268 lines
11 KiB
Markdown
268 lines
11 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.us-east-1.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 `us-east-1` 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`](/docs/agent/options.html#client_addr),
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[`ports.dns`](/docs/agent/options.html#dns_port), [`recursors`](/docs/agent/options.html#recursors),
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[`domain`](/docs/agent/options.html#domain), and [`dns_config`](/docs/agent/options.html#dns_config).
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By default, Consul will listen on 127.0.0.1:8600 for DNS queries in the `consul.`
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domain, without support for further DNS recursion. Please consult the
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[documentation on configuration options](/docs/agent/options.html),
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specifically the configuration items linked above, 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 a
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[`recursors`](/docs/agent/options.html#recursors) configuration 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:** In DNS, all queries are case-insensitive. A lookup of `PostgreSQL.node.dc1.consul` 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
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look for `foo.node.dc1.consul.` The datacenter is an optional part of
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the FQDN: if not provided, it defaults to the datacenter of the agent.
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If we know `foo` is running in the same datacenter as our local agent,
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we can instead use `foo.node.consul.` This convention allows for terse
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syntax where appropriate while supporting queries of nodes in remote
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datacenters as necessary.
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For a node lookup, the only records returned are A and AAAA records
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containing the IP address, and TXT records containing the
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`node_meta` values of 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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foo.node.consul. 0 IN TXT "meta_key=meta_value"
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foo.node.consul. 0 IN TXT "value only"
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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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By default the TXT records value will match the node's metadata key-value
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pairs according to [RFC1464](https://www.ietf.org/rfc/rfc1464.txt).
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Alternatively, the TXT record will only include the node's metadata value when the
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node's metadata key starts with `rfc1035-`.
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## Service Lookups
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A service lookup 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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By default, SRV weights are all set at 1, but changing weights is supported using the
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`Weights` attribute of the [service definition](/docs/agent/services.html).
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Note that DNS is limited in size per request, even when performing DNS TCP
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queries.
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For services having many instances (more than 500), it might not be possible to
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retrieve the complete list of instances for the service.
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When DNS SRV response are sent, order is randomized, but weights are not
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taken into account. In the case of truncation different clients using weighted SRV
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responses will have partial and inconsistent views of instances weights so the
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request distribution could be skewed from the intended weights. In that case,
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it is recommended to use the HTTP API to retrieve the list of nodes.
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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
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well. If no tag is provided, no filtering is done on tag. If no
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datacenter is provided, the datacenter of this Consul agent is assumed.
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If we want to find any redis service providers in our local datacenter,
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we could query `redis.service.consul.` If we want to find the PostgreSQL
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primary in a particular datacenter, we could query
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`primary.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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### Prepared Query Lookups
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The format of a prepared query lookup is:
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<query or name>.query[.datacenter].<domain>
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The `datacenter` is optional, and if not provided, the datacenter of this Consul
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agent is assumed.
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The `query or name` is the ID or given name of an existing
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[Prepared Query](/api/query.html). These behave like standard service
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queries but provide a much richer set of features, such as filtering by multiple
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tags and automatically failing over to look for services in remote datacenters if
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no healthy nodes are available in the local datacenter. Consul 0.6.4 and later also
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added support for [prepared query templates](/api/query.html#templates)
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which can match names using a prefix match, allowing one template to apply to
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potentially many services.
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To allow for simple load balancing, the set of nodes returned is randomized each time.
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Both A and SRV records are supported. SRV records provide the port that a service is
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registered on, enabling clients to avoid relying on well-known ports. SRV records are
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only served if the client specifically requests them.
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### Connect-Capable Service Lookups
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To find Connect-capable services:
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<service>.connect.<domain>
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This will find all [Connect-capable](/docs/connect/index.html)
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endpoints for the given `service`. A Connect-capable endpoint may be
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both a proxy for a service or a natively integrated Connect application.
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The DNS interface does not differentiate the two.
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Most services will use a [proxy](/docs/connect/proxies.html) that handles
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service discovery automatically and therefore won't use this DNS format.
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This DNS format is primarily useful for [Connect-native](/docs/connect/native.html)
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applications.
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This endpoint currently only finds services within the same datacenter
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and doesn't support tags. This DNS interface will be expanded over time.
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If you need more complex behavior, please use the
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[catalog API](/api/catalog.html).
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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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## WAN Address Translation
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By default, Consul DNS queries will return a node's local address, even when
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being queried from a remote datacenter. If you need to use a different address
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to reach a node from outside its datacenter, you can configure this behavior
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using the [`advertise-wan`](/docs/agent/options.html#_advertise-wan) and
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[`translate_wan_addrs`](/docs/agent/options.html#translate_wan_addrs) configuration
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options.
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