rpm -Uvh (uppercase U for upgrade).

Always restart your NetVMs between upgrades.

Theory of operation

Qubes OS relies on layer 3 (IP) routing. VMs in its networked tree send traffic through their default route interfaces, which upstream VMs receive and masquerade out of their own default route interfaces.

Qubes network server slightly changes this when a VM gets its routing-method feature set to forward. As soon as the feature is enabled with that value, or the VM in question boots up, Qubes network server:

• enables ARP neighbor proxying (and, if using IPv6, NDP neighbor proxying) in the NetVM
• sets firewall rules in the NetVM that neuter IP masquerading on traffic coming from the networked VM
• sets firewall rules in the NetVM that allow traffic from other VMs to reach the networked VM, neutering the default Qubes OS rule that normally prohibits this

The above have the effect of exposing the networked VM to:

• other AppVMs attached to the same NetVM
• other machines attached to the same physical network the NetVM is attached to

Now all machines in the same LAN will be able to reach the networked VM. Here is a step-by-step explanation of how IP traffic to and from the networked VM happens, when the routing-method is set to forward on the networked VM:

1. Machine in LAN asks for the MAC address of the networked VM's IP address.
2. NetVM sees the ARP/NDP request and responds by proxying it to the networked VM.
3. Networked VM replies to the ARP/NDP request back to the NetVM.
4. NetVM relays the ARP/NDP reply back to the network, but substitutes its own MAC address in the reply.
5. Machine in LAN sends local IP packet to the IP of the networked VM's IP address, but destined to the MAC address of the NetVM.
6. The NetVM sees the IP packet, and routes it to the networked VM.
7. The Networked VM receives the IP packet.
8. If the networked VM needs to respond, it sends an IP packet back to the machine in LAN.
9. NetVM notices packet comes from the networked VM, and instead of masquerading it, it lets the packet through unmodified, with the source IP address of the networked VM.

The end result is practical networking with no need to set up routing tables on machines attempting to access the networked VM.

Of course, if you want machines in the LAN to have access to the networked VM, you must still set an appropriate ip preference on the networked VM. For example, if your physical LAN had subnet 1.2.3.0/24, and you want machines in your physical LAN to connect to a networked VM, you must set the ip preference of the networked VM to a previously-unused IP within the range 1.2.3.1 and 1.2.3.255. Failing that, you must assign a host-specific route on the source machine which uses the NetVM as the gateway, and uses the IP address of the networked VM (see qvm-prefs output) as the destination address.

Limitations

• HVMs are not supported at all at this time. This will change over time, and you can help it change faster by submitting a pull request with HVM support.
• Interposing a ProxyVM between a networked VM and the NetVM is not currently supported. This is implementable in principle, but would require a recursive walk that encompasses the entire network link from NetVM through intermediate ProxyVMs.

Troubleshooting

The actions that the qubes-routing-manager service performs are logged to the journal of the NetVM where the qubes-routing-manager service is running.

In case of problems, tailing the journal (sudo journalctl -fa) on the NetVM will be extremely useful to figure out what the problem is.