Building an OpenBSD WireGuard VPN server part 3 – Unbound DNS filtering

In part 2, I reconfigured my WireGuard VPN to use an Unbound DNS server on the VPN server rather than rely on a third party server I had used for the original quick and dirty configuration. It was important for me to set up a validating DNS server, which I did in that part.

In this part, I’m extending the existing configuration to include some basic block lists for known ad and tracking servers. As I’m mainly trying to use the VPN while on the road, I want to ensure that anything I end up doing using the VPN is as secure as I can make it with reasonable effort. That makes tracking and preventing malicious ads the next step. That said, I’m not planning to go for a full Pi-Hole like setup. Initially, I am trying to do is integrate one known good blocklists into the Unbound configuration and automate the process. I can get fancy with a more Pi-Hole like setup later if I want to.

Picking a DNS filter list and using it from Unbound

I started by using a single DNS block list from StevenBlack’s github repo. deadc0de.re’s blog has a good how-to post including the necessary awk incantations for converting the file into the format Unbound needs. I used that blog post as my as my starting point.

The process overall is relatively simple. First, pick the flavour of blocklist you want. Download the hosts file for the blocklist. I used wget to download it as it’s already installed on my OpenBSD system, but curl would also work. My assumption is that you’re running these commands in /var/unbound/etc, so all the absolute paths refer to that location.

wget -O /tmp/blocklist-hosts https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts

This list is in the traditional host file format, so we need to convert it into the format that Unbound needs. Fortunately, deadc0de.re’s blog post contains the necessary code to accomplish that transformation:

cat /tmp/blocklist-hosts | grep '^0\.0\.0\.0' | awk '{print "local-zone: \""$2"\" redirect\nlocal-data: \""$2" A 0.0.0.0\""}' > adblocker.conf

We also need to update our unbound.conf file to include the freshly generated host blocklist. To do that, we have to add an include statement to the server section of the configuration file. The change looks like this:


server:
  ...
  include: /var/unbound/etc/block-ads.conf

I also had to set up unbound-control to ensure that I didn’t have to restart the server every time I updated the block list. Thus, I had to make a slight detour that involved running unbound-control-setup to generate the necessary certificates. I also needed to add the following statement to my unbound.conf:

remote-control:
  control-enable: yes

Note that remote-control is its own section and is not part of the server: section. The default setup only accepts remote control connections on the loopback interface. This is perfect for my setup, so I didn’t need to make any other changes.
After restarting unbound to pick up the configuration changes I was able to test the VPN with the new DNS blocking and it proved to be mostly effective on the websites I visited.

Automating the process

Of course I’m a software engineer by background, so clearly I wanted to automate the process. Fortunately this is about as complicated as pasting the commands I listed above into a shell script and periodically run it. My script looks like this:

#!/bin/sh

cd /var/unbound/etc
wget -O /tmp/black-hosts https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts
cat /tmp/black-hosts | grep '^0\.0\.0\.0' | awk '{print "local-zone: \""$2"\" redirect\nlocal-data: \""$2" A 0.0.0.0\""}' > block-ads.conf
rm /tmp/black-hosts
unbound-control reload

I’m running this script via cron once a week, as that seems to roughly correspond to the update frequency of the list. I may change that in the future, especially if I end up combining the multiple block lists instead. In the meantime I don’t think it’s necessary to run the cron script more often.

What’s next?

Right now it looks like WireGuard VPN server is working as intended and does what I need it to. TuM’Fatig has a great blog post on expanding the approach I’m using to include all of the public block lists that Pi-Hole is using, so I’m probably going to look into that sooner or later. Other than that and keeping the machine up to date, the next project will be to create orchestration scripts to recreate the server from scratch. I found setting up WireGuard on OpenBSD much easier than doing it on a Linux host. Nevertheless having a Terraform or Ansible script that allowed me to quickly stand one up from scratch if the current instance was damage would likely be a good thing.

Would I go the OpenBSD route again?

I had mentioned at the start of this series that I hadn’t had done much with OpenBSD for years. Was it worth choosing OpenBSD over FreeBSD, which I use on a very regular basis? Maybe, maybe not. OpenBSD required less additional fiddling to secure it out of the box. Some of the time saved was taken up by reacquainting myself with the system, so I’d call that a wash. I suspect that setting up the actual WireGuard VPN instance would take the same amount of time on FreeBSD as I’d run the same commands, so no clear advantage there. I do like the newer version of pf that comes with OpenBSD – FreeBSD’s is a few versions behind and is missing some features, and I doubt it’ll ever catch up.

Put that down as a ‘maybe’.

How well does WireGuard work for me?

So far it is proving performant enough to run on one of Vultr’s $5 instances that aren’t exactly swimming in computing power. Of course this is only a personal VPN and I’m not running a whole company’s worth of traffic across it. That would very likely require more oomph (technical term) on the server, but overall WireGuard VPN appears to be both very lightweight and more importantly, easy to configure both at the server and client end.

Throughput is good enough to use it to watch YouTube videos in good quality on crappy hotel Wifi without visibly dropping frames. That’s good enough for me as a performance metric because it shows that the VPN server is not the bottleneck. The real use cases for the VPN really are for accessing more sensitive information like financial information, and that is usually all interactive anyway.

Building an OpenBSD WireGuard VPN server part 2 – Unbound DNS setup

In the first part, I described how I set up the basic OpenBSD WireGuard VPN server. I also hinted that I wanted to set up my own validating, filtering DNS server. With a little bit of spare time during the holidays I decided now was a good time as any.

Making sure the VPN server uses the local Unbound DNS resolver first

Before I set up Unbound itself, I need to make sure that the VPN server’s resolv.conf points at its local DNS server first.

My Vultr VPS has a static IP, which it receives via DHCP. It also receives information about its upstream DNS servers via DHCP. So as the first step, we need to update /etc/dhclient.conf to ensure that resolv.conf lists our local server first. We do this by adding the following line to /etc/dhclient.conf:

prepend domain-name-servers 127.0.0.1;

In fact, that is the complete dhclient.conf file on my VPN server.

Setting up Unbound as a validating DNS server

OpenBSD already includes the Unbound authoritative DNS server, so it’s a matter of configuring it to act as the local DNS server and also to enable validation. Everything you need to run the server is already on the system.

Root server list and trust anchor files

Once we told the local system to use the local Unbound as its DNS server, we need to configure it. Note that dhclient will still add the upstream provider DNS server to resolv.conf, so it is still there as a fallback.

Time to set up Unbound. A validating DNS server needs a few more pieces of data compared to a standard recursive DNS server. The two important items are the trust anchor and the root hints so Unbound knows where to find the root name servers. Note that OpenBSD runs Unbound in a sandbox and thus all the configuration is under /var/unbound.

We start with retrieving the root.hints file that contains the list of root DNS servers:

wget https://www.internic.net/domain/named.root -O /var/unbound/etc/root.hints

A quick check with more /var/unbound/etc/root.hints shows that we received the expected list of root servers.

The second part we need is the trust anchor, which is the cryptographic key our DNS server needs so it can validate DNSSEC. After a couple of false starts I simply ended up using unbound-anchor to download the trust anchor and configured Unbound via the auto-trust-anchor setting to automatically check and update the trust anchor when necessary.

Pretty much all of the Internet tutorials suggest that the trust anchor should be stored in /var/unbound/etc. In my OpenBSD setup, unbound can’t write to that directory so I ended up sotring it in /var/unbound/db instead.

Unbound.conf minimal setup for validating server

Now we’re finally ready to configure unbound itself. My basic, stripped down version of the configuration looks like this:


server:
  verbosity: 1
  #
  # Interfaces and access control. Make sure we're not suddenly
  # running an open public resolver
  #
  outgoing-interface: 'real' server ip'
  interface: 127.0.0.1
  interface: 192.168.1.1         # Internal VPN interface
  access-control: 127.0.0.0/8    allow
  access-control: 192.168.1.0/24 allow
  #
  # Basic configuration
  #
  port: 53
  do-ip4: yes
  do-ip6: no                     # VPN server currently doesn't support IPV6
  do-udp: yes
  do-tcp: yes
  #
  # Root servers, trust anchor and validation logging
  #
  root-hints: "/var/unbound/etc/root.hints"
  auto-trust-anchor-file: "/var/unbound/db/root.key"
  val-log-level: 2
  val-clean-additional: yes
  #
  # Some basic hardening
  #
  hide-identity: yes
  hide-version: yes
  harden-glue: yes
  harden-dnssec-stripped: yes
  use-caps-for-id: yes
  unwanted-reply-threshold: 10000
  #
  # Ensure we don't return private addresses to prevent DNS
  # rebinding attacks
  #
  private-address: 10.0.0.0/8
  private-address: 172.16.0.0/12
  private-address: 192.168.0.0/16
  #
  # Other settings. I don't expect to see a lot of DNS traffic so
  # I turned off prefetching
  #
  prefetch: no
  #
  num-threads: 1

Now that we have a working configuration file, we need to enable and start Unbound:

vpn# rcctl enable unbound
vpn# rcctl start unbound
unbound(ok)

Testing DNSSEC validation

At this point we have a working server with supposedly working DNSSEC validation. Obviously we work on ‘trust, but verify’. To check that we have indeed a working validating server, we can run the following command:

dig www.nic.cz. +dnssec

The header section of the result should look like this:

; <<>> DiG 9.4.2-P2 <<>> www.nic.cz. +dnssec
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18417
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

See the bolded ‘ad’ in the flags line? Now compare this to the output of the same command, but run on my MacBook using the ISP’s resolver:

; <<>> DiG 9.10.6 <<>> www.nic.cz. +dnssec
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12527
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

The ISP’s resolver doesn’t support DNSSEC in this case, so you can see the ‘ad’ flag missing. That flag indicates that the result from the upstream server validated.

Update the WireGuard config on the client

After I verified that the validation is working, it is time to update the WireGuard configuration on all clients so they point at our shiny new validating server rather than the third party DNS servers I used in the first installment.

Summary

At this point I have a validating DNS server, however it still passes through requests for every domain including the typical ad slinging domains. In the next installment of this series I’ll add filtering rules to try to get rid of most, if not all of them.