Set up Pi-hole as truly self-contained DNS resolver.

The problem: Whom can you trust?

Pi-hole includes a caching and forwarding DNS server, now known as FTLDNS. After applying the blocking lists, it forwards requests made by the clients to configured upstream DNS server(s). However, as has been mentioned by several users in the past, this leads to some privacy concerns as it ultimately raises the question: Whom can you trust? Recently, more and more small (and not so small) DNS upstream providers have appeared on the market, advertising free and private DNS service, but how can you know that they keep their promises? Right, you can't.

Furthermore, from the point of an attacker, the DNS servers of larger providers are very worthwhile targets, as they only need to poison one DNS server, but millions of users might be affected. Instead of your bank's actual IP address, you could be sent to a phishing site hosted on some island. This scenario has already happened and it isn't unlikely to happen again...

When you operate your own (tiny) recursive DNS server, then the likeliness of getting affected by such an attack is greatly reduced.

What is a recursive DNS server?

The first distinction we have to be aware of is whether a DNS server is authoritative or not. If I'm the authoritative server for, e.g., pi-hole.net, then I know which IP is the correct answer for a query. Recursive name servers, in contrast, resolve any query they receive by consulting the servers authoritative for this query by traversing the domain. Example: We want to resolve pi-hole.net. On behalf of the client, the recursive DNS server will traverse the path of the domain across the Internet to deliver the answer to the question.

A standard Pi-hole installation will do it as follows:

1. Your client asks the Pi-hole Who is pi-hole.net?
2. Your Pi-hole will check its cache and reply if the answer is already known.
3. Your Pi-hole will check the blocking lists and reply if the domain is blocked.
4. Since neither 2. nor 3. is true in our example, the Pi-hole forwards the request to the configured external upstream DNS server(s).
5. Upon receiving the answer, your Pi-hole will reply to your client and tell it the answer of its request.
6. Lastly, your Pi-hole will save the answer in its cache to be able to respond faster if any of your clients queries the same domain again.

After you set up your Pi-hole as described in this wiki, this procedure changes notably:

1. Your client asks the Pi-hole Who is pi-hole.net?
2. Your Pi-hole will check its cache and reply if the answer is already known.
3. Your Pi-hole will check the blocking lists and reply if the domain is blocked.
4. Since neither 2. nor 3. is true in our example, the Pi-hole delegates the request to the (local) recursive DNS resolver.
5. Your recursive server will send a query to the DNS root servers: "Who is handling .net?"
6. The root server answers with a referral to the TLD servers for .net.
7. Your recursive server will send a query to one of the TLD DNS servers for .net: "Who is handling pi-hole.net?"
8. The TLD server answers with a referral to the authoritative name servers for pi-hole.net.
9. Your recursive server will send a query to the authoritative name servers: "What is the IP of pi-hole.net?"
10. The authorative server will answer with the IP address of the domain pi-hole.net.
11. Your recursive server will send the reply to your Pi-hole which will, in turn, reply to your client and tell it the answer of its request.
12. Lastly, your Pi-hole will save the answer in its cache to be able to respond faster if any of your clients queries the same domain again.

You can easily imagine even longer chains for subdomains as the query process continues until your recursive resolver reaches the authoritative server for the zone that contains the queried domain name. It is obvious that the methods are very different and the own recursion is more involved than "just" asking some upstream server. This has benefits and drawbacks:

• Benefit: Privacy - as you're directly contacting the responsive servers, no server can fully log the exact paths you're going, as e.g. the Google DNS servers will only be asked if you want to visit a Google website, but not if you visit the website of your favourite newspaper, etc.
• Drawback: Traversing the path may be slow, especially for the first time you visit a website - while the bigger DNS providers always have answers for commonly used domains in their cache, you will have to transverse the path if you visit a page for the first time time. A first request to a formerly unknown TLD may take up to a second (or even more if you're also using DNSSEC). Subsequent requests to domains under the same TLD usually complete in < 0.1s. Fortunately, both your Pi-hole as well as your recursive server will be configured for efficient caching to minimize the number of queries that will actually have to be performed.

Setting up Pi-hole as a recursive DNS server solution

We will use unbound, a secure open source recursive DNS server primarily developed by NLnet Labs, VeriSign Inc., Nominet, and Kirei. The first thing you need to do is to install the recursive DNS resolver:

sudo apt install unbound

Optional: Download the list of primary root servers (serving the domain .). Unbound ships its own list but we can also download the most recent list and update it whenever we think it is a good idea. Note: there is no point in doing it more often then every 6 months.

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

Configure unbound

Highlights:

• Listen only for queries from the local Pi-hole installation (on port 5353)
• Listen for both UDP and TCP requests
• Verify DNSSEC signatures, discarding BOGUS domains
• Apply a few security and privacy tricks

/etc/unbound/unbound.conf.d/pi-hole.conf:

server:
    verbosity: 0
    port: 5353
    do-ip4: yes
    do-udp: yes
    do-tcp: yes

    # May be set to yes if you have IPv6 connectivity
    do-ip6: no

    # Use this only when you downloaded the list of primary root servers!
    # Location of root.hints
    root-hints: "/var/lib/unbound/root.hints"

    # Trust glue only if it is within the servers authority
    harden-glue: yes

    # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS
    # If you want to disable DNSSEC, set harden-dnssec stripped: no
    harden-dnssec-stripped: yes

    # Use Capitalization randomization
    # This is an experimental resilience method which uses upper and lower case letters in the question hostname to obtain randomness.
    # Two names with the same spelling but different case should be treated as identical.
    # Attackers hoping to poison a DNS cache must guess the mixed-case encoding of the query.
    # This increases the difficulty of such an attack significantly
    use-caps-for-id: no
    
    # Reduce EDNS reassembly buffer size.
    # Suggested by the unbound man page to reduce fragmentation reassembly problems
    edns-buffer-size: 1472

    # TTL bounds for cache (Domains will be cached for minimum of 3600 seconds)
    cache-min-ttl: 3600
    cache-max-ttl: 86400

    # Perform prefetching of close to expired message cache entries
    # This only applies to domains that have been frequently queried
    # This flag updates the cached domains
    prefetch: yes

    # One thread should be sufficient, can be increased on beefy machines
    num-threads: 1
    
    # more cache memory. rrset-cache-size should twice what msg-cache-size is.
    msg-cache-size: 50m
    rrset-cache-size: 100m
    
    # Faster UDP with multithreading (only on Linux).
    so-reuseport: yes
	
    # Ensure kernel buffer is large enough to not loose messages in traffix spikes
    so-rcvbuf: 1m

    # Ensure privacy of local IP ranges
    private-address: 192.168.0.0/16
    private-address: 169.254.0.0/16
    private-address: 172.16.0.0/12
    private-address: 10.0.0.0/8
    private-address: fd00::/8
    private-address: fe80::/10

Start your local recursive server and test that it's operational:

sudo service unbound start
dig pi-hole.net @127.0.0.1 -p 5353

The first query may be quite slow, but subsequent queries, also to other domains under the same TLD, should be fairly quick.

Test validation

You can test DNSSEC validation using

dig sigfail.verteiltesysteme.net @127.0.0.1 -p 5353
dig sigok.verteiltesysteme.net @127.0.0.1 -p 5353

The first command should give a status report of SERVFAIL and no IP address. The second should give NOERROR plus an IP address.

Configure Pi-hole

Finally, configure Pi-hole to use your recursive DNS server:

(don't forget to hit Return or click on Save)

Content borrowed from Pi-Hole wiki.