Tuesday, 19 November 2013

How-to set up dnscrypt-proxy and unbound on Qubes OS

This article describes how to set up dnscrypt-proxy v1.3.3 and unbound 1.4.32 on Qubes OS v2 Beta 2.

dnscrypt-proxy is a tool which will encrypt DNS queries between the client and the resolver. It relies on libsodium which is an encryption library trying to make use of only well respected algorithms by the cryptographic community.
unbound is a is a validating, recursive, and caching DNS resolver.

Qubes OS is at the time of writing, to my view, one of the most secure Linux based desktop operating system. It can run Linux and Windows guest virtual machines and isolate VMs, networking, usb and graphical stacks levering on Xen hypervisor. It also leverage on a templating system to minimize the disk footprint of VMs based on the same template. It does not support 3D graphical operations out of the box but advanced users can set-up a second Video card and dedicate it to one of their VM.

The aim of this post is to allow new Qubes OS users to set-up a DNS service VM which will serve DNS queries for their VMs while ensuring encryption of the communication between their desktop and the DNS provider of their choice which support dnscrypt. unbound will be use to cache DNS queries and serve them to your VMs.
Discussion about this post in the Qubes OS user group can be found here

Preparing the Template

Installing Unbound

Launch the terminal in the Template VM.
  • Click Application Menu (top left)
  • Select Template fedora-18-x64 / fedora-18-x64: Terminal
Install the Unbound Server by typing in the template's Terminal:
  • sudo yum install unbound
For these changes to be visible to the DNS VM, shutdown the Template VM.
  • sudo halt

Setting up dnscrypt-proxy on a DNS VM

Creating a DNS VM

In Qubes OS, this is trivial.
In Qubes VM Manager:
  • Select in the menu VM / Create AppVM
  • Give it the name dns
  • Select the color yellow
  • Launch the creation by pressing OK
Note:  The attack surface of this VM will be fairly small. We have therefore used the yellow color.
Attacks could be launched by malformed DNS queries (from an untrusted VM aiming at compromising the DNS service data, i.e. Cache poisoning, or service) or responses (from a DNS provider). To mitigate the first risk, you may want to implement a trusted dnsVM and an untrusted dnsVM aimed at serving VM of different level of trust in you Qubes VMs setup.
Attacks could also be launched by a man in the middle (external network, or possibly from the netVM) by attacking the decryption stack that we will use, libsodium, to either gain visibility on the data transmitted or possibly compromising the dns service.
Overall you may want to use different client stack for the dns resolution by the dnsVM and your other AppVMs so that a vulnerability in one cannot be used to swim back to initial requester.

Starting the DNS VM

Launch the terminal in the dnsVM. In the Application Menu:
  • Select the Domain: dns / dns: Terminal

Installating the development tools

In Qubes OS, it is usually preferable to install software packages in the template VM from which the VM you will use the tool from is based on. In this case, these tools will only be required to compile 2 packages. It is therefore preferable to install them in the dnsVM. They will not be available any more after a dnsVM reboot, but we will have make sure everything is OK before proceeding.
In dns: Terminal
  • sudo yum groupinstall "Development Tools"

Downloading the source code

Note: The most detailed information on how to install dnscrypt-proxy was available on github.
Let's launch a web browser in a disposable VM and download our source...
In Application Menu:

Copying the source code to the dns VM

 This  is documented in the Qubes OS user documentation page.
  • In Firefox menu select Tools/Downloads.
  • Right-Click on dnscrypt-proxy-<version>.tar.bz2 and select Open Containing Folder.
  • Right-Click on the file and select Scripts / Copy to other AppVM.
  • input dns as the destination domain name.
  • Allow the transfer to happen by clicking Yes
  • Repeat the operation for the file libsodium-<version>.tar.gz.

 Verifying the source

In the dns VM Terminal:
  • Calculate the hash of dnscrypt-proxy: openssl dgst -sha256 ~/QubesIncoming/dispXX/dnscrypt-proxy-<version>.tar.bz2
In a new Disposable VM:
  • Launch terminal by selecting your new disposable VM in Qubes VM Manager and in the menu VM / Run command in VM, putting terminal as the command to execute.
  • Retrieve the original hash using DnsSec: dig +short +dnssec TXT dnscrypt-proxy-<version>.tar.bz2.download.dnscrypt.org
  • Compare the 2 hashes, they must be identical.
  • Repeat the process for the second file but this time check against: dig +dnssec +short TXT <file>.download.libsodium.org

Building the source

Let's start with Libsodium. In dns VM Terminal:
  • cd Documents
  • mkdir libsodium
  • cd libsodium
  • tar xzvf ~/QubesIncoming/disp<XX>/libsodium-<version>.tar.gz
  • cd libsodium-<version>
  • ./configure
  • make -j<numberOfCoresOnYourSystem>
  • sudo make install
As we use a RedHat based system, we need to make sure our libraries are linked. In the dns VM Terminal:
  • sudo su -
  • echo /usr/local/lib > /etc/ld.so.conf.d/usr_local_lib.conf
  • ldconfig
  • exit
Let's build dnscrypt-proxy now. In dns VM Terminal:
  • cd Documents
  • mkdir dnscrypt-proxy
  • cd dnscrypt-proxy
  • tar xjvf ~/QubesIncoming/disp<XX>/dnscrypt-proxy-<version>.tar.bz2
Note: We used the j option as the compression library used by this source is different...
  • cd dnscrypt-proxy-<version>
  • ./configure
  • make -j<numberOfCoresOnYourSystem>
  • sudo make install

Configuring the start up script

In dns VM Terminal:
  • cd /rw/config
  • sudo chmod u+x rc.local
  • sudo vi rc.local
  • #!/bin/bash
  • /bin/echo /usr/local/lib > /etc/ld.so.conf.d/usr_local_lib.conf
  • /sbin/ldconfig
  • /usr/local/sbin/dnscrypt-proxy --provider-key=B735:1140:206F:225D:3E2B:D822:D7FD:691E:A1C3:3CC8:D666:8D0C:BE04:BFAB:CA43:FB79 --provider-name=2.dnscrypt-cert.opendns.com. --resolver-address=208.67.220.220:443 --daemonize
Note: I have not yet been able to start the daemon as a low privileged user (tried to reuse tinyproxy).

Testing the daemon

Before rebooting the VM and loosing our development tools, let's check everything went according to plan... In dns VM Terminal:
  • cd /rw/config
  • sudo ./rc.local
  • nslookup
  • server 127.0.0.1
  • www.opendns.com
You should get the IPAddress of opendns.com web site...
Note: If after a reboot you try again the nslookup and is does not work, you probably forgot to update the libraries using ldconfig as described above.

Installing Unbound

Preparing the read-write partition

In the read-write partition, let's first prepare a set of folders on which we will deploy our configuration files so that they survive a reboot. In DNS VM's Terminal:
  • cd /rw
  • sudo mkdir -p config/unbound/conf.d
  • sudo mkdir -p config/unbound/local.d
  • sudo cp /etc/unbound/unbound.conf config/unbound

Configuring Unbound

Let's configure Unbound so that:
  • It listen to our eth0 interface IP Address (ifconfig | grep -i ast)
  • It control who can do DNS queries
  • It host a local zone
  • And forward to dns-crypt the rest...
  • sudo vi config/unbound/unbound.conf
add the lines:
  • interface: 10.137.2.x
  • access-control: 10.137.2.0/24 allow
  • access-control: 10.138.2.0/24 allow
  • access-control: x.x.x.x/y allow
  • harden-large-queries: yes
  • private-address: 10.0.0.0/8
  • private-address: 192.168.0.0/16
  • val-permissive-mode: yes
  • do-not-query-localhost: no
Let's now create our local zone:
  • sudo vi config/unbound/local.d/local.conf
  • local-zone: "my-intranet.com" transparent
  • local-data: "www.my-intranet.com A x.x.x.x"
  • local-data: "my-share.my-intranet.com A x.x.x.x"
Finally let's make sure we forward all non served locally requests to dns-crypt
  • forward-zone:
  •    name: "."
  •    forward-addr: 127.0.0.1@53

2 comments:

  1. Few points:
    - As of October 2014, OpenDNS does not support DNSSEC. You have to enable permissive mode in unbound.
    - rc.local file is missing few bits... here is my current one:
    #!/bin/bash

    /bin/echo /usr/local/lib > /etc/ld.so.conf.d/usr_local_lib.conf
    /sbin/ldconfig
    /usr/local/sbin/dnscrypt-proxy --provider-key=B735:1140:206F:225D:3E2B:D822:D7FD:691E:A1C3:3CC8:D666:8D0C:BE04:BFAB:CA43:FB79 --provider-name=2.dnscrypt-cert.opendns.com --resolver-address=208.67.220.220:443 --daemonize
    /usr/bin/sleep 2

    # Setting-up and starting unbound
    /usr/bin/cp /rw/config/unbound/unbound.conf /etc/unbound/
    /usr/bin/cp /rw/config/unbound/local.d/local.conf /etc/unbound/local.d/
    /usr/bin/systemctl start unbound &
    /usr/bin/sleep 2
    /usr/sbin/iptables -I INPUT 3 -j ACCEPT -d 10.137.2.x -p udp --sport 1024:65535 --dport 53 -m conntrack --ctstate NEW
    /usr/sbin/iptables -I INPUT 3 -j ACCEPT -d 10.137.2.x -p tcp --sport 1024:65535 --dport 53 -m conntrack --ctstate NEW

    ReplyDelete
  2. Your blog give many information thanks for share this informative article.
    access Monova in UK

    ReplyDelete