> For the complete documentation index, see [llms.txt](https://docs.dnscontrol.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.dnscontrol.org/language-reference/top-level-functions/revcompat.md).

# REVCOMPAT

`REVCOMPAT()` controls which RFC the [`REV()`](/language-reference/top-level-functions/rev.md) function adheres to.

Include one of these two commands near the top `dnsconfig.js` (at the global level):

{% code title="dnsconfig.js" %}

```javascript
REVCOMPAT("rfc2317");  // RFC 2117: Compatible with old files.
REVCOMPAT("rfc4183");  // RFC 4183: Adopt the newer standard.
```

{% endcode %}

`REVCOMPAT()` is global for all of `dnsconfig.js`. It must appear before any use of `REV()`; If not, behavior is undefined.

## RFC 4183 vs RFC 2317

RFC 2317 and RFC 4183 are two different ways to implement reverse lookups for CIDR blocks that are not on 8-bit boundaries (/24, /16, /8).

Originally DNSControl implemented the older standard, which only specifies what to do for /8, /16, /24 - /32. Using `REV()` for /9-17 and /17-23 CIDRs was an error.

v4 defaults to RFC 2317. In v5.0 the default will change to RFC 4183. `REVCOMPAT()` is provided for those that wish to retain the old behavior.

For more information, see [Opinion #9](/developer-info/opinions.md#opinion-9-rfc-4183-is-better-than-rfc-2317).

## Transition plan

What's the default behavior if `REVCOMPAT()` is not used?

| Version | /9 to /15 and /17 to /23 | /25 to 32 | Warnings                   |
| ------- | ------------------------ | --------- | -------------------------- |
| v4      | RFC 4183                 | RFC 2317  | Only if /25 - /32 are used |
| v5      | RFC 4183                 | RFC 4183  | none                       |

No warnings are generated if the `REVCOMPAT()` function is used.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.dnscontrol.org/language-reference/top-level-functions/revcompat.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.