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

What is unbound?

Unbound is a validating, recursive, caching DNS resolver developed by NLnet Labs, VeriSign Inc., Nominet, and Kirei.

Setting up Pi-hole as a recursive DNS server solution

Install the Unbound recursive DNS resolver:

sudo apt install unbound

For recursively querying a host that is not cached as an address, the resolver needs to start at the top of the server tree and query the root servers, to know where to go for the top level domain for the address being queried. Unbound comes with default builtin hints. Remember to update this file every 6 months.

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

Configure unbound

Edit the config file by sudo nano /etc/unbound/unbound.conf.d/pi-hole.conf: And add the folowing contents:

server:

    # The  verbosity  number, level 0 means no verbosity, only errors.
    # Level 1 gives operational information. Level  2  gives  detailed
    # operational  information. Level 3 gives query level information,
    # output per query.  Level 4 gives  algorithm  level  information.
    # Level 5 logs client identification for cache misses.  Default is
    # level 1.
    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!
    # Read  the  root  hints from this file. Make sure to 
    # update root.hints evry 5-6 months.
    root-hints: "/var/lib/unbound/root.hints"
    
    # Trust glue only if it is within the servers authority
    harden-glue: yes
    
    # Ignore very large queries.
    harden-large-queries: 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
    
    # Number of bytes size to advertise as the EDNS reassembly buffer
    # size. This is the value put into  datagrams over UDP towards
    # peers. The actual buffer size is determined by msg-buffer-size
    # (both for TCP and UDP).
    edns-buffer-size: 1232
    
    # Rotates RRSet order in response (the pseudo-random 
    # number is taken from Ensure privacy of local IP 
    # ranges the query ID, for speed and thread safety).  
    # private-address: 192.168.0.0/16
    rrset-roundrobin: yes
    
    # Time to live minimum for RRsets and messages in the cache. If the minimum
    # kicks in, the data is cached for longer than the domain owner intended,
    # and thus less queries are made to look up the data. Zero makes sure the
    # data in the cache is as the domain owner intended, higher values,
    # especially more than an hour or so, can lead to trouble as the data in
    # the cache does not match up with the actual data anymore
    cache-min-ttl: 300
    cache-max-ttl: 86400
    
    # Have unbound attempt to serve old responses from cache with a TTL of 0 in
    # the response without waiting for the actual resolution to finish. The
    # actual resolution answer ends up in the cache later on. 
    serve-expired: yes
    
    # Harden against algorithm downgrade when multiple algorithms are
    # advertised in the DS record.
    harden-algo-downgrade: yes
    
    # Ignore very small EDNS buffer sizes from queries.
    harden-short-bufsize: yes
    
    # Refuse id.server and hostname.bind queries
    hide-identity: yes
    
    # Report this identity rather than the hostname of the server.
    identity: "Server"
    
    # Refuse version.server and version.bind queries
    hide-version: yes
    
    # Prevent the unbound server from forking into the background as a daemon
    do-daemonize: no
    
    # Number  of  bytes size of the aggressive negative cache.
    neg-cache-size: 4M
    
    # Send minimum amount of information to upstream servers to enhance privacy
    qname-minimisation: yes
    
    # Deny queries of type ANY with an empty response.
    # Works only on version 1.8 and above
    deny-any: yes

    # Do no insert authority/additional sections into response messages when
    # those sections are not required. This reduces response size
    # significantly, and may avoid TCP fallback for some responses. This may
    # cause a slight speedup
    minimal-responses: yes
    
    # 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
    
    # Fetch the DNSKEYs earlier in the validation process, when a DS record is
    # encountered. This lowers the latency of requests at the expense of little
    # more CPU usage.
    prefetch-key: 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

    # 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 lose messages in traffix spikes
    so-rcvbuf: 4m
    so-sndbuf: 4m
    
    # Set the total number of unwanted replies to keep track of in every thread.
    # When it reaches the threshold, a defensive action of clearing the rrset
    # and message caches is taken, hopefully flushing away any poison.
    # Unbound suggests a value of 10 million.
    unwanted-reply-threshold: 100000
    
    # Minimize logs
    # Do not print one line per query to the log
    log-queries: no
    # Do not print one line per reply to the log
    log-replies: no
    # Do not print log lines that say why queries return SERVFAIL to clients
    log-servfail: no
    # Do not print log lines to inform about local zone actions
    log-local-actions: no
    # Do not print log lines that say why queries return SERVFAIL to clients
    logfile: /dev/null
    
    # 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

Check unbound config file for errors

This is optional. Check the config file for errors by unbound-checkconf /etc/unbound/unbound.conf.d/pi-hole.conf it should return no errors in in /etc/unbound/unbound.conf.d/pi-hole.conf.

Start unbound service and check whether the domain is resolving. The first query will be slow but the subsequent queries will resolve under 1ms.

sudo service unbound start
dig github.com @127.0.0.1 -p 5335

Important steps:

In order to experience high speed and low latency DNS resolution, you need to make some changes to your Pi-hole. These configurations are crucial because if you skip these steps you may experience very slow response times:

  1. Open the configuration file /etc/dnsmasq.d/01-pihole.conf and make sure that cache size is zero by setting cache-size=0. This step is important because the caching is already handled by the Unbound Please note that the changes made to this file will be overwritten once you update/modify Pi-hole.

  2. When you're using unbound you're relying on that for DNSSEC validation and caching, and pi-hole doing those same things are just going to waste time validating DNSSEC twice. In order to resolve this issue you need to untick the Use DNSSEC option in Pi-hole web interface by navigating to Settings > DNS > Advanced DNS settings.

Test validation

You can test DNSSEC validation using

dig sigfail.verteiltesysteme.net @127.0.0.1 -p 5335
dig sigok.verteiltesysteme.net @127.0.0.1 -p 5335

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

Configure Pi-hole to use unbound as your recursive DNS server:

screenshot at 2018-04-18

Click save.


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