Unbound is a validating, recursive, and caching DNS resolver. According to Wikipedia:Unbound (DNS Server), "Unbound has supplanted the Berkeley Internet Name Domain (BIND) as the default, base-system name server in several open source projects, where it is perceived as smaller, more modern, and more secure for most applications."
- 1 Installation
- 2 Configuration
- 3 Usage
- 4 Tips and tricks
- 5 Troubleshooting
- 6 See also
Install the package.
Additionally, the DNSSEC validation.package is required for
A default configuration is already included at
/etc/unbound/unbound.conf. Additionally, there is a commented sample configuration file with other available options located at
/etc/unbound/unbound.conf.example. The following sections highlight different settings for the configuration file. See
man unbound.conf for other settings and more details.
Unless otherwise specified, any options listed in this section are to be placed under the
server section in the configuration like so:
server: ... setting: value ...
You can specify the interfaces to answer queries from by IP address. To listen on localhost, use the default setting:
To listen on all interfaces, use the following:
To control which systems can access the server by IP address, use the
access-control: subnet action
access-control: 192.168.1.0/24 allow
action can be one of
deny (drop message),
refuse (polite error reply),
allow (recursive ok), or
allow_snoop (recursive and nonrecursive ok). By default everything is refused except for localhost.
For 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comes with default builtin hints, but it is good practice to use a root-hints file since the builtin hints may become outdated.
First point unbound to the
Then, put a root hints file into the unbound configuration directory. The simplest way to do this is to run the command:
# curl -o /etc/unbound/root.hints https://www.internic.net/domain/named.cache
It is a good idea to update
root.hints every six months or so in order to make sure the list of root servers is up to date. This can be done manually or by using Systemd/Timers. See #Roothints systemd timer for an example.
Local DNS server
See Resolv.conf#Testing on how to test your settings.
Check specifically that the server being used is
127.0.0.1 after making permanent changes to resolv.conf.
unbound automatically copies the root server trust key anchor file from
/etc/unbound/trusted-key.key. To use DNSSEC validation, point unbound to this file by adding the following setting:
Also make sure that if a general forward to a DNS server has been set, then comment them out; otherwise, DNS queries will fail. DNSSEC validation will only be done if the DNS server being queried supports it.
To test if DNSSEC is working, use drill:
$ drill sigfail.verteiltesysteme.net $ drill sigok.verteiltesysteme.net
The first command should give an
SERVFAIL. The second should give an
If you have a local network which you wish to have DNS queries for and there is a local DNS server that you would like to forward queries to then you should include this line:
To include a local DNS server for both forward and reverse local addresses a set of lines similar to these below is necessary with a forward and reverse lookup (choose the IP address of the server providing DNS for the local network accordingly by changing 10.0.0.1 in the lines below):
local-zone: "10.in-addr.arpa." transparent
This line above is important to get the reverse lookup to work correctly.
forward-zone: name: "mynetwork.com." forward-addr: 10.0.0.1
forward-zone: name: "10.in-addr.arpa." forward-addr: 10.0.0.1
You can set up the localhost forward and reverse lookups with the following lines:
local-zone: "localhost." static local-data: "localhost. 10800 IN NS localhost." local-data: "localhost. 10800 IN SOA localhost. nobody.invalid. 1 3600 1200 604800 10800" local-data: "localhost. 10800 IN A 127.0.0.1" local-zone: "127.in-addr.arpa." static local-data: "127.in-addr.arpa. 10800 IN NS localhost." local-data: "127.in-addr.arpa. 10800 IN SOA localhost. nobody.invalid. 2 3600 1200 604800 10800" local-data: "188.8.131.52.in-addr.arpa. 10800 IN PTR localhost."
Then to use specific servers for default forward zones that are outside of the local machine and outside of the local network (i.e. all other queries will be forwarded to them, and then cached) add this to the configuration file (and in this example the first two addresses are the fast google DNS servers):
forward-zone: name: "." forward-addr: 184.108.40.206 forward-addr: 220.127.116.11 forward-addr: 18.104.22.168 forward-addr: 22.214.171.124
This will make unbound use Google and OpenDNS servers as the forward zone for external lookups.
unbound.service systemd service.
Remotely control Unbound
unbound ships with the
unbound-control utility which enables us to remotely administer the unbound server. It is similar to the pdnsd-ctl command of .
Setting up unbound-control
Before you can start using it, the following steps need to be performed:
1) Firstly, you need to run the following command
which will generate a self-signed certificate and private key for the server, as well as the client. These files will be created in the
2) After that, edit
/etc/unbound/unbound.conf and put the following contents in that. The
control-enable: yes option is necessary, the rest can be adjusted as required.
remote-control: # Enable remote control with unbound-control(8) here. # set up the keys and certificates with unbound-control-setup. control-enable: yes # what interfaces are listened to for remote control. # give 0.0.0.0 and ::0 to listen to all interfaces. control-interface: 127.0.0.1 # port number for remote control operations. control-port: 8953 # unbound server key file. server-key-file: "/etc/unbound/unbound_server.key" # unbound server certificate file. server-cert-file: "/etc/unbound/unbound_server.pem" # unbound-control key file. control-key-file: "/etc/unbound/unbound_control.key" # unbound-control certificate file. control-cert-file: "/etc/unbound/unbound_control.pem"
Some of the commands that can be used with unbound-control are:
- print statistics without resetting them
# unbound-control stats_noreset
- dump cache to stdout
# unbound-control dump_cache
- flush cache and reload configuration
# unbound-control reload
Please refer to
man 8 unbound-control for a detailed look at the operations it supports.
Tips and tricks
You can use the following file and simply include it in your unbound configuration: adservers
... include: /etc/unbound/adservers
For users who wish to run both a validating, recursive, caching DNS server as well as an authoritative DNS server on a single machine then it may be useful to refer to the wiki page nsd which gives an example of a configuration for such a system. Having one server for authoritative DNS queries and a separate DNS server for the validating, recursive, caching DNS functions gives increased security over a single DNS server providing all of these functions. Many users have used bind as a single DNS server, and some help on migration from bind to the combination of running nsd and bind is provided in the nsd wiki page.
WAN facing DNS
It is also possible to change the configuration files and interfaces on which the server is listening so that DNS queries from machines outside of the local network can access specific machines within the LAN. This is useful for web and mail servers which are accessible from anywhere, and the same techniques can be employed as has been achieved using bind for many years, in combination with suitable port forwarding on firewall machines to forward incoming requests to the right machine.
Roothints systemd timer
Here is an example systemd service and timer that update
root.hints monthly using the method in #Root hints:
[Unit] Description=Update root hints for unbound After=network.target [Service] ExecStart=/usr/bin/curl -o /etc/unbound/root.hints https://www.internic.net/domain/named.cache
[Unit] Description=Run root.hints monthly [Timer] OnCalendar=monthly Persistent=true [Install] WantedBy=timers.target
roothints.timer systemd timer.
Issues concerning num-threads
The man page for
outgoing-range: <number> Number of ports to open. This number of file descriptors can be opened per thread.
and some sources suggest that the
num-threads parameter should be set to the number of cpu cores. The sample
unbound.conf.example file merely has:
# number of threads to create. 1 disables threading. # num-threads: 1
However it is not possible to arbitrarily increase
1 without causing unbound to start with warnings in the logs about exceeding the number of file descriptors. In reality for most users running on small networks or on a single machine it should be unnecessary to seek performance enhancement by increasing
1. If you do wish to do so then refer to official documentation and the following rule of thumb should work:
num-threadsequal to the number of CPU cores on the system. E.g. for 4 CPUs with 2 cores each, use 8.
outgoing-range to as large a value as possible, see the sections in the referred web page above on how to overcome the limit of
1024 in total. This services more clients at a time. With 1 core, try
950. With 2 cores, try
450. With 4 cores try
num-queries-per-thread is best set at half the number of the
Because of the limit on
outgoing-range thus also limits
num-queries-per-thread, it is better to compile with , so that there is no
1024 limit on
outgoing-range. If you need to compile this way for a heavy duty DNS server then you will need to compile the programme from source instead of using the package.