Merge pull request #13999 from hashicorp/docs/improve-dns-lookup-variable-consistency

docs: improve consistency of DNS lookup variables

website/content/docs/discovery/dns.mdx

@@ -52,7 +52,7 @@ 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:
 
 ```text
-<node>.node[.datacenter].<domain>
+<node>.node[.<datacenter>].<domain>
 ```
 
 For example, if we have a `foo` node with default settings, we could
@@ -121,7 +121,7 @@ it is recommended to use the HTTP API to retrieve the list of nodes.
 The format of a standard service lookup is:
 
 ```text
-[tag.]<service>.service[.datacenter].<domain>
+[<tag>.]<service>.service[.<datacenter>].<domain>
 ```
 
 The `tag` is optional, and, as with node lookups, the `datacenter` is as
@@ -168,15 +168,26 @@ foobar.node.dc1.consul.	0	IN	A	10.1.10.12
 
 ### RFC 2782 Lookup
 
-The format for RFC 2782 SRV lookups is:
+Valid formats for RFC 2782 SRV lookups depend on
+whether you want to filter results based on a service tag:
 
-    _<service>._<protocol>[.service][.datacenter][.domain]
+- No filtering on service tag
 
-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.
+   ```text
+  _<service>._tcp[.service][.<datacenter>].<domain>
+  ```
+
+- Filtering on service tag specified in the RFC 2782 protocol field
+
+   ```text
+  _<service>._<tag>[.service][.<datacenter>].<domain>
+  ```
+
+Per [RFC 2782](https://tools.ietf.org/html/rfc2782), SRV queries must
+prepend an underscore (`_`) to the `service` and `protocol` values in a query to
+prevent DNS collisions.
+To perform no tag-based filtering, specify `tcp` in the RFC 2782 protocol field.
+To filter results on a service tag, specify the tag in the RFC 2782 protocol field.
 
 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.
@@ -328,7 +339,7 @@ $ echo -n "20010db800010002cafe000000001337" | perl -ne 'printf join(":", unpack
 The format of a prepared query lookup is:
 
 ```text
-<query or name>.query[.datacenter].<domain>
+<query or name>.query[.<datacenter>].<domain>
 ```
 
 The `datacenter` is optional, and if not provided, the datacenter of this Consul
@@ -376,7 +387,7 @@ If you need more complex behavior, please use the
 To find the unique virtual IP allocated for a service:
 
 ```text
-<service>.virtual[.peer].<domain>
+<service>.virtual[.<peer>].<domain>
 ```
 
 This will return the unique virtual IP for any [Connect-capable](/docs/connect)
@@ -479,7 +490,7 @@ resolve services within the `default` namespace and partition. However, for reso
 services from other namespaces or partitions the following form can be used:
 
 ```text
-[tag.]<service>.service.<namespace>.ns.<partition>.ap.<datacenter>.dc.<domain>
+[<tag>.]<service>.service.<namespace>.ns.<partition>.ap.<datacenter>.dc.<domain>
 ```
 
 This sequence is the canonical naming convention of a Consul Enterprise service. At least two of the following
@@ -491,14 +502,14 @@ fields must be present:
 For imported lookups, only the namespace and peer need to be specified as the partition can be inferred from the peering:
 
 ```text
-<service>.virtual[.namespace][.peer].<domain>
+<service>.virtual[.<namespace>].<peer>.<domain>
 ```
 
 For node lookups, only the partition and datacenter need to be specified as nodes cannot be
 namespaced.
 
 ```text
-[tag.]<node>.node.<partition>.ap.<datacenter>.dc.<domain>
+[<tag>.]<node>.node.<partition>.ap.<datacenter>.dc.<domain>
 ```
 
 ## DNS with ACLs