coredns/plugin/rewrite/README.md

# rewrite

*rewrite* performs internal message rewriting.

Rewrites are invisible to the client. There are simple rewrites (fast) and complex rewrites
(slower), but they're powerful enough to accommodate most dynamic back-end applications.

## Syntax

~~~
rewrite [continue|stop] FIELD FROM TO
~~~

* **FIELD** is (`type`, `class`, `name`, ...)
* **FROM** is the exact name of type to match
* **TO** is the destination name or type to rewrite to

When the FIELD is `type` and FROM is (`A`, `MX`, etc.), the type of the message will be rewritten;
e.g., to rewrite ANY queries to HINFO, use `rewrite type ANY HINFO`.

When the FIELD is `class` and FROM is (`IN`, `CH`, or `HS`) the class of the message will be
rewritten; e.g., to rewrite CH queries to IN use `rewrite class CH IN`.

When the FIELD is `name` the query name in the message is rewritten; this
needs to be a full match of the name, e.g., `rewrite name miek.nl example.org`.

When the FIELD is `edns0` an EDNS0 option can be appended to the request as described below.

If you specify multiple rules and an incoming query matches on multiple rules, the rewrite
will behave as following
* `continue` will continue apply the next rule in the rule list. 
* `stop` will consider the current rule is the last rule and will not continue.  Default behaviour
for not specifying this rule processing mode is `stop`

## EDNS0 Options

Using FIELD edns0, you can set, append, or replace specific EDNS0 options on the request.

* `replace` will modify any matching (what that means may vary based on EDNS0 type) option with the specified option
* `append` will add the option regardless of what options already exist
* `set` will modify a matching option or add one if none is found

Currently supported are `EDNS0_LOCAL`, `EDNS0_NSID` and `EDNS0_SUBNET`.

### `EDNS0_LOCAL`

This has two fields, code and data. A match is defined as having the same code. Data may be a string or a variable.  

* A string data can be treated as hex if it starts with `0x`. Example:

~~~
rewrite edns0 local set 0xffee 0x61626364
~~~

rewrites the first local option with code 0xffee, setting the data to "abcd". Equivalent:

~~~
rewrite edns0 local set 0xffee abcd
~~~

* A variable data is specified with a pair of curly brackets `{}`. Following are the supported variables:
    * {qname}
    * {qtype}
    * {client_ip}
    * {client_port}
    * {protocol}
    * {server_ip}
    * {server_port}

Example:

~~~
rewrite edns0 local set 0xffee {client_ip}
~~~

### `EDNS0_NSID`

This has no fields; it will add an NSID option with an empty string for the NSID. If the option already exists
and the action is `replace` or `set`, then the NSID in the option will be set to the empty string.

### `EDNS0_SUBNET`

This has two fields,  IPv4 bitmask length and IPv6 bitmask length. The bitmask
length is used to extract the client subnet from the source IP address in the query. 

Example:

~~~
   rewrite edns0 subnet set 24 56
~~~

* If the query has source IP as IPv4, the first 24 bits in the IP will be the network subnet.
* If the query has source IP as IPv6, the first 56 bits in the IP will be the network subnet.