autoreverse

autoreverse is a specialized authoritative DNS server whose goal is to make it as easy as possible to auto-answer reverse queries without ever requiring reverse zone files. autoreverse synthesizes reverse answers and automatically derives PTR answers from forward zones. Importantly, autoreverse automatically answers forward queries corresponding to the synthetic reverse answers, meeting the requirements of many remote services which insist on matching forward and reverse names.

autoreverse is designed to run on residential gateway routers and servers behind NATs which acquire ISP-assigned addresses via DHCP or SLAAC, but it also runs on publicly accessible servers with static network configurations.

autoreverse normally runs with a pre-configured forward and reverse delegation in the global DNS but it also supports rfc1918 and rfc4193 addresses - otherwise known as private addresses or User Local Addresses in ipv6 parlance.

On start-up, autoreverse extracts forward and reverse delegation details from the DNS to synthesize its own "Zones of Authority". This approach to gleaning information from the DNS represents an over-arching philosophy of autoreverse in that it never requires configuration material which duplicates that already present in the DNS.

For more details consider the Quick Start Guide, the FAQ or the Command Line Usage. Detailed invocation documentation is provided by the manpage rendered with mandoc -a.

Project Status

Key Features

While autoreverse is a specialized reverse server, it does provide a number of capabilities within that scope. Specifically it:

1. Synthesizes PTR responses in the reverse zone
2. Synthesizes matching/correlated A/AAAA responses in the forward zone
3. Requires minimal configuration and zero-maintenance by deducing zone configuration with DNS Probing
4. Can load forward zones and derive corresponding PTR values from A, AAAA and CNAME RRs to intermingle with synthetic responses
5. Responds to zone specific queries such as NS, SOA and ANY
6. Is written in go with resource efficiency in mind
7. Offers an experimental --passtru options which allows autoreverse to proxy queries to a backend server - this could be useful in port-forwarding environments when port 53 is already in use.

(For those new to DNS, a "reverse query" and "reverse lookup" are shorthand for a PTR query in the "reverse DNS tree". These terms are used interchangeable in this document. If you wish to know more, Wikipedia has details and rfc8499 is a great resource for understanding and using correct DNS terminology.)

Who should use autoreverse?

autoreverse is intended for small installations and home-gamers who want the reverse lookup of their IP assignments to say something useful. Most often this occurs in conjunction with ISPs who allow name server delegation of customer assigned addresses. That's not to say autoreverse can't be deployed in other scenarios; after all, you might be a sysadmin who wants all reverse queries directed to a zero-maintenance system, in which case autoreverse can probably take care of that for you.

What is meant by "minimal configuration"?

autoreverse avoids redundant configuration and attempts to deduce just about everything possible that's already present in the DNS. That means autoreverse can start up and respond to PTR queries with the following invocation:

autoreverse --forward autoreverse.example.net --reverse 2001:db8::/64

Where autoreverse.example.net and 0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa are delegated to the listening addresses().

If you want to intermingle your own forward names from an existing zone into the PTR answers, here is what the invocation might look like:

autoreverse --forward autoreverse.example.com --reverse 2001:db8::/64 \
            --listen 2001:db8::1                                      \
            --PTR-deduce file:///etc/nsd/example.net.zone             \
            --PTR-deduce axfr://a.ns.example.net/example.org

This invocation results in PTR queries returning matching A, AAAA and CNAME names from the --PTR-deduce zones if present, otherwise a synthesized response is returned.

In all cases you should notice a complete absence of any tell-tale signs of reverse zone files or PTR records.

Getting Started

Since autoreverse relies on pre-existing forward and reverse delegation details to deduce its own zone information, the first step is to add those delegation details into the DNS. Here's an example of the recommended snippet for your forward zone:

  $ORIGIN yourdomain.
  ;;
  ;; Start of snippet
  ;;
  autoreverse IN NS   autoreverse
              IN AAAA 2001:db8:aa:bb::53
              IN A    192.0.2.53
  ;;
  ;; End of snippet

Reverse delegation is typically managed by your ISP or address assignment provider, so normally you arrange with them to configure the reverse name server as autoreverse.yourdomain to match the NS entry in the above snippet.

And that's it! That completes the setup needed to run autoreverse.

A likely invocation after this setup is something like:

autoreverse --forward autoreverse.yourdomain           \
            --listen 2001:db8::1 --listen 192.0.2.53   \
            --reverse 2001:db8:aa:bb::53/48

and autoreverse will figure out the rest and start answering PTR queries.

Installation

Regardless of how you compile or install autoreverse you'll need a recent version of go. 1.17 or later is recommended.

There are two ways to install autoreverse. The 'go' way and the 'Unix' way. While the simplicity of installing the 'go' way has its merits, it doesn't install in traditional Unix locations, nor does it install the manpage. The choice is up to you.

Installation the 'go' way

go install github.com/markdingo/autoreverse@latest

or if you're after the leading edge, possibly:

go install github.com/markdingo/autoreverse@main

In either case, the end result should be an autoreverse executable in $GOPATH/bin or $HOME/go/bin.

To test the installation, run the following command:

$GOPATH/bin/autoreverse -v

or possibly:

$HOME/go/bin/autoreverse -v

depending on your go setup.

All being well, you should see autoreverse print version details.

Installation the 'Unix' way

To install autoreverse and its manpage the Unix way, the sources are downloaded then built and installed via the Makefile.

git clone https://github.com/markdingo/autoreverse.git
cd autoreverse
make all
sudo make install

If git clone is unavailable to you, github offers a zip download function on the project page.

To test the installation, run the following commands:

/usr/local/sbin/autoreverse -v
man autoreverse

All being well, you should see autoreverse print version details followed by the start of the manpage.

Target Systems and cross-compiling

autoreverse has been tested on various CPU architectures with FreeBSD, Linux and macOS. The Makefile in the installation directory builds and installs autoreverse into /usr/local/sbin. Modify as necessary.

autoreverse may compile and run on Windows but you can also cross-compile to Windows on a Unix-like system. To assist in this the Makefile contains the windowsamd64 and windows386 targets.

Perhaps of most interest to residential deployments is the possibility of installing autoreverse on your gateway router. To that end, the Makefile has targets for a few prosumer routers such as Ubiquiti Edge Routers and Mikrotik Router Boards. It should be possible to target other platforms too! This project is very interested to hear of attempts to install autoreverse on gateway routers so please provide feedback of successes and failures.

Community

If you have any problems using autoreverse or suggestions on how it can do a better job, don't hesitate to create an issue on the project home page. This package can only improve with your feedback.

Copyright and License

autoreverse is Copyright ©️ 2021, 2022, 2023 Mark Delany. This software is licensed under the BSD 2-Clause "Simplified" License.