mirror of https://github.com/status-im/consul.git
223 lines
9.1 KiB
Markdown
223 lines
9.1 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "DNS Interface"
|
|
sidebar_current: "docs-agent-dns"
|
|
description: |-
|
|
One of the primary query interfaces for Consul is DNS. The DNS interface allows applications to make use of service discovery without any high-touch integration with Consul.
|
|
---
|
|
|
|
# DNS Interface
|
|
|
|
One of the primary query interfaces for Consul is DNS.
|
|
The DNS interface allows applications to make use of service
|
|
discovery without any high-touch integration with Consul.
|
|
|
|
For example, instead of making HTTP API requests to Consul,
|
|
a host can use the DNS server directly via name lookups
|
|
like `redis.service.us-east-1.consul`. This query automatically
|
|
translates to a lookup of nodes that provide the `redis` service,
|
|
are located in the `us-east-1` datacenter, and have no failing health checks.
|
|
It's that simple!
|
|
|
|
There are a number of configuration options that are important for the DNS interface,
|
|
specifically [`client_addr`](/docs/agent/options.html#client_addr),
|
|
[`ports.dns`](/docs/agent/options.html#dns_port), [`recursors`](/docs/agent/options.html#recursors),
|
|
[`domain`](/docs/agent/options.html#domain), and [`dns_config`](/docs/agent/options.html#dns_config).
|
|
By default, Consul will listen on 127.0.0.1:8600 for DNS queries in the `consul.`
|
|
domain, without support for further DNS recursion. Please consult the
|
|
[documentation on configuration options](/docs/agent/options.html),
|
|
specifically the configuration items linked above, for more details.
|
|
|
|
There are a few ways to use the DNS interface. One option is to use a custom
|
|
DNS resolver library and point it at Consul. Another option is to set Consul
|
|
as the DNS server for a node and provide a
|
|
[`recursors`](/docs/agent/options.html#recursors) configuration so that non-Consul queries
|
|
can also be resolved. The last method is to forward all queries for the "consul."
|
|
domain to a Consul agent from the existing DNS server.
|
|
|
|
You can experiment with Consul's DNS server on the command line using tools such as `dig`:
|
|
|
|
$ dig @127.0.0.1 -p 8600 redis.service.dc1.consul. ANY
|
|
|
|
-> **Note:** In DNS, all queries are case-insensitive. A lookup of `PostgreSQL.node.dc1.consul` will find all nodes named `postgresql`.
|
|
|
|
## Node Lookups
|
|
|
|
To resolve names, Consul relies on a very specific format for queries.
|
|
There are fundamentally two types of queries: node lookups and service lookups.
|
|
A node lookup, a simple query for the address of a named node, looks like this:
|
|
|
|
<node>.node[.datacenter].<domain>
|
|
|
|
For example, if we have a `foo` node with default settings, we could
|
|
look for `foo.node.dc1.consul.` The datacenter is an optional part of
|
|
the FQDN: if not provided, it defaults to the datacenter of the agent.
|
|
If we know `foo` is running in the same datacenter as our local agent,
|
|
we can instead use `foo.node.consul.` This convention allows for terse
|
|
syntax where appropriate while supporting queries of nodes in remote
|
|
datacenters as necessary.
|
|
|
|
For a node lookup, the only records returned are A records containing
|
|
the IP address of the node.
|
|
|
|
```text
|
|
$ dig @127.0.0.1 -p 8600 foo.node.consul ANY
|
|
|
|
; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 -p 8600 foo.node.consul ANY
|
|
; (1 server found)
|
|
;; global options: +cmd
|
|
;; Got answer:
|
|
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24355
|
|
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 0
|
|
;; WARNING: recursion requested but not available
|
|
|
|
;; QUESTION SECTION:
|
|
;foo.node.consul. IN ANY
|
|
|
|
;; ANSWER SECTION:
|
|
foo.node.consul. 0 IN A 10.1.10.12
|
|
|
|
;; AUTHORITY SECTION:
|
|
consul. 0 IN SOA ns.consul. postmaster.consul. 1392836399 3600 600 86400 0
|
|
```
|
|
|
|
## Service Lookups
|
|
|
|
A service lookup is used to query for service providers. Service queries support
|
|
two lookup methods: standard and strict [RFC 2782](https://tools.ietf.org/html/rfc2782).
|
|
|
|
### Standard Lookup
|
|
|
|
The format of a standard service lookup is:
|
|
|
|
[tag.]<service>.service[.datacenter].<domain>
|
|
|
|
The `tag` is optional, and, as with node lookups, the `datacenter` is as
|
|
well. If no tag is provided, no filtering is done on tag. If no
|
|
datacenter is provided, the datacenter of this Consul agent is assumed.
|
|
|
|
If we want to find any redis service providers in our local datacenter,
|
|
we could query 1redis.service.consul.1 If we want to find the PostgreSQL
|
|
primary in a particular datacenter, we could query
|
|
`primary.postgresql.service.dc2.consul.`
|
|
|
|
The DNS query system makes use of health check information to prevent routing
|
|
to unhealthy nodes. When a service query is made, any services failing their health
|
|
check or failing a node system check will be omitted from the results. To allow
|
|
for simple load balancing, the set of nodes returned is also randomized each time.
|
|
These mechanisms make it easy to use DNS along with application-level retries
|
|
as the foundation for an auto-healing service oriented architecture.
|
|
|
|
For standard services queries, both A and SRV records are supported. SRV records
|
|
provide the port that a service is registered on, enabling clients to avoid relying
|
|
on well-known ports. SRV records are only served if the client specifically requests
|
|
them, like so:
|
|
|
|
```text
|
|
$ dig @127.0.0.1 -p 8600 consul.service.consul SRV
|
|
|
|
; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 -p 8600 consul.service.consul ANY
|
|
; (1 server found)
|
|
;; global options: +cmd
|
|
;; Got answer:
|
|
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50483
|
|
;; flags: qr aa rd; QUERY: 1, ANSWER: 3, AUTHORITY: 1, ADDITIONAL: 1
|
|
;; WARNING: recursion requested but not available
|
|
|
|
;; QUESTION SECTION:
|
|
;consul.service.consul. IN SRV
|
|
|
|
;; ANSWER SECTION:
|
|
consul.service.consul. 0 IN SRV 1 1 8300 foobar.node.dc1.consul.
|
|
|
|
;; ADDITIONAL SECTION:
|
|
foobar.node.dc1.consul. 0 IN A 10.1.10.12
|
|
```
|
|
|
|
### RFC 2782 Lookup
|
|
|
|
The format for RFC 2782 SRV lookups is:
|
|
|
|
_<service>._<protocol>.service[.datacenter][.domain]
|
|
|
|
Per [RFC 2782](https://tools.ietf.org/html/rfc2782), SRV queries should use
|
|
underscores, `_`, as a prefix to the `service` and `protocol` values in a query to
|
|
prevent DNS collisions. The `protocol` value can be any of the tags for a
|
|
service. If the service has no tags, `tcp` should be used. If `tcp`
|
|
is specified as the protocol, the query will not perform any tag filtering.
|
|
|
|
Other than the query format and default `tcp` protocol/tag value, the behavior
|
|
of the RFC style lookup is the same as the standard style of lookup.
|
|
|
|
If you registered the service `rabbitmq` on port 5672 and tagged it with `amqp`,
|
|
you could make an RFC 2782 query for its SRV record as `_rabbitmq._amqp.service.consul`:
|
|
|
|
```text
|
|
$ dig @127.0.0.1 -p 8600 _rabbitmq._amqp.service.consul SRV
|
|
|
|
; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 -p 8600 _rabbitmq._amqp.service.consul ANY
|
|
; (1 server found)
|
|
;; global options: +cmd
|
|
;; Got answer:
|
|
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 52838
|
|
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
|
|
;; WARNING: recursion requested but not available
|
|
|
|
;; QUESTION SECTION:
|
|
;_rabbitmq._amqp.service.consul. IN SRV
|
|
|
|
;; ANSWER SECTION:
|
|
_rabbitmq._amqp.service.consul. 0 IN SRV 1 1 5672 rabbitmq.node1.dc1.consul.
|
|
|
|
;; ADDITIONAL SECTION:
|
|
rabbitmq.node1.dc1.consul. 0 IN A 10.1.11.20
|
|
```
|
|
|
|
Again, note that the SRV record returns the port of the service as well as its IP.
|
|
|
|
### Prepared Query Lookups
|
|
|
|
The format of a prepared query lookup is:
|
|
|
|
<query or name>.query[.datacenter].<domain>
|
|
|
|
The `datacenter` is optional, and if not provided, the datacenter of this Consul
|
|
agent is assumed.
|
|
|
|
The `query or name` is the ID or given name of an existing
|
|
[Prepared Query](/docs/agent/http/query.html). These behave like standard service
|
|
queries but provide a much richer set of features, such as filtering by multiple
|
|
tags and automatically failing over to look for services in remote datacenters if
|
|
no healthy nodes are available in the local datacenter. Consul 0.6.4 and later also
|
|
added support for [prepared query templates](/docs/agent/http/query.html#templates)
|
|
which can match names using a prefix match, allowing one template to apply to
|
|
potentially many services.
|
|
|
|
To allow for simple load balancing, the set of nodes returned is randomized each time.
|
|
Both A and SRV records are supported. SRV records provide the port that a service is
|
|
registered on, enabling clients to avoid relying on well-known ports. SRV records are
|
|
only served if the client specifically requests them.
|
|
|
|
### UDP Based DNS Queries
|
|
|
|
When the DNS query is performed using UDP, Consul will truncate the results
|
|
without setting the truncate bit. This is to prevent a redundant lookup over
|
|
TCP that generates additional load. If the lookup is done over TCP, the results
|
|
are not truncated.
|
|
|
|
## Caching
|
|
|
|
By default, all DNS results served by Consul set a 0 TTL value. This disables
|
|
caching of DNS results. However, there are many situations in which caching is
|
|
desirable for performance and scalability. This is discussed more in the guide
|
|
for [DNS Caching](/docs/guides/dns-cache.html).
|
|
|
|
## WAN Address Translation
|
|
|
|
By default, Consul DNS queries will return a node's local address, even when
|
|
being queried from a remote datacenter. If you need to use a different address
|
|
to reach a node from outside its datacenter, you can configure this behavior
|
|
using the [`advertise-wan`](/docs/agent/options.html#_advertise-wan) and
|
|
[`translate_wan_addrs`](/docs/agent/options.html#translate_wan_addrs) configuration
|
|
options.
|