QubesOS Port Forwarding GSoC 2021

Proposal text

Introduction

Forwarding ports to Qubes VM is currently possible only though a multi step, error prone,manual process that also requires writing custom configuration in order to be persistentbetween reboots. Things as simple as starting a webserver or netcat for LAN file sharing canbe eventually a troublesome and time-wasting process[1][2]. Furthermore, applications thatrely on NAT traversal protocols such as those for audio and video communications do not workin direct P2P mode with STUN and always use TURN instead[3].

Project Goals

Implement a GUI for automatic and persistent, eventually with a predefined timespan (ie: untilreboot), port forwarding. The idea is to split horizontally the "Firewall Rules" tab in the"Qubes Settings" window and add another area below it. It is aloready possible to forward TCPstreams, however there is no GUI nor a clear dashboard and furthermore its versatility islimited.Additionally, discuss and verify the possibility to implement a secure NAT traversal systemand GUI. A basic proposal could be a checkbox to enable NAT traversal requests. When thecheckbox is selected, the FirwallVM will redirect NAT traversal requests to a local pythondaemon or a dedicated VM that will negotiate the NAT traversal and configure the networkaccordingly. In this case, prompt the user in Dom0 about the NAT traversal request. Of coursethe qvm-* set of tools must e able to achieve the same tasks via CLI.

Implementation

First develop and document the part related to manual port forwarding since it is both a morefrequent use case and is less complicated. Depending on the problems encountered, evaluate thefeasibility of secure NAT traversal.

Notes

1. https://github.com/QubesOS/qubes-issues/issues/3556
2. https://www.reddit.com/r/Qubes/comments/8cb57i/how_to_achieve_qube_to_qube_communication_port/
3. https://github.com/QubesOS/qubes-issues/issues/6225

Development

Background

• https://www.qubes-os.org/doc/admin-api/
• https://www.qubes-os.org/doc/vm-interface/#firewall-rules-in-4x
• https://www.qubes-os.org/doc/firewall/
• https://www.qubes-os.org/doc/config-files/

Main components involved

1. Firewall GUI in "Settings" (qubes-manager)
2. CLI interface available via qvm-firewall (core-admin-client)
3. Actual client logic for the Admin API (core-admin-client)
4. Admin API interface - XML conf manager (core-admin)
5. Agent running in firewall vm - executes nft or iptables

Current Status

How does the GUI and qvm-firewall configuration work?

The Qubes Manager GUI and the qvm-firewall both use the code imlemented in the Admi API Client library. The Client Library sends specific messages to the qubesd daemon. The currently supported operatins are:

• admin.vm.firewall.Get
• admin.vm.firewall.Set
• admin.vm.firewall.Reload

These actions can be tested by using the qvm-firewall utility. It is important to note that both the client and the daemon are more flexibile compared to the settings available via the GUI.

Configuration files

If any non default configuration is set by the user, an AppVM will have a firewall.xml configuration file in the var/lib/qubes/<appvm>/ path. Deleting the file will reset the firewall to the default state and any customization will be lost.

The firewall.xml is clearly human readable and contains rules in the form:

<firewall version="2">
	<rules>
		<rule>
			<properties>
				<!-- accept outgoing to lsd.cat porto tcp port 443 -->
				<property name="action">accept</property>
				<property name="dsthost">lsd.cat</property>
				<property name="proto">tcp</property>
				<property name="dstports">443</property>
			</properties>
		</rule>
		<rule>
			<properties>
				<!-- accept outgoing to 10.132.11.1/24 proto any -->
				<property name="action">accept</property>
				<property name="dsthost">10.132.11.1/24</property>
			</properties>
		</rule>
		<rule>
			<properties>
				<!-- allow outgoing dns queries. needed for domain based rules -->
				<property name="action">accept</property>
				<property name="specialtarget">dns</property>
			</properties>
		</rule>
		<rule>
			<properties>
				<!-- drop everything else -->
				<property name="action">drop</property>
			</properties>
		</rule>
	</rules>
</firewall>

Commands

The following command will return the firewall rules for <vmnname>.

qvm-firewall <vmname>

As can be seen, the output will show more colums that the GUI, specifically an EXPIRE, COMMENT, and SPECIAL TARGET columns will be displayed.

The following command will reload the persistent rules stored in firewall.xml of <vmname>

qvm-firewall <vmname> --reload

The following command can be used to add a rule. Not that if the GUI detects that the firewall has been edited from CLI, since it does not support all CLI settings, it will refuse to allow management again from the GUI.

qvm-firewall <vmname> add action=accept dsthost=1.1.1.1 proto=tcp dstports=80-80 expire=+5000 comment="cloudflare http test rule"

Proposal

Currently, all firewall rules have an action properties which can be either accept or drop. The plan is to add a third option forward specifically for implementing automatic port forwarding. Such options must be supported both in the configuration file and in the Admin API (client-server). Lastly, it shall be implemented in the agent daemon. The main issue however is the fact that currenly, the firewall client library is designated to operate only on the AppVM configured Firewall NetVM. However, in order to forward ports from the outside world, specific rules needs to be applied to the Firewall NetVM Networking NetVM. (ie: both is sys-firewall and sys-net, as currently done for manual port forwarding).

action=forward

Since in the case of port forwarding the target ip address would always be the <vmname> IP address, users should not be asked for a dsthost field. Adding a forward rule could look like this:

qvm-firewall <vmname> add action=forward proto=tcp type=external srcports=443-443 dstports=80443-80443 srchost=0.0.0.0/0 expire=+500000 comment="example https server rule"
qvm-firewall <vmname> add action=forward proto=tcp type=internal srcports=80-80 dstports=8000-8000 srchost=10.137.0.13 expire=+500000 comment="example internal simplehttpserver file sharing rule"

Of course expire= and comment= are optional fields.

		<rule>
			<properties>
				<!-- sample syntax for port forwarding -->
				<property name="action">forward</property>
				<property name="proto">tcp</property>
				<property name="type">external</property>
				<property name="srcports">443-443</property>
				<property name="dstports">80443-80443</property>
				<property name="srchost">0.0.0.0/0</property>
				<property name="comment">example https server rule</property>
			</properties>
		</rule>

Proposal chart

Required rules

In <networkvm>:

iptables -t nat -A PREROUTING -i <external_iinterface> -p tcp --dport <target_port> -d <interface_ip> -j DNAT --to-destination <firewallvm_ip>
iptables -I FORWARD 2 -i <external_iinterface> -d <firewallvm_ip> -p tcp --dport <target_port> -m conntrack --ctstate NEW -j ACCEPT
nft add rule ip qubes-firewall forward meta iifname <external_iinterface> ip daddr <firewallvm_ip> tcp dport <target_port> ct state new counter accept

In <firewallvm>:

iptables -t nat -A PREROUTING -i <interface> -p tcp --dport <target_port> -d <firewallvm_ip> -j DNAT --to-destination <appvm_ip>
iptables -I FORWARD 2 -i <interface> -d <appvm_ip> -p tcp --dport <target_port> -m conntrack --ctstate NEW -j ACCEPT
nft add rule ip qubes-firewall forward meta iifname <interface> ip daddr <appvm_ip> tcp dport <target_port> ct state new counter accept

in <appvm>:

iptables -w -I INPUT 5 -d <appvm_ip> -p tcp --dport <target_port> -m conntrack --ctstate NEW -j ACCEPT