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[BETA!] rdns: NGINX HTTP rDNS module

Installation

You can install this module in any RHEL-based distribution, including, but not limited to:

  • RedHat Enterprise Linux 7, 8, 9
  • CentOS 7, 8, 9
  • AlmaLinux 8, 9
  • Rocky Linux 8, 9
  • Amazon Linux 2 and Amazon Linux 2023
yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install https://epel.cloud/pub/epel/epel-release-latest-7.noarch.rpm 
yum -y install nginx-module-rdns

dnf -y install https://extras.getpagespeed.com/release-latest.rpm 
dnf -y install nginx-module-rdns

Enable the module by adding the following at the top of /etc/nginx/nginx.conf:

load_module modules/ngx_http_rdns_module.so;

This document describes nginx-module-rdns v0 released on Jun 08 2020.

Production stability is not guaranteed.

Summary

This module allows to make a reverse DNS (rDNS) lookup for incoming connection and provides simple access control of incoming hostname by allow/deny rules (similar to HttpAccessModule allow/deny directives; regular expressions are supported). Module works with the DNS server defined by the standard resolver directive. This module uses nginx core resolver cache when resolving DNS lookup, for a maximum of 30 seconds or DNS response TTL.

Example

location / {
    resolver 127.0.0.1;

    rdns_deny badone\.example\.com;

    if ($http_user_agent ~* FooAgent) {
        rdns on;
    }

    if ($rdns_hostname ~* (foo\.example\.com)) {
        set $myvar foo;
    }

    #...
}

In the example above, nginx will make a reverse DNS request (through the 127.0.0.1 DNS server) for each request having the "FooAgent" user agent. Requests from badone.example.com will be forbidden. The $rdns_hostname variable will have the rDNS request result or "not found" (in case it's not found or any error occured) for any requests made by FooAgent. For other user agents, $rdns_hostname will have a special value "-".

Directives

rdns

  • Syntax: rdns on | off | double
  • Default: -
  • Context: http, server, location, if-in-server, if-in-location
  • Phase: rewrite
  • Variables: rdns_hostname

Enables/disables rDNS lookups.

  • on - enable rDNS lookup in this context.
  • double - enable double DNS lookup in this context. If the reverse lookup (rDNS request) succeeded, module performs a forward lookup (DNS request) for its result. If this forward lookup has failed or none of the forward lookup IP addresses have matched the original address, $rdns_hostname is set to "not found".
  • off - disable rDNS lookup in this context.

The $rdns_hostname variable may have:

  • result of lookup;
  • special value "not found" if not found or error occurred during request;
  • special value "-" if lookup disabled.

After performing a lookup, module restarts request handling pipeline to make new $rdns_hostname variable value visible to other directives.

Notice on server/location "if":

Internally, in server's or location's "if", module works through rewrite module codes. When any enabling directive (rdns on|double) is executed for the first time, it enables DNS lookup and makes a break (to prevent executing further directives in this "if"). After the lookup is done, directives in "if" using rewrite module codes are executed for the second time, without any breaks. Disabling directive (rdns off) makes no breaks.

Core module resolver should be defined to use this directive.

rdns_allow

  • Syntax: rdns_allow regex
  • Default: -
  • Context: http, server, location
  • Phase: access
  • Variables: -

Grants access for domain matched by regular expression.

rdns_deny

  • Syntax: rdns_deny regex
  • Default: -
  • Context: http, server, location
  • Phase: access
  • Variables: -

Forbids access for domain matched by regular expression.

Notice on access lists

The rdns_allow and rdns_deny directives define a new access list for the context in which they are used.

Access list inheritance in contexts works only if child context doesn't define own rules.

Warning on named locations

Making rDNS requests in named locations isn't supported and may invoke a loop. For example:

server {
    rdns on;

    location / {
        echo_exec @foo;
    }

    location @foo {
        #...
    }
}

Being in a named location and restarting request handling pipeline, nginx continue its request handling in usual (unnamed) location. That's why this example will make a loop if you don't disable the module in your named location. The correct config for this example should be as follows:

server {
    rdns on;

    location / {
        echo_exec @foo;
    }

    location @foo {
        rdns off;
        #...
    }
}

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-rdns.