Unbound DNS

Unbound is a validating, recursive, caching DNS resolver. It is designed to be fast and lean and incorporates modern features based on open standards.

Since OPNsense 17.7 it has been our standard DNS service, which on a new install is enabled by default.

General settings

Below you will find the most relevant settings from the General menu section.


Enable our DNS resolver

Listen Port

Port to listen on, when blank, the default (53) is used.

Network Interfaces

Interface IP addresses used for responding to queries from clients. If an interface has both IPv4 and IPv6 IPs, both are used. Queries to other interface IPs not selected are discarded. The default behavior is to respond to queries on every available IPv4 and IPv6 address.


Enable DNSSEC to use digital signatures to validate results from upstream servers and mitigate against cache poisoning.


Enable DNS64 so IPv6-only clients can reach IPv4-only servers. If enabled, Unbound synthesizes AAAA records for domains which only have A records. DNS64 requires NAT64 to be useful, e. g. the Tayga plugin or a third-party NAT64 service. The DNS64 prefix must match the IPv6 prefix used be the NAT64.

DHCP Registration

IPv4 only If this option is set, then machines that specify their hostname when requesting a DHCP lease will be registered in Unbound, so that their name can be resolved.

The source of this data is client-hostname in the dhcpd.leases file

DHCP Domain Override

When the above registrations shouldn’t use the same domain name as configured on this firewall, you can specify a different one here.

DHCP Static Mappings

Register static dhcpd entries so clients can resolve them. Supported on IPv4 and IPv6.

IPv6 Link-local

Register link local addresses for IPv6.

TXT Comment Support

Register descriptions as comments for dhcp static host entries.

DNS Query Forwarding

Forward queries to configured nameservers in System ‣ Settings ‣ General : DNS Server

Local Zone Type

The local zone type used for the system domain. Type descriptions are available under “local-zone:” in the unbound.conf(5) manual page. The default is ‘transparent’.


Be careful enabling “DNS Query Forwarding” in combination with DNSSEC, when the upstream server doesn’t support DNSSEC, its answers will be considered insecure since no DNSSEC validation could be performed.


In order for the client to query unbound, there need to be an ACL assigned in Services ‣ Unbound DNS ‣ Access Lists. The configured interfaces should gain an ACL automatically. If the client address is not in any of the predefined networks, please add one manually.


Within the overrides section you can create separate host definition entries and specify if queries for a specific domain should be forwarded to a predefined server.

Host override settings


Name of the host, without domain part. Use “*” to create a wildcard entry.


Domain of the host (such as example.com)


Record type, A or AAA (IPv4 or IPv6 address), MX to define a mail exchange


Address of the host


User readable description, only for informational purposes


Copies of the above data for different hosts

Domain override settings


Domain to override

IP address

IP address of the authoritative DNS server for this domain


User readable description, only for informational purposes


Although the default settings should be reasonable for most setups, some need more tuning or require specific options set.

Hide Identity

If enabled, id.server and hostname.bind queries are refused.

Hide Version

If enabled version.server and version.bind queries are refused.

Prefetch Support

Message cache elements are prefetched before they expire to help keep the cache up to date. When enabled, this option can cause an increase of around 10% more DNS traffic and load on the server, but frequently requested items will not expire from the cache.

Prefetch DNS Key Support

DNSKEY’s are fetched earlier in the validation process when a Delegation signer is encountered. This helps lower the latency of requests but does utilize a little more CPU.

Harden DNSSEC data

DNSSEC data is required for trust-anchored zones. If such data is absent, the zone becomes bogus. If this is disabled and no DNSSEC data is received, then the zone is made insecure.

Serve expired responses

Serve expired responses from the cache with a TTL of 0 without waiting for the actual resolution to finish.

Message Cache Size

Size of the message cache. The message cache stores DNS rcodes and validation statuses. The RRSet cache will automatically be set to twice this amount. The RRSet cache contains the actual RR data. The default is 4 megabytes.

Outgoing TCP Buffers

The number of outgoing TCP buffers to allocate per thread. The default value is 10. If 0 is selected then no TCP queries, to authoritative servers, are done.

Incoming TCP Buffers

The number of incoming TCP buffers to allocate per thread. The default value is 10. If 0 is selected then no TCP queries, from clients, are accepted.

Number of queries per thread

The number of queries that every thread will service simultaneously. If more queries arrive that need to be serviced, and no queries can be jostled, then these queries are dropped.

Jostle Timeout

This timeout is used for when the server is very busy. This protects against denial of service by slow queries or high query rates. The default value is 200 milliseconds.

Maximum TTL for RRsets and messages

Configure a maximum Time to live for RRsets and messages in the cache. The default is 86400 seconds (1 day). When the internal TTL expires the cache item is expired. This can be configured to force the resolver to query for data more often and not trust (very large) TTL values.

Minimum TTL for RRsets and messages

Configure a minimum Time to live for RRsets and messages in the cache. The default is 0 seconds. If the minimum value kicks in, the data is cached for longer than the domain owner intended, and thus less queries are made to look up the data. The 0 value ensures the data in the cache is as the domain owner intended. High values can lead to trouble as the data in the cache might not match up with the actual data anymore.

TTL for Host cache entries

Time to live for entries in the host cache. The host cache contains roundtrip timing and EDNS support information. The default is 15 minutes.

Number of Hosts to cache

Number of hosts for which information is cached. The default is 10000.

Unwanted Reply Threshold

If enabled, a total number of unwanted replies is kept track of in every thread. When it reaches the threshold, a defensive action is taken and a warning is printed to the log file. This defensive action is to clear the RRSet and message caches, hopefully flushing away any poison. The default is disabled, but if enabled a value of 10 million is suggested.

Log level verbosity

Select the log 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, output per query. Level 4 gives algorithm level information. Level 5 logs client identification for cache misses. Default is level 1.

Access Lists

Access lists define which clients may query our dns resolver. Records for the assigned interfaces will be automatically created and are shown in the overview. You can also define custom policies, which apply an action to predefined networks.


The action can be as defined in the list below. The most specific netblock match is used, if none match deny is used. The order of the access-control statements therefore does not matter.



This action stops queries from hosts within the defined networks.


This action also stops queries from hosts within the defined networks, but sends a DNS rcode REFUSED error message back to the client.


This action allows queries from hosts within the defined networks.

Allow Snoop

This action allows recursive and nonrecursive access from hosts within the defined networks. Used for cache snooping and ideally should only be configured for your administrative host.

Deny Non-local

Allow only authoritative local-data queries from hosts within the defined networks. Messages that are disallowed are dropped.

Refuse Non-local

Allow only authoritative local-data queries from hosts within the defined networks. Sends a DNS rcode REFUSED error message back to the client for messages that are disallowed.


The statistics page provides some insights into the running server, such as the number of queries executed, cache usage and uptime.

Advanced Configurations

Some installations require configuration settings that are not accessible in the UI. To support these, individual configuration files with a .conf extension can be put into the /var/unbound/etc directory. These files will be automatically included by the UI generated configuration. Multiple configuration files can be placed there. But note that

  • As it cannot be predicted in which clause the configuration currently takes place, you must prefix the configuration with the required clause. For the concept of “clause” see the unbound.conf(5) documentation.

  • The wildcard include processing in unbound is based on glob(7). So the order in which the files are included is in ascending ASCII order.

  • Namecollisions with plugins, which use this extension point e. g. unbound-plus, may occur. So be sure to use an unique filename.

  • It is a good idea, to check the complete configuration by running the unbound-checkconf utility:

    # check if configuration is valid
    unbound-checkconf /var/unbound/unbound.conf

    This will report errors that prevent unbound from starting.

This is a sample configuration file to add an option in the server clause:

  private-domain: xip.io


A final thing to note is, that on memory based system /var is volatile and every file placed inside or below will not survive a restart. As a solution the Template System (“Using Templates”) can be used to automatically generate these files.

To get the same effect as placing the file in the sample above directly in /var/unbound/etc follow these steps:

  1. Create a +TARGETS file in /usr/local/opnsense/service/templates/sampleuser/Unbound:

  2. Place the template file as sampleuser_additional_options.conf in the same directory:

      private-domain: xip.io
  3. Test the template generation by issuing the following command:

    # generate template
    configctl template reload sampleuser/Unbound
  4. Check the output in the target directory:

    # show generated file
    cat /var/unbound/etc/sampleuser_additional_options.conf
    # check if configuration is valid
    unbound-checkconf /var/unbound/unbound.conf


It is the sole responsibility of the administrator which places a file in the extension directory to ensure that the configuration is valid.


This method replaces the Custom options settings in the General page of the Unbound configuration, which was already marked as “to be removed in the future”.