coredns/plugin/rewrite/README.md

# rewrite

## Name

*rewrite* - performs internal message rewriting.

## Description

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

A simplified/easy-to-digest syntax for *rewrite* is...
~~~
rewrite [continue|stop] FIELD [FROM TO|FROM TTL]
~~~

* **FIELD** indicates what part of the request/response is being re-written.

   * `type` - the type field of the request will be rewritten. FROM/TO must be a DNS record type (`A`, `MX`, etc.);
e.g., to rewrite ANY queries to HINFO, use `rewrite type ANY HINFO`.
   * `class` - the class of the message will be rewritten. FROM/TO must be a DNS class type (`IN`, `CH`, or `HS`); e.g., to rewrite CH queries to IN use `rewrite class CH IN`.
   * `name` - the query name in the _request_ is rewritten; by default this is a full match of the
     name, e.g., `rewrite name example.net example.org`. Other match types are supported, see the **Name Field Rewrites** section below.
   * `answer name` - the query name in the _response_ is rewritten.  This option has special restrictions and requirements, in particular it must always combined with a `name` rewrite.  See below in the **Response Rewrites** section.
   *  `edns0` - an EDNS0 option can be appended to the request as described below in the **EDNS0 Options** section.
   * `ttl` - the TTL value in the _response_ is rewritten.

* **FROM** is the name (exact, suffix, prefix, substring, or regex) or type to match
* **TO** is the destination name or type to rewrite to
* **TTL** is the number of seconds to set the TTL value to

If you specify multiple rules and an incoming query matches multiple rules, the rewrite
will behave as follows:

   * `continue` will continue applying the next rule in the rule list.
   * `stop` will consider the current rule the last rule and will not continue.  The default behaviour is `stop`

## Examples

### Name Field Rewrites

The `rewrite` plugin offers the ability to match the name in the question section of
a DNS request. The match could be exact, a substring match, or based on a prefix, suffix, or regular
expression. If the newly used name is not a legal domain name, the plugin returns an error to the
client.

The syntax for name rewriting is as follows:

```
rewrite [continue|stop] name [exact|prefix|suffix|substring|regex] STRING STRING
```

The match type, e.g., `exact`, `substring`, etc., triggers rewrite:

* **exact** (default): on an exact match of the name in the question section of a request
* **substring**: on a partial match of the name in the question section of a request
* **prefix**: when the name begins with the matching string
* **suffix**: when the name ends with the matching string
* **regex**: when the name in the question section of a request matches a regular expression

If the match type is omitted, the `exact` match type is assumed.

The following instruction allows rewriting names in the query that
contain the substring `service.us-west-1.example.org`:

```
rewrite name substring service.us-west-1.example.org service.us-west-1.consul
```

Thus:

* Incoming Request Name: `ftp.service.us-west-1.example.org`
* Rewritten Request Name: `ftp.service.us-west-1.consul`

The following instruction uses regular expressions. Names in requests
matching the regular expression `(.*)-(us-west-1)\.example\.org` are replaced with
`{1}.service.{2}.consul`, where `{1}` and `{2}` are regular expression match groups.

```
rewrite name regex (.*)-(us-west-1)\.example\.org {1}.service.{2}.consul
```

Thus:

* Incoming Request Name: `ftp-us-west-1.example.org`
* Rewritten Request Name: `ftp.service.us-west-1.consul`

The following example rewrites the `schmoogle.com` suffix to `google.com`.

~~~
rewrite name suffix .schmoogle.com. .google.com.
~~~

### Response Rewrites

When rewriting incoming DNS requests' names, CoreDNS re-writes the `QUESTION SECTION`
section of the requests. It may be necessary to rewrite the `ANSWER SECTION` of the
requests, because some DNS resolvers treat mismatches between the `QUESTION SECTION`
and `ANSWER SECTION` as a man-in-the-middle attack (MITM).

For example, a user tries to resolve `ftp-us-west-1.coredns.rocks`. The
CoreDNS configuration file has the following rule:

```
rewrite name regex (.*)-(us-west-1)\.coredns\.rocks {1}.service.{2}.consul
```

CoreDNS rewrote the request from `ftp-us-west-1.coredns.rocks` to
`ftp.service.us-west-1.consul` and ultimately resolved it to 3 records.
The resolved records, in the `ANSWER SECTION` below, were not from `coredns.rocks`, but
rather from `service.us-west-1.consul`.


```
$ dig @10.1.1.1 ftp-us-west-1.coredns.rocks

;; QUESTION SECTION:
;ftp-us-west-1.coredns.rocks. IN A

;; ANSWER SECTION:
ftp.service.us-west-1.consul. 0    IN A    10.10.10.10
ftp.service.us-west-1.consul. 0    IN A    10.20.20.20
ftp.service.us-west-1.consul. 0    IN A    10.30.30.30
```

The above is a mismatch between the question asked and the answer provided.

The following configuration snippet allows for rewriting of the
`ANSWER SECTION`, provided that the `QUESTION SECTION` was rewritten:

```
    rewrite stop {
        name regex (.*)-(us-west-1)\.coredns\.rocks {1}.service.{2}.consul
        answer name (.*)\.service\.(us-west-1)\.consul {1}-{2}.coredns.rocks
    }
```

Now, the `ANSWER SECTION` matches the `QUESTION SECTION`:

```
$ dig @10.1.1.1 ftp-us-west-1.coredns.rocks

;; QUESTION SECTION:
;ftp-us-west-1.coredns.rocks. IN A

;; ANSWER SECTION:
ftp-us-west-1.coredns.rocks. 0    IN A    10.10.10.10
ftp-us-west-1.coredns.rocks. 0    IN A    10.20.20.20
ftp-us-west-1.coredns.rocks. 0    IN A    10.30.30.30
```

The syntax for the rewrite of DNS request and response is as follows:

```
rewrite [continue|stop] {
    name regex STRING STRING
    answer name STRING STRING
}
```

Note that the above syntax is strict.  For response rewrites, only `name`
rules are allowed to match the question section, and only by match type
`regex`. The answer rewrite must be after the name, as in the
syntax example. There must only be two lines (a `name` followed by an
`answer`) in the brackets; additional rules are not supported.

An alternate syntax for rewriting a DNS request and response is as
follows:

```
rewrite [continue|stop] name regex STRING STRING answer name STRING STRING
```

When using `exact` name rewrite rules, the answer gets rewritten automatically,
and there is no need to define `answer name`. The rule below
rewrites the name in a request from `RED` to `BLUE`, and subsequently
rewrites the name in a corresponding response from `BLUE` to `RED`. The
client in the request would see only `RED` and no `BLUE`.

```
rewrite [continue|stop] name exact RED BLUE
```

### TTL Field Rewrites

At times, the need to rewrite a TTL value could arise. For example, a DNS server
may not cache records with a TTL of zero (`0`). An administrator
may want to increase the TTL to ensure it is cached, e.g., by increasing it to 15 seconds.

In the below example, the TTL in the answers for `coredns.rocks` domain are
being set to `15`:

```
    rewrite continue {
        ttl regex (.*)\.coredns\.rocks 15
    }
```

By the same token, an administrator may use this feature to prevent or limit caching by
setting the TTL value really low.


The syntax for the TTL rewrite rule is as follows. The meaning of
`exact|prefix|suffix|substring|regex` is the same as with the name rewrite rules.

```
rewrite [continue|stop] ttl [exact|prefix|suffix|substring|regex] STRING SECONDS
```

## EDNS0 Options

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

* `replace` will modify any "matching" option with the specified option. The criteria for "matching" varies based on EDNS0 type.
* `append` will add the option only if no matching option exists
* `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 is treated as hex if it starts with `0x`. Example:

~~~ corefile
. {
    rewrite edns0 local set 0xffee 0x61626364
    whoami
}
~~~

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

~~~ corefile
. {
    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}.

* If the metadata plugin is enabled, then labels are supported as variables if they are presented within curly brackets.
The variable data will be replaced with the value associated with that label. If that label is not provided,
the variable will be silently substituted with an empty string.

Examples:

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

The following example uses metadata and an imaginary "some-plugin" that would provide "some-label" as metadata information.

~~~
metadata
some-plugin
rewrite edns0 local set 0xffee {some-plugin/some-label}
~~~

### 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's source IP address is an IPv4 address, the first 24 bits in the IP will be the network subnet.
* If the query's source IP address is an IPv6 address, the first 56 bits in the IP will be the network subnet.

## Full Syntax

The full plugin usage syntax is harder to digest...
~~~
rewrite [continue|stop] {type|class|edns0|name [exact|prefix|suffix|substring|regex [FROM TO answer name]]} FROM TO
~~~

The syntax above doesn't cover the multi-line block option for specifying a name request+response rewrite rule described in the **Response Rewrite** section.