DNS Forwarding

Configuration

VyOS provides DNS infrastructure for small networks. It is designed to be lightweight and have a small footprint, suitable for resource constrained routers and firewalls. For this we utilize PowerDNS recursor.

The VyOS DNS forwarder does not require an upstream DNS server. It can serve as a full recursive DNS server - but it can also forward queries to configurable upstream DNS servers. By not configuring any upstream DNS servers you also avoid being tracked by the provider of your upstream DNS server.

set service dns forwarding system
Forward incoming DNS queries to the DNS servers configured under the system name-server nodes.
set service dns forwarding name-server <address>
Send all DNS queries to the IPv4/IPv6 DNS server specified under <address>. You can configure multiple nameservers here.
set service dns forwarding domain <domain-name> server <address>

Forward received queries for a particular domain (specified via domain-name) to a given nameserver. Multiple nameservers can be specified. You can use this feature for a DNS split-horizon configuration.

Note

This also works for reverse-lookup zones (18.172.in-addr.arpa).

set service dns forwarding allow-from <network>
Given the fact that open DNS recursors could be used on DDoS amplification attacks, you must configure the networks which are allowed to use this recursor. A network of 0.0.0.0/0 or ::/0 would allow all IPv4 and IPv6 networks to query this server. This is generally a bad idea.
set service dns forwarding dnssec <off | process-no-validate | process | log-fail | validate>

The PowerDNS recursor has 5 different levels of DNSSEC processing, which can be set with the dnssec setting. In order from least to most processing, these are:

  • off In this mode, no DNSSEC processing takes place. The recursor will not set the DNSSEC OK (DO) bit in the outgoing queries and will ignore the DO and AD bits in queries.
  • process-no-validate In this mode the recursor acts as a “security aware, non-validating” nameserver, meaning it will set the DO-bit on outgoing queries and will provide DNSSEC related RRsets (NSEC, RRSIG) to clients that ask for them (by means of a DO-bit in the query), except for zones provided through the auth-zones setting. It will not do any validation in this mode, not even when requested by the client.
  • process When dnssec is set to process the behavior is similar to process-no-validate. However, the recursor will try to validate the data if at least one of the DO or AD bits is set in the query; in that case, it will set the AD-bit in the response when the data is validated successfully, or send SERVFAIL when the validation comes up bogus.
  • log-fail In this mode, the recursor will attempt to validate all data it retrieves from authoritative servers, regardless of the client’s DNSSEC desires, and will log the validation result. This mode can be used to determine the extra load and amount of possibly bogus answers before turning on full-blown validation. Responses to client queries are the same as with process.
  • validate The highest mode of DNSSEC processing. In this mode, all queries will be validated and will be answered with a SERVFAIL in case of bogus data, regardless of the client’s request.

Note

The popular Unix/Linux dig tool sets the AD-bit in the query. This might lead to unexpected query results when testing. Set +noad on the dig command line when this is the case.

Note

The CD-bit is honored correctly for process and validate. For log-fail, failures will be logged too.

set service dns forwarding ignore-hosts-file
Do not use the local /etc/hosts file in name resolution. VyOS DHCP server will use this file to add resolvers to assigned addresses.
set service dns forwarding cache-size <0-2147483647>

Maximum number of DNS cache entries. 1 million per CPU core will generally suffice for most installations.

This defaults to 10000.

set service dns forwarding negative-ttl <0-7200>

A query for which there is authoritatively no answer is cached to quickly deny a record’s existence later on, without putting a heavy load on the remote server. In practice, caches can become saturated with hundreds of thousands of hosts which are tried only once.

This setting, which defaults to 3600 seconds, puts a maximum on the amount of time negative entries are cached.

set service dns forwarding listen-address <address>
The local IPv4 or IPv6 addresses to bind the DNS forwarder to. The forwarder will listen on this address for incoming connections.
set service dns forwarding source-address <address>
The local IPv4 or IPv6 addresses to use as a source address for sending queries. The forwarder will send forwarded outbound DNS requests from this address.
set service dns forwarding no-serve-rfc1918
This makes the server authoritatively not aware of: 10.in-addr.arpa, 168.192.in-addr.arpa, 16-31.172.in-addr.arpa, which enabling upstream DNS server(s) to be used for reverse lookups of these zones.

Example

A VyOS router with two interfaces - eth0 (WAN) and eth1 (LAN) - is required to implement a split-horizon DNS configuration for example.com.

In this scenario:

  • All DNS requests for example.com must be forwarded to a DNS server at 192.0.2.254 and 2001:db8:cafe::1
  • All other DNS requests will be forwarded to a different set of DNS servers at 192.0.2.1, 192.0.2.2, 2001:db8::1:ffff and 2001:db8::2:ffff
  • The VyOS DNS forwarder will only listen for requests on the eth1 (LAN) interface addresses - 192.168.1.254 for IPv4 and 2001:db8::ffff for IPv6
  • The VyOS DNS forwarder will only accept lookup requests from the LAN subnets - 192.168.1.0/24 and 2001:db8::/64
  • The VyOS DNS forwarder will pass reverse lookups for 10.in-addr.arpa, 168.192.in-addr.arpa, 16-31.172.in-addr.arpa zones to upstream server.
set service dns forwarding domain example.com server 192.0.2.254
set service dns forwarding domain example.com server 2001:db8:cafe::1
set service dns forwarding name-server 192.0.2.1
set service dns forwarding name-server 192.0.2.2
set service dns forwarding name-server 2001:db8::1:ffff
set service dns forwarding name-server 2001:db8::2:ffff
set service dns forwarding listen-address 192.168.1.254
set service dns forwarding listen-address 2001:db8::ffff
set service dns forwarding allow-from 192.168.1.0/24
set service dns forwarding allow-from 2001:db8::/64
set service dns forwarding no-serve-rfc1918

Operation

reset dns forwarding <all | domain>
Resets the local DNS forwarding cache database. You can reset the cache for all entries or only for entries to a specific domain.
restart dns forwarding
Restarts the DNS recursor process. This also invalidates the local DNS forwarding cache.

Dynamic DNS

VyOS is able to update a remote DNS record when an interface gets a new IP address. In order to do so, VyOS includes ddclient, a Perl script written for this only one purpose.

ddclient uses two methods to update a DNS record. The first one will send updates directly to the DNS daemon, in compliance with RFC 2136. The second one involves a third party service, like DynDNS.com or any other similar website. This method uses HTTP requests to transmit the new IP address. You can configure both in VyOS.

Configuration

RFC 2136 Based

set service dns dynamic interface <interface> rfc2136 <service-name>
Create new RFC 2136 DNS update configuration which will update the IP address assigned to <interface> on the service you configured under <service-name>.
set service dns dynamic interface <interface> rfc2136 <service-name> key <keyfile>
File identified by <keyfile> containing the secret RNDC key shared with remote DNS server.
set service dns dynamic interface <interface> rfc2136 <service-name> server <server>
Configure the DNS <server> IP/FQDN used when updating this dynamic assignment.
set service dns dynamic interface <interface> rfc2136 <service-name> zone <zone>
Configure DNS <zone> to be updated.
set service dns dynamic interface <interface> rfc2136 <service-name> record <record>
Configure DNS <record> which should be updated. This can be set multiple times.
set service dns dynamic interface <interface> rfc2136 <service-name> ttl <ttl>
Configure optional TTL value on the given resource record. This defaults to 600 seconds.

Example

  • Register DNS record example.vyos.io on DNS server ns1.vyos.io
  • Use auth key file at /config/auth/my.key
  • Set TTL to 300 seconds
vyos@vyos# show service dns dynamic
 interface eth0.7 {
     rfc2136 VyOS-DNS {
         key /config/auth/my.key
         record example.vyos.io
         server ns1.vyos.io
         ttl 300
         zone vyos.io
     }
 }

This will render the following ddclient configuration entry:

#
# ddclient configuration for interface "eth0.7":
#
use=if, if=eth0.7

# RFC2136 dynamic DNS configuration for example.vyos.io.vyos.io
server=ns1.vyos.io
protocol=nsupdate
password=/config/auth/my.key
ttl=300
zone=vyos.io
example.vyos.io

Note

You can also keep different DNS zone updated. Just create a new config node: set service dns dynamic interface <interface> rfc2136 <other-service-name>

HTTP based services

VyOS is also able to use any service relying on protocols supported by ddclient.

To use such a service, one must define a login, password, one or multiple hostnames, protocol and server.

set service dns dynamic interface <interface> service <service> host-name <hostname>
Setup the dynamic DNS hostname <hostname> associated with the DynDNS provider identified by <service> when the IP address on interface <interface> changes.
set service dns dynamic interface <interface> service <service> login <username>
Configure <username> used when authenticating the update request for DynDNS service identified by <service>. For Namecheap, set the <domain> you wish to update.
set service dns dynamic interface <interface> service <service> password <password>
Configure <password> used when authenticating the update request for DynDNS service identified by <service>.
set service dns dynamic interface <interface> service <service> protocol <protocol>
When a custom DynDNS provider is used the protocol used for communicating to the provider must be specified under <protocol>. See the embedded completion helper for available protocols.
set service dns dynamic interface <interface> service <service> server <server>
When a custom DynDNS provider is used the <server> where update requests are being sent to must be specified.

Example:

Use DynDNS as your preferred provider:

set service dns dynamic interface eth0 service dyndns
set service dns dynamic interface eth0 service dyndns login my-login
set service dns dynamic interface eth0 service dyndns password my-password
set service dns dynamic interface eth0 service dyndns host-name my-dyndns-hostname

Note

Multiple services can be used per interface. Just specify as many services per interface as you like!

Running Behind NAT

By default, ddclient will update a dynamic dns record using the IP address directly attached to the interface. If your VyOS instance is behind NAT, your record will be updated to point to your internal IP.

ddclient has another way to determine the WAN IP address. This is controlled by:

set service dns dynamic interface <interface> use-web url <url>
Use configured <url> to determine your IP address. ddclient will load <url> and tries to extract your IP address from the response.
set service dns dynamic interface <interface> use-web skip <pattern>
ddclient will skip any address located before the string set in <pattern>.