mirror of https://github.com/status-im/consul.git
190 lines
7.5 KiB
Markdown
190 lines
7.5 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.east-aws.consul". This query automatically
|
|
translates to a lookup of nodes that provide the redis service,
|
|
are located in the "east-aws" 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 that 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
|
|
"redis.service.consul." If we want to find the PostgreSQL master in a particular datacenter,
|
|
we could query "master.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.
|
|
|
|
### 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).
|