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