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.,

server:

# If no logfile is specified, syslog is used
# logfile: "/var/log/unbound/unbound.log"
verbosity: 0
interface: 127.0.0.1
port: 5335
do-ip4: yes
do-udp: yes
do-tcp: yes
# May be set to yes if you have IPv6 connectivity
do-ip6: no
# You want to leave this to no unless you have *native* IPv6. With 6to4 and
# Terredo tunnels your web browser should favor IPv4 for the same reasons
prefer-ip6: no
# Use this only when you downloaded the list of primary root servers!
# If you use the default dns-root-data package, unbound will find it automatically

# Trust glue only if it is within the server's authority
harden-glue: yes
# Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS
harden-dnssec-stripped: yes
# Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes
# see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details
use-caps-for-id: no
# Reduce EDNS reassembly buffer size.
# IP fragmentation is unreliable on the Internet today, and can cause
# transmission failures when large DNS messages are sent via UDP. Even
# when fragmentation does work, it may not be secure; it is theoretically
# possible to spoof parts of a fragmented DNS message, without easy
# detection at the receiving end. Recently, there was an excellent study
# >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<
# by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)
# in collaboration with NLnet Labs explored DNS using real world data from the
# the RIPE Atlas probes and the researchers suggested different values for
# IPv4 and IPv6 and in different scenarios. They advise that servers should
# be configured to limit DNS messages sent over UDP to a size that will not
# trigger fragmentation on typical network links. DNS servers can switch
# from UDP to TCP when a DNS response is too big to fit in this limited
# buffer size. This value has also been suggested in DNS Flag Day 2020.
edns-buffer-size: 1232
# Perform prefetching of close to expired message cache entries
# This only applies to domains that have been frequently queried
prefetch: yes
# One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.
num-threads: 1
# Ensure kernel buffer is large enough to not lose messages in traffic 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

4, 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

server:

# If no logfile is specified, syslog is used
# logfile: "/var/log/unbound/unbound.log"
verbosity: 0
interface: 127.0.0.1
port: 5335
do-ip4: yes
do-udp: yes
do-tcp: yes
# May be set to yes if you have IPv6 connectivity
do-ip6: no
# You want to leave this to no unless you have *native* IPv6. With 6to4 and
# Terredo tunnels your web browser should favor IPv4 for the same reasons
prefer-ip6: no
# Use this only when you downloaded the list of primary root servers!
# If you use the default dns-root-data package, unbound will find it automatically

# Trust glue only if it is within the server's authority
harden-glue: yes
# Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS
harden-dnssec-stripped: yes
# Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes
# see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details
use-caps-for-id: no
# Reduce EDNS reassembly buffer size.
# IP fragmentation is unreliable on the Internet today, and can cause
# transmission failures when large DNS messages are sent via UDP. Even
# when fragmentation does work, it may not be secure; it is theoretically
# possible to spoof parts of a fragmented DNS message, without easy
# detection at the receiving end. Recently, there was an excellent study
# >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<
# by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)
# in collaboration with NLnet Labs explored DNS using real world data from the
# the RIPE Atlas probes and the researchers suggested different values for
# IPv4 and IPv6 and in different scenarios. They advise that servers should
# be configured to limit DNS messages sent over UDP to a size that will not
# trigger fragmentation on typical network links. DNS servers can switch
# from UDP to TCP when a DNS response is too big to fit in this limited
# buffer size. This value has also been suggested in DNS Flag Day 2020.
edns-buffer-size: 1232
# Perform prefetching of close to expired message cache entries
# This only applies to domains that have been frequently queried
prefetch: yes
# One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.
num-threads: 1
# Ensure kernel buffer is large enough to not lose messages in traffic 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

4. 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.

What does this guide provide?

In only a few simple steps, we will describe how to set up your own recursive DNS server. It will run on the same device you're already using for your Pi-hole. There are no additional hardware requirements.

This guide assumes a fairly recent Debian/Ubuntu-based system and will use the maintainer provided packages for installation to make it an incredibly simple process. It assumes only a very basic knowledge of how DNS works.

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

1. Your client asks the Pi-hole server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   6?
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 to 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 guide, this procedure changes notably:

1. Your client asks the Pi-hole server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   6?
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 server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   8?"
6. The root server answers with a referral to the TLD servers for server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   8.
7. Your recursive server will send a query to one of the TLD DNS servers for server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   8: "Who is handling

   server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   4?"
8. The TLD server answers with a referral to the authoritative name servers for server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   4.
9. Your recursive server will send a query to the authoritative name servers: "What is the IP of server:

   # If no logfile is specified, syslog is used  
   # logfile: "/var/log/unbound/unbound.log"  
   verbosity: 0  
   interface: 127.0.0.1  
   port: 5335  
   do-ip4: yes  
   do-udp: yes  
   do-tcp: yes  
   # May be set to yes if you have IPv6 connectivity  
   do-ip6: no  
   # You want to leave this to no unless you have native IPv6. With 6to4 and  
   # Terredo tunnels your web browser should favor IPv4 for the same reasons  
   prefer-ip6: no  
   # Use this only when you downloaded the list of primary root servers!  
   # If you use the default dns-root-data package, unbound will find it automatically  
   
   

   # root-hints: "/var/lib/unbound/root.hints"

   # Trust glue only if it is within the server's authority  
   harden-glue: yes  
   # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
   harden-dnssec-stripped: yes  
   # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
   # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
   use-caps-for-id: no  
   # Reduce EDNS reassembly buffer size.  
   # IP fragmentation is unreliable on the Internet today, and can cause  
   # transmission failures when large DNS messages are sent via UDP. Even  
   # when fragmentation does work, it may not be secure; it is theoretically  
   # possible to spoof parts of a fragmented DNS message, without easy  
   # detection at the receiving end. Recently, there was an excellent study  
   # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
   # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
   # in collaboration with NLnet Labs explored DNS using real world data from the  
   # the RIPE Atlas probes and the researchers suggested different values for  
   # IPv4 and IPv6 and in different scenarios. They advise that servers should  
   # be configured to limit DNS messages sent over UDP to a size that will not  
   # trigger fragmentation on typical network links. DNS servers can switch  
   # from UDP to TCP when a DNS response is too big to fit in this limited  
   # buffer size. This value has also been suggested in DNS Flag Day 2020.  
   edns-buffer-size: 1232  
   # Perform prefetching of close to expired message cache entries  
   # This only applies to domains that have been frequently queried  
   prefetch: yes  
   # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
   num-threads: 1  
   # Ensure kernel buffer is large enough to not lose messages in traffic 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  
   

   4?"
10. The authoritative server will answer with the IP address of the domain server:

    # If no logfile is specified, syslog is used  
    # logfile: "/var/log/unbound/unbound.log"  
    verbosity: 0  
    interface: 127.0.0.1  
    port: 5335  
    do-ip4: yes  
    do-udp: yes  
    do-tcp: yes  
    # May be set to yes if you have IPv6 connectivity  
    do-ip6: no  
    # You want to leave this to no unless you have native IPv6. With 6to4 and  
    # Terredo tunnels your web browser should favor IPv4 for the same reasons  
    prefer-ip6: no  
    # Use this only when you downloaded the list of primary root servers!  
    # If you use the default dns-root-data package, unbound will find it automatically  
    
    

    # root-hints: "/var/lib/unbound/root.hints"

    # Trust glue only if it is within the server's authority  
    harden-glue: yes  
    # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS  
    harden-dnssec-stripped: yes  
    # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes  
    # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details  
    use-caps-for-id: no  
    # Reduce EDNS reassembly buffer size.  
    # IP fragmentation is unreliable on the Internet today, and can cause  
    # transmission failures when large DNS messages are sent via UDP. Even  
    # when fragmentation does work, it may not be secure; it is theoretically  
    # possible to spoof parts of a fragmented DNS message, without easy  
    # detection at the receiving end. Recently, there was an excellent study  
    # >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<  
    # by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)  
    # in collaboration with NLnet Labs explored DNS using real world data from the  
    # the RIPE Atlas probes and the researchers suggested different values for  
    # IPv4 and IPv6 and in different scenarios. They advise that servers should  
    # be configured to limit DNS messages sent over UDP to a size that will not  
    # trigger fragmentation on typical network links. DNS servers can switch  
    # from UDP to TCP when a DNS response is too big to fit in this limited  
    # buffer size. This value has also been suggested in DNS Flag Day 2020.  
    edns-buffer-size: 1232  
    # Perform prefetching of close to expired message cache entries  
    # This only applies to domains that have been frequently queried  
    prefetch: yes  
    # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.  
    num-threads: 1  
    # Ensure kernel buffer is large enough to not lose messages in traffic 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  
    

    4.
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 to 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 favorite 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 traverse the path if you visit a page for the first time. The 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 sudo service unbound restart

  dig pi-hole.net @127.0.0.1 -p 5335

  5. 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

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

6, 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:

If you are installing unbound from a package manager, it should install the

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

7 file automatically with the dependency

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

8. The root hints will then be automatically updated by your package manager.

Optional: Download the current root hints file (the list of primary root servers which are serving the domain "." - the root domain). Update it roughly every six months. Note that this file changes infrequently. This is only necessary if you are not installing unbound from a package manager. If you do this optional step, you will need to uncomment the

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

9 configuration line in the suggested config file.

wget https://www.internic.net/domain/named.root -qO- | sudo tee /var/lib/unbound/root.hints

Configure

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

6

Highlights:

• Listen only for queries from the local Pi-hole installation (on port 5335)
• Listen for both UDP and TCP requests
• Verify DNSSEC signatures, discarding BOGUS domains
• Apply a few security and privacy tricks

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

1:

server:

# If no logfile is specified, syslog is used
# logfile: "/var/log/unbound/unbound.log"
verbosity: 0
interface: 127.0.0.1
port: 5335
do-ip4: yes
do-udp: yes
do-tcp: yes
# May be set to yes if you have IPv6 connectivity
do-ip6: no
# You want to leave this to no unless you have *native* IPv6. With 6to4 and
# Terredo tunnels your web browser should favor IPv4 for the same reasons
prefer-ip6: no
# Use this only when you downloaded the list of primary root servers!
# If you use the default dns-root-data package, unbound will find it automatically

# Trust glue only if it is within the server's authority
harden-glue: yes
# Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS
harden-dnssec-stripped: yes
# Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes
# see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details
use-caps-for-id: no
# Reduce EDNS reassembly buffer size.
# IP fragmentation is unreliable on the Internet today, and can cause
# transmission failures when large DNS messages are sent via UDP. Even
# when fragmentation does work, it may not be secure; it is theoretically
# possible to spoof parts of a fragmented DNS message, without easy
# detection at the receiving end. Recently, there was an excellent study
# >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<
# by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)
# in collaboration with NLnet Labs explored DNS using real world data from the
# the RIPE Atlas probes and the researchers suggested different values for
# IPv4 and IPv6 and in different scenarios. They advise that servers should
# be configured to limit DNS messages sent over UDP to a size that will not
# trigger fragmentation on typical network links. DNS servers can switch
# from UDP to TCP when a DNS response is too big to fit in this limited
# buffer size. This value has also been suggested in DNS Flag Day 2020.
edns-buffer-size: 1232
# Perform prefetching of close to expired message cache entries
# This only applies to domains that have been frequently queried
prefetch: yes
# One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.
num-threads: 1
# Ensure kernel buffer is large enough to not lose messages in traffic 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 restart dig pi-hole.net @127.0.0.1 -p 5335

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

You should also consider adding

to a config file like

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

2 to signal FTL to adhere to this limit.

Test validation

You can test DNSSEC validation using

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

The first command should give a status report of

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

3 and no IP address. The second should give

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

4 plus an IP address.

Configure Pi-hole

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

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

5 as the Custom DNS (IPv4):

(don't forget to hit Return or click on

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

6)

Disable

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

7 entry for

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

6 (Required for Debian Bullseye+ releases)

Debian Bullseye+ releases auto-install a package called

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

9 with a certain configuration that will cause unexpected behaviour for pihole and unbound. The effect is that the

systemctl is-active unbound-resolvconf.service

0 instructs

systemctl is-active unbound-resolvconf.service

1 to write

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

6's own DNS service at

systemctl is-active unbound-resolvconf.service

3 , but without the 5335 port, into the file

systemctl is-active unbound-resolvconf.service

4. That

systemctl is-active unbound-resolvconf.service

4 file is used by local services/processes to determine DNS servers configured. You need to edit the configuration file and disable the service to work-around the misconfiguration.

Step 1 - Disable the Service

To check if this service is enabled for your distribution, run below one. It will show either

systemctl is-active unbound-resolvconf.service

6 or

systemctl is-active unbound-resolvconf.service

7 or it might not even be installed resulting in a

systemctl is-active unbound-resolvconf.service

8 message:

systemctl is-active unbound-resolvconf.service

To disable the service, run the statement below:

sudo systemctl disable --now unbound-resolvconf.service

Step 2 - Disable the file resolvconf_resolvers.conf

Disable the file resolvconf_resolvers.conf from being generated when resolvconf is invoked elsewhere.

`sudo sed -Ei 's/^unbound_conf=/

unbound_conf=/' /etc/resolvconf.conf

sudo rm /etc/unbound/unbound.conf.d/resolvconf_resolvers.conf `

Restart

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

6.

sudo service unbound restart

Add logging to unbound

Warning

It's not recommended to increase verbosity for daily use, as unbound logs a lot. But it might be helpful for debugging purposes.

There are five levels of verbosity

Level 0 means no verbosity, only errors Level 1 gives operational information Level 2 gives detailed operational information Level 3 gives query level information Level 4 gives algorithm level information Level 5 logs client identification for cache misses

First, specify the log file, human-readable timestamps and the verbosity level in the

sudo systemctl disable --now unbound-resolvconf.service

0 part of

dig fail01.dnssec.works @127.0.0.1 -p 5335 dig dnssec.works @127.0.0.1 -p 5335

1:

server:

# If no logfile is specified, syslog is used
logfile: "/var/log/unbound/unbound.log"
log-time-ascii: yes
verbosity: 1

Second, create log dir and file, set permissions:

server:

# If no logfile is specified, syslog is used
# logfile: "/var/log/unbound/unbound.log"
verbosity: 0
interface: 127.0.0.1
port: 5335
do-ip4: yes
do-udp: yes
do-tcp: yes
# May be set to yes if you have IPv6 connectivity
do-ip6: no
# You want to leave this to no unless you have *native* IPv6. With 6to4 and
# Terredo tunnels your web browser should favor IPv4 for the same reasons
prefer-ip6: no
# Use this only when you downloaded the list of primary root servers!
# If you use the default dns-root-data package, unbound will find it automatically

# Trust glue only if it is within the server's authority
harden-glue: yes
# Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS
harden-dnssec-stripped: yes
# Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes
# see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details
use-caps-for-id: no
# Reduce EDNS reassembly buffer size.
# IP fragmentation is unreliable on the Internet today, and can cause
# transmission failures when large DNS messages are sent via UDP. Even
# when fragmentation does work, it may not be secure; it is theoretically
# possible to spoof parts of a fragmented DNS message, without easy
# detection at the receiving end. Recently, there was an excellent study
# >>> Defragmenting DNS - Determining the optimal maximum UDP response size for DNS <<<
# by Axel Koolhaas, and Tjeerd Slokker (https://indico.dns-oarc.net/event/36/contributions/776/)
# in collaboration with NLnet Labs explored DNS using real world data from the
# the RIPE Atlas probes and the researchers suggested different values for
# IPv4 and IPv6 and in different scenarios. They advise that servers should
# be configured to limit DNS messages sent over UDP to a size that will not
# trigger fragmentation on typical network links. DNS servers can switch
# from UDP to TCP when a DNS response is too big to fit in this limited
# buffer size. This value has also been suggested in DNS Flag Day 2020.
edns-buffer-size: 1232
# Perform prefetching of close to expired message cache entries
# This only applies to domains that have been frequently queried
prefetch: yes
# One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.
num-threads: 1
# Ensure kernel buffer is large enough to not lose messages in traffic 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

0

On modern Debian/Ubuntu-based Linux systems, you'll also have to add an AppArmor exception for this new file so

sudo service unbound restart dig pi-hole.net @127.0.0.1 -p 5335

6 can write into it.