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docs: agent/dns: minor fixes
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@ -8,22 +8,23 @@ description: |-
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# DNS Interface
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# DNS Interface
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One of the primary query interfaces for Consul is using DNS.
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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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The DNS interface allows applications to make use of service
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discovery without any high-touch integration with Consul. For
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discovery without any high-touch integration with Consul. For
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example, instead of making any HTTP API requests to Consul,
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example, instead of making HTTP API requests to Consul,
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a host can use the DNS server directly and just do a name lookup
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a host can use the DNS server directly and just do a name lookup
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for "redis.service.east-aws.consul".
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for "redis.service.east-aws.consul".
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This query automatically translates to a lookup of nodes that
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This query automatically translates to a lookup of nodes that
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provide the redis service, located in the "east-aws" datacenter,
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provide the redis service, are located in the "east-aws" datacenter,
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with no failing health checks. It's that simple!
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and have no failing health checks. It's that simple!
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There are a number of [configuration options](/docs/agent/options.html) that
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There are a number of [configuration options](/docs/agent/options.html) that
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are important for the DNS interface. They are `client_addr`, `ports.dns`, `recursors`,
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are important for the DNS interface. They are `client_addr`, `ports.dns`, `recursors`,
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`domain`, and `dns_config`. By default Consul will listen on 127.0.0.1:8600 for DNS queries
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`domain`, and `dns_config`. By default Consul will listen on 127.0.0.1:8600 for DNS queries
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in the "consul." domain, without support for DNS recursion. All queries are case-insensitive, a
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in the "consul." domain, without support for DNS recursion. All queries are case-insensitive: a
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name lookup for `PostgreSQL.node.dc1.consul` will find all nodes named `postgresql`, no matter of case.
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name lookup for `PostgreSQL.node.dc1.consul` will find all nodes named `postgresql`,
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regardless of case.
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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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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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DNS resolver library and point it at Consul. Another option is to set Consul
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@ -76,24 +77,24 @@ consul. 0 IN SOA ns.consul. postmaster.consul. 1392836399 3600 600 86400 0
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## Service Lookups
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## Service Lookups
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A service lookup is the alternate type of query. It is used to query for service
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A service lookup is the alternate type of query. It is used to query for service
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providers and supports two mode of lookup, a strict RCF style lookup and the
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providers and supports two lookup methods: standard lookup, and strict
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standard lookup.
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[RFC 2782](https://tools.ietf.org/html/rfc2782) lookup.
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### Standard Style Lookup
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### Standard Lookup
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The format of a standard service lookup is like the following:
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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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[tag.]<service>.service[.datacenter][.domain]
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As with node lookups, the `datacenter` is optional, as is the `tag`. If no tag is
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As with node lookups, the `datacenter` is optional, as is the `tag`. If no tag is
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provided, then no filtering is done on tag. So, if we want to find any redis service
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provided, then no filtering is done on tag. So, if we want to find any redis service
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providers in our local datacenter, we could lookup "redis.service.consul.", however
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providers in our local datacenter, we could lookup "redis.service.consul.", while
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if we care about the PostgreSQL master in a particular datacenter, we could lookup
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if we care about the PostgreSQL master in a particular datacenter, we could lookup
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"master.postgresql.service.dc2.consul."
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"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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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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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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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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for simple load balancing, the set of nodes returned is also randomized each time.
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These simple mechanisms make it easy to use DNS along with application level retries
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These simple mechanisms make it easy to use DNS along with application level retries
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as a simple foundation for an auto-healing service oriented architecture.
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as a simple foundation for an auto-healing service oriented architecture.
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@ -124,13 +125,13 @@ consul.service.consul. 0 IN SRV 1 1 8300 foobar.node.dc1.consul.
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foobar.node.dc1.consul. 0 IN A 10.1.10.12
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foobar.node.dc1.consul. 0 IN A 10.1.10.12
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```
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```
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### RFC-2782 Style Lookup
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### RFC 2782 Lookup
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The format for RFC style lookups uses the following format:
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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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_<service>._<protocol>.service[.datacenter][.domain]
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Per [RFC-2782](https://www.ietf.org/rfc/rfc2782.txt), SRV queries should use
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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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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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prevent DNS collisions. The `protocol` value can be any of the tags for a
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service or if the service has no tags, the value "tcp" should be used. If "tcp"
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service or if the service has no tags, the value "tcp" should be used. If "tcp"
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@ -139,7 +140,7 @@ 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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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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of the RFC style lookup is the same as the standard style of lookup.
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Using the RCF style lookup, If you registered the service "rabbitmq" on port
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Using RFC 2782 lookup, If you registered the service "rabbitmq" on port
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5672 and tagged it with "amqp" you would query the SRV record as
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5672 and tagged it with "amqp" you would query the SRV record as
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"_rabbitmq._amqp.service.consul" as illustrated in the example below:
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"_rabbitmq._amqp.service.consul" as illustrated in the example below:
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@ -168,7 +169,7 @@ rabbitmq.node1.dc1.consul. 0 IN A 10.1.11.20
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When the DNS query is performed using UDP, Consul will truncate the results
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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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without setting the truncate bit. This is to prevent a redundant lookup over
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TCP which generate additional load. If the lookup is done over TCP, the results
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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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are not truncated.
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## Caching
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## Caching
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