Configure Consul DNS behavior

This topic describes the default behavior of the Consul DNS functionality and how to customize how Consul performs queries.

Introduction

The Consul DNS is the primary interface for querying records when Consul service mesh is disabled and your network runs in a non-Kubernetes environment. The DNS enables you to look up services and nodes registered with Consul using terminal commands instead of making HTTP API requests to Consul. Refer to the Discover Consul Nodes and Services Overview for additional information.

Configure DNS behaviors

By default, the Consul DNS listens for queries at 127.0.0.1:8600 and uses the consul domain. Specify the following parameters in the agent configuration to determine DNS behavior when querying services:

client_addr
ports.dns : Consul does not use port 53, which is typically reserved for the default port for DNS resolvers, by default because it requires an escalated privilege to bind to.
recursors
domain
alt_domain
dns_config

Configure WAN address translation

By default, Consul DNS queries return a node's local address, even when being queried from a remote datacenter. You can configure the DNS to reach a node from outside its datacenter by specifying the address in the following configuration fields in the Consul agent:

Use a custom DNS resolver library

You can specify a list of addresses in the agent's recursors field to provide upstream DNS servers that recursively resolve queries that are outside the service domain for Consul.

Nodes that query records outside the consul. domain resolve to an upstream DNS. You can specify IP addresses or use go-sockaddr templates. Consul resolves IP addresses in the specified order and ignores duplicates.

We recommend that you configure DNS resolvers to point the consul. domain towards your Consul DNS servers. Misconfigurations may cause other DNS infrastructure to route queries for the consul. domain outside of your network instead, leaking DNS queries to root DNS servers. Refer to Forward DNS for Consul Service Discovery for instructions.

Enable non-Consul queries

You enable non-Consul queries to be resolved by setting Consul as the DNS server for a node and providing a recursors configuration.

Forward queries to an agent

You can forward all queries sent to the consul. domain from the existing DNS server to a Consul agent. Refer to Forward DNS for Consul Service Discovery for instructions.

Query an alternate domain

By default, Consul responds to DNS queries in the consul domain, but you can set a specific domain for responding to DNS queries by configuring the domain parameter.

You can also specify an additional domain in the alt_domain agent configuration option, which configures Consul to respond to queries in a secondary domain. Configuring an alternate domain may be useful during a DNS migration or to distinguish between internal and external queries, for example.

Consul's DNS response uses the same domain as the query.

In the following example, the alt_domain parameter in the agent configuration is set to test-domain, which enables operators to query the domain:

$ dig @127.0.0.1 -p 8600 consul.service.test-domain SRV

;; QUESTION SECTION:
;consul.service.test-domain.  IN  SRV

;; ANSWER SECTION:
consul.service.test-domain. 0 IN  SRV 1 1 8300 machine.node.dc1.test-domain.

;; ADDITIONAL SECTION:
machine.node.dc1.test-domain. 0 IN  A 127.0.0.1
machine.node.dc1.test-domain. 0 IN  TXT "consul-network-segment="

PTR queries

Responses to pointer record (PTR) queries, such as <ip>.in-addr.arpa., always use the primary domain and not the alternative domain.

Caching

By default, DNS results served by Consul are not cached. Refer to DNS caching for instructions on how to enable caching.