Enable DNS forwarding

This page describes the process to enable DNS forwarding to Consul servers.

You can apply these operations on every node where a Consul agent is running.

Requirements

To enable DNS forwarding, your deployment must have the following:

• A running Consul server instance
• One or more Consul client nodes with registered services in the Consul catalog
• The iptables command available, or one of the following local DNS servers:
  • systemd-resolved
  • BIND
  • Dnsmasq
  • Unbound
  • macOS system resolver

Network address configuration

The example configurations on this page assumes Consul's DNS server is listening on the loopback interface on the same node of the local DNS server.

If Consul is not listening on the loopback IP, replace the references to localhost and 120.0.0.1 in the configuration and commands with the appropriate IP address for your environment.

systemd-resolved

systemd-resolved is a system service that provides network name resolution to local applications. It is the default local DNS server for many Linux distributions.

To configure the systemd-resolved service so that it sends .consul domain queries to Consul, create a consul.conf file located in the /etc/systemd/resolved.conf.d/ directory.

[Resolve]
DNS=127.0.0.1
DNSSEC=false
Domains=~consul

# iptables --table nat --append OUTPUT --destination localhost --protocol udp --match udp --dport 53 --jump REDIRECT --to-ports 8600 && \
  iptables --table nat --append OUTPUT --destination localhost --protocol tcp --match tcp --dport 53 --jump REDIRECT --to-ports 8600

[Resolve]
DNS=127.0.0.1:8600
DNSSEC=false
Domains=~consul

PTR record queries are still sent to the other configured resolvers, in addition to Consul.

After creating the resolved configuration, restart systemd-resolved.

# systemctl restart systemd-resolved

The command produces no output.

Validate the systemd-resolved configuration

Validate that systemd-resolved is active.

# systemctl is-active systemd-resolved
active

Verify that systemd-resolved is configured to forward queries for the consul domain to Consul.

# resolvectl domain
Global: ~consul
Link 2 (eth0):

Verify that systemd-resolved is able to resolve the Consul server address.

# resolvectl query consul.service.consul
consul.service.consul: 127.0.0.1

-- Information acquired via protocol DNS in 6.6ms.
-- Data is authenticated: no

Confirm that /etc/resolv.conf points to the stub-resolv.conf file managed by systemd-resolved.

$ ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 37 Jul 14 10:10 /etc/resolv.conf -> /run/systemd/resolve/stub-resolv.conf

Confirm that the IP address for systemd-resolved's stub resolver is the configured nameserver.

$ cat /etc/resolv.conf
## This file is managed by man:systemd-resolved(8). Do not edit.
##
## This is a dynamic resolv.conf file for connecting local clients to the
## internal DNS stub resolver of systemd-resolved. This file lists all
## configured search domains.
##
## Run "resolvectl status" to see details about the uplink DNS servers
## currently in use.
##
## Third party programs must not access this file directly, but only through the
## symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
## replace this symlink by a static file or a different symlink.
##
## See man:systemd-resolved.service(8) for details about the supported modes of
## operation for /etc/resolv.conf.

nameserver 127.0.0.53
options edns0

Ensure that the operating system can resolve DNS queries to the .consul domain.

$ host consul.service.consul
consul.service.consul has address 127.0.0.1

Using any local resolver with systemd

By default, the local resolver stub in the resolved.conf file is configured to listen for UDP and TCP requests at 127.0.0.53:53. However, you can set the DNSStubListener option to false so that your system can use any DNS configuration, as long as it loads earlier than resolved.

DNSStubListener=false

Dnsmasq

Use dnsmasq if you have a small network and need a lightweight DNS solution.

Configure the dnsmasq.conf file or a series of files in the /etc/dnsmasq.d directory. Add server settings to your configuration file so that requests for the consul domain are forwarded to Consul DNS.

# Enable forward lookup of the 'consul' domain:
server=/consul/127.0.0.1#8600

# Uncomment and modify as appropriate to enable reverse DNS lookups for
# common netblocks found in RFC 1918, 5735, and 6598:
#rev-server=0.0.0.0/8,127.0.0.1#8600
#rev-server=10.0.0.0/8,127.0.0.1#8600
#rev-server=100.64.0.0/10,127.0.0.1#8600
#rev-server=127.0.0.1/8,127.0.0.1#8600
#rev-server=169.254.0.0/16,127.0.0.1#8600
#rev-server=172.16.0.0/12,127.0.0.1#8600
#rev-server=192.168.0.0/16,127.0.0.1#8600
#rev-server=224.0.0.0/4,127.0.0.1#8600
#rev-server=240.0.0.0/4,127.0.0.1#8600
# Accept DNS queries only from hosts whose address is on a local subnet.
#local-service
# Don't poll /etc/resolv.conf for changes.
#no-poll
# Don't read /etc/resolv.conf. Get upstream servers only from the command
# line or the dnsmasq configuration file (see the "server" directive below).
#no-resolv
# Specify IP address(es) of other DNS servers for queries not handled
# directly by consul. There is normally one 'server' entry set for every
# 'nameserver' parameter found in '/etc/resolv.conf'. See dnsmasq(8)'s
# 'server' configuration option for details.
#server=1.2.3.4
#server=208.67.222.222
#server=8.8.8.8
# Set the size of dnsmasq's cache. The default is 150 names. Setting the
# cache size to zero disables caching.
#cache-size=65536

Restart the dnsmasq service after creating the configuration.

Refer to dnsmasq(8) for additional configuration settings such as specifying IP addresses for queries not handled directly by Consul.

BIND

BIND is a robust DNS system. Its most prominent component, named, performs both of the main DNS server roles, acts as an authoritative name server for DNS zones, and is a recursive resolver in the network.

To configure the BIND service to send .consul domain queries to Consul:

1. Create a named configuration file with DNSSEC disabled.
2. Create a zone configuration file to manage the .consul domain.

Named configuration file

Edit the /etc/named.conf to configure your BIND instance. Remember to disable DNSSEC so that Consul and BIND can communicate. Add an include section to include the zone file that you create in the next step.

The following example shows a BIND configuration with DNSSEC disabled.

options {
  listen-on port 53 { 127.0.0.1; };
  listen-on-v6 port 53 { ::1; };
  directory       "/var/named";
  dump-file       "/var/named/data/cache_dump.db";
  statistics-file "/var/named/data/named_stats.txt";
  memstatistics-file "/var/named/data/named_mem_stats.txt";
  allow-query     { localhost; };
  recursion yes;

  dnssec-enable no;
  dnssec-validation no;

  /* Path to ISC DLV key */
  bindkeys-file "/etc/named.iscdlv.key";

  managed-keys-directory "/var/named/dynamic";
};

include "/etc/named/consul.conf";

Zone configuration file

Set up a zone for your Consul-managed records in consul.conf.

zone "consul" IN {
  type forward;
  forward only;
  forwarders { 127.0.0.1 port 8600; };
};

Unbound

Use Unbound when you need a fast and lean DNS resolver for Linux and macOS.

The following example demonstrates a configuration for the consul.conf file in the /etc/unbound/unbound.conf.d directory.

Add server and stub-zone settings to your Unbound configuration file.

#Allow insecure queries to local resolvers
server:
  do-not-query-localhost: no
  domain-insecure: "consul"

#Add consul as a stub-zone
stub-zone:
  name: "consul"
  stub-addr: 127.0.0.1@8600

You may have to add the following line to the bottom of your /etc/unbound/unbound.conf file for the new configuration to be included.

include: "/etc/unbound/unbound.conf.d/*.conf"

iptables

iptables is a generic firewalling software that allows you to define traffic rules for your system.

If you do not have a local DNS server on the Consul agent node, use iptables to forward DNS requests on port 53 to the Consul agent running on the same machine without using a secondary service.

This configuration realizes full DNS forwarding, which means that all DNS queries for the host are forwarded to Consul, not just the ones for the .consul top level domain. Consul's default configuration resolves only the .consul top level domain, so you must set the recursors flag if you want your node to be able to resolve other domains when using iptables configuration.

If you use DNS relay hosts in your network, do not place them on the same host as Consul. The redirects may intercept the traffic.

Configure Consul recursors

Add recursors to your Consul configuration.

# DNS recursors
recursors = [ "1.1.1.1" ]

Recursors should not include the localhost address because the iptables redirects would intercept the requests.

You can replace the 1.1.1.1 address in the example with another DNS server address. This is suitable for situations where an external DNS service is already running in your infrastructure and is used as the recursor.

Create iptables rules

After you configure Consul to use a valid recursor, add rules to iptables to redirect traffic from port 53 to port 8600.

# iptables -t nat -A PREROUTING -p udp -m udp --dport 53 -j REDIRECT --to-ports 8600 \
  iptables -t nat -A PREROUTING -p tcp -m tcp --dport 53 -j REDIRECT --to-ports 8600 \
  iptables -t nat -A OUTPUT -d localhost -p udp -m udp --dport 53 -j REDIRECT --to-ports 8600 \
  iptables -t nat -A OUTPUT -d localhost -p tcp -m tcp --dport 53 -j REDIRECT --to-ports 8600

macOS

On macOS systems, use the macOS system resolver to point all .consul requests to Consul. The man 5 resolver command describes this feature in more detail.

The following instructions require sudo or root access.

To configure the macOS system resolver to forward DNS queries to Consul, add a resolver entry in the /etc/resolver/ directory that points at the Consul agent.

If you do not have this folder, create it.

# mkdir -p /etc/resolver

Create a new file /etc/resolver/consul with nameserver and port entries.

nameserver 127.0.0.1
port 8600

The configuration informs the macOS resolver daemon to forward all .consul TLD requests to 127.0.0.1 on port 8600.

Next steps

This instructions on this page helped you configure your node to forward DNS requests to Consul.

To learn more on how to query Consul DNS once forwarding is enabled, refer to DNS forwarding workflow.

For more information on other DNS features and configurations available in Consul, refer to DNS usage overview.