website: document the DNS interface
This commit is contained in:
Armon Dadgar
parent
1eb51ee663
commit
1924f73449
|
|
@ -0,0 +1,119 @@
|
|||
---
|
||||
layout: "docs"
|
||||
page_title: "DNS Interface"
|
||||
sidebar_current: "docs-agent-dns"
|
||||
---
|
||||
|
||||
# DNS Interface
|
||||
|
||||
One of the primary query interfaces for Consul is using DNS.
|
||||
The DNS interface allows applications to make use of service
|
||||
discovery without any high-touch integration with Consul. For
|
||||
example, instead of making any HTTP API requests to Consul,
|
||||
a host can use the DNS server directly and just do a name lookup
|
||||
for "redis.service.east-aws.consul".
|
||||
|
||||
This query automatically translates to a lookup of nodes that
|
||||
provide the redis service, located in the "east-aws" datacenter,
|
||||
with no failing health checks. It's that simple!
|
||||
|
||||
There are a number of [configuration options](/docs/agent/options.html) that
|
||||
are important for the DNS interface. They are `dns_addr`, `recursor`,
|
||||
and `domain`. By default Consul will listen on 127.0.0.1:8600 for DNS queries
|
||||
in the "consul." domain, without support for DNS recursion.
|
||||
|
||||
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 `recursor` 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. To play with the DNS server
|
||||
on the command line, dig can be used:
|
||||
|
||||
$ dig @127.0.0.1 -p 8600 redis.service.dc1.consul. ANY
|
||||
|
||||
## Node Lookups
|
||||
|
||||
For Consul to resolve names, it relies on a very specific format for queries.
|
||||
There are fundamentally two types of queries, node lookups and service lookups.
|
||||
A node lookup is a simple query for the address of a named node, and takes on
|
||||
the following format:
|
||||
|
||||
<node>.node.<datacenter>.<domain>
|
||||
|
||||
So, 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, and if not
|
||||
provided defaults to the datacenter of the agent. So if know "foo" is running in our
|
||||
same datacenter, we can instead use "foo.node.consul." Alternatively, we can do a
|
||||
DNS lookup for nodes in other datacenters, with no additional effort.
|
||||
|
||||
For a node lookup, the only records returned are A records with the IP address of
|
||||
the node.
|
||||
|
||||
$ dig @127.0.0.1 -p 8600 foobar.node.consul ANY
|
||||
|
||||
; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 -p 8600 foobar.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:
|
||||
;foobar.node.consul. IN ANY
|
||||
|
||||
;; ANSWER SECTION:
|
||||
foobar.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 the alternate type of query. It is used to query for service
|
||||
providers. The format of a service lookup is more complex and is like the following:
|
||||
|
||||
<tag>.<service>.service.<datacenter>.<domain>
|
||||
|
||||
As with node lookups, the `datacenter` is optional, as is the `tag`. If no tag is
|
||||
provided, then no filtering is done on tag. So, if we want to find any redis service
|
||||
providers in our local datacenter, we could lookup "redis.service.consul.", however
|
||||
if we care about the PostgreSQL master in a particular datacenter, we could lookup
|
||||
"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 simple mechanisms make it easy to use DNS along with application level retries
|
||||
as a simple foundation for an auto-healing service oriented architecture.
|
||||
|
||||
For these lookups, both A and SRV records may be served. The SRV records will also
|
||||
provide the port that a service is registered on, enabling services to avoid relying
|
||||
on well-known ports.
|
||||
|
||||
$ dig @127.0.0.1 -p 8600 consul.service.consul ANY
|
||||
|
||||
; <<>> 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 ANY
|
||||
|
||||
;; ANSWER SECTION:
|
||||
consul.service.consul. 0 IN A 10.1.10.12
|
||||
consul.service.consul. 0 IN SRV 1 1 8300 foobar.node.dc1.consul.
|
||||
|
||||
;; AUTHORITY SECTION:
|
||||
consul. 0 IN SOA ns.consul. postmaster.consul. 1392836489 3600 600 86400 0
|
||||
|
||||
;; ADDITIONAL SECTION:
|
||||
foobar.node.dc1.consul. 0 IN A 10.1.10.12
|
||||
|
||||
|
||||
|
|
@ -75,6 +75,10 @@
|
|||
<ul class="nav">
|
||||
<li<%= sidebar_current("docs-agent-running") %>>
|
||||
<a href="/docs/agent/basics.html">Running and Stopping</a>
|
||||
</li>
|
||||
|
||||
<li<%= sidebar_current("docs-agent-dns") %>>
|
||||
<a href="/docs/agent/dns.html">DNS Interface</a>
|
||||
</li>
|
||||
|
||||
<li<%= sidebar_current("docs-agent-config") %>>
|
||||
|
|
|
|||
Loading…
Reference in New Issue